Laser Types and Operation

A. Basic Laser Operation:

The term LASER is an acronym. It stands for Light Amplification by Stimulated Emission of Radiation. Thus the laser is a device which produces and amplifies light. The mechanism by which this is accomplished, stimulated emission, was first postulated by Albert Einstein in 1917. The light which the laser produces is unique, for it is characterized by properties which are very desirable, but almost impossible to obtain by any means other than the laser.

To gain a better understanding of the laser and what it can do, a review is included of some of the phenomena involved.

B. Energy Levels:

Light can be produced by atomic processes, and it is these processes which are responsible for the generation of laser light. Let’s look first at atomic energy levels and then see how changes in these energy levels can lead to the production of laser light.

A number of simplifications will be made regarding the concept of the atom. It can be assumed, for the purposes of this discussion, that an atom consists of a small dense nucleus and one or more electrons in motion about the nucleus.

The relationship between the electrons and the nucleus is described in terms of energy levels. Quantum mechanics predicts that these energy levels are discrete.

C. Radiative Transitions:

The electrons normally occupy the lowest available energy levels. When this is the case, the atom is said to be in its ground state. However, electrons can occupy higher energy levels, leaving some of the lower energy states vacant or sparsely populated.

One way that electrons and atoms can change from one energy state to another is by the absorption or emission of light energy, via a process called a radiative transition.

D. Absorption:

An electron can absorb energy from a variety of external sources. From the point of view of laser action, two methods of supplying energy to the electrons are of prime importance. The first of these is the transfer of all the energy of a photon directly to an orbital electron. The increase in the energy of the electron causes it to “jump” to a higher energy level; the atom is then said to be in an “excited” state. It is important to note that an electron can accept only the precise amount of energy that is needed to move it from one allowable energy level to another. Only photons of the exact energy acceptable to the electron can be absorbed. Photons of slightly more (or slightly less) energy will not be absorbed.

Another means often used to excite electrons is an electrical discharge. In this technique, the energy is supplied by collisions with electrons which have been accelerated by an electric field. The result of either type of excitation is that through the absorption of energy, an electron has been placed in a higher energy level than it originally resided. As a result, the atom of which it is a part is said to be excited.

E. Spontaneous Emission:

The nature of all matter is such that atomic and molecular structures tend to exist in the lowest energy state possible. Thus, an excited electron in a higher energy level will soon attempt to DE-EXCITE itself by any of several means. Some of the energy may be converted to heat.

Another means of de-excitation is the spontaneous emission of a photon. The photon released by an atom as it is de-excited will have a total energy exactly equal to the difference in energy between the excited and lower energy levels. This release of a photon is called spontaneous emission. One example of spontaneous emission is the common neon sign. Atoms of neon are excited by an electrical discharge through the tube. They de-excite themselves by spontaneously emitting photons of visible light.

NOTE: The exciting force is not of a unique energy, so that the electrons may be excited to any one of several allowable levels.

Now let’s look at the third, and probably the least familar, type of radiative transition.

F. Stimulated Emission:

In 1917, Einstein postulated that a photon released from an excited atom could, upon interacting with a second, similarly excited atom, trigger the second atom into de-exciting itself with the release of another photon. The photon released by the second atom would be identical in frequency, energy, direction, and phase with the triggering photon, and the triggering photon would continue on its way, unchanged. Where there was one photon now there are two. These two photons could then proceed to trigger more through the process of stimulated emission.

If an appropriate medium contains a great many excited atoms and de-excitation occurs only by spontaneous emission, the light output will be random and approximately equal in all directions. The process of stimulated emission, however, can cause an amplification of the number of photons traveling in a particular direction – a photon cascade if you will.

A preferential direction is established by placing mirrors at the ends of an optical cavity. Thus the number of photons traveling along the axis of the two mirrors increases greatly and Light Amplification by the Stimulated Emission of Radiation may occur. If enough amplification occurs, LASER beam is created.

G. Population Inversion:

Practically speaking, the process of stimulated emission will not produce a very efficient or even noticeable amplification of light unless a condition called “population inversion” occurs. If only a few atoms of several million are in an excited state, the chances of stimulated emission occurring are small. The greater the percentage of atoms in an excited state, the greater the probability of stimulated emission. In the normal state of matter the population of electrons will be such that most of the electrons reside in the ground or lowest levels, leaving the upper levels somewhat depopulated. When electrons are excited and fill these upper levels to the extent that there are more atoms excited than not excited, the population is said to be inverted.

H. Laser Components:

A generalized laser consists of a lasing medium, a “pumping” system and an optical cavity. The laser material must have a metastable state in which the atoms or molecules can be trapped after receiving energy from the pumping system. Each of these laser components are discussed below:

1. Pumping Systems:

The pumping system imparts energy to the atoms or molecules of the lasing medium enabling them to be raised to an excited “metastable state” creating a population inversion. Optical pumping uses photons provided by a source such as a Xenon gas flash lamp or another laser to transfer energy to the lasing material. The optical source must provide photons which correspond to the allowed transition levels of the lasing material.

Collision pumping relies on the transfer of energy to the lasing material by collision with the atoms (or molecules) of the lasing material. Again, energies which correspond to the allowed transitions must be provided. This is often done by electrical discharge in a pure gas or gas mixture in a tube.

Chemical pumping systems use the binding energy released in chemical reactions to state.

2. Optical Cavity:

An optical cavity is required to provide the amplification desired in the laser and to select the photons which are traveling in the desired direction. As the first atom or molecule in the metastable state of the inverted population decays, it triggers via stimulated emission, the decay of another atom or molecule in the metastable state. If the photons are traveling in a direction which leads to the walls of the lasing material, which is usually in the form of a rod or tube, they are lost and the amplification process terminates. They may actually be reflected at the wall of the rod or tube, but sooner or later they will be lost in the system and will not contribute to the beam.

If, on the other hand, one of the decaying atoms or molecules releases a photon parallel to the axis of the lasing material, it can trigger the emission of another photon and both will be reflected by the mirror on the end of the lasing rod or tube. The reflected photons then pass back through the material triggering further emissions along exactly the same path which are reflected by the mirrors on the ends of the lasing material. As this amplification process continues, a portion of the radiation will always escape through the partially reflecting mirror. When the amount of amplification or gain through this process exceeds the losses in the cavity, laser oscillation is said to occur. In this way, a narrow concentrated beam of coherent light is formed.

The mirrors on the laser optical cavity must be precisely aligned for light beams parallel to the axis. The optical cavity itself, i.e., the lasing medium material must not be a strong absorber of the light energy.

3. Laser Media:

Lasers are commonly designated by the type of lasing material employed. There are four types which are: solid state, gas, dye, and semiconductor. The characteristics of each type will be described.

Solid State Lasers employ a lasing material distributed in a solid matrix. One example is the Neodymium: YAG laser (Nd:YAG). The term: YAG is an abbreviation for the crystal: Yttrium Aluminum Garnet which serves as the host for the Neodymium ions. This laser emits an infrared beam at the wavelength of 1.064 µm (µm = 10(-6) meters). Accessory devices that may be internal or external to the cavity may be used to convert the output to visible or ultraviolet wavelength.

Gas Lasers use a gas or a mixture of gases within a tube. The most common gas laser uses a mixture of helium and neon (HeNe), with a primary output of 632.8 nm (nm = 10(-9) meter) which is a visible red color. It was first developed in 1961 and has proved to be the forerunner of a whole family of gas lasers. All gas lasers are quite similar in construction and behavior. For example, the carbon dioxide (CO(2)) gas laser radiates at 10.6 µm in the far-infrared spectrum. Argon and krypton gas lasers operate with multiple frequency emissions principally in the visible spectra. The main emission wavelengths of an argon laser are 488 and 514 nm.

Dye Lasers use a laser medium that is usually a complex organic dye in liquid solution or suspension. The most striking feature of these lasers is their “tunability.”

Proper choice of the dye and its concentration allows the production of laser light over a broad range of wavelengths in or near the visible spectrum.

Dye lasers commonly employ optical pumping although some types have used chemical reaction pumping. The most commonly used dye is Rhodamine 6G which provides tunability over 200 nm bandwidth in the red portion (620 nm) of the spectrum.

Semiconductor Lasers (sometimes referred to as diode lasers) are not to be confused with solid state lasers. Semiconductor devices consist of two layers of semiconductor material sandwiched together. These lasers are generally very small physically, and individually of only modest power. However, they may be built into larger arrays. The most common diode laser is the Gallium Arsenide diode laser with a central emission of 840 nm.

4. Time Modes of Operation:

The different time modes of operation of a laser are distinguished by the rate at which energy is delivered.

Continuous Wave (CW) lasers operate with a stable average beam power. In most higher power systems, one is able to adjust the power. In low power gas lasers, such as HeNe, the power level is fixed by design and performance usually degrades with long term use.

Single Pulsed (normal mode) lasers generally have pulse durations of a few hundred microseconds to a few milliseconds. This mode of operation is sometimes referred to as long pulse or normal mode.

Single Pulsed Q-Switched lasers are the result of an intracavity delay (Q-switch cell) which allows the laser media to store a maximum of potential energy. Then, under optimum gain conditions, emission occurs in single pulses; typically of 10(-8) second time domain. These pulses will have high peak powers often in the range from 10(6) to 10(9) Watts peak.

Repetitively Pulsed or scanning lasers generally involve the operation of pulsed laser performance operating at a fixed (or variable) pulse rates which may range from a few pulses per second to as high as 20,000 pulses per second. The direction of a CW laser can be scanned rapidly using optical scanning systems to produce the equivalent of a repetitively pulsed output at a given location.

Mode Locked lasers operate as a result of the resonant modes of the optical cavity which can effect the characteristics of the output beam. When the phases of different frequency modes are synchronized, i.e., “locked together,” the different modes will interfere with one another to generate a beat effect. The result is a laser output which is observed as regularly spaced pulsations. Lasers operating in this mode-locked fashion, usually produce a train of regularly spaced pulses, each having a duration of 10(-15) (femto) to 10(-12) (pico) sec. A mode-locked laser can deliver extremely high peak powers than the same laser operating in the Q-switched mode. These pulses will have enormous peak powers often in the range from 10(12) Watts peak.

I. Specific Laser Types:

1. Helium Neon Laser:

The first CW system was the helium neon (HeNe) gas mixture. Although its first successful operation was at an infrared wavelength of 1.15 µm, the HeNe laser is most well known operating at the red 633 nm transition. Some HeNe lasers today also can emit operate at other wavelengths (594 nm, 612 nm, 543 nm). Some earlier HeNe lasers were excited by radio frequency (RF) discharge but virtually all HeNe lasers today are driven by a small DC discharge between electrodes in the laser tube.

