U.S. patent application number 11/188398 was filed with the patent office on 2005-12-08 for high intensity and high power solid state laser amplifying system and method.
This patent application is currently assigned to JMAR Research, Inc.. Invention is credited to Cambeau, Serge, Rieger, Harry.
Application Number | 20050271111 11/188398 |
Document ID | / |
Family ID | 25423602 |
Filed Date | 2005-12-08 |
United States Patent
Application |
20050271111 |
Kind Code |
A1 |
Rieger, Harry ; et
al. |
December 8, 2005 |
High intensity and high power solid state laser amplifying system
and method
Abstract
Systems and methods are provided for achieving high power and
high intensity laser amplification. In a four-pass optical
amplifying system, a linear polarized optical beam is directed by
various optical elements four times through an optical amplifier.
The optical amplifier is transversely pumped by a pumping energy
source that includes laser diode arrays. The pumping module and the
other optical components are provided to counteract thermal lensing
effects, induced thermal birefringence effects and to achieve
enhanced amplification and efficiencies.
Inventors: |
Rieger, Harry; (San Diego,
CA) ; Cambeau, Serge; (San Diego, CA) |
Correspondence
Address: |
BAKER & MCKENZIE
PATENT DEPARTMENT
2001 ROSS AVENUE
SUITE 2300
DALLAS
TX
75201
US
|
Assignee: |
JMAR Research, Inc.
|
Family ID: |
25423602 |
Appl. No.: |
11/188398 |
Filed: |
July 25, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11188398 |
Jul 25, 2005 |
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09907154 |
Jul 16, 2001 |
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09907154 |
Jul 16, 2001 |
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09689539 |
Oct 12, 2000 |
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60159521 |
Oct 15, 1999 |
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Current U.S.
Class: |
372/69 ;
372/75 |
Current CPC
Class: |
H01S 3/0064 20130101;
H01S 3/0606 20130101; H01S 3/005 20130101; H01S 3/1103 20130101;
H01S 3/2341 20130101; H01S 3/0941 20130101; H01S 3/094076 20130101;
H01S 3/1611 20130101; H01S 3/1643 20130101; H01S 3/063 20130101;
H01S 3/08072 20130101; H01S 3/1106 20130101; H01S 3/094084
20130101; H01S 3/2316 20130101 |
Class at
Publication: |
372/069 ;
372/075 |
International
Class: |
H01S 003/09; H01S
003/091; H01S 003/094 |
Claims
What is claimed is:
1. A pumping module for substantially uniformly pumping a laser
crystal rod, the laser crystal rod having a longitudinal axis,
comprising: a set of laser diode arrays disposed around the laser
crystal rod, the laser diode arrays being approximately equally
spaced around the laser crystal rod, each of the laser diode arrays
including laser diodes that are disposed in a plane that is
substantially orthogonal to the longitudinal axis of the laser
crystal rod, the laser diodes emitting light that substantially
uniformly pumps the laser crystal rod.
2. The pumping module according to claim 1, wherein the set of
laser diode arrays includes an odd number of laser diode
arrays.
3. The pumping module according to claim 1, wherein the set of
laser diode arrays includes at least five laser diode arrays.
4. A pumping module according to claim 1, further comprising: a
spherical lensing module disposed along the optical axis, the
lensing module being adapted to approximately negate thermal
spherical lensing effects present in the laser crystal rod during
the substantially uniform pumping of the laser crystal rod.
5. The pumping module according to claim 4, wherein the spherical
lensing module includes a negative lens.
6. The pumping module according to claim 4, wherein the spherical
lensing module includes a positive lens.
7. A pumping module according to claim 1, further comprising: a
reflecting element disposed in the plane and opposite a respective
laser diode array with respect to the laser crystal rod, the
reflecting element reflecting, back to the laser crystal rod, light
emitted from the respective laser diode array that passes through
the laser crystal rod without contributing to the pumping of the
laser diode.
8. A pumping module according to claim 1, further comprising: at
least one electrical load coupled to at least one laser diode
array, wherein the electrical load is configured to match a power
level of the at least one laser diode array to a power level of the
laser diode array with a lowest power level in the set.
9. A pumping module according to claim 1, further comprising: power
control means coupled to at least one laser diode array, the power
control means are configured to adjust light intensity of the at
least one laser diode array to match a laser diode array with a
lowest pumping light intensity.
10. A pumping module according to claim 1, wherein each laser diode
has a light emitting surface that has a substantially rectangular
shape characterized by a long side and a short side, each laser
diode being disposed relative to the laser crystal rod such that
the short side extends in a substantially parallel direction as the
longitudinal axis of the laser crystal rod.
11. A pumping module according to claim 1, further comprising:
cylindrical lenses disposed between the laser diode arrays and the
laser crystal rod, the cylindrical lenses being configured to guide
the emitted light toward the laser crystal rod, the cylindrical
lenses being adapted such that focal points of the lenses are not
within the laser crystal rod.
12. A pumping module according to claim 1, further comprising:
cylindrical lenses disposed between the laser diode arrays and the
laser crystal rod, the cylindrical lenses being configured to guide
the emitted light toward the laser crystal rod, the cylindrical
lenses being integrally formed with a cladding layer that is
disposed around the laser crystal rod.
Description
[0001] This is a divisional patent application of co-pending U.S.
patent application Ser. No. 09/907,154, filed Jul. 16, 2001,
entitled "High Intensity and High Power Solid State Laser
Amplifying System and Method", which is a continuation-in-part of
co-pending U.S. patent application Ser. No. 09/689,539, filed Oct.
12, 2000, entitled "Beam Correcting Laser Amplifier", which itself
is based on U.S. Provisional Patent Application Ser. No.
60/159,521, filed Oct. 15, 1999, entitled "Beam Correcting Laser
Amplifier". Priority is claimed to the above-identified co-pending
U.S. Patent Application(s) and to the above-identified U.S.
Provisional Patent Application. The disclosures of all of the
above-mentioned applications are incorporated herein by reference
in their entirety for all purposes.
FIELD OF THE INVENTION
[0002] The present invention generally relates to a system and a
method for amplifying coherent light and, more specifically, to a
system and a method for amplifying coherent light of a laser
system.
BACKGROUND OF THE INVENTION
[0003] High power and high intensity laser systems are very
desirable. However, such high power and high intensity lasers are
very hard to obtain with a high quality beam and a short pulse
duration. Laser amplifiers take an input laser beam from an
external laser oscillator and amplify the input laser beam.
Increasingly more intense and more powerful laser beams are
achieved by increasing amplification power. However, conventional
laser amplifiers have design and performance aspects that limit and
even reduce achievable power and intensity gains. At high power and
high intensity, heat generated by the laser pump light can create
thermal optical effects and thermal stresses in laser and
amplifying systems which distort the light beam, making
conventional laser and amplifying systems inefficient or even
inoperable. Furthermore, the energy contained in high power and
high intensity laser beams can permanently damage, if not instantly
vaporize, components of conventional laser and amplifying
systems.
[0004] A limit on high power and high intensity laser amplification
is the B-integral effect. The B-integral effect describes the
relationship between the refractive index of a material and the
intensity of illumination. Thus, a light beam with a non-uniform
intensity distribution, such as a Gaussian intensity profile, has
higher indices of refraction in areas of higher light intensity.
Varying illumination intensities and thus varying indices of
refraction also occur due to non-uniform energy densities resulting
from laser pumping sources. The refractive index of the material
determines the phase velocity of light through it, and thus the
effective optical path length. As a result, phase delays occur in
the regions of higher intensity, distort the focus of the light
beam and limit the gains in intensity and power. A varying index of
refraction also alters the optical path of affected portions of the
beam, causing the whole beam or portions of the beam to collapse
into focus points. The B-integral effect becomes more pronounced
under high power and high intensity amplification because of the
greater variances in illumination levels.
[0005] As a result of the B-integral effects and other sources of
distortion to the light beam (such as optical imperfections in the
laser path), high power and high intensity amplification in
conventional laser amplifiers creates regions of heat accumulation
(i.e., hot spots). Hot spots occur in areas of imperfections that
disrupt the laser, dissipating energy into the surrounding regions.
Hot spots may also form as a result of non-uniform pumping that
causes varying levels of heat (e.g., heat gradients) to develop in
different regions within the amplifier. As a region heats, it
further distorts the refractive index profile in the laser
amplifier, leading to still greater heat accumulation. This cycle
of increasing heat and distortion continues until either the laser
amplifier breaks down or destructive optical interference due to,
for example, phase delays, prevents further gains in intensity and
power.
[0006] For at least the above reasons, conventional laser amplifier
designs are prone to hot spot formation and are limited in
achievable gains in intensity and power. Hot spot formation also
enhances inefficiencies in conventional laser amplifiers since much
of the amplifying laser light energy is lost as it is converted
into waste heat. Furthermore, conventional laser amplifiers are not
well designed to withstand hot spots and rapidly break down under
high power and high intensity amplification, requiring expensive
repair and replacement of parts.
[0007] To manage these high temperatures, a means of active heat
removal is generally advantageous. Conventionally, the non-optical
surfaces of the laser crystal rod are cooled by the forced
convection of a fluid, which is usually water. Alternatively, these
surfaces can be thermally connected to a heat sink of sufficient
mass to absorb the waste heat. However, due to the geometry of the
active laser volume and the relatively low thermal conductivity of
the laser crystal rod, high temperatures and large temperature
gradients may persist.
[0008] Accordingly, there is a need for systems and methods for
amplifying light that effectively produces high power and high
intensity laser beams, but minimizes the formation of harmful hot
spots and/or is robust enough to withstand the hot spots that do
develop.
SUMMARY OF THE INVENTION
[0009] The present invention alleviates to a great extent the
disadvantages of conventional systems and methods for amplifying
light. In an exemplary embodiment, the present invention provides a
four pass laser amplifier that receives a polarized laser input
beam from an external source, directs the input beam through four
amplifying passes in an amplifier, and then allows the beam that
has been so amplified to exit as a desired output. The amplifier
may provide for a first polarizing beam splitter (PBS), a second
PBS, a directional polarization rotator (DPR), a non-directional
polarization rotator (NPR), a first reflector, a second reflector
and/or a pumping module. Furthermore, although the exemplary
embodiment employs four passes through the amplifier, the present
invention also contemplates other even or odd numbers of amplifying
passes. Furthermore, passes that are made by the laser beam through
the amplifier need not be collinear.
