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
2x10(-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 1x10(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).
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