The HeNe laser operates by an excitation of the helium atoms from the ground state. This energy excess is coupled to an unexcited neon atom by a collisional process with the net result of an inversion in the neon atom population, thus allowing laser action to begin. Power levels available from the HeNe laser ranges from a fraction of a milliwatt to about 75 milliwatts in the largest available systems. The HeNe laser is noted for its high-frequency stability and TEM(oo) (single mode) operation. The HeNe laser is one of the most widely used laser in existence today. Its pencil-thin beam is used in surveying work, to align pipelines, as a sawing guide in sawmills, and is also used to “align” patients in medical X-ray units, just to name a few of its many applications. It is also used in many retail scanners, lecture hall pointers and display devices. In addition, holograms are often made using the coherent light of HeNe lasers.

2. Argon, Krypton, and Xenon Ion Lasers

The family of ion lasers utilize argon, krypton, xenon, and neon gases to provides a source for over 35 different laser frequencies, ranging from the near ultraviolet (neon at 0.322 µm) to the near-infrared (krypton at 0.799 µm). It is possible to mix the gases, for example, argon and krypton, to produce either single frequency or simultaneous emission at ten different wavelengths, ranging from the violet through the red end of the spectrum.

The basic design of an ion gas laser is similar to the HeNe. The major difference is that the electrical current flowing in the laser tube will be 10-20 amperes; sufficient to ionize the gas. Population inversion is obtained only in the ionized state of the gas. An important feature of these lasers is the very stable (0.2%) high output power of up to 20 Watts/CW. Commercial models will normally have a wavelength selector (a prism) within the cavity to allow for operation at any one of the wavelengths available. In addition, approximately single frequency operation can be achieved by placing an etalon inside the optical resonator cavity.

Argon ion lasers produce the highest visible power levels and have up to 10 lasing wavelengths in the blue-green portion of the spectrum. These lasers are normally rated by the power level (typically 1-10 Watts) produced by all of the six major visible wavelengths from 458 to 514 nm. The most prominent argon wavelengths are the 514 and 488 nm lines. Wavelengths in the ultraviolet spectrum at 351 and 364 nm available by changing resonator mirrors. To dissipate the large amount of generated heat, the larger argon ion laser tubes are water cooled. Although some lasers have separate heat exchangers, most use tap water.

Simple pulsed versions of argon ion lasers also are available. Since the duty cycle (“on” time divided by the time between pulses) is low, the heat energy generated is small, and usually only convective cooling is needed. The average power output may be as high as several Watts, thought the peak powers can be as high as several kilowatts. Pulse widths are approximately five to fifty microseconds, with repetition rates as high as 60 Hz.

3. Carbon Dioxide Laser

The carbon dioxide laser is the most efficient and powerful of all CW laser devices. Continuous powers have been reported above 30 kilowatts at the far infrared 10.6 µm wavelength.

An electrical discharge is initiated in a plasma tube containing carbon dioxide gas. CO(2) molecules are excited by electron collisions to higher vibrational levels, from which they decay to the metastable vibrational level occurs; which has a lifetime of approximately 2×10(-3) seconds at low pressure of a few Torr. Establishing a population inversion between certain vibrational levels leads to lasing transitions at 10.6 µm, while a population inversion between other vibrational levels can result in lasing transitions at 9.6 µm. Although lasing can be obtained in a plasma tube containing CO(2) gas alone, various gases usually added, including N(2), He, Xe, CO(2) and H(2)O. Such additives are used to increase the operating efficiency of CO(2) lasers. The most common gas composition in CO(2) lasers is a mixture of He, N(2) and CO(2).

Carbon dioxide lasers are capable of producing tremendous amounts of output power, primarily because of the high efficiency of about 30%, as compared to less than 0.1% for most HeNe lasers. The principal difference between the CO(2) and other gas lasers is that the optics must be coated, or made of special materials, to be reflective or transmissive at the far infrared wavelength of 10.6 µm. The output mirror can be made of germanium, which, if cooled, has very low loss at 10.6 µm.

There are three common laser cavity configurations of the CO(2) laser. The first is the gas discharge tube encountered with the discussion of the HeNe laser. Secondly is the axial gas flow, where the gas mixture is pumped into one end of the tube and taken out the other. The gas flow allows for the replacement of the CO(2) molecules depleted (disassociated CO(2) molecules) by the electrical discharge. Nitrogen is added to the CO(2) to increase the efficiency of the pumping process and transfers energy by collisions. Associated effects enhance the de-excitation process. Helium is added to the mixture to further increase the efficiency of the process of pumping and stimulated emissions. The third method is the transverse gas flow. This technique can produce CO(2) laser emissions at power levels approaching 25 kW.

The CO(2) laser has a strong emission wavelength at 10.6 micrp m. There is another strong line at 9.6 miceo m and a multitude of lines between 9 and 11 µm. CO(2) lasers are highly efficient (10-30%), give high output powers (used for welding and cutting), and applications out-of-doors can take advantage of low transmission loss atmospheric window at about 10 µm.

4. ND:YAG Laser Systems:

One of the most widely used laser sources for moderate to high power uses a neodymium doped crystal Yttrium Aluminum Garnet (YAG), commonly designated Nd:YAG. In addition, other hosts can be used with Nd, such as calcium tungstate and glass. The Nd:YAG laser is optically pumped either by tungsten or krypton pump lamps and is capable of CW outputs approaching 2000 W at the 1.06 µm wavelength. The ends of the crystal, which is usually in the form of a rod, are lapped, polished, and may be coated to provide the cavity mirrors.

Nd:YAG lasers belong to the class of solid state lasers. Solid state lasers occupy a unique place in laser development. The first operational laser medium was a crystal of pink ruby (a sapphire crystal doped with chromium); since that time, the term “solid state laser” usually has been used to describe a laser whose active medium is a crystal doped with an impurity ion. Solid state lasers are rugged, simple to maintain, and capable of generating high powers.

Although solid state lasers offer some unique advantages over gas lasers, crystals are not ideal cavities or perfect laser media. Real crystals contain refractive index variations that distort the wavefront and mode structure of the laser. High power operation causes thermal expansion of the crystal that alters the effective cavity dimensions and thus changes the modes. The laser crystals are cooled by forced air or liquids, particularly for high repetition rates.

The most striking aspect of solid state lasers is that the output is usually not continuous, but consists of a large number of often separated power bursts. Normal mode and Q-Switched solid-state lasers are often designed for a high repetition-rate operation. Usually the specific parameters of operation are dictated by the application.

For example, pulsed YAG lasers operating 1 Hz at 150 Joules per pulse are used in metal removal applications. As the repetition rate increases, the allowable exit energy per pulse necessarily decreases. Systems are in operation, for example, which produce up to ten Joules per pulse at a repetition rate of 10 Hz. A similar laser, operated in the Q-Switched mode, could produce a one megawatt per pulse at a rate up to ten pulses a minute.

5. Excimer Lasers:

High power ultraviolet (UV) lasers have been the desire of many in the laser applications community for over twenty-five years. Theoretically, such a laser could produce a focused beam of sub-micrometer size and, therefore, be useful in laser microsurgery and industrial microlithography. Also, photochemical processes which are dependent upon the shorter UV wavelength would be possible at significantly greater speeds because of the enormous UV photon flux presented by a laser beam.

In 1975 the first of a family of new UV laser devices was discovered by Searles and Hart. This type laser was to be referred to as an excimer laser, an abbreviation for the term: Excited Dimer. It has taken about a decade for these devices to move from the development lab into real world applications.

Excimer lasers operate using reactive gases such as chlorine and fluorine mixed with inert gases such as argon, krypton or xenon. The various gas combinations, when electrically excited, produce a pseudo molecule (called a “dimer”) with an energy level configuration that causes the generation of a specific laser wavelength emission which falls in the UV spectrum as given in Table II-2. The reliability of excimer lasers has made significant strides over the past several years. Now, systems operating at average powers from 50-100 Watts are commercially available. A typical excimer operates in a repetitively pulsed mode of 30-40 ns pulses at pulse rates up to 50 Hz with pulse energies of 1-2 Joules/pulse. Some systems use x-rays to preionize the excimer laser’s gas mixture so-as-to enhance lasing efficiency and increase the overall output power.

Until the late 1980s, excimer lasers were more commonly found in the research laboratory where they are used either as a specific UV source or, in many cases, to serve as a “pumping” or exciting source to generate visible laser emissions. In the latter case, the excimer’s UV output is directed into a tunable dye laser or Raman shifter module and converted into a modestly high power visible frequency emission.

Excimer lasers are now making the transition from the lab to the production area for a few unique uses in industry or in the operating room for exploratory surgical applications.

6. Semiconductor Diode Lasers

The semiconductor or diode injection laser is another type of solid state laser. The energy level scheme is constructed by charge carriers in the semiconductor. They may be pumped optically or by electron beam bombardment, but most commonly, they are pumped by an externally applied current. Although all of these devices operate in the near infrared spectral region,visible laser diodes are being made today. A useful feature is that many are tunable by varying the applied current, changing temperature, or by applying an external magnetic field. Laser diodes are used extensively for communications, in compact disc players, retail scanners, printer, and are beginning to be used in ophthalmology.

Semiconductor lasers are used in distance detectors and remote sensing systems, rangefinders, and for voice and data communications. Many of the diode lasers may be operated on a continuous wave basis. The most common diode uses a gallium-arsenide junction which emits a fan shaped infrared beam at 840 nm.

7. Other Lasers:

Dye Lasers were the first true tunable laser. Using different organic dyes, a dye laser is capable of producing emission from the ultraviolet to near infrared. Most are operated in the visible with tunable emissions of red, yellow, green, or blue laser emission at almost any wavelength. The more common organic dye lasers are optically pumped. The most common dye used is Rhodamine-6G in solution. Such lasers may either be flashlamp pumped, or more commonly pumped with another laser such as an Argon or Nitrogen laser. To obtain CW reliable operation the dye is made to flow through a thin cell. Using the appropriate dye solutions, an argon-ion laser as a pump, and a prism, the dye laser is tunable across most of the visible spectrum. Tunable dye lasers are now widely used in high resolution atomic and molecular spectroscopy.

J. Laser Beam Parameters:

The following seven properties are common to the beams emitted from all laser types and are the factors which, when combined together, distinguish laser outputs from other sources of electromagnetic radiation:

  1. A nearly single frequency operation of low bandwidth (i.e., an almost pure monochromatic light beam).
  2. A beam with a Gaussian beam intensity profile.
  3. A beam of small divergence.
  4. A beam of enormous intensity.
  5. A beam which maintains a high degree of temporal and spatial coherence.
  6. A beam that is, in many laser devices, highly plane polarized.
  7. A beam with enormous electromagnetic field strengths.