[0010] Amplification of the input beam occurs in the pumping module
that is disposed in the beam path, downstream from the two PBS's
and the DPR. The pumping module includes one or more pump sources
(e.g., light sources) that add optical energy to the input beam to
increase its intensity and power. The one or more pump sources may
include, for example, a flash pump, another type of lamp and/or a
laser diode. Laser diodes can be adapted to emit in a relatively
small frequency band centered around a desired frequency.
Accordingly, efficiency is increased since, for example, light
energy is not substantially wasted at other frequencies that might
not provide effective pumping.
[0011] The pumping module may include an optical pathway that
itself includes an elongated laser crystal rod including, for
example, a solid state material such as yttrium-aluminum-garnet
(YAG), doped with active materials such as, for example, neodymium
(Nd), ytterbium (Yb), holmium (Ho), and/or erbium (Er). The active
materials can be optically excited, for example, by light impinging
on them from the laser diodes, so that certain electrons within
atoms of such materials are temporarily excited (i.e., temporarily
raised to elevated energy states). If the input beam impinges upon
such excited materials before, for example, the electrons
spontaneously revert to their normal, stable energy states, then
that reversion can be triggered (i.e., stimulated), thereby causing
a massive release of photons as a result of the electrons returning
to the lower energy state. In other words, the laser crystal rod is
pumped by the laser diodes, thereby creating corresponding excited
atoms that give up quanta to the radiation field via induced
emission. The stimulated or induced emission provides a
phase-coherent amplification mechanism for the input beam.
[0012] Individual laser diodes are used in the laser amplifiers and
emit, for example, a rectangular-shaped beam of light, although
other shapes are also contemplated by the present invention. These
diodes may be mounted in longitudinal rows comprising diode bars. A
plurality of such bars are mounted adjacent to each other (with the
diodes mutually oriented in the same direction in a matrix) forming
a laser diode array. These arrays are then positioned in a desired
configuration opposite the laser crystal rod so that the diode
light can impinge upon and pump the rod. Each laser crystal rod is
paired with at least one set of laser diode arrays within an
amplifier stage. Multi-stage amplifiers, including, for example, a
pre-amplifier stage and amplifier stages arranged in series and/or
parallel, are contemplated.
[0013] In an exemplary embodiment, the present invention provides
that the pumping efficiency is enhanced by using an odd number of
laser diode arrays disposed around the laser crystal rod. For
example, in an amplifier stage having an odd number of
regularly-spaced, circumferentially disposed arrays, no two arrays
are disposed opposite each other (i.e., at 180.degree. to each
other) around the longitudinal axis of the laser crystal rod. In
addition, reflectors disposed between the laser diode arrays can
reflect pumping energy that passes unabsorbed through the laser
crystal rod, back into the active medium.
[0014] The present invention has an advantage in supplying
substantially uniform pumping energy in the laser amplifier.
Pumping uniformity may be improved by increasing the number of
laser diode arrays disposed around the circumference of the laser
crystal rod, thereby decreasing the radial angle between adjacent
laser diode arrays. Furthermore, increasing the number of laser
diode arrays increases the total amount of available pumping energy
(by increasing the number of input energy sources) and enhances
amplification. Accordingly, in an exemplary embodiment, the present
invention may provide, for example, five or more laser diode
arrays.
[0015] The present invention may also provide one or more
cylindrical lenses and/or mirrors that direct the emissions from
the laser diodes. As a result, amplification efficiency improves
since more pumping energy reaches the laser crystal rod where it
can amplify the input beam. Pumping uniformity is further improved
by disposing the lenses and/or mirrors so that the focal points of
the pumping energy are, for example, at a distance away from the
surface of and outside of the laser crystal rod. In this manner,
when the pumping light reaches the laser crystal rods, it is
unfocused and accordingly, dispersed evenly across the diameter of
the laser crystal rod. The present invention also contemplates the
use of aspherical lenses to further improve amplifier
performance.
[0016] In an exemplary embodiment, the present invention uses a
plurality of laser crystal rods aligned along their long axes. This
multi-stage configuration allows for increased amplification of the
input beam by increasing the number of pumping energy sources
successively amplifying the same light beam. In addition,
multi-stage configurations may benefit by adjusting for
distortions, for example, resulting from thermal lensing effects
and/or birefringence effects.
[0017] In an exemplary embodiment, the present invention provides
laser diode bars and/or individual laser diodes that are selected
to have substantially identical characteristics (e.g.,
substantially similar peak output intensities and wavelengths). The
laser diode bars and/or the laser diodes are then incorporated into
laser diode arrays and wired so that each laser diode receives
substantially identical electrical input. Such laser diode arrays
do not vary significantly in output power and average peak output
wavelength.
[0018] The present invention also has an advantage in that the
output power of the laser diode arrays may be controlled so as to
insubstantially vary. In an exemplary embodiment, the present
invention provides an electrical power supply for individual laser
diode arrays so that output power of the laser diode arrays in each
of the stages of the laser amplifier can be empirically matched.
The electrical power supplies include power controlling means
(e.g., a rheostat, tunable transistor, etc.) which may be monitored
and controlled manually or automatically such as, for example, by a
computer system employing feedback loop circuitry using, for
example, sensor circuitry. In another exemplary embodiment, the
present invention provides that the electrical input to each of the
arrays is limited to the level used by the least powerful laser
diode array. Laser diode arrays that exhibit higher power than the
least powerful laser diode array may have electrical loads placed
in parallel with the higher power arrays, thereby draining the
proper amount of power from the higher power arrays. Thus, all of
the laser diode arrays produce substantially similar output
power.
[0019] In an exemplary embodiment, the present invention provides
that the laser diode arrays may be oriented such that the short
side of the rectangular light emitting surface of each laser diodes
is disposed in parallel with the longitudinal axis of the laser
crystal rod. The rectangular light emitting surface of the laser
diodes has two sets of sides, the short sides and the long sides.
The length of a short side is smaller than the length of a long
side. In viewing the intensity patterns of light emitted from the
rectangular emitting surface of a particular individual laser
diode, there is a large optical angular dispersion in a direction
parallel to the short sides (i.e., the short axis or the fast axis)
of the rectangular light emitting surface. Conversely, there is a
small optical angular dispersion in a direction parallel to the
long sides (i.e., the long axis or slow axis) of the rectangular
light emitting surface. Such optical angular dispersion can be
approximately analyzed as a two-dimensional single slit diffraction
pattern in which optical angular dispersion is approximately
proportional to the wavelength of the emitted light and
approximately is inversely proportional to the width of the slit in
a particular dimension. By perpendicularly orienting the individual
laser diodes relative to the longitudinal axis of the laser crystal
rod (i.e., the short axes or the fast axes of the laser diodes are
parallel to the longitudinal axis of the laser crystal rod), the
dispersion away from their long axes or slow axes substantially
overlaps each other and effectively smoothes out the pumping
intensity impinging on the laser crystal rod. The smoothing out of
the pumping intensity enhances the uniformity with which the laser
crystal rod is pumped. The present invention also contemplates
employing other laser diode assemblies or individual laser diodes
that take advantage of the above-described smoothing effects and
uniformity in pumping intensity. Further pumping uniformity may be
achieved by decreasing the spacing between the laser diodes in the
array. Thus, for example, the exemplary embodiment naturally
provides decreased spacing between laser diodes, and thus provides
increased uniformity in the pumping intensities impinging upon the
laser crystal rod. In addition, the exemplary embodiment also uses
a larger number of laser diodes. These latter aspects improve the
overall amplification and efficiency of the laser amplifier.
[0020] In an exemplary embodiment, the present invention improves
the durability and robustness of the laser amplifier. The pumping
source, and thus the laser amplifier, continue to operate
effectively even if an isolated laser diode or an isolated laser
diode bar fails. This is due, in part, to the substantial overlap
of pumping intensity between substantially adjacent individual
laser diodes and/or substantially adjacent laser diode bars. Thus,
optical angular dispersion from the substantially adjacent laser
diodes and/or the substantially adjacent laser diode bars
sufficiently illuminates the portion of the laser crystal opposite
the dark (damaged) bar and/or diode to compensate for the lost
pumping energy. Alternatively, the pumping intensity can be
selectively increased via control circuitry which may be coupled to
a computer that monitors and adjusts pumping intensity (e.g.,
adjusts power supplied to individual laser diodes, laser bars
and/or diode arrays) to compensate for the lost pumping energy in
the portion of the laser crystal rod opposite the dark (damaged)
laser bar and/or laser diode.
[0021] In an exemplary embodiment, the present invention achieves
greater uniformity in pumping energy by providing a plurality of
sets of laser diode arrays along the longitudinal axes of the laser
crystal rods. Each successive set of arrays is rotated with respect
to the orientation of an adjacent set of arrays, so that the
pumping energy reaches the laser crystal rod or laser crystal rods
in each amplifier stage from different radial directions. For
example, with laser amplifiers using sets of five laser diode
arrays for transverse pumping, each successive set of laser diode
arrays may be rotated relative to adjacent set of arrays by
36.degree.. Thus, laser diode arrays in adjacent amplifier stages
are disposed in a radially staggered relative orientation along the
laser crystal rod longitudinal axes. Note, however, that although
such staggering yields symmetrical (and therefore optimum)
distribution of the five arrays around the 360.degree.
circumference, other, non-symmetrical staggering arrangements can
also be used.
[0022] In an exemplary embodiment, the present invention provides a
rotator such as, for example, a 90.degree. rotator, disposed
between successive laser crystal rods in a multi-stage amplifier.
The rotator compensates for phase and polarization changes in the
input beam resulting, in part, from refractive index gradients for
different polarization orientations in the laser crystal rods. In
particular, the rotator changes the polarization of the light so
that the refractive index gradients in one laser crystal rod
counteract (e.g., substantially cancel out) the polarization
changes that occur in the other laser crystal rod.
[0023] Under high average pumping conditions, the laser crystal rod
may exhibit an internal heat distribution across its cross section
with the highest temperatures at its axial core. Since the
refractive index of the material is a function of the temperature,
the refractive index within the laser crystal rod may take on a
similar distribution. As a result, after reaching steady state
operation, the laser crystal rod may behave like a positive lense,
altering the propagation characteristics of the beam. However, it
is advantageous for the beam to retain a parallel profile across
the beam throughout the multiple passes through the laser amplifier
in order to achieve an amplified beam having uniform power and
intensity gains. Accordingly, in an exemplary embodiment, the
present invention provides uniform pumping conditions across the
laser crystal rods to achieve simple spherical lensing for which
compensation may be achieved via, for example, a spherical lense.