Each of these laser beam properties are briefly reviewed in the following sections.

K. Single Frequency Operation (Monochromacity):

The frequency of any electro-magnetic wave is related to the number of cycles the electric or magnetic field undergo each second. A completely coherent, monochromatic wave oscillates exactly at a constant frequency. Most laser systems display a narrow multifrequency characteristic. This frequency spread is, however, very narrow when compared to the average laser frequency.

In most lasers, the frequency degeneracy is solely dependent upon the quantum transition characteristics of the active media, and the geometry of the laser resonator cavity (Fabry-Perot). In this sense, the laser media may be considered as a high number of isolated light generators placed between two mirrors. The electromagnetic field developed between the mirrors may be regarded as a superposition of plane waves at each of the slightly different frequencies which the laser media generates and allows to oscillate. These different frequencies are termed the “modes” of the laser resonator. The off-axis modes result from plane waves propagating at an angle with respect to the axis of the resonator. These different modes are produced by diffraction effects in the Fabry-Perot cavity. The lowest order axial mode is designated as the TEM(oo) mode. This mode has the lowest diffraction losses and often will be the predominant mode of oscillation.

For each transverse mode, there will be many longitudinal modes which can oscillate; hence the output of a multimode laser will actually contain a superposition of plane waves oscillating at many discrete frequencies. However, as previously mentioned, this frequency spread will be very small. In each laser, there will be specific “allowed” frequencies of the resonator cavity (Fabry-Perot modes). For most cases, the average wavelength at which the laser oscillates is sufficient to describe it’s operation. If more precision is needed, then the frequency spread or bandwidth is given. Depending on the type of laser, bandwidths range typically from 10(-4) to 10(-9) times the average frequency of the laser; although bandwidths as low as 0.1 Hz. have been reported for stabilized gas lasers.

The wave nature of light most often allows adequate description of the output of the laser, and for most cases it will be sufficient to use geometrical optics to describe the output as a beam with well defined edges and some beam divergence. The beam is emitted from the laser with a beam diameter (alpha) and a beam divergence (PHI), as though it came from a small point source far behind the laser output aperture.

L. Point Source Emission:

The emission from most lasers can be considered as emanating from a “virtual point source” located within or behind the laser device. A “virtual point source” is one which really doesn’t exist, but the properties of the emitted beam are such that there appears to be a source at this position. Thus, the virtual point source is located two meters behind the exit mirror.

M. Gaussian Distribution of the Beam:

The intensity profile across a TEM(oo) laser beam will be in the form of a bell-shaped (Gaussian) distribution. The decrease in intensity at the edge of the beam is the result of diffraction effects produced at the edges.

The spatial intensity distribution of this mode may be expressed by equation: (see printed copy) where R is the radius and W is a constant which defines the mean radius and is commonly referred to as the “spot size.” At this point the intensity has fallen to e(-2) of the peak intensity at the center of distribution.

In fact, the edges f the laser beam are not well defined. If one were to measure the energy or power per unit area point by point across the center of the output aperture, a Gaussian beam distribution is defined. The peak intensity is in the center of the beam and approaches zero as one moves from the center. This shape is maintained as the beam propagates through space subject to broadening and distortion by atmospheric effects.

Important points on the distribution curve are the e(-1) and e(-2) intensity points since they are used as standard quantities to define the laser beam divergence parameter. (The e is the natural number associated with the natural logarithm and is equal to: e= 2.7183). The e(-1) point is where the intensity is reduced by the factor (see printed copy) or approximately 63% of the energy (or power) is contained within the aperture of diameter (a) centered in the beam.

Note: In most all laser safety and compliance standards, the output aperture, a, and the beam divergence, (PHI), are defined relative to the e(-1) points. The total laser energy (Q(t)) or beam power (PHI) is defined as that which is collected from the entire beam (total values).

Many manufactures specifications use the e(-2) i points to define beam divergence. In this case, e(-2) i = 0.1353, or the total power (energy) is: 100% – 0.13 3 x 100% = 86.47% or approximately 86% of the total energy /power is within the e(-2) aperture. In some cases, they specify relative to the 90% point. Note that the beam divergence is larger at the e(-2) or 90% point. Hazard calculations are sensitive to the beam divergence and conversions from e(-2) points to e(-2) power points are often performed on beam sizes.

The beam diameters at the two points are related: (see printed copy). Departure from the Gaussian distribution arise when independent oscillation occurs within the resonator at higher order modes. For example, some gas lasers may be designed to have sufficient gain to support simultaneous oscillation in many different transverse modes. Mode selection may often be accomplished by slight adjustment of the mirror alignments. With this technique, one can observe the different complex intensity distributions of each mode.

The lowest order TEM(oo) mode with the nearly Gaussian intensity distribution has the lowest cavity losses and hence will generally be the dominant mode of oscillation.

Optically pumped solid-state lasers such as the normal mode Nd:YAG laser usually display a randomly varying mode output. Thermal gradients in the optical media (i.e., the crystal) caused by nonuniform absorption of the pump light give rise to lens effects in the crystal which change during the pumping cycle. The result is a sporadic switching of transverse modes during the laser pulse. The time average is generally a bell-shaped distribution which is dependent upon the optical purity of the laser crystal, the pumping scheme, and the level at which the system is operated above lasing threshold.

Some pumping schemes produce pronounced “hot spots” in the intensity distributions. For long range transmission, atmospheric effects can also produce intensity variations by a factor of ten over localized regions of the beam. Such non-uniformities distribution make it difficult to specify the cross sectional area of the beam. As a result, an average value of beam radius must be chosen. Typically, this is often (1) the half-power point; (2) the e(-1) power point; or (3) the e(-2) power point.

A more precise laboratory practice is to measure the diameter at the stated power point on a densitometer recording obtained from a photographic negative of the output beam distribution.

In the case of optically pumped solid-state lasers, the size of the beam cross section is generally a function of the pumping level of the laser. In general, the higher the pumping level, the wider the beam size. Only when pulsed lasers are operated near threshold, or in special cavity conditions, will the zero order mode (lowest beam spread) predominate.

N. Beam Divergence:

Beam divergence is a very important laser parameter and is often expressed in units of milliradians. The symmetry of the laser beam allows the geometry to be reduced to the two dimensions of a plane. The angle (phi), in radians can be related to degrees by noting that for a full circle, phi is 360 degrees.

For some smaller angle, the arc length(s) intercepted along the circumference of the circle can be used to define the angle as: (see printed copy). The minimum beam divergence, called the diffraction limited beam divergence, is related by an equation: (see printed copy). This concept is expanded to three dimensions by introducing the concept of solid angle. The solid angle (OMEGA) is expressed in units of steradians (sr) and is determined by using the area cut out of a surface of a sphere divided by the square of the distance to that surface; that is: (see printed copy). For a sphere, the solid angle may be opened up to include the entire sphere surface area (A = 4 pi R(2)), therefore, the output of a typical laser will be confined to less than 10(-6) sr.

O. Intensity of Laser Emission:

In many applications, the most important laser beam charactertic is the enormous intensity of the beam. Intensity is related to the beam power the cross sectional area and the manner in which the beam spreads from one point in space to the next.

Power, by definition, is the time-rate at which work is done; specifically, it is the rate at which energy is used or produced. Energy relates the ability to do work. As with other forms of energy (eg, chemical, mechanical, electrical), electromagnetic energy (light energy) is a conserved quantity. The intensity of the laser is usually expressed by the IRRADIANCE (power/area) of the beam. This is determined by dividing the average value of beam power by the average value of the beam cross section. Irradiance units are expressed in Watts per square centimeter.

In pulsed laser operation, instantaneous (peak) Irradiances in excess of 100,000 W/cm(2) are quite easily generated in an unfocused high energy pulsed solid state laser pulse. If this output were contained within a typical beam divergence of 20 milliradians and focused by only moderate power optics, the Irradiance at the focal plane would be increased at least one-hundred fold.

A CW laser is rated in Watts and a pulsed laser is normally rated according to the total energy (Joules) per pulse. Pulsed outputs are also expressed as a RADIANT EXPOSURE in units of Joules per square centimeter.

In order to determine the peak power of pulsed laser, it is necessary to know the pulse shape and duration. The peak power may be closely approximated by assuming a triangular pulse shape and dividing the energy per pulse by the pulse duration at half power. Average power is an important factor for high PRF lasers when determining the laser classification and maximum permissible exposure levels.

The radiometric units of RADIANCE and INTEGRATED RADIANCE are used to describe the diffuse reflection of a continuous wave or pulsed laser beam.

Radiance is expressed, by definition, as the Irradiance per unit solid angle (Watts per square centimeter per steradian).

Integrated Radiance is expressed as the Radiant Exposure per unit solid angle (Joules per square centimeter per steradian).

The unit of solid angle is defined such that all space about a point source (i.e., the source of light) will encompass 4 pi sr.

P. Focused Laser Beams:

The beam from an ideal laser, i.e., a laser which emits a coherent wave, can be considered as a diffraction-limited beam. In this case, divergence of the beam is limited to the effects of diffraction at the beam edges. The emission from such a laser will display a far-field diffraction pattern at a distance (see printed copy) where a is the diameter of the emergent laser radiation.

The TEM(oo) beam from a typical helium neon laser will display a 0.5-1.0 milliradian beam spread at a distance of 1.0-2.0 meters from the laser.

Due to the high degree of coherence of a laser beam, it is theoretically possible to focus the beam to the diffraction limit of the wavelength of light. Typically, however, the laser will have a finite beam spread and can be expressed by the simple equations of geometrical optics.

The spot diameter (d) is given by the simple equation: d = f phi where: d = spot diameter at focus f = focal length of lens phi = laser beam divergence (radians).

As an example, one can calculate the spot size of a beam focused on the human retina. For this case, consider a “typical” HeNe laser where: phi = 1.0 milliradian and assume that the effective focal length (f) of the human eye is 1.7 cm.

Thus: d = f phi = (1.7 cm) x (1.0 x 10(-3) rad.) = 17 x 10(-4) cm = 17 µm

To give some idea of how small this focused spot is, consider that 17 micrometers is approximately the size of two or three human blood cells stacked end-to-end.

Using the equation for the area of a circle (see printed copy), one can now calculate the focused beam area (see printed copy).

As the spot diameter approaches the wavelength of light, the spot becomes diffraction-limited. For example, the beam from a highly coherent single transverse mode (TEM(oo)) gas laser will produce a Gaussian intensity pattern when focused. This distribution may be described mathematically by an equation where it is considered that the beam energy will be contained in a diameter defined at the e(-2) power point (see printed copy).