The present invention may also provide a compensating lensing
module that may include, for example, a negative lens or, if
applicable, a positive lens disposed between the first amplifying
module and the second amplifying module. In an exemplary
embodiment, the amplifying modules include the laser crystal rod,
which includes a material that exhibits, for example, positive
lensing effects under high average pumping power. The negative lens
is adapted to compensate for the positive thermal lensing effects
of the two laser crystal rods disposed in the amplifying modules so
that the beam remains substantially parallel and collimated
throughout the multiple passes. The negative lens is selected not
only to inversely match the positive thermal lensing effects of the
two laser crystal rods in amplifying modules, but also to withstand
the rigors of laser amplification.
[0024] To minimize polarization effects within the laser crystal
rods, the successive laser crystal rods in a given amplifier should
have substantially similar physical characteristics. In an
exemplary embodiment, the present invention provides for
manufacturing the laser crystal rods for a given multi-stage
amplifier from the same crystal boule. Laser crystal boules may be
grown from a melt containing the active elements, which are
subsequently embedded into the crystal. Some dopant concentration
gradients may be established along the longitudinal direction of
the crystal boule. For example, the highest dopant concentrations
may be at the bottom end of the crystal boule. Coring the rods from
the same longitudinal region (e.g., a transverse slice of the
crystal boule) in the same crystal boule thus yields laser crystal
rods having substantially identical dopant concentration profiles
and other similar physical characteristics, thereby achieving even
greater similarity between the laser crystal rods. Accordingly, the
similarity between the laser crystal rods provides similar stress
birefringence (e.g., thermal stress birefringence) that is
advantageous, for example, in canceling the effects of the similar
stress birefringence.
[0025] The first and the second PBS's are disposed in the laser
path to transmit the original input beam, but deflect light having
a polarization that is perpendicular (normal) to the original input
beam polarization. The first PBS allows the original input beam to
enter the laser amplifier, but directs away the fully amplified
beam as output, because its polarization at that point is
perpendicular to that of the input beam. As a result, the first PBS
allows the first amplifying pass of the laser beam through the
amplifier to occur, and finishes the fourth and final pass through
the amplifier. The second PBS transmits the original input beam and
(in the opposite direction) the fully amplified beam, but deflects
the partially amplified beam after the second pass and prior to the
third pass through the amplifier, so that it undergoes the third
pass (followed by the fourth pass) for more amplification. Thus,
the second PBS deflects the second pass to initiate the third pass,
while not directly affecting the first or fourth passes.
[0026] A DPR is disposed in the beam path between the first and the
second PBS's. The DPR includes a component or set of components
that has no polarization effect on the beam travelling from the
first PBS to the second PBS, while rotating the polarization of the
beam travelling (after full amplification) from the second PBS back
to the first PBS. As a result, the second PBS transmits light
received from the first PBS, while the first PBS deflects light
received from the second PBS. The DPR allows the original input
beam to enter the amplifier, while rotating the polarization of the
fully amplified beam so that the first PBS deflects it away as the
output beam. The DPR may include a Faraday rotator, a wave plate or
some combination thereof. With respect to light traveling in the
input direction, these elements are selected to impose mutually
opposite polarizations so that they cancel each other out. With
respect to light traveling in the opposite direction the Faraday
rotator and the wave plate have an additive polarizing effect.
[0027] The NPR is in the beam path, downstream from the pumping
module. The NPR rotates the polarization of the input beam 900 on
two passes. As a result, the beam is orthogonally polarized in the
second pass (after a cumulative 900 of rotation) so that the second
PBS deflects it away to begin the third pass. The NPR also returns
the beam to its initial polarization in the fourth pass (after a
cumulative 1800 of rotation), allowing the second PBS to transmit
(rather than deflect) the beam. The NPR may include, for example, a
wave plate. In an exemplary embodiment, the present invention
provides a Faraday rotator which also may achieve residual
birefringence compensation of, for example, the dual laser rod
system.
[0028] The four-pass amplifier also may include two reflectors. The
first reflector is in the beam path downstream from the NPR and
returns the beam after the first and third passes to begin the
second and the fourth passes. The second reflector is disposed in
the deflection path of the second PBS to receive and return the
beam between the second and third passes. The reflectors include
optical mirrors that receive and return the input beam. Laser
reflector materials in high power and high intensity applications
can be fabricated from materials that operate to optically reflect
and to transmit light, that can dissipate heat efficiently and/or
that are structurally robust. Hard boundary reflectors such as
metal-backed mirrors may be disadvantageous for certain
applications since the amplified laser light may destroy the
metal-backed mirrors.
[0029] In an exemplary embodiment, the present invention provides
that one or both of the reflectors include a Porro prism. Porro
prisms include 45.degree.-90.degree.-45.degree. solid structures
constructed of substantially clear materials. Porro prisms
efficiently reflect the entering light energy without the need for
reflective coatings. Furthermore, Porro prisms increase the
uniformity of the laser beam by inverting the beam after the first
and third passes through the amplifier. As a result, the beam
passes through the four-pass amplifier twice in one orientation,
and twice in an inverted orientation to homogenize (e.g., smooth,
counteract or cancel out) imperfections in the amplification
profile within the laser crystal rods.
[0030] In an exemplary embodiment, the present invention provides
that the first and the second PBS's have polarizing coatings on an
outside surface. In another exemplary embodiment, the present
invention provides that the first and the second PBS's are solid
PBS's having polarizing coatings on an interior surface. For
example, the PBS might constitute a solid, cubed-shaped
optically-transparent device that has a polarizing coating on an
interior diagonal plane. The solid PBS's are especially applicable
in the field of high-powered laser applications. High power and
high intensity laser systems may produce beam intensities of, for
example, a few gigawatts per square centimeter (GW/cm.sup.2), which
when focused, may exceed, for example, 10.sup.15 W/cm.sup.2 at a
particular focal plane. The positioning of the polarizing coating
in a high-powered laser amplifier avoids the formation of hot spots
at the air-coating boundary since the polarizing coating no longer
substantially contacts air.
[0031] In an exemplary embodiment, the present invention provides a
cladding layer around the circumference of the laser crystal rod to
reduce diffraction effects and to improve amplification
performance. The cladding layer may be a substantially clear
solid-state material (e.g., a material similar to that of the laser
crystal rod, but without the active element dopants). The cladding
layer may be configured to improve beam quality by reducing
diffraction effects of the beam that is exiting the laser crystal
rod especially where, for example, the beam diameter exceeds the
laser crystal rod diameter without the cladding layer.
[0032] In an exemplary embodiment, the present invention provides a
fluid (e.g., water) that cools the laser crystal rod. The fluid may
be contained within a housing (casing) including a substantially
clear material (e.g., glass or plastic). The casing and fluid are
disposed between the laser crystal rod and the pumping source. The
fluid layer may undergo forced circulation to help deter localized
heat accumulation. In an another exemplary embodiment, the present
invention provides that the case is adapted to include or to form a
lens (e.g., a positive lense) that focuses the pumping energy from
the laser diodes and directs it toward the optical pathway within
the laser crystal rod. The adapted case achieves the same increased
efficiency that can be produced by employing lens systems and
mirrors, but at reduced materials expense. The adapted casing may
be an elongated unitary lens spanning the length of the laser
crystal rod and surrounded by the laser diode arrays. The shape of
the wall structure of the casing can then be configured using
fundamental lens principles to achieve the desired lensing. Such a
casing can be fabricated, for example, by using extrusion, molding
or machining techniques.
[0033] In an exemplary embodiment, the present invention provides
that the substrates of the first and the second reflective optical
mirrors include transparent materials (e.g., sapphire or diamond)
characterized by high heat conductivity. Artificial analogs (e.g.,
cubic zirconium) can also be used. Sapphire and diamond have the
physical property of rapidly diffusing localized heat. Thus,
sapphire and diamond substrates help prevent the accumulation of
heat that causes hot spots. In addition, both sapphire and diamond
are durable materials, thus increasing the reliability of the
mirrors.
[0034] These and other features and advantages of the present
invention will be appreciated from review of the following detailed
description of the present invention, along with the accompanying
figures in which like reference numerals refer to like parts
throughout.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a schematic representation illustrating a light
amplifying system according to the present invention;
[0036] FIG. 2 is a schematic representation illustrating a light
amplifying system according to the present invention;
[0037] FIG. 3 is a perspective view of a polarizing beam splitter
according to the present invention;
[0038] FIG. 4 is a perspective view of a polarizing beam splitter
according to the present invention;
[0039] FIG. 5 is a perspective view of a Porro prism according to
the present invention;
[0040] FIG. 6 is a schematic representation illustrating an
amplifying module according to the present invention in which the
laser diodes are in a conventional orientation;
[0041] FIG. 7 is a schematic representation illustrating an
amplifying module according to the present invention;
[0042] FIG. 8 is a cross-sectional view illustrating an amplifying
module according to the present invention;
[0043] FIG. 9 is a cross-sectional view illustrating an amplifying
module according to the present invention;
[0044] FIG. 10 is a cross-sectional view illustrating an amplifying
module according to the present invention;
[0045] FIG. 11 is a schematic representation illustrating a laser
diode bar according to the present invention;
[0046] FIG. 12 is a schematic representation illustrating a laser
diode array according to the present invention;
[0047] FIG. 13 is a schematic representation illustrating selected
portions of a pumping module according to the present
invention;
[0048] FIG. 14 is a schematic representation illustrating a laser
diode according to the present invention;
[0049] FIG. 15 is a schematic representation illustrating selected
portions of a pumping module according to the present
invention;
[0050] FIG. 16 is a schematic representation illustrating a master
oscillator according to the present invention;
[0051] FIG. 17 is a block diagram illustrating an amplifying system
according to the present invention;
[0052] FIG. 18 is a block diagram illustrating a lithography system
according to the present invention;
[0053] FIG. 19 is a schematic representation illustrating a
cladding layer and a laser crystal rod according to the present
invention;
[0054] FIG. 20 is a schematic representation illustrating selected
portions of a pumping module according to the present invention;
and
[0055] FIG. 21 is a schematic representation illustrating a
lithography system according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0056] In accordance with the present invention, a system and a
method are provided that produce high power and high intensity
laser beams of high quality.
[0057] I. System and Method Overview
[0058] In an exemplary embodiment as illustrated in FIG. 1, the
present invention provides an external laser source 10 (e.g., a
master oscillator) and a laser amplifier 5. The external laser
source 10 generates an input beam 20 that is amplified by the laser
amplifier 5. The laser amplifier 5 includes a first polarizing beam
splitter (PBS) 30, a second PBS 40, a directional polarization
rotator (DPR) 50, a non-directional polarization rotator (NPR) 60,
a first reflector 70, a second reflector 80 and a pumping module
90. In the illustrated embodiment, the laser amplifier 5 is
configured as a four-pass optical amplifier. However, the present
invention also contemplates an n-pass optical amplifier in which n
is a cardinal number. Furthermore, the present invention
contemplates modifications by those skilled in the art including
modifying the quantities of each of the components and orienting
the components in other configurations such as, for example, in
which the optical paths are collinear in part, in whole, or not at
all.