Therefore, the smallest possible spot size of a focused laser beam will approach the dimensions the wavelength of light which is being focused.

Combining the equations above can yield an expression for the spot area (see printed copy).

Thus, the Irradiance (power per unit area) of a focused laser beam will vary inversely with the square of the focal length of the lens and with the square of the beam divergence angle. Hence, these two factors have dramatic effects on the power distribution at the focal plane of the lens.

Consequently, either a reduction in the focal length of the lens used to focus the beam or a reduction in the beam spread by a factor of ten will produce a one-hundred fold increase in the irradiance at the focal plane of the lens. Simultaneous reduction of both by a factor of ten would increase the Irradiance at the focal plane by a factor of 10(4).

In practice, however, it is usually the beam divergence value that limits the focal spot diameter. This is especially true with pulsed laser systems. To achieve high power outputs, the laser crystal is usually pumped well over threshold; consequently, the beam will contain a conglomerate of high order “off-axis” modes which subsequently increase the beam size.

Typical beam divergence values for gas lasers (helium-neon, argon, etc.) will be about one milliradian, (1 milliradian = 3.44 minutes of arc). Solid-state ruby and neodymium lasers generally have a higher beam spread (1- 30 milliradians), due primarily to the high beam divergence associated with the random multimode operation of such devices.

Q. Scanning Lasers:

Some laser applications employ electro-mechanical or electro-optical scanner units to allow a raster-scan capability to the beam. In this way, the beam can be scanned over a large area (such as in a laser print maker) or over a small area (such as a laser UPC label reader) in a repeated geometry. (equations, see printed copy).

R. Coherence:

The coherency of a laser beam relates to the constancy of the spatial and temporal variations in the radiation wavefronts. A high degree of coherence implies a constant phase different between two points on a series of equal-amplitude wavefronts (spatial coherence), and in a correlation in time between the same points on different wavefronts (temporal coherence). The two coherence terms are a part of the overall four-dimensional coherence function which completely describes the degree of coherency of the beam.

If the laser beam is considered as a plane wave traveling in one direction, it will be spatially coherent due to the perpendicularity of the wavefronts in the direction of propagation. Also, due to the monochromatic nature of the laser light, the beam will be temporally coherent; that is, it will display a fixed-phase relation between a part of the beam emitted at one time and portion emitted at another. Should the wavelength (or frequency) change, then the temporal coherency would degrade.

In 1802, Thomas Young performed his classic double-slit experiment to demonstrate the wave nature of light. Sunlight though one pinhole was allowed to illuminate two closely spaced pinholes. Each pinhole acted as a “new source” for light and the waves from each of the two pinholes interfered with one another so-as-to produce corresponding light and dark regions (or fringes – as they are called) at the observation screen. If light was not a wave, it would travel in a straight line from one pinhole to another to fall at two points on the screen. As a wave, however, it is diffracted and bent about the edges of the pinholes such that each pinhole illuminates the entire screen.

In part, if it were not for the diffraction effects produced as a light wave passes through a finite aperture, the plane wave output of laser could theoretically be focused by a lens (such as the human eye) to a real point with minimal spot diameter. Thus, the image Irradiance , (see printed copy) would have an infinite value. That would indeed be hazardous!

Due to the wave nature of light and the corresponding diffraction effects produced by finite apertures, the image a point source of light (provided by any real optical system) is not actually a point.

Such a distribution has a bright central area surrounded by light and dark rings. When an optical system’s resolution is only limited by the diffraction effects, it is said to be diffraction limited. Even in this condition, a point is “spread out” as it is imaged. The more defects and aberrations introduced by the optical system, the more the spreading. Each lens images a point with some spreading and the manner which it behaves is defined as the point spread function.

Since an optical system spreads a point source image, there is a corresponding limit to its resolution or ability to separate two points close together. This is especially important when looking at stars through a telescope. There are at least two criteria used to define the resolution of two points close together. A simple one, and the one most commonly used, is the Rayleigh criterion. This states that when the peak of one Airy disk is over the first dark ring of the other, the points are resolved. This is normally defined in terms of the apparent angle between the two points.

Lasers are often referred to as coherent sources, but in fact, they really only partially coherent. Only absolutely monochromatic or single frequency waves are truly coherent; however, lasers are so close, relative to anything else, that a loose definition may seem justified. The degree to which two waves are coherent determines how well they interfere when brought together at some point in space.

A thorough treatment of the subject of partial coherence is far beyond the scope of this guideline; however, there are a few properties worth discussing. One frequently encounters the terms spatial and temporal coherence. Temporal coherence effects are those which arise from the finiteness of the spectral band. An increase in fringe visibility with a decrease in source size is a measure of the spatial coherence. An important measure of coherence is the coherence length, dL, which can be conceptually related to the duration of an uninterrupted wavetrain. Even in the beam from an “ideal” laser, there will be random fluctuations in the phase difference of the electromagnetic fields at two separate points on a wavefront. The distance between points on the wavefront for which the average of this phase different is equal to (equation, see printed copy) adians is generally defined as the lateral coherence distance. Recombination of the light samples from points separated by a distance equal to, or less than, this amount can produce interference fringes. The distance is a classical measure of the spatial coherence of a light beam as observed in the famous “double slit” experiment of Young.

The temporal coherence is a measure of the length of time that the beam is truly monochromatic. This may be considered as the time during which the amplitude of the electromagnetic field will remain constant at a given point in space while the phase varies linearly with time. During this time, the beam will travel a length dL = c dT defined as the coherence length (where c = 3 x 10(8) m/sec., the velocity of light). Thus the coherence time is the time required for light to travel the coherence length in the direction of travel of the beam.

By virtue of this argument, it is seen that the frequency bandwidth is actually a measure of temporal coherence. Thus a frequency stabilized HeNe gas laser (d(v) = 3-5 hertz) will have a coherence time of several hundred milliseconds and a corresponding coherence length of 10(5) km.

In contrast to the high spectral purity of gas lasers, the coherence lengths of pulsed ruby lasers are in the order of 15 meters with corresponding coherence times in the order of only 100 nanoseconds.

S. Polarization of the Laser Output:

The polarization of most lasers is directly related to the nature of the resonator. For example, many high power gas lasers are built with Brewster’s angle windows on both ends of the gas discharge tube. Such windows present virtually no losses to a beam which has a linear polarization component lying in the plane of incident. Hence the output will be linearly polarized in this plane.

In some solid-state crystal lasers, for example, the ruby laser, the output will be linearly polarized. This is a result of the birefringent nature of the crystal in which the slower “ordinary” polarized photons will have a longer time to interact with the excited chromium ions, thereby favoring a polarized output in this plane. This is generally only true for ruby crystals operating near lasing threshold unless Brewster’s angles are fabricated on the ends of the crystal. This latter practice is often necessary for very high power Q-switched laser systems.

In diode lasers, linear polarized light is also observed. This may be attributed to the linear symmetry of the junction region.

T. Electrical Field Strength:

The electromagnetic theory of light depicts a light wave as having instantaneous electric and magnetic fields which oscillate at the same frequency. The electrical (E) and magnetic (H) fields are fixed at right angles and are mutually perpendicular to the direction of propagation of the wave. Of particular importance in the description of laser beam interactions is the magnitude of the electric field associated with the beam.

From classical considerations (using Maxwell’s equations) the electric field (E) in volts per centimeter associated with a light beam in a vacuum (or air) of average power (PHI) in Watts, spread over a cross-sectional area (A) in cm(2) is given by (equation, see printed copy).

Prior to lasers, the electric fields associated with commonly occurring light sources were most nominal. For example, the electric field of sunlight occurring at the earth’s surface is about (equation, see printed copy). This constitutes an average field spread over all the wavelengths present in the “white light” of the sun.

In contrast, the instantaneous electric field associated with an unfocused “Q- Switched” Nd:YAG laser burst operating at a level of 100 megawatts and confined to 3 mm beam diameter will approach (Equation, see printed copy).

Such strong fields are also found elsewhere in nature, as they are at the magnitude of the electrostatic cohesive forces which bind atomic structures. Consequently, when a laser beam with a field of comparable magnitude enters a transparent structure, an instantaneous massive redistribution of the electric system of the material can occur due to the interaction of the fields. At the present, the interaction of these enormous electromagnetic fields is not fully understood, to be sure. The production of free electrons, ionized atoms, and X-rays have been detected in the reaction association with the interaction of high power laser beams.

U. Comparison With Other Sources:

Light from conventional thermal sources is emitted over a wide spectral band. The polarizations of the photons are distributed over all possible states of polarization and leave the source in all possible directions (Lambertian source). In contrast, a laser source has a very narrow spectral linewidth even in comparison to special, narrow band thermal sources; the photons may have, essentially, the same polarization and they are highly directional as they leave the laser cavity.

Conventional optical sources can most certainly constitute a hazard to the human eye and/or skin, particularly close up and when focused. For example, one’s first introduction to optical physics might well have been using a magnifying glass to focus the sun’s rays on dry leaves to start a fire. However, even a relatively small laser is capable of producing power/energy distributions much greater that conventional sources. In addition, the hazard can exist at very long ranges due to the highly directional nature of the laser output.

A conventional thermal source will emit light into a sphere or hemisphere. The power/energy per unit area (intensity) may be large at the source; however, the intensity at the observer falls off rapidly as the observer moves away from the source. The intensity at the observer can be dramatically increased by using optics to reduce the divergence, making a searchlight; however, the effect is limited by the size of the source.

The output of the laser has a very small divergence, typically less that 1 milliradian (1 mrad = 0.0573 degrees), and the intensity decreases very slowly as the distance to the observer increases. It would take a very powerful thermal source to put as much power into as tight a beam as offered by even the smaller lasers. If one were to insert a very narrow bandpass filter into the searchlight (in order to approximate the spectral purity of monochromatic nature of the laser output), the laser would be brighter than any thermal source by an enormous factor, where brightness is defined as the power output per steradian of solid angle.

To illustrate the relative brightness of the laser over its narrow band, one notes that the sun emits, at its surface, approximately 10(4) W/cm(2)/sr/µm and lasers can produce greater than 10(10)W/cm(2)/sr/µm in single pulse. Therefore, it is not difficult for a laser to be a million times brighter that the sun. Indeed, a laser can not only burn dry leaves, but some are used to weld metal. The most significant factor is not total power, but rather the power per unit area, where the laser may be focused to an extremely small spot (approximately a wavelength in diameter). For example, a one milliwatt laser focused to a one micrometer spot will produce a focused irradiance greater than 1×10(5) W/cm(2).