[0059] In operation, the beam 20 enters the laser amplifier 5 and
passes four times through the pumping module 90. On each pass, the
pumping module 90 amplifies the beam 20 resulting, for example, in
a more intense and more powerful beam. After being amplified four
times, the beam 20 exits the laser amplifier 5 as an output beam
15. Note that although FIG. 1 illustrates the four passes as not
overlapping, the beam 20 may overlap or even be collinear during
some or all of the passes.
[0060] In operation, the laser source 10 which may include, for
example, a pulsed laser oscillator or a laser diode, generates the
beam 20. In this example, for ease in discussion, assume that the
beam 20 generated by the laser source 10 is horizontally polarized
(p-polarized) and oscillates as a wave parallel to the plane of the
paper in FIG. 1. The first PBS 30, which has been adapted to pass
p-polarized light, allows the beam 20 to pass. The beam 20 passes
through the DPR 50 without a change in polarization since the DPR
50 has been adapted not to affect the polarization of the beam 20
in a direction of propagation from the first PBS 30 to the second
PBS 40. Thus, in this example, the beam 20 remains p-polarized
after exiting the DPR 50 and passes through the second PBS 40,
which has been adapted to pass p-polarized light, without a change
in polarization. The beam 20 then enters the pumping module 90 in
which the pumping module 90 amplifies the beam 20 by, for example,
increasing the power and the intensity of the beam 20. Upon exiting
the pumping module 90, the polarization of the beam 20 may be
rotated, for example, 45.degree. by the NPR 60 over one pass (but
is more generally rotated 90.degree. over two passes through the
NPR 60). The first pass is completed by the beam 20 being reflected
by the first reflector 70.
[0061] In a second pass, the NPR 60 then rotates the polarization
of the beam 20 for a total of 90.degree. from p-polarization. As a
result, in this example, the beam 20 is now vertically polarized
(i.e., s-polarized). Then the pumping module 90 amplifies the beam
20 for a second time. Since the beam 20 is no longer p-polarized,
the beam 20 will not pass though the second PBS 40, but instead the
second PBS 40 diverts the beam 20 to the second reflector 80.
[0062] In a third pass, the beam 20 is reflected by the second
reflector 80 and subsequently reflected by the second PBS 40. Then
the pumping module 90 amplifies the beam 20 for a third time and
the NPR 60 rotates the polarization of the beam 20, for example, by
yet another 45.degree. before being reflected by the first
reflector 70. The NPR 60 need not rotate the polarization of the
beam 45.degree. each pass. Instead, the NPR 60 may rotate the
polarization of the beam 90.degree. over two passes.
[0063] In a fourth and final pass, the polarization of the beam 20
may be rotated, for example, by another 45.degree. by the NPR 60 or
by 90.degree. over last two (i.e., the third and fourth) passes,
thereby returning the polarization of the beam 20 to the
p-polarization. The pumping module 90 then amplifies the beam 20
for the fourth time. Because the beam 20 is again p-polarized, the
beam 20 passes through the second PBS 40. Then, the beam 20 passes
through the DPR 50. The DPR 50, which has been adapted to affect
the polarization of the beam 20 in a direction of propagation from
the second PBS 40 to the first PBS 30, rotates the polarization of
the beam 20 by 90.degree.. Thus, although the beam 20 entered the
DPR 50 as p-polarized, the beam 20 exits the DPR 50 as s-polarized.
Accordingly, the first PBS 30 does not pass the beam 20, but
instead deflects the beam 20 that then exits the laser amplifier 5
as the output beam 15.
[0064] II. Master Oscillator
[0065] In an exemplary embodiment, the present invention provides
the master oscillator 10 that generates the beam 20 that is
subsequently amplified by the laser amplifier 5. As illustrated in
FIG. 16, the master oscillator 10 may include a first high
reflectivity mirror 210, a cavity dumper 220, a polarizer 230, a
first etalon 240, a second etalon 250, a second high reflectivity
mirror 260, a Q-switch 270, a mode locker 280, an Nd:YAG medium
290, a high reflectivity coating 300 and a laser diode array 310. A
resonator cavity is defined by an optical path 320, the two mirrors
210, 260, the polarizer 230 and the coating 300.
[0066] In operation, the Nd:YAG medium 290 is pumped by the laser
diode array 310. The laser diode array 310 may include lenses that
assist by focusing emitted light for longitudinal pumping of the
Nd:YAG medium 290 (e.g., a Nd:YAG laser crystal rod). The diode
array may operate, for example, at approximately 808 nm and
approximately 250 Watts (W) (peak) at approximately 1 kilohertz
(kHz), and with an approximately 20% duty cycle. The Q-switch 270,
the mode locker 280 and the cavity dumper 220 are employed to
generate short pulse duration with near diffraction limited beam
quality and high energy per pulse. The Q-switch 270 opens the
resonator at the end of the pump pulse to form, for example, an
approximately 150 picosecond (ps) laser pulse. The mode-locker
forces the laser modes to form a short pulse that circulates in the
resonator cavity during the Q-switch time duration. At the moment
the pulse reaches its peak intensity, the cavity dumper 220 rotates
the polarization of the light by 900 and the entire pulse (which
becomes the beam 20) exits the resonator cavity through the
polarizer 230. Such a configuration has an advantage in that almost
the entire stored energy in the resonator cavity can exit the
resonator cavity in the short pulse.
[0067] In an exemplary embodiment, the present invention provides
the master oscillator 10 that includes an intracavity spectral
filter 330. The intracavity spectral filter 330 may include the
first etalon 240 and the second etalon 250. The first etalon 240
includes an etalon that is, for example, approximately 15
millimeters (mm) thick. The second etalon 250 includes an etalon
that is, for example, approximately 10 mm thick. However, these
values are merely exemplary (i.e., not limitations) and the present
invention contemplates other values for the etalons 240, 250. The
etalon 240, 250 may include, for example, a piece of glass or
quartz formed like a window in which both surfaces of the window
are almost perfectly parallel and flat with respect to each other.
The thickness of the etalon 240, 250 relates to which particular
set of wavelengths passes through the etalon 240, 250. The
reflectivity of the surfaces determines the modulation depth. The
etalons 240, 250 may form a spectral filter that controls the laser
modes. Proper combinations of the etalons 240, 250 may provide, for
example, the proper modes with which to generate a smooth pulse. A
smooth pulse is advantageous in operations at high peak power since
sharp spikes may damage the laser crystal rods and/or the optical
components. By having a consistent pulse shape, the laser crystal
rods and the optical components can operate at high intensity and
high power without exceeding a damage threshold.
[0068] Empirical data indicates that approximately 0.5 millijoule
(mJ) to approximately 3 mJ per pulse up at 1 kHz may be achieved
using the intracavity spectral filter. Without the two etalons 240,
250, the pulse duration may not be smooth and may vary from pulse
to pulse due to, for example, mode beating. A single etalon 240 or
250 may stabilize the pulse duration to approximately 700-800 ps
full-width-half-maximum (FWHM), but fixed higher modulations still
persist. The two etalons 240, 250 working together eliminate the
fixed higher modulations and a smooth approximately 700-800 ps
pulse is obtained.
[0069] III. Polarizing Beam Splitter (PBS)
[0070] In an exemplary embodiment, the present invention provides
that the first PBS 30 and the second PBS 40 are substantially
similar to each other. However, in another example, the first PBS
30 and the second PBS 40 may differ substantially without
significantly affecting the performance of the laser amplifier
5.
[0071] The first PBS 30 and the second PBS 40 may include devices
that have been adapted to allow light having a desired polarization
to pass substantially unhindered while deflecting other
polarizations. In an exemplary embodiment, the present invention
employs polarized beam splitters that are available in a number of
models from CVI Technology Inc of Albuquerque, N. Mex.
[0072] An exemplary embodiment of the PBS 30, 40 in accordance with
the present invention is illustrated in FIG. 3. The illustrated PBS
30, 40 includes a thin film polarizer in which a polarizing coating
35 is applied on an outside surface of a substantially clear
material 45. Under certain conditions, the beam 20 may cause the
PBS 30, 40 to suffer from heat accumulation at the air-coating
boundary. This heat accumulation generates thermal optical effects
that may be sufficient, for example, to deflect the beam 20 from
its intended direction and misalign the optical pathways of the
laser amplifier 5.
[0073] Another exemplary embodiment of the PBS 30, 40 in accordance
with the present invention is illustrated in FIGS. 2 and 4. In this
exemplary embodiment, the present invention provides that the PBS
30, 40 include a solid structure that has an internal polarizing
coating layer. In particular, FIG. 4 illustrates the polarizing
coating 35 disposed along an internal diagonal plane within the
substantially clear cube-shaped material 45. Accordingly, this
exemplary embodiment does not have a substantial air-coating
boundary, and thus does not suffer from heat accumulation at the
air-coating boundary. Thus, this exemplary embodiment of the PBS
30, 40 according to the present invention does not suffer from the
above-described thermal optical effects and provides enhanced
reliability and durability.
[0074] IV. Directional Polarization Rotator (DPR)
[0075] In an exemplary embodiment, the present invention provides
that the DPR 50, as illustrated in FIGS. 1 and 2, has been adapted
to have no substantial effect on the polarization of the beam 20
travelling in a direction of propagation from the first PBS 30 to
the second PBS 40. The DPR 50 has also been adapted, for example,
to rotate by 900 the polarization of the beam 20 travelling in a
direction of propagation from the second PBS 40 to the first PBS
30. As a result, in the above-described example, the second PBS 40
transmits light received from the first PBS 30 while the first PBS
30 deflects light received from the second PBS 40.
[0076] The DPR 50 may include, for example, a half-wave plate 100
(e.g., a retardation plate) and a Faraday rotator 110. The
half-wave plate 100 includes an optical element having two mutually
orthogonal axes (i.e., a slow axis and a fast axis). The optical
element transforms the polarization of the beam 20 by introducing a
relative phase retardation between mutually orthogonal components
of the polarization of the beam 20. In operation, the half-wave
plate 100, which is appropriately oriented with respect to the
polarization of the beam 20, rotates the linearly polarized beam,
for example, by 45.degree.. In an exemplary embodiment, the present
invention employs the half-wave plate 100 that is available in a
number of models from CVI Laser Corp. of Albuquerque, N. Mex. The
selection of a particular model is well within the understanding of
one skilled in the art and depends at least on the desired size of
the half-wave plate 100 and the wavelength of the beam 20.