Guidelines for Laser Safety and Hazard Assessment
Source: Occupational Safety & Health Administration, Guidelines for Laser Safety and Hazard Assessment PUB 8-1.7 (tablular data and equation illustrations have been omitted)

A. Basic Laser Operation:

The term LASER is an acronym. It stands for Light Amplification by Stimulated Emission of Radiation. Thus the laser is a device which produces and amplifies light. The mechanism by which this is accomplished, stimulated emission, was first postulated by Albert Einstein in 1917. The light which the laser produces is unique, for it is characterized by properties which are very desirable, but almost impossible to obtain by any means other than the laser.

To gain a better understanding of the laser and what it can do, a review is included of some of the phenomena involved.

B. Energy Levels:
Light can be produced by atomic processes, and it is these processes which are responsible for the generation of laser light. Let’s look first at atomic energy levels and then see how changes in these energy levels can lead to the production of laser light.

A number of simplifications will be made regarding the concept of the atom. It can be assumed, for the purposes of this discussion, that an atom consists of a small dense nucleus and one or more electrons in motion about the nucleus.

The relationship between the electrons and the nucleus is described in terms of energy levels. Quantum mechanics predicts that these energy levels are discrete.

C. Radiative Transitions:
The electrons normally occupy the lowest available energy levels. When this is the case, the atom is said to be in its ground state. However, electrons can occupy higher energy levels, leaving some of the lower energy states vacant or sparsely populated.

One way that electrons and atoms can change from one energy state to another is by the absorption or emission of light energy, via a process called a radiative transition.

D. Absorption:
An electron can absorb energy from a variety of external sources. From the point of view of laser action, two methods of supplying energy to the electrons are of prime importance. The first of these is the transfer of all the energy of a photon directly to an orbital electron. The increase in the energy of the electron causes it to “jump” to a higher energy level; the atom is then said to be in an “excited” state. It is important to note that an electron can accept only the precise amount of energy that is needed to move it from one allowable energy level to another. Only photons of the exact energy acceptable to the electron can be absorbed. Photons of slightly more (or slightly less) energy will not be absorbed.

Another means often used to excite electrons is an electrical discharge. In this technique, the energy is supplied by collisions with electrons which have been accelerated by an electric field. The result of either type of excitation is that through the absorption of energy, an electron has been placed in a higher energy level than it originally resided. As a result, the atom of which it is a part is said to be excited.

E. Spontaneous Emission:
The nature of all matter is such that atomic and molecular structures tend to exist in the lowest energy state possible. Thus, an excited electron in a higher energy level will soon attempt to DE-EXCITE itself by any of several means. Some of the energy may be converted to heat.

Another means of de-excitation is the spontaneous emission of a photon. The photon released by an atom as it is de-excited will have a total energy exactly equal to the difference in energy between the excited and lower energy levels. This release of a photon is called spontaneous emission. One example of spontaneous emission is the common neon sign. Atoms of neon are excited by an electrical discharge through the tube. They de-excite themselves by spontaneously emitting photons of visible light.

NOTE: The exciting force is not of a unique energy, so that the electrons may be excited to any one of several allowable levels.

Now let’s look at the third, and probably the least familar, type of radiative transition.

F. Stimulated Emission:
In 1917, Einstein postulated that a photon released from an excited atom could, upon interacting with a second, similarly excited atom, trigger the second atom into de-exciting itself with the release of another photon. The photon released by the second atom would be identical in frequency, energy, direction, and phase with the triggering photon, and the triggering photon would continue on its way, unchanged. Where there was one photon now there are two. These two photons could then proceed to trigger more through the process of stimulated emission.

If an appropriate medium contains a great many excited atoms and de-excitation occurs only by spontaneous emission, the light output will be random and approximately equal in all directions. The process of stimulated emission, however, can cause an amplification of the number of photons traveling in a particular direction – a photon cascade if you will.

A preferential direction is established by placing mirrors at the ends of an optical cavity. Thus the number of photons traveling along the axis of the two mirrors increases greatly and Light Amplification by the Stimulated Emission of Radiation may occur. If enough amplification occurs, LASER beam is created.

G. Population Inversion:
Practically speaking, the process of stimulated emission will not produce a very efficient or even noticeable amplification of light unless a condition called “population inversion” occurs. If only a few atoms of several million are in an excited state, the chances of stimulated emission occurring are small. The greater the percentage of atoms in an excited state, the greater the probability of stimulated emission. In the normal state of matter the population of electrons will be such that most of the electrons reside in the ground or lowest levels, leaving the upper levels somewhat depopulated. When electrons are excited and fill these upper levels to the extent that there are more atoms excited than not excited, the population is said to be inverted.

H. Laser Components:
A generalized laser consists of a lasing medium, a “pumping” system and an optical cavity. The laser material must have a metastable state in which the atoms or molecules can be trapped after receiving energy from the pumping system. Each of these laser components are discussed below:

1. Pumping Systems:

The pumping system imparts energy to the atoms or molecules of the lasing medium enabling them to be raised to an excited “metastable state” creating a population inversion. Optical pumping uses photons provided by a source such as a Xenon gas flash lamp or another laser to transfer energy to the lasing material. The optical source must provide photons which correspond to the allowed transition levels of the lasing material.

Collision pumping relies on the transfer of energy to the lasing material by collision with the atoms (or molecules) of the lasing material. Again, energies which correspond to the allowed transitions must be provided. This is often done by electrical discharge in a pure gas or gas mixture in a tube.

Chemical pumping systems use the binding energy released in chemical reactions to state.

2. Optical Cavity:

An optical cavity is required to provide the amplification desired in the laser and to select the photons which are traveling in the desired direction. As the first atom or molecule in the metastable state of the inverted population decays, it triggers via stimulated emission, the decay of another atom or molecule in the metastable state. If the photons are traveling in a direction which leads to the walls of the lasing material, which is usually in the form of a rod or tube, they are lost and the amplification process terminates. They may actually be reflected at the wall of the rod or tube, but sooner or later they will be lost in the system and will not contribute to the beam.

If, on the other hand, one of the decaying atoms or molecules releases a photon parallel to the axis of the lasing material, it can trigger the emission of another photon and both will be reflected by the mirror on the end of the lasing rod or tube. The reflected photons then pass back through the material triggering further emissions along exactly the same path which are reflected by the mirrors on the ends of the lasing material. As this amplification process continues, a portion of the radiation will always escape through the partially reflecting mirror. When the amount of amplification or gain through this process exceeds the losses in the cavity, laser oscillation is said to occur. In this way, a narrow concentrated beam of coherent light is formed.

The mirrors on the laser optical cavity must be precisely aligned for light beams parallel to the axis. The optical cavity itself, i.e., the lasing medium material must not be a strong absorber of the light energy.

3. Laser Media:

Lasers are commonly designated by the type of lasing material employed. There are four types which are: solid state, gas, dye, and semiconductor. The characteristics of each type will be described.

Solid State Lasers employ a lasing material distributed in a solid matrix. One example is the Neodymium: YAG laser (Nd:YAG). The term: YAG is an abbreviation for the crystal: Yttrium Aluminum Garnet which serves as the host for the Neodymium ions. This laser emits an infrared beam at the wavelength of 1.064 µm (µm = 10(-6) meters). Accessory devices that may be internal or external to the cavity may be used to convert the output to visible or ultraviolet wavelength.

Gas Lasers use a gas or a mixture of gases within a tube. The most common gas laser uses a mixture of helium and neon (HeNe), with a primary output of 632.8 nm (nm = 10(-9) meter) which is a visible red color. It was first developed in 1961 and has proved to be the forerunner of a whole family of gas lasers. All gas lasers are quite similar in construction and behavior. For example, the carbon dioxide (CO(2)) gas laser radiates at 10.6 µm in the far-infrared spectrum. Argon and krypton gas lasers operate with multiple frequency emissions principally in the visible spectra. The main emission wavelengths of an argon laser are 488 and 514 nm.

Dye Lasers use a laser medium that is usually a complex organic dye in liquid solution or suspension. The most striking feature of these lasers is their “tunability.”

Proper choice of the dye and its concentration allows the production of laser light over a broad range of wavelengths in or near the visible spectrum.

Dye lasers commonly employ optical pumping although some types have used chemical reaction pumping. The most commonly used dye is Rhodamine 6G which provides tunability over 200 nm bandwidth in the red portion (620 nm) of the spectrum.

Semiconductor Lasers (sometimes referred to as diode lasers) are not to be confused with solid state lasers. Semiconductor devices consist of two layers of semiconductor material sandwiched together. These lasers are generally very small physically, and individually of only modest power. However, they may be built into larger arrays. The most common diode laser is the Gallium Arsenide diode laser with a central emission of 840 nm.

4. Time Modes of Operation:
The different time modes of operation of a laser are distinguished by the rate at which energy is delivered.

Continuous Wave (CW) lasers operate with a stable average beam power. In most higher power systems, one is able to adjust the power. In low power gas lasers, such as HeNe, the power level is fixed by design and performance usually degrades with long term use.

Single Pulsed (normal mode) lasers generally have pulse durations of a few hundred microseconds to a few milliseconds. This mode of operation is sometimes referred to as long pulse or normal mode.

Single Pulsed Q-Switched lasers are the result of an intracavity delay (Q-switch cell) which allows the laser media to store a maximum of potential energy. Then, under optimum gain conditions, emission occurs in single pulses; typically of 10(-8) second time domain. These pulses will have high peak powers often in the range from 10(6) to 10(9) Watts peak.

Repetitively Pulsed or scanning lasers generally involve the operation of pulsed laser performance operating at a fixed (or variable) pulse rates which may range from a few pulses per second to as high as 20,000 pulses per second. The direction of a CW laser can be scanned rapidly using optical scanning systems to produce the equivalent of a repetitively pulsed output at a given location.

Mode Locked lasers operate as a result of the resonant modes of the optical cavity which can effect the characteristics of the output beam. When the phases of different frequency modes are synchronized, i.e., “locked together,” the different modes will interfere with one another to generate a beat effect. The result is a laser output which is observed as regularly spaced pulsations. Lasers operating in this mode-locked fashion, usually produce a train of regularly spaced pulses, each having a duration of 10(-15) (femto) to 10(-12) (pico) sec. A mode-locked laser can deliver extremely high peak powers than the same laser operating in the Q-switched mode. These pulses will have enormous peak powers often in the range from 10(12) Watts peak.

I. Specific Laser Types:

1. Helium Neon Laser:
The first CW system was the helium neon (HeNe) gas mixture. Although its first successful operation was at an infrared wavelength of 1.15 µm, the HeNe laser is most well known operating at the red 633 nm transition. Some HeNe lasers today also can emit operate at other wavelengths (594 nm, 612 nm, 543 nm). Some earlier HeNe lasers were excited by radio frequency (RF) discharge but virtually all HeNe lasers today are driven by a small DC discharge between electrodes in the laser tube.

The HeNe laser operates by an excitation of the helium atoms from the ground state. This energy excess is coupled to an unexcited neon atom by a collisional process with the net result of an inversion in the neon atom population, thus allowing laser action to begin. Power levels available from the HeNe laser ranges from a fraction of a milliwatt to about 75 milliwatts in the largest available systems. The HeNe laser is noted for its high-frequency stability and TEM(oo) (single mode) operation. The HeNe laser is one of the most widely used laser in existence today. Its pencil-thin beam is used in surveying work, to align pipelines, as a sawing guide in sawmills, and is also used to “align” patients in medical X-ray units, just to name a few of its many applications. It is also used in many retail scanners, lecture hall pointers and display devices. In addition, holograms are often made using the coherent light of HeNe lasers.

2. Argon, Krypton, and Xenon Ion Lasers
The family of ion lasers utilize argon, krypton, xenon, and neon gases to provides a source for over 35 different laser frequencies, ranging from the near ultraviolet (neon at 0.322 µm) to the near-infrared (krypton at 0.799 µm). It is possible to mix the gases, for example, argon and krypton, to produce either single frequency or simultaneous emission at ten different wavelengths, ranging from the violet through the red end of the spectrum.

The basic design of an ion gas laser is similar to the HeNe. The major difference is that the electrical current flowing in the laser tube will be 10-20 amperes; sufficient to ionize the gas. Population inversion is obtained only in the ionized state of the gas. An important feature of these lasers is the very stable (0.2%) high output power of up to 20 Watts/CW. Commercial models will normally have a wavelength selector (a prism) within the cavity to allow for operation at any one of the wavelengths available. In addition, approximately single frequency operation can be achieved by placing an etalon inside the optical resonator cavity.

Argon ion lasers produce the highest visible power levels and have up to 10 lasing wavelengths in the blue-green portion of the spectrum. These lasers are normally rated by the power level (typically 1-10 Watts) produced by all of the six major visible wavelengths from 458 to 514 nm. The most prominent argon wavelengths are the 514 and 488 nm lines. Wavelengths in the ultraviolet spectrum at 351 and 364 nm available by changing resonator mirrors. To dissipate the large amount of generated heat, the larger argon ion laser tubes are water cooled. Although some lasers have separate heat exchangers, most use tap water.

Simple pulsed versions of argon ion lasers also are available. Since the duty cycle (“on” time divided by the time between pulses) is low, the heat energy generated is small, and usually only convective cooling is needed. The average power output may be as high as several Watts, thought the peak powers can be as high as several kilowatts. Pulse widths are approximately five to fifty microseconds, with repetition rates as high as 60 Hz.

3. Carbon Dioxide Laser
The carbon dioxide laser is the most efficient and powerful of all CW laser devices. Continuous powers have been reported above 30 kilowatts at the far infrared 10.6 µm wavelength.

An electrical discharge is initiated in a plasma tube containing carbon dioxide gas. CO(2) molecules are excited by electron collisions to higher vibrational levels, from which they decay to the metastable vibrational level occurs; which has a lifetime of approximately 2×10(-3) seconds at low pressure of a few Torr. Establishing a population inversion between certain vibrational levels leads to lasing transitions at 10.6 µm, while a population inversion between other vibrational levels can result in lasing transitions at 9.6 µm. Although lasing can be obtained in a plasma tube containing CO(2) gas alone, various gases usually added, including N(2), He, Xe, CO(2) and H(2)O. Such additives are used to increase the operating efficiency of CO(2) lasers. The most common gas composition in CO(2) lasers is a mixture of He, N(2) and CO(2).

Carbon dioxide lasers are capable of producing tremendous amounts of output power, primarily because of the high efficiency of about 30%, as compared to less than 0.1% for most HeNe lasers. The principal difference between the CO(2) and other gas lasers is that the optics must be coated, or made of special materials, to be reflective or transmissive at the far infrared wavelength of 10.6 µm. The output mirror can be made of germanium, which, if cooled, has very low loss at 10.6 µm.

There are three common laser cavity configurations of the CO(2) laser. The first is the gas discharge tube encountered with the discussion of the HeNe laser. Secondly is the axial gas flow, where the gas mixture is pumped into one end of the tube and taken out the other. The gas flow allows for the replacement of the CO(2) molecules depleted (disassociated CO(2) molecules) by the electrical discharge. Nitrogen is added to the CO(2) to increase the efficiency of the pumping process and transfers energy by collisions. Associated effects enhance the de-excitation process. Helium is added to the mixture to further increase the efficiency of the process of pumping and stimulated emissions. The third method is the transverse gas flow. This technique can produce CO(2) laser emissions at power levels approaching 25 kW.

The CO(2) laser has a strong emission wavelength at 10.6 micrp m. There is another strong line at 9.6 miceo m and a multitude of lines between 9 and 11 µm. CO(2) lasers are highly efficient (10-30%), give high output powers (used for welding and cutting), and applications out-of-doors can take advantage of low transmission loss atmospheric window at about 10 µm.

4. ND:YAG Laser Systems:
One of the most widely used laser sources for moderate to high power uses a neodymium doped crystal Yttrium Aluminum Garnet (YAG), commonly designated Nd:YAG. In addition, other hosts can be used with Nd, such as calcium tungstate and glass. The Nd:YAG laser is optically pumped either by tungsten or krypton pump lamps and is capable of CW outputs approaching 2000 W at the 1.06 µm wavelength. The ends of the crystal, which is usually in the form of a rod, are lapped, polished, and may be coated to provide the cavity mirrors.

Nd:YAG lasers belong to the class of solid state lasers. Solid state lasers occupy a unique place in laser development. The first operational laser medium was a crystal of pink ruby (a sapphire crystal doped with chromium); since that time, the term “solid state laser” usually has been used to describe a laser whose active medium is a crystal doped with an impurity ion. Solid state lasers are rugged, simple to maintain, and capable of generating high powers.

Although solid state lasers offer some unique advantages over gas lasers, crystals are not ideal cavities or perfect laser media. Real crystals contain refractive index variations that distort the wavefront and mode structure of the laser. High power operation causes thermal expansion of the crystal that alters the effective cavity dimensions and thus changes the modes. The laser crystals are cooled by forced air or liquids, particularly for high repetition rates.

The most striking aspect of solid state lasers is that the output is usually not continuous, but consists of a large number of often separated power bursts. Normal mode and Q-Switched solid-state lasers are often designed for a high repetition-rate operation. Usually the specific parameters of operation are dictated by the application.

For example, pulsed YAG lasers operating 1 Hz at 150 Joules per pulse are used in metal removal applications. As the repetition rate increases, the allowable exit energy per pulse necessarily decreases. Systems are in operation, for example, which produce up to ten Joules per pulse at a repetition rate of 10 Hz. A similar laser, operated in the Q-Switched mode, could produce a one megawatt per pulse at a rate up to ten pulses a minute.

5. Excimer Lasers:
High power ultraviolet (UV) lasers have been the desire of many in the laser applications community for over twenty-five years. Theoretically, such a laser could produce a focused beam of sub-micrometer size and, therefore, be useful in laser microsurgery and industrial microlithography. Also, photochemical processes which are dependent upon the shorter UV wavelength would be possible at significantly greater speeds because of the enormous UV photon flux presented by a laser beam.

In 1975 the first of a family of new UV laser devices was discovered by Searles and Hart. This type laser was to be referred to as an excimer laser, an abbreviation for the term: Excited Dimer. It has taken about a decade for these devices to move from the development lab into real world applications.

Excimer lasers operate using reactive gases such as chlorine and fluorine mixed with inert gases such as argon, krypton or xenon. The various gas combinations, when electrically excited, produce a pseudo molecule (called a “dimer”) with an energy level configuration that causes the generation of a specific laser wavelength emission which falls in the UV spectrum as given in Table II-2. The reliability of excimer lasers has made significant strides over the past several years. Now, systems operating at average powers from 50-100 Watts are commercially available. A typical excimer operates in a repetitively pulsed mode of 30-40 ns pulses at pulse rates up to 50 Hz with pulse energies of 1-2 Joules/pulse. Some systems use x-rays to preionize the excimer laser’s gas mixture so-as-to enhance lasing efficiency and increase the overall output power.

Until the late 1980s, excimer lasers were more commonly found in the research laboratory where they are used either as a specific UV source or, in many cases, to serve as a “pumping” or exciting source to generate visible laser emissions. In the latter case, the excimer’s UV output is directed into a tunable dye laser or Raman shifter module and converted into a modestly high power visible frequency emission.

Excimer lasers are now making the transition from the lab to the production area for a few unique uses in industry or in the operating room for exploratory surgical applications.

6. Semiconductor Diode Lasers
The semiconductor or diode injection laser is another type of solid state laser. The energy level scheme is constructed by charge carriers in the semiconductor. They may be pumped optically or by electron beam bombardment, but most commonly, they are pumped by an externally applied current. Although all of these devices operate in the near infrared spectral region,visible laser diodes are being made today. A useful feature is that many are tunable by varying the applied current, changing temperature, or by applying an external magnetic field. Laser diodes are used extensively for communications, in compact disc players, retail scanners, printer, and are beginning to be used in ophthalmology.

Semiconductor lasers are used in distance detectors and remote sensing systems, rangefinders, and for voice and data communications. Many of the diode lasers may be operated on a continuous wave basis. The most common diode uses a gallium-arsenide junction which emits a fan shaped infrared beam at 840 nm.

7. Other Lasers:
Dye Lasers were the first true tunable laser. Using different organic dyes, a dye laser is capable of producing emission from the ultraviolet to near infrared. Most are operated in the visible with tunable emissions of red, yellow, green, or blue laser emission at almost any wavelength. The more common organic dye lasers are optically pumped. The most common dye used is Rhodamine-6G in solution. Such lasers may either be flashlamp pumped, or more commonly pumped with another laser such as an Argon or Nitrogen laser. To obtain CW reliable operation the dye is made to flow through a thin cell. Using the appropriate dye solutions, an argon-ion laser as a pump, and a prism, the dye laser is tunable across most of the visible spectrum. Tunable dye lasers are now widely used in high resolution atomic and molecular spectroscopy.

J. Laser Beam Parameters:
The following seven properties are common to the beams emitted from all laser types and are the factors which, when combined together, distinguish laser outputs from other sources of electromagnetic radiation:

1. A nearly single frequency operation of low bandwidth (i.e., an almost pure monochromatic light beam).

2. A beam with a Gaussian beam intensity profile.
3. A beam of small divergence.
4. A beam of enormous intensity.
5. A beam which maintains a high degree of temporal and spatial coherence.
6. A beam that is, in many laser devices, highly plane polarized.
7. A beam with enormous electromagnetic field strengths.
Each of these laser beam properties are briefly reviewed in the following sections.