[0077] The Faraday rotator may include, for example, a glass rod
having a high Verdet constant that is disposed within an axial
magnetic field that affects the polarization of the beam 20.
However, other types of Faraday rotators may be employed. Since
light is electromagnetic radiation, this strong magnetic field
rotates the polarization of the beam 20. However, the sign of the
rotation is a function of the propagation direction of the beam 20
and the polarity of the magnetic field. Thus, for example, the
Faraday rotator 110 as integrated into the amplifier illustrated in
FIG. 2 rotates (e.g., clockwise) the polarization of beam 20 prior
to the first pass through the amplifier 5; however, the Faraday
rotator 110 rotates (e.g., counter-clockwise) the polarization of
the beam 20 following the fourth pass through the amplifier. In an
exemplary embodiment, the present invention employs Faraday
rotators which are available in numerous models from Electro-Optics
Technology, Inc. of Traverse City, Mich.
[0078] In operation, as the beam 20 propagates from the first PBS
30 to the second PBS 40, the DPR 50 has no substantial effect on
the polarization of the beam 20 because the half-wave plate 100
rotates the polarization by 450 in one direction and the Faraday
rotator 110 rotates the polarization by 450 in the opposite
direction. Thus, the half-wave plate 100 and the Faraday rotator
110 effectively cancel each other out (i.e., operate
destructively). On the other hand, as the beam 20 propagates from
the second PBS 40 to the first PBS 30, the DPR rotates the
polarization of the beam 20 by 900 transforming, for example, a
p-polarized beam into an s-polarized beam. In this case, the
half-wave plate 100 and the Faraday rotator 110 each rotates the
polarization by 450 in the same direction. The net effect is that
the half-wave plate 100 and the Faraday rotator 110 rotate the
polarization of the beam 20 by 900. In other words, the half-wave
plate 100 and the Faraday rotator 110 effectively operate
constructively.
[0079] V. Non-Directional Polarization Rotator (NPR)
[0080] As illustrated in FIGS. 1 and 2, the laser amplifier 5
includes the NPR 60. As previously described, the NPR 60 may rotate
the polarization of the beam 20 by 45.degree. on each pass, but
more generally, the NPR 60 may rotate the polarization of the beam
20 by 90.degree. over two passes. In an exemplary embodiment, the
present invention provides that the NPR 60 include a wave plate
(e.g., a quarter-wave plate) that is appropriately oriented with
respect to the polarization of the beam 20 to produce a 900
rotation over a round trip. However, the properly oriented wave
plate accomplishes proper polarization rotation only if the
incoming polarization of the input beam 20 is linear and known.
Unfortunately, the polarization of the beam 20 may not be perfectly
linear nor necessarily known due to, for example, birefringence
effects during pumping. Although some birefringence can be
minimized through the careful design of the laser amplifier 5,
birefringence can become substantial during high power and high
intensity operation. Empirical data confirms that high average
power operation of the laser amplifier 5 while employing the wave
plate as the NPR 60 may be unsuccessful due, in part, because the
laser amplifier 5 then behaves as a free running laser without the
proper rotation of the polarization of the beam 20.
[0081] In another exemplary embodiment, the present invention
provides that the NPR 60 include a Faraday rotator to rotate the
polarization, for example, by 45.degree. over per pass or, more
generally, by 90.degree. over two passes. The Faraday rotator has
an advantage in that the residual non-uniform polarization rotation
not compensated for in the first pass through the Faraday rotator
can be compensated for on the second pass through the Faraday
rotator. Empirical data shows that the laser amplifier 5 with the
NPR 60 that includes the Faraday rotator can operate beyond a power
input level of approximately 100 W.
[0082] VI. Reflectors
[0083] In an exemplary embodiment, the present invention includes
the first reflector 70 and the second reflector 80 as illustrated
in FIGS. 1 and 2. The reflectors 70 and 80 may be optical mirrors
that are adapted to withstand and redirect the energy and intensity
of the beam 20. In another exemplary embodiment, the present
invention provides that the optical mirror include a substrate
layer on which is applied a repeated pattern of alternating layers
of two different types of coatings. The two different coating
layers include materials that are transparent at least at the
wavelength of the beam 20, have different indices of diffraction
and may have different thicknesses. When the beam 20 impinges on
the repeated pattern of alternating layers, constructive and
destructive interference results from the internal reflection and
transmission at each coating layer interface. Only certain
wavelengths that satisfy constructive interference are reflected by
the mirror. Furthermore, the reflectivity of the mirrors may be
enhanced, for example, by increasing the number of alternating
coating layers.
[0084] In an exemplary embodiment, the present invention provides
that the mirror substrate is not an opaque material such as a metal
plate, but rather is optically transparent. Under certain
circumstances (e.g., under high power and high intensity laser
operation), the beam 20 can instantly vaporize an opaque substrate
material. Furthermore, internal birefringence within optical
mirrors can create sufficient heat to itself destroy or distort the
optical properties of the mirror. Accordingly, in another exemplary
embodiment, the present invention provides at least one reflector
70, 80 that includes a substrate material that is transparent and
has excellent heat conductivity characteristics. Thus, in yet
another exemplary embodiment, the present invention provides that
the substrate material include materials such as sapphire
(Al.sub.2O.sub.3) or diamond (C). Such materials provide efficient
heat dissipation and added structural stability and durability.
Furthermore, such materials provide added resilience by not
becoming distorted under extreme thermal conditions.
[0085] In another exemplary embodiment, the present invention
provides the reflector 70, 80 that includes a Porro prism. As
illustrated in FIG. 5, a Porro prism is a
45.degree.-90.degree.-45.degree. prism that reflects the beam 20
with a total directional change of 180.degree.. The Porro prism is
composed of substantially clear material such as, for example,
glass or plastic. The beam 20 enters the Porro prism though a long
plane 73, reflects off of a first short plane 76 and then reflects
off a second short plane 79, the first short plane 76 and the
second short plane 79 being orthogonal.
[0086] In operation, the Porro prism inverts the cross section of
the beam 20 (either vertically or horizontally). Hence, upon return
passage through the amplifier, the beam 20 is inverted with respect
to its orientation during the preceding amplifier pass. Such
inversion causes the beam to be exposed to an inverted pumping
energy profile within the laser crystal rod upon successive passes
through the amplifier. Accordingly, the Porro prism has an
advantage of substantially canceling many pumping irregularities,
thereby helping to homogenize much of the non-uniformity in the
amplifying process as well as the birefringence and power level
across the cross-section of the beam 20.
[0087] VII. Pumping Module
[0088] In an exemplary embodiment, the present invention provides
the pumping module 90 that includes a pumping energy source and an
optical path disposed through a laser crystal rod. The pumping
energy source generates, for example, light energy which is
absorbed by the laser crystal rod, thereby creating a population of
atoms with electrons in excited states. As the beam 20 passes
through the laser crystal rod along the optical path, the energy
stored in the population is transformed into, for example,
stimulated emission, thereby amplifying the beam 20.
[0089] A. Laser Crystal Rod
[0090] As illustrated in FIGS. 6-10, the optical path is disposed
through the laser crystal rod 160. In an exemplary embodiment, the
present invention provides that the laser crystal rod 160 include a
solid state material such as, for example, YAG, doped with active
elements such as, for example, Nd, Yb, Ho, Er or the like. Thus,
for example, the laser crystal rod 160 may include Nd:YAG solid
state material. The solid state material has an advantage in that
the laser crystal rod 160 then allows for high powered and
efficient pumping. Furthermore, laser crystal rods 160 such as
Nd:YAG and the like commercially are widely available. One of
ordinary skill in the art understands that changes in the
characteristics of the laser crystal rod 160 such as composition or
physical dimensions, alters the performance characteristics of the
laser crystal rod 160. Accordingly, one of ordinary skill in the
art would appreciate that laser crystal rod 160 is selected to
provide desired performance.
[0091] In an exemplary embodiment as illustrated in FIG. 2, the
present invention provides that the pumping module 90 include a
first amplifying module 120, a rotator 150, a lensing module 140
and a second amplifying module 130. Each amplifying module 120, 130
includes, for example, its own laser crystal rod 160 and its own
pumping energy source. Although illustrated with two amplifying
modules 120, 130, the present invention also contemplates a pumping
module have any desired number of amplifying modules. The
amplifying modules 120, 130 may be optically positioned in series,
in parallel or some combination thereof to achieve the
amplification power range and profile desired for particular
applications.
[0092] In an exemplary embodiment, the present invention provides
that the laser crystal rods 160 in the amplifying modules 120, 130
be substantially identical. For example, the laser crystal rods 160
may have substantially similar compositional profiles provide
substantially similar thermal stress characteristics which is
advantageous in properly correcting the birefringence. Furthermore,
localized irregularities, which may be caused by, for example,
variations in doping and/or composition profiles, are factors in
creating differential heating which, in turn, causes undesired
optical effects. Accordingly, it is advantageous that the
substantially similar compositional profiles of the crystal
composition of the laser crystal rod 160 also be highly
uniform.
[0093] In an exemplary embodiment, the present invention provides
for a method that produces substantially similar laser crystal
rods. A crystal boule that provides laser crystal rods 160 may be
grown from a melt that includes active elements. In an example in
which dopant concentrations gradients develop along the
longitudinal direction of the crystal boule, the laser crystal rods
should be taken from the same longitudinal region of the crystal
boule. A plurality (e.g., a pair) of substantially similar laser
crystal rods may be machined, for example, by coring from the same
boule, and more preferably, from the same longitudinal region
(e.g., a transverse boule slice) in the same boule, thereby
yielding crystal rods having substantially similar dopant
concentration profiles and substantial uniformity.
[0094] Certain crystal compositions may themselves include
significant birefringence. In these applications, it may further be
necessary or advantageous to orient the matched laser crystal rods
in a multi-stage amplifier with respect to each other to compensate
for or at least reduce this birefringence. For example, two laser
crystal rods machined from the same transverse boule slice can be
radially oriented (e.g., 90.degree.) with respect to each other in
order to compensate for laser crystal-induced birefringence.
[0095] B. Lensing Module
[0096] Under high average power pumping conditions, the laser
crystal rod 160 that includes Nd:YAG material or like materials may
exhibit an internal heat distribution across its cross section with
the highest temperatures at its axial core. The heat distribution
affects the refractive index of the laser crystal rod material
which is a function of temperature. Hence, light propagates slowest
at the axial core, and gradually faster toward the radial perimeter
of the laser crystal rod 160. As a result, at steady state
operation, the laser crystal rod behaves like a positive lense,
altering the propagation characteristics of the beam 5. However, it
is advantageous for the beam 20 to retain a parallel profile across
the beam 20 throughout the four passes through the laser amplifier
5 in order to achieve an amplified beam having uniform power and
intensity gains. Accordingly, in an exemplary embodiment, the
present invention provides a lensing module 140 and uniform pumping
across the cross section of the laser crystal rods.
[0097] In an exemplary embodiment, the present invention provides
uniform pumping for simple spherical lensing in the laser crystal
rods 160 and the lensing module 140 that includes a negative lens
(e.g., a spherical lense). In FIG. 2, the negative lens 140 is
disposed between the first amplifying module 120 and the second
amplifying module 130. The amplifying modules 120, 130 include the
laser crystal rod 160, which under at least high average pumping
power develops positive lensing effects. The negative lens 140 is
adapted to compensate for the positive thermal lensing effects of
the two laser crystal rods 160 disposed in the amplifying modules
120, 130 so that the beam 20 remains substantially parallel and
collimated throughout the four passes. The negative lens 140 is
selected not only to inversely match the positive thermal lensing
effects of the two laser crystal rods 160 in amplifying modules
120, 130, but also to withstand the rigors of laser amplification.
The present invention also contemplates that laser crystal rods of
other elemental compositions may generate different heat profiles
across their cross sections and accordingly, different types of
lenses adapted to compensate for particular thermal lensing effects
may be provided. For example, in the case of a laser crystal rod
having a composition that generates a negative Gaussian profile, a
positive lens would be advantageous.
[0098] C. Rotator
[0099] The high average pumping of the laser crystal rods 160
creates thermal stress which manifests itself as a birefringence in
the laser crystal rods 160. The heat-induced birefringence can
rotate the polarization unevenly within the cross section of the
beam 20. The profile of this uneven polarization rotation affects
each of the four radial quadrants, and looks like a four-leaf
clover. In an exemplary embodiment, the present invention provides
the pumping module 90 with the rotator 150 to compensate for this
uneven rotation in the polarization of the beam 20. In another
exemplary embodiment, the present invention provides that the
rotator 150 is a 90.degree. rotator which is disposed between the
first amplifying module 120 and the second amplifying module 130 as
illustrated in FIG. 2. By using the rotator 150 to rotate the beam
20 by 90.degree. between the amplifying modules 120, 130,
birefringence that occurs in one amplifying module is countered and
neutralized in the other amplifying module. However, under some
circumstances, the effectiveness of the rotator 150 is a function
of the matching of the laser crystal rods 160 which, due to the
matching, are disposed in the same orientation with respect to the
beam 20 and exhibit substantially identical thermal stress. The
direction of the rotation caused by 90.degree. rotator 150 is not a
significant factor for this consideration. Accordingly, the present
invention contemplates that the rotator 150 may include other
devices that rotate by 90.degree. the polarization of the beam 20.
For example, the rotator 150 may include a wave plate, a Faraday
rotator or some combination thereof.
[0100] D. Cooling System
[0101] In an exemplary embodiment, the present invention provides
that the laser crystal rod 160 is cooled by a fluid 190 housed in a
tube 170 which, at least partially, surrounds the laser crystal rod
160 as illustrated in FIGS. 6-10. The fluid cooling has an
advantage in that it improves the high-powered operation
characteristics of the laser crystal rod 160. The fluid 190 is a
substantially clear fluid (e.g., water) that substantially does not
optically absorb or reflect light impinging from the laser diodes
or otherwise substantially disrupt optical pumping. Similarly, the
tube 170 is a substantially clear material (e.g., glass or plastic)
that also does not substantially affect pumping. In an exemplary
embodiment, the fluid 190 that surrounds the laser crystal rod 160
may be stagnant according to the present invention. However, in
another exemplary embodiment, the present invention provides more
stable temperatures by allowing the fluid 190 to flow around the
laser crystal rod 160 to prevent substantial heat accumulation. For
example, laser amplifier 5 may include a mechanical pump that
transports the fluid 190. However, it should be appreciated that
the present invention also contemplates other means for causing the
flow of the fluid 190. Additionally, conventional means may be
employed to cool the fluid 190.
[0102] E. Cladding Layer
[0103] In an exemplary embodiment, a beam 20 may exhibit, for
example, a Gaussian profile. Although a Gaussian profile is typical
of, for example, laser operation in the fundamental mode, the
present invention contemplates other profiles as well. For
efficient amplification, the beam 20 should optically overfill the
aperture of laser crystal rod 160. Thus, the low intensity fringe
regions of the Gaussian profile are not amplified in the laser
crystal rod 160. However, such overfilling of the aperture causes
the beam 20, which enters with a Gaussian profile, to exit with an
accentuated "top hat" profile and a concentric, ringed pattern
resulting from diffraction by the circular aperture of the laser
crystal rod 160. Such distortions of the Gaussian profile can limit
the achievable power and intensity gains of the beam 20.
[0104] Thus, in an exemplary embodiment, the present invention
provides a cladding layer 165 around the laser crystal rod 160 as
illustrated in FIG. 19. The cladding layer 165 is advantageous
because the Gaussian profile of the beam 20 can be better preserved
if the aperture of the laser crystal rod 160 is optically softened,
thereby reducing the effect of the diffraction ring patterns. The
cladding layer 165 includes an optically clear material that is
adapted to transmit, but not substantially amplify and/or otherwise
substantially affect, the low intensity fringe regions of the
Gaussian profile. In an exemplary embodiment, the cladding layer
165 includes material having, for example, a substantially similar
refractive index as the material of the laser crystal rod 160. The
fringe light remains concentric with the amplified light and the
diffraction effect is reduced.
[0105] In another exemplary embodiment, the cladding layer 165
includes material having a refractive index with respect to the
refractive index of the laser crystal rod 160 such that fringe
light is directed towards the laser crystal rod 160. Thus, light
that otherwise would not have been amplified is amplified, thereby
improving the efficiency of the laser amplifier 5 and increasing
the intensity of the beam 20.
[0106] The cladding layer 165 may be fabricated, for example, from
the same composition as the laser crystal rod 160, but undoped.
After the laser crystal rod 160 is prepared, the cladding layer 165
can be applied using optical diffusion bonding. Alternatively, the
cladding layer may also be applied onto the laser crystal rod 160
using conventional techniques such as conventional deposition
and/or conventional growth techniques. The cladding layer 165 has a
further advantage in that the cladding layer can improve the
uniformity of the cooling of the laser by buffering fluctuations in
the temperature of the fluid 190.
[0107] F. Pumping Source
[0108] As illustrated in FIGS. 6-12, the pumping energy source in
pumping module 90 includes one or more laser diodes 180. The use of
emissions from the laser diodes 180 as the source for pumping
energy is advantageous in that the laser diodes 180 efficiently
produce high quality pumping energy having a desired wavelength.
The laser diodes 180 may include semiconductor devices that have
been adapted to receive energy (e.g., electrical) and emit
electromagnetic (e.g., light) energy. The energy may travel along
the exemplary paths 162 and 164.
[0109] The laser diodes 180 may be fabricated into a monolithic
longitudinal row that includes a laser diode bar 182 as illustrated
in FIG. 11. The laser diode bar 182 may include a plurality of the
laser diodes 180 disposed end to end with the diodes all facing in
the same direction so that emitted light from all of them
propagates in substantially the same general direction. Although
illustrated in FIG. 11 as including three laser diodes 180, the
present invention contemplates the laser diode bar 182 including
other quantities of the laser diodes 180. As illustrated in FIG.
12, a plurality of the laser diode bars 182 can be subsequently
assembled (e.g., with one laser diode bar 182 disposed on top of
another laser diode bar 182), thus forming a laser diode array 184.
Although illustrated in FIG. 12 as the laser diode array 184
including three laser diode bars 182, the present invention
contemplates other quantities of the laser diode bars 182. In an
exemplary embodiment, the laser diode array 184 according to the
present invention measures, for example, approximately 1 cm by
approximately 2.5 cm by approximately 6 cm. The light-emitting face
of each laser diode 180 may be, for example, approximately 100
.mu.m by approximately 1 .mu.m. Such dimensions are merely
exemplary and are not intended to limit, in any way or manner, the
scope of the present invention.
[0110] The laser diode array 184 includes a housing 186 into which
are mounted either the assembled laser diode bars 182, the
individual laser diode bars 182 and/or individual laser diodes 180.
The housing 186 includes circuitry and wiring needed to supply
electrical power to the laser diodes 180. The housing 186 can also
incorporate cooling systems such as, for example, cooling fluid
conduits, through which a suitable cooling fluid can be circulated
during operation to dissipate heat away from the laser diodes 180.
The wavelength of the laser diodes 180 is, for example,
approximately 805.5 nanometers (nm) with a bandwidth of less than
approximately 5 nm. Other wavelengths and bandwidths generated by
the laser diodes 180 are also contemplated by the present
invention.
[0111] To effectively and efficiently pump the laser crystal rods
160 optically, one or more laser diode arrays 184 are disposed so
that light emitted from the laser diodes 180 impinges on the laser
crystal rod 160. As illustrated in FIGS. 8-10, the amplifying
module 120, 130 includes a plurality of laser diode arrays 184
centered around the laser crystal rod 160 with each laser diode
array 184 with the emitting surfaces of the laser diodes 180
disposed toward the laser crystal rod 160. In an exemplary
embodiment, the present invention provides, in a particular plane
that is perpendicular to the longitudinal axis of the laser crystal
rod 160, an even number of laser diode arrays 184 equally spaced
around the laser crystal rod 160. In another exemplary embodiment,
the present invention provides, in a particular plane that is
perpendicular to the longitudinal axis of the laser crystal rod
160, an odd number of the laser diode arrays 184 equally spaced
around the laser crystal rod 160, thereby minimizing the
possibility that pumping light passing through or around the laser
crystal rod 160 will directly impinge on another laser diode array
184. Regardless of whether there is an even or an odd number of the
laser diode arrays 184, they should be evenly spaced around the
laser crystal rod 160 as illustrated, for example, in FIGS. 8-10.
Such a configuration prevents the formation of hot spots by
providing greater uniformity in applying the pumping energy to the
laser crystal rod 160. However, it should be appreciated that the
present invention also contemplates that the laser diode arrays 180
may be disposed in any other suitable configuration around laser
crystal rod 160 depending upon the application.
[0112] In an exemplary embodiment, the present invention provides
reflective elements 188 such as, for example, optical- or
metal-backed mirrors that are disposed on the other side of the
laser crystal rod 160 from corresponding laser diode arrays 184.