K. Single Frequency Operation (Monochromacity):
The frequency of any electro-magnetic wave is related to the number of cycles the electric or magnetic field undergo each second. A completely coherent, monochromatic wave oscillates exactly at a constant frequency. Most laser systems display a narrow multifrequency characteristic. This frequency spread is, however, very narrow when compared to the average laser frequency.

In most lasers, the frequency degeneracy is solely dependent upon the quantum transition characteristics of the active media, and the geometry of the laser resonator cavity (Fabry-Perot). In this sense, the laser media may be considered as a high number of isolated light generators placed between two mirrors. The electromagnetic field developed between the mirrors may be regarded as a superposition of plane waves at each of the slightly different frequencies which the laser media generates and allows to oscillate. These different frequencies are termed the “modes” of the laser resonator. The off-axis modes result from plane waves propagating at an angle with respect to the axis of the resonator. These different modes are produced by diffraction effects in the Fabry-Perot cavity. The lowest order axial mode is designated as the TEM(oo) mode. This mode has the lowest diffraction losses and often will be the predominant mode of oscillation.

For each transverse mode, there will be many longitudinal modes which can oscillate; hence the output of a multimode laser will actually contain a superposition of plane waves oscillating at many discrete frequencies. However, as previously mentioned, this frequency spread will be very small. In each laser, there will be specific “allowed” frequencies of the resonator cavity (Fabry-Perot modes). For most cases, the average wavelength at which the laser oscillates is sufficient to describe it’s operation. If more precision is needed, then the frequency spread or bandwidth is given. Depending on the type of laser, bandwidths range typically from 10(-4) to 10(-9) times the average frequency of the laser; although bandwidths as low as 0.1 Hz. have been reported for stabilized gas lasers.

The wave nature of light most often allows adequate description of the output of the laser, and for most cases it will be sufficient to use geometrical optics to describe the output as a beam with well defined edges and some beam divergence. The beam is emitted from the laser with a beam diameter (alpha) and a beam divergence (PHI), as though it came from a small point source far behind the laser output aperture.

L. Point Source Emission:
The emission from most lasers can be considered as emanating from a “virtual point source” located within or behind the laser device. A “virtual point source” is one which really doesn’t exist, but the properties of the emitted beam are such that there appears to be a source at this position. Thus, the virtual point source is located two meters behind the exit mirror.

M. Gaussian Distribution of the Beam:
The intensity profile across a TEM(oo) laser beam will be in the form of a bell-shaped (Gaussian) distribution. The decrease in intensity at the edge of the beam is the result of diffraction effects produced at the edges.

The spatial intensity distribution of this mode may be expressed by equation: (see printed copy) where R is the radius and W is a constant which defines the mean radius and is commonly referred to as the “spot size.” At this point the intensity has fallen to e(-2) of the peak intensity at the center of distribution.

In fact, the edges f the laser beam are not well defined. If one were to measure the energy or power per unit area point by point across the center of the output aperture, a Gaussian beam distribution is defined. The peak intensity is in the center of the beam and approaches zero as one moves from the center. This shape is maintained as the beam propagates through space subject to broadening and distortion by atmospheric effects.

Important points on the distribution curve are the e(-1) and e(-2) intensity points since they are used as standard quantities to define the laser beam divergence parameter. (The e is the natural number associated with the natural logarithm and is equal to: e= 2.7183). The e(-1) point is where the intensity is reduced by the factor (see printed copy) or approximately 63% of the energy (or power) is contained within the aperture of diameter (a) centered in the beam.

Note: In most all laser safety and compliance standards, the output aperture, a, and the beam divergence, (PHI), are defined relative to the e(-1) points. The total laser energy (Q(t)) or beam power (PHI) is defined as that which is collected from the entire beam (total values).

Many manufactures specifications use the e(-2) i points to define beam divergence. In this case, e(-2) i = 0.1353, or the total power (energy) is: 100% – 0.13 3 x 100% = 86.47% or approximately 86% of the total energy /power is within the e(-2) aperture. In some cases, they specify relative to the 90% point. Note that the beam divergence is larger at the e(-2) or 90% point. Hazard calculations are sensitive to the beam divergence and conversions from e(-2) points to e(-2) power points are often performed on beam sizes.

The beam diameters at the two points are related: (see printed copy). Departure from the Gaussian distribution arise when independent oscillation occurs within the resonator at higher order modes. For example, some gas lasers may be designed to have sufficient gain to support simultaneous oscillation in many different transverse modes. Mode selection may often be accomplished by slight adjustment of the mirror alignments. With this technique, one can observe the different complex intensity distributions of each mode.

The lowest order TEM(oo) mode with the nearly Gaussian intensity distribution has the lowest cavity losses and hence will generally be the dominant mode of oscillation.

Optically pumped solid-state lasers such as the normal mode Nd:YAG laser usually display a randomly varying mode output. Thermal gradients in the optical media (i.e., the crystal) caused by nonuniform absorption of the pump light give rise to lens effects in the crystal which change during the pumping cycle. The result is a sporadic switching of transverse modes during the laser pulse. The time average is generally a bell-shaped distribution which is dependent upon the optical purity of the laser crystal, the pumping scheme, and the level at which the system is operated above lasing threshold.

Some pumping schemes produce pronounced “hot spots” in the intensity distributions. For long range transmission, atmospheric effects can also produce intensity variations by a factor of ten over localized regions of the beam. Such non-uniformities distribution make it difficult to specify the cross sectional area of the beam. As a result, an average value of beam radius must be chosen. Typically, this is often (1) the half-power point; (2) the e(-1) power point; or (3) the e(-2) power point.

A more precise laboratory practice is to measure the diameter at the stated power point on a densitometer recording obtained from a photographic negative of the output beam distribution.

In the case of optically pumped solid-state lasers, the size of the beam cross section is generally a function of the pumping level of the laser. In general, the higher the pumping level, the wider the beam size. Only when pulsed lasers are operated near threshold, or in special cavity conditions, will the zero order mode (lowest beam spread) predominate.

N. Beam Divergence:
Beam divergence is a very important laser parameter and is often expressed in units of milliradians. The symmetry of the laser beam allows the geometry to be reduced to the two dimensions of a plane. The angle (phi), in radians can be related to degrees by noting that for a full circle, phi is 360 degrees.

For some smaller angle, the arc length(s) intercepted along the circumference of the circle can be used to define the angle as: (see printed copy). The minimum beam divergence, called the diffraction limited beam divergence, is related by an equation: (see printed copy). This concept is expanded to three dimensions by introducing the concept of solid angle. The solid angle (OMEGA) is expressed in units of steradians (sr) and is determined by using the area cut out of a surface of a sphere divided by the square of the distance to that surface; that is: (see printed copy). For a sphere, the solid angle may be opened up to include the entire sphere surface area (A = 4 pi R(2)), therefore, the output of a typical laser will be confined to less than 10(-6) sr.

O. Intensity of Laser Emission:
In many applications, the most important laser beam charactertic is the enormous intensity of the beam. Intensity is related to the beam power the cross sectional area and the manner in which the beam spreads from one point in space to the next.

Power, by definition, is the time-rate at which work is done; specifically, it is the rate at which energy is used or produced. Energy relates the ability to do work. As with other forms of energy (eg, chemical, mechanical, electrical), electromagnetic energy (light energy) is a conserved quantity. The intensity of the laser is usually expressed by the IRRADIANCE (power/area) of the beam. This is determined by dividing the average value of beam power by the average value of the beam cross section. Irradiance units are expressed in Watts per square centimeter.

In pulsed laser operation, instantaneous (peak) Irradiances in excess of 100,000 W/cm(2) are quite easily generated in an unfocused high energy pulsed solid state laser pulse. If this output were contained within a typical beam divergence of 20 milliradians and focused by only moderate power optics, the Irradiance at the focal plane would be increased at least one-hundred fold.

A CW laser is rated in Watts and a pulsed laser is normally rated according to the total energy (Joules) per pulse. Pulsed outputs are also expressed as a RADIANT EXPOSURE in units of Joules per square centimeter.

In order to determine the peak power of pulsed laser, it is necessary to know the pulse shape and duration. The peak power may be closely approximated by assuming a triangular pulse shape and dividing the energy per pulse by the pulse duration at half power. Average power is an important factor for high PRF lasers when determining the laser classification and maximum permissible exposure levels.

The radiometric units of RADIANCE and INTEGRATED RADIANCE are used to describe the diffuse reflection of a continuous wave or pulsed laser beam.

Radiance is expressed, by definition, as the Irradiance per unit solid angle (Watts per square centimeter per steradian).

Integrated Radiance is expressed as the Radiant Exposure per unit solid angle (Joules per square centimeter per steradian).

The unit of solid angle is defined such that all space about a point source (i.e., the source of light) will encompass 4 pi sr.

P. Focused Laser Beams:
The beam from an ideal laser, i.e., a laser which emits a coherent wave, can be considered as a diffraction-limited beam. In this case, divergence of the beam is limited to the effects of diffraction at the beam edges. The emission from such a laser will display a far-field diffraction pattern at a distance (see printed copy) where a is the diameter of the emergent laser radiation.

The TEM(oo) beam from a typical helium neon laser will display a 0.5-1.0 milliradian beam spread at a distance of 1.0-2.0 meters from the laser.

Due to the high degree of coherence of a laser beam, it is theoretically possible to focus the beam to the diffraction limit of the wavelength of light. Typically, however, the laser will have a finite beam spread and can be expressed by the simple equations of geometrical optics.

The spot diameter (d) is given by the simple equation: d = f phi where: d = spot diameter at focus f = focal length of lens phi = laser beam divergence (radians).

As an example, one can calculate the spot size of a beam focused on the human retina. For this case, consider a “typical” HeNe laser where: phi = 1.0 milliradian and assume that the effective focal length (f) of the human eye is 1.7 cm.

Thus: d = f phi = (1.7 cm) x (1.0 x 10(-3) rad.) = 17 x 10(-4) cm = 17 µm

To give some idea of how small this focused spot is, consider that 17 micrometers is approximately the size of two or three human blood cells stacked end-to-end.

Using the equation for the area of a circle (see printed copy), one can now calculate the focused beam area (see printed copy).

As the spot diameter approaches the wavelength of light, the spot becomes diffraction-limited. For example, the beam from a highly coherent single transverse mode (TEM(oo)) gas laser will produce a Gaussian intensity pattern when focused. This distribution may be described mathematically by an equation where it is considered that the beam energy will be contained in a diameter defined at the e(-2) power point (see printed copy).

Therefore, the smallest possible spot size of a focused laser beam will approach the dimensions the wavelength of light which is being focused.