FIG. 13 illustrates an exemplary reflective element 188 disposed on
the other side of the laser crystal rod 160 from the corresponding
laser diode array 184. The laser diode array 184 emits light 192
which is mostly absorbed by the laser crystal rod, but some of
which passes through the laser crystal rod 160. The light 192 that
passes through the laser crystal rod 160 may then be reflected by
the corresponding reflective element 188. The reflected light 192
impinges a second time on the laser crystal rod 160 in which the
reflected light 192 may be absorbed, thereby adding to the
population inversion of the laser crystal rod 160. Note that FIG.
13 merely shows one reflective element 186 and its corresponding
laser diode array 184, a plurality of reflective elements 188
corresponding to a plurality of laser diode arrays 184 may be
provided. In yet another exemplary embodiment, the reflective
elements are disposed between adjacent laser diode arrays 184. In
still yet another exemplary embodiment as illustrated in FIG. 20,
the reflective element 188 is either mounted on or integrated with
the tubing 170 that houses the cooling fluid 190. The deployment of
properly oriented reflective elements 186 increases the pumping
efficiency of the laser diode arrays 184 since emitted light that
would otherwise not have been used by the laser crystal rod 160, in
fact, assists in the pumping of the laser crystal rod 160.
[0113] In FIGS. 8-10, exemplary embodiments of the amplifying
modules 120, 130 are illustrated according to the present
invention. The amplifying modules 120, 130 each include at least
one laser crystal rod 160 in which each of the laser crystal rods
160 is pumped by corresponding sets of, for example, five laser
diode arrays 184. Although five laser diode arrays 184 are
illustrated in FIGS. 8-10, the present invention also contemplates
other quantities of laser diode arrays 184. Configurations of the
amplifying modules 120, 130, each with sets of five laser diode
arrays 184, deliver higher average power than configurations with
lesser quantities of laser diode arrays 184 per set, for example,
three laser diode arrays 184 per set. Furthermore, pumping
uniformity may be improved by increasing the quantity of laser
diode arrays 184 to an optimum quantity per set that surround each
laser crystal rod 160.
[0114] In FIG. 9, an exemplary embodiment of the amplifying module
120, 130 is illustrated according to the present invention. In the
amplifying module 120, 130, the pumping uniformity is further
improved by mounting each set of the laser diode arrays 184 at an
angle around the longitudinal axis of the laser crystal rod 160
with respect to the angular disposition of the other sets of laser
diode arrays 184. For example, in a configuration in which a first
set of five laser diode arrays 184 is disposed around a first laser
crystal rod 160 in the first amplifying module 120 and in which a
second set of five laser diode arrays 184 is disposed around a
second laser crystal rod 160 in the second amplifying module 120,
the first set may be set at 360 around the longitudinal axes of the
laser crystal rods 160 with respect to the second set.
[0115] Proper positioning of the laser diode arrays 184 around the
laser crystal rod 160 does not alone ensure even deposition of
pumping energy. Another cause for uneven pumping is variances in
the laser diodes 180. For example, the laser diodes 180 often vary
in power, peak wavelength and bandwidth (e.g., approximate range of
emitted wavelengths). It is very hard to obtain from commercial
laser diode bar and array suppliers substantially similar
performance specifications for the laser diode arrays 184 at a
reasonable cost.
[0116] The non-uniformity in the output energy of the laser diode
arrays 184 degrades performance of the laser amplifier 5,
especially in high power and high intensity amplification. For
example, the use of the laser diode array 184 having a center
wavelength that is close to the peak absorption in the laser
crystal rod 160 would result in more pumping energy absorption at
the radial perimeter of the rod than the center of the rod. Another
laser diode array 184 having a center wavelength that is farther
away from the peak absorption wavelength would result in a more
even distribution of pumping energy absorption throughout the laser
crystal rod. Similarly, variances in the output energy intensity of
the laser diode arrays 184 are undesirable because the laser diode
arrays 184 having higher power output result in stronger energy
absorption in the adjacent portion of the laser crystal rod 160,
distorting the amplified beam and potentially creating hot spots in
the laser crystal rod 160. Furthermore, such non-uniform pumping or
heating may result in non-spherical lensing effects for which
compensation via non-spherical lenses has proven costly.
[0117] In an exemplary embodiment, the present invention improves
amplifier performance uniformity by selecting laser diode arrays
184 that closely correspond in power, wavelength and bandwidth. The
laser diode arrays 184 can be adapted to match the power of the
lowest power laser diode array 184. For those laser diode arrays
184 that exhibit power that is higher than that of the lowest power
laser diode array 184, an electrical load may be placed in parallel
with the higher power laser diode arrays 184. The parallel load
drains an appropriate amount of power from the higher power arrays
184 so that all the arrays 184 use substantially the same power
(i.e., all the arrays 184 are matched in power with the lowest
power array 184). Parallel electrical loads may include a rheostat
or a transistor or other circuitry adapted to drain the proper
amount of power. The laser diode arrays 184 may be coupled in
series, parallel or any combination thereof. Furthermore, the laser
diode arrays 184 may share a power source or may have individual
power sources. The present invention also contemplates that such
systems can be manually adjusted or automatically controlled (e.g.,
using computers), and that such systems may include feedback loop
circuitry that, for example, measures empirical output levels and
then automatically adjusts the levels of electrical power delivered
to each laser diode array 184 to yield balanced pumping light
emissions among the operating laser diode arrays 184. The present
invention also contemplates that the above-described schemes for
matching power, wavelength and bandwidth of the laser diode arrays
184 are also equally applicable for matching power, wavelength and
bandwidth of the laser diode bars 182 and/or the laser diodes
180.
[0118] In another exemplary embodiment, during the set up of the
high power and high intensity laser system, the laser crystal rod
(or the laser crystal rods) 160 can be pumped by the laser diode
arrays 184 or the set of laser diode arrays 184 without the input
beam 20. The light that exits the laser crystal rod 160 along its
longitudinal axis can be analyzed and/or viewed to determine
whether uniform pumping has been achieved. For example, the light
that exits the laser crystal rod can be displayed on a visual
display such that a user can see whether or not uniform pumping is
being achieved. The user can then change the power levels received
by, for example, the individual laser diode arrays 184 until a
uniform light pattern is shown on the visual display. In one
example, each laser diode array 184 is tested to determine which of
the laser diode arrays 184 provides the weakest light that exits
the laser crystal rod 160. Thus, for example, one laser diode array
184 pumps the laser crystal rod 160 without the input beam and the
light that exits the laser crystal rod 160 along the longitudinal
axis is measured and/or viewed by the user and/or analyzed by, for
example, a computer. The power supplies supplying power to each of
the laser diode arrays or the circuitry that controls the amount of
power that reaches the laser diode arrays (e.g., power draining
circuitry) can be adjusted so that light exiting the laser crystal
rod 160 along the longitudinal axis has the same intensity for each
of the laser diode arrays 184. Furthermore, the orientation of the
laser diode arrays 184 and the set of laser diode arrays 184 around
the longitudinal axis of the laser crystal rod 160 can be
optimized. For example, the set of laser diode arrays 184 can pump
the laser crystal rod without an input beam 20 and the light that
exits the laser crystal rod 160 can be viewed and/or analyzed to
determine whether the set of laser diode arrays 184 are optimally
disposed with respect to the laser crystal rod 160. Furthermore,
the laser diode arrays 184 can pump the laser crystal rod 160
without an input beam 20 and the light that exits the laser crystal
rod 160 can be viewed and/or analyzed to determine whether or not
uniform pumping is being achieved via optimally positioning each
laser diode array at a particular radial angle and/or radial
distance around the longitudinal axis of the laser crystal rod 160.
Accordingly, uniform pumping of the laser crystal rod 160 can be
set up even before the entire high power and high intensity laser
system has been fully activated.
[0119] Pumping performance can be further improved by adjusting the
temperature of the laser diode array 184. This adjustment is
accomplished, for example, by providing a chiller that cools the
fluid 190 to a controlled temperature. The wavelength of the
pumping energy emitted by laser diodes 180 changes as a function of
the temperature of the laser diode array 184. For example,
empirical data indicates that the peak wavelength for the pumping
output of arrays 184 fabricated from one type of laser diode 180
shifts by approximately 1 nm per approximately 3.5.degree. C.
change in the average diode temperature. Thus, adjustments to the
average temperature of the diodes in an array allows for the tuning
of the peak wavelength of the output of the laser diode arrays 184
to correspond to the optimum wavelength for pumping a particular
laser crystal rod 160. When the peak wavelength of the pumping
energy corresponds to the desired absorption wavelength of the
laser crystal rod 160, pumping efficiency and uniformity
improve.
[0120] The orientation of the laser diodes also may substantially
impact the performance of the amplifier 5. As previous described,
FIG. 6 illustrates a configuration for the diodes in a laser diode
array 184 in which the long sides of the laser diodes 180 are
oriented substantially parallel to the long sides of the face of
the laser diode array 184, and the array 184 is positioned parallel
to the longitudinal axis of the laser crystal rod 160. FIG. 6 can
also be described as a configuration in which the long sides of the
laser diodes 180 are substantially parallel to the longitudinal
axis of the laser crystal rod 160.
[0121] As illustrated in FIG. 14, the shape of the light-emitting
surface 194 of each laser diode 180 is typically an elongated
rectangle. The rectangular light emitting surface of the laser
diodes has two sets of sides, the short sides and the long sides.
The length of the short sides is a smaller than the length of the
long sides. A rectangular aperture diffracts monochromatic light
with at least two diffraction angles that are roughly proportional
to the wavelength of the monochromatic light and that roughly are
inversely proportional to the length of a respective side of the
rectangular aperture. Light dispersion through a rectangular
surface beyond the "footprint" of the light emitting surface 194 is
far greater along directions parallel to the short axis 196 (i.e.,
parallel to the short sides of the laser diode 180) than along
directions parallel to the long axis 198 (i.e., parallel to the
long sides of the laser diode 180). In fact, at far enough
distances away from the light emitting surface 194, the dispersion
due to diffraction becomes the most significant factor in
determining the illumination footprint on the laser crystal rod
160. In an exemplary embodiment, the laser diode 180 is disposed
with respect to the laser crystal rod 160 such that the diffraction
angle of the light dispersion is, for example, between
approximately 20.degree. to approximately 60.degree. along
directions parallel to the short axis 196 and is, for example,
approximately 5.degree. to approximately 15.degree. along
directions parallel to the long axis 198.