Combining the equations above can yield an expression for the spot area (see printed copy).

Thus, the Irradiance (power per unit area) of a focused laser beam will vary inversely with the square of the focal length of the lens and with the square of the beam divergence angle. Hence, these two factors have dramatic effects on the power distribution at the focal plane of the lens.

Consequently, either a reduction in the focal length of the lens used to focus the beam or a reduction in the beam spread by a factor of ten will produce a one-hundred fold increase in the irradiance at the focal plane of the lens. Simultaneous reduction of both by a factor of ten would increase the Irradiance at the focal plane by a factor of 10(4).

In practice, however, it is usually the beam divergence value that limits the focal spot diameter. This is especially true with pulsed laser systems. To achieve high power outputs, the laser crystal is usually pumped well over threshold; consequently, the beam will contain a conglomerate of high order “off-axis” modes which subsequently increase the beam size.

Typical beam divergence values for gas lasers (helium-neon, argon, etc.) will be about one milliradian, (1 milliradian = 3.44 minutes of arc). Solid-state ruby and neodymium lasers generally have a higher beam spread (1- 30 milliradians), due primarily to the high beam divergence associated with the random multimode operation of such devices.

Q. Scanning Lasers:
Some laser applications employ electro-mechanical or electro-optical scanner units to allow a raster-scan capability to the beam. In this way, the beam can be scanned over a large area (such as in a laser print maker) or over a small area (such as a laser UPC label reader) in a repeated geometry. (equations, see printed copy).

R. Coherence:
The coherency of a laser beam relates to the constancy of the spatial and temporal variations in the radiation wavefronts. A high degree of coherence implies a constant phase different between two points on a series of equal-amplitude wavefronts (spatial coherence), and in a correlation in time between the same points on different wavefronts (temporal coherence). The two coherence terms are a part of the overall four-dimensional coherence function which completely describes the degree of coherency of the beam.

If the laser beam is considered as a plane wave traveling in one direction, it will be spatially coherent due to the perpendicularity of the wavefronts in the direction of propagation. Also, due to the monochromatic nature of the laser light, the beam will be temporally coherent; that is, it will display a fixed-phase relation between a part of the beam emitted at one time and portion emitted at another. Should the wavelength (or frequency) change, then the temporal coherency would degrade.

In 1802, Thomas Young performed his classic double-slit experiment to demonstrate the wave nature of light. Sunlight though one pinhole was allowed to illuminate two closely spaced pinholes. Each pinhole acted as a “new source” for light and the waves from each of the two pinholes interfered with one another so-as-to produce corresponding light and dark regions (or fringes – as they are called) at the observation screen. If light was not a wave, it would travel in a straight line from one pinhole to another to fall at two points on the screen. As a wave, however, it is diffracted and bent about the edges of the pinholes such that each pinhole illuminates the entire screen.

In part, if it were not for the diffraction effects produced as a light wave passes through a finite aperture, the plane wave output of laser could theoretically be focused by a lens (such as the human eye) to a real point with minimal spot diameter. Thus, the image Irradiance , (see printed copy) would have an infinite value. That would indeed be hazardous!

Due to the wave nature of light and the corresponding diffraction effects produced by finite apertures, the image a point source of light (provided by any real optical system) is not actually a point.

Such a distribution has a bright central area surrounded by light and dark rings. When an optical system’s resolution is only limited by the diffraction effects, it is said to be diffraction limited. Even in this condition, a point is “spread out” as it is imaged. The more defects and aberrations introduced by the optical system, the more the spreading. Each lens images a point with some spreading and the manner which it behaves is defined as the point spread function.

Since an optical system spreads a point source image, there is a corresponding limit to its resolution or ability to separate two points close together. This is especially important when looking at stars through a telescope. There are at least two criteria used to define the resolution of two points close together. A simple one, and the one most commonly used, is the Rayleigh criterion. This states that when the peak of one Airy disk is over the first dark ring of the other, the points are resolved. This is normally defined in terms of the apparent angle between the two points.

Lasers are often referred to as coherent sources, but in fact, they really only partially coherent. Only absolutely monochromatic or single frequency waves are truly coherent; however, lasers are so close, relative to anything else, that a loose definition may seem justified. The degree to which two waves are coherent determines how well they interfere when brought together at some point in space.

A thorough treatment of the subject of partial coherence is far beyond the scope of this guideline; however, there are a few properties worth discussing. One frequently encounters the terms spatial and temporal coherence. Temporal coherence effects are those which arise from the finiteness of the spectral band. An increase in fringe visibility with a decrease in source size is a measure of the spatial coherence. An important measure of coherence is the coherence length, dL, which can be conceptually related to the duration of an uninterrupted wavetrain. Even in the beam from an “ideal” laser, there will be random fluctuations in the phase difference of the electromagnetic fields at two separate points on a wavefront. The distance between points on the wavefront for which the average of this phase different is equal to (equation, see printed copy) adians is generally defined as the lateral coherence distance. Recombination of the light samples from points separated by a distance equal to, or less than, this amount can produce interference fringes. The distance is a classical measure of the spatial coherence of a light beam as observed in the famous “double slit” experiment of Young.

The temporal coherence is a measure of the length of time that the beam is truly monochromatic. This may be considered as the time during which the amplitude of the electromagnetic field will remain constant at a given point in space while the phase varies linearly with time. During this time, the beam will travel a length dL = c dT defined as the coherence length (where c = 3 x 10(8) m/sec., the velocity of light). Thus the coherence time is the time required for light to travel the coherence length in the direction of travel of the beam.

By virtue of this argument, it is seen that the frequency bandwidth is actually a measure of temporal coherence. Thus a frequency stabilized HeNe gas laser (d(v) = 3-5 hertz) will have a coherence time of several hundred milliseconds and a corresponding coherence length of 10(5) km.

In contrast to the high spectral purity of gas lasers, the coherence lengths of pulsed ruby lasers are in the order of 15 meters with corresponding coherence times in the order of only 100 nanoseconds.

S. Polarization of the Laser Output:
The polarization of most lasers is directly related to the nature of the resonator. For example, many high power gas lasers are built with Brewster’s angle windows on both ends of the gas discharge tube. Such windows present virtually no losses to a beam which has a linear polarization component lying in the plane of incident. Hence the output will be linearly polarized in this plane.

In some solid-state crystal lasers, for example, the ruby laser, the output will be linearly polarized. This is a result of the birefringent nature of the crystal in which the slower “ordinary” polarized photons will have a longer time to interact with the excited chromium ions, thereby favoring a polarized output in this plane. This is generally only true for ruby crystals operating near lasing threshold unless Brewster’s angles are fabricated on the ends of the crystal. This latter practice is often necessary for very high power Q-switched laser systems.

In diode lasers, linear polarized light is also observed. This may be attributed to the linear symmetry of the junction region.

T. Electrical Field Strenght:
The electromagnetic theory of light depicts a light wave as having instantaneous electric and magnetic fields which oscillate at the same frequency. The electrical (E) and magnetic (H) fields are fixed at right angles and are mutually perpendicular to the direction of propagation of the wave. Of particular importance in the description of laser beam interactions is the magnitude of the electric field associated with the beam.

From classical considerations (using Maxwell’s equations) the electric field (E) in volts per centimeter associated with a light beam in a vacuum (or air) of average power (PHI) in Watts, spread over a cross-sectional area (A) in cm(2) is given by (equation, see printed copy).

Prior to lasers, the electric fields associated with commonly occurring light sources were most nominal. For example, the electric field of sunlight occurring at the earth’s surface is about (equation, see printed copy). This constitutes an average field spread over all the wavelengths present in the “white light” of the sun.

In contrast, the instantaneous electric field associated with an unfocused “Q- Switched” Nd:YAG laser burst operating at a level of 100 megawatts and confined to 3 mm beam diameter will approach (Equation, see printed copy).

Such strong fields are also found elsewhere in nature, as they are at the magnitude of the electrostatic cohesive forces which bind atomic structures. Consequently, when a laser beam with a field of comparable magnitude enters a transparent structure, an instantaneous massive redistribution of the electric system of the material can occur due to the interaction of the fields. At the present, the interaction of these enormous electromagnetic fields is not fully understood, to be sure. The production of free electrons, ionized atoms, and X-rays have been detected in the reaction association with the interaction of high power laser beams.

U. Comparison With Other Sources:
Light from conventional thermal sources is emitted over a wide spectral band. The polarizations of the photons are distributed over all possible states of polarization and leave the source in all possible directions (Lambertian source). In contrast, a laser source has a very narrow spectral linewidth even in comparison to special, narrow band thermal sources; the photons may have, essentially, the same polarization and they are highly directional as they leave the laser cavity.

Conventional optical sources can most certainly constitute a hazard to the human eye and/or skin, particularly close up and when focused. For example, one’s first introduction to optical physics might well have been using a magnifying glass to focus the sun’s rays on dry leaves to start a fire. However, even a relatively small laser is capable of producing power/energy distributions much greater that conventional sources. In addition, the hazard can exist at very long ranges due to the highly directional nature of the laser output.

A conventional thermal source will emit light into a sphere or hemisphere. The power/energy per unit area (intensity) may be large at the source; however, the intensity at the observer falls off rapidly as the observer moves away from the source. The intensity at the observer can be dramatically increased by using optics to reduce the divergence, making a searchlight; however, the effect is limited by the size of the source.

The output of the laser has a very small divergence, typically less that 1 milliradian (1 mrad = 0.0573 degrees), and the intensity decreases very slowly as the distance to the observer increases. It would take a very powerful thermal source to put as much power into as tight a beam as offered by even the smaller lasers. If one were to insert a very narrow bandpass filter into the searchlight (in order to approximate the spectral purity of monochromatic nature of the laser output), the laser would be brighter than any thermal source by an enormous factor, where brightness is defined as the power output per steradian of solid angle.

To illustrate the relative brightness of the laser over its narrow band, one notes that the sun emits, at its surface, approximately 10(4) W/cm(2)/sr/µm and lasers can produce greater than 10(10)W/cm(2)/sr/µm in single pulse. Therefore, it is not difficult for a laser to be a million times brighter that the sun. Indeed, a laser can not only burn dry leaves, but some are used to weld metal. The most significant factor is not total power, but rather the power per unit area, where the laser may be focused to an extremely small spot (approximately a wavelength in diameter). For example, a one milliwatt laser focused to a one micrometer spot will produce a focused irradiance greater than 1×10(5) W/cm(2).

Guidelines for Laser Safety and Hazard Assessment
Source: Occupational Safety & Health Administration, Guidelines for Laser Safety and Hazard Assessment PUB 8-1.7 (tablular data and equation illustrations have been omitted)