[0122] The laser diode array 184 as illustrated in FIG. 6 is
mounted facing the laser crystal rod 160 so that each laser diode
has a long side that is parallel to the longitudinal axis of the
laser crystal rod 160. This configuration suffers from light
dispersion from the rectangular light emitting surface 194 as
described above. Thus, a substantial portion of the light that is
dispersed along directions parallel to the short axis does not
efficiently reach the laser crystal rod 160. Furthermore, because
of the insubstantial overlap of the illumination footprints along
directions parallel to the long axis from adjacent diode lasers 180
on a laser diode bar 182, non-uniformities can occur, resulting in
localized hot spots in the laser crystal rod 160 that degrade the
performance, and possibly the operability, of the laser amplifier
5. Accordingly, the configuration may employ the use of a lensing
system 200 (illustrated in FIGS. 8 and 9) to achieve more uniform
pumping of the laser crystal rod 160. It is advantageous to design
amplifying modules 120, 130 to operate without the lensing system
200 since lensing systems 200 increase the cost and complexity of
the amplifying modules 120, 130. Furthermore, this configuration
offers no redundancy in the pumping output. Accordingly, the
failure of any individual laser diode 180 of laser diode array 184
may result in non-uniform pumping of the laser crystal rod 160. In
such a situation, the only remedy available, which is also the most
costly, might be the replacing of the entire array 184.
[0123] As illustrated in FIG. 7, an exemplary embodiment of the
laser diode array 184 is illustrated according to the present
invention. The laser diodes 180 in the laser diode array 184 are
oriented with their short axes (i.e., a direction parallel with the
short sides of the laser diodes 180) parallel to the longitudinal
axis of the laser crystal rod 160. The configuration illustrated in
FIG. 7 has an advantage in that the laser diodes 180 are more
closely spaced than the configuration illustrated in FIG. 6, thus
resulting in the pumping radiation being more substantially uniform
due to the wide dispersion of pumping radiation in directions
parallel to the short axes, which are parallel to the longitudinal
axis of the laser crystal rod 160. In addition, the closer spacing
allows for more laser diodes 180 to be placed in the laser array
184, thus more pumping energy may be delivered to the laser crystal
rod 160. As previously discussed, illuminating the laser crystal
rod 160 with more uniform pumping radiation minimizes hot spots and
reduces thermal stress, non-uniform gain and other undesirable
thermal optical effects. Furthermore, the need is reduced for
employing the lensing system 200 in the configuration of laser
diode arrays 184 as illustrated in FIG. 7 because of the
substantial uniformity of the pump radiation. This configuration
for the laser diode arrays 184 is also more durable than the
configuration illustrated in FIG. 6 due to the substantial
overlapping of the dispersed light. Accordingly, the failure of any
individual laser diode 180 may be offset by power adjustments to,
for example, the remaining laser diodes 180, laser diode arrays 184
or sets of laser diode arrays 184.
[0124] Empirical data indicate that the exemplary embodiment
illustrated in FIG. 7 allows for higher powered pumping and greater
gains from the same levels of pump energy as obtained from the
exemplary embodiment illustrated in FIG. 6. For example, according
to empirical data, a free running laser operation yielded a
conversion rate of over approximately 35% from laser diode output
power to laser output power, indicating very efficient pumping.
Furthermore, high average power empirical data indicates that a
water-cooled package with the configuration illustrated in FIG. 7
can handle a duty cycle exceeding approximately 20% (i.e., the
laser diodes can be emitting light for approximately 20% of the
time). In tests performed by Applicants, the laser diodes 180 in
such a configuration were operated for approximately 200 .mu.s and
at repetition rates up to approximately 1000 Hz.
[0125] E. Lensing System
[0126] As previously described and as illustrated in FIGS. 8 and 9,
the performance of the laser amplifier 5 can be further improved by
using amplifying modules 120, 130 including a lensing system 200
juxtaposed to the light emitting surfaces 194 of the laser diodes
180. The lensing system 200 is adapted to guide the pump radiation
from the laser diode array 184 toward the laser crystal rod 160. In
an exemplary embodiment, the lensing system 200 includes a
cylindrical lens (e.g., a positive cylindrical lense) and a
reflector (e.g., a flat reflector). The positive cylindrical lens
is configured to extend in front of the light emitting surfaces 194
of the laser diode array 184. The positive cylindrical lens focuses
the slow axis of the array 184, while the flat reflector reflects
the fast axis. This combination ensures that all the radiation from
the array gets to the laser crystal rod 160. Thus pumping
efficiency is increased, allowing greater intensity and power with
the same pumping power. The lensing system 200 may include, for
example, spherical lenses or aspherical lenses.
[0127] Proper positioning of the lensing system 200 is advantageous
in achieving the desired pumping performance gains. For example,
when the laser crystal rod 160 is at the focal points of the
lensing system 200, there is strong deposition of the pumping
energy in the center of the laser crystal rod 160. As a result of
this strong beam intensity at the center of the laser crystal rod
160, pumping at the center is maximized, but damage to the laser
crystal rod 160 and poor beam characteristics may occur above
approximately 20 W. By setting the laser diodes 180 and the lensing
system 200 so that the focal point of the pumping energy from the
laser diodes 180 is in front of the laser crystal rod 160 rather
than within it, the pumping energy diverges as it enters the laser
crystal rod 160. As a result of this divergence, the deposition of
pumping energy into laser crystal rod 160 is substantially more
uniform. For example, tests show that the laser amplifiers 5 with
this configuration can achieve output power greater than
approximately 100 W without damaging the laser crystal rod 160.
Further increases in the divergence of the pumping energy result in
increasingly uniform deposition at the expense under certain
circumstances of further limiting the total overall pumping power.
With a perpendicular laser diode orientation (i.e., the short side
of the individual laser diodes are parallel to the longitudinal
axis of the laser crystal rod), the laser amplifier may, under
certain circumstances, provide sufficiently uniform pumping energy
to perform effectively even without employing the lenses and/or the
mirrors.
[0128] FIG. 10 illustrates an exemplary embodiment of the present
invention in which the lensing system 200 includes a unitary lens
that is integrated with and runs the entire length of the tube 170
that holds the cooling fluid 190. Such a lens can be created by
shaping the portion of the tube wall through which the pumping
light passes to the laser crystal rod 160. Accordingly, the
geometry (e.g., shape and/or thickness) and material of the tube
170 may be adapted to form a desired lens (i.e., a positive lens
that counteracts dispersion of the light emitted from the diodes).
Providing such a unitary lens may provide a more robust system and
may be more economical to design and fabricate.
[0129] Another exemplary embodiment of the present invention is
illustrated in FIG. 15 in which the lensing system 200 is mounted
on the tube 170. In such a configuration, the diode array 184 may
or may not be in direct contact with the lensing system 200.
Furthermore, although FIG. 15 illustrates one lensing system 200
corresponding to a respective laser diode array 184, more than one
lensing system 200 may be used per respective laser diode array
184. Furthermore, although FIG. 16 illustrates only one laser diode
array 184, more than one laser diode array 184 may be employed
according to the present invention.
[0130] VIII. Applications
[0131] In an exemplary embodiment as illustrated in block form in
FIG. 17, the present invention provides for the modular development
of high power and high intensity coherent light. The present
invention provides that the master oscillator 10 is coupled to a
pre-amplifier 340 which, in turn, is coupled to an amplifier 350 or
multiple amplifiers in parallel, series or some combination
thereof.
[0132] In operation, the master oscillator 10 provides, for
example, approximately 1 mJ per pulse up to approximately 1 kHz.
The beam profile is, for example, Gaussian with an approximately
1.2 times diffraction limited (DL) beam quality. The pulse duration
ranges from approximately 150 ps to approximately several
nanoseconds. For stable and consistent pulse duration, the master
oscillator 10 includes the intracavity spectral filter 330 as shown
in FIG. 16. Accordingly, for example, a smooth approximately 800 ps
pulse with less than an approximately 5% fluctuation may be
generated by the master oscillator 10. The pre-amplifier 340 is
configured as the above-described laser amplifier 5 with, for
example, one laser crystal rod 160. The pre-amplifier may amplify
the beam 20 to over approximately 100 mJ per pulse with negligible
degradation in beam quality (e.g., less than approximately 1.3
times DL beam quality). The beam 20 may then be split, if
applicable, to multiple beams of, for example, approximately 10 mJ
to approximately 15 mJ. The amplifier 350 is configured as the
above-described laser amplifier 5 with, for example, two laser
crystal rods 160. The amplifier 350 amplifies the pre-amplified
beam 20 to, for example, approximately 250 mJ per pulse from an
input beam of, for example, approximately 10 mJ to approximately 15
ml, with better than approximately 2 times DL beam quality.
Although FIG. 17 illustrates an exemplary embodiment in which only
one amplifier 350 is provided, the present invention also
contemplates additional amplifying stages coupled to the amplifier
350 where applicable. For example, in an exemplary embodiment, four
parallel amplifiers are coupled to generate soft x-rays and/or
extreme ultraviolet rays for use in, for example, lithography. Such
high power and high intensity lasers according to the present
invention may find application in many fields such as in, for
example, lithography, tomography, microsopy and spectroscopy.
[0133] In an exemplary embodiment, the present invention provides a
high energy point source of light for generating soft x-ray
radiation and/or extreme ultraviolet (EUV) radiation (e.g.,
radiation having a wavelength from approximately 0.5 nm to
approximately 50 nm). As shown in FIG. 18, the amplified beam 20
from the amplifier 350 with a power density exceeding approximately
10.sup.14 W/cm.sup.2 can be guided to a radiation generator 360.
The radiation generator 360 generates soft x-ray radiation and/or
EUV radiation. The radiation generator 360 includes a constantly
replenished metallic target (e.g., a moving copper tape) on which
impinges the amplified beam 20. The resulting plasma then emits
soft x-ray radiation and/or EUV radiation. Such soft x-ray
radiation and/or EUV radiation finds application in many fields.
The soft x-ray radiation and/or the EUV radiation generated
according to the present invention can be used in, for example,
lithography, tomography, microscopy and spectroscopy.
[0134] In an exemplary embodiment, the present invention employs
the soft x-ray radiation and/or the EUV radiation in sub-micron
lithography. As illustrated in FIG. 18, after the radiation
generator 360 generates the soft x-ray radiation and/or the EUV
radiation, it is used in a lithographic system 370. The
lithographic system 370 might include, for example, a mask and
optical components. The radiation impinges on the mask or optical
components that direct the radiation to the mask. Further optical
components may be used to guide the resulting image onto the target
380, for example, a prepared semiconductor wafer. With wavelengths
in the soft x-ray and/or EUV range, lithographic machining of
structures having a size of less than approximately 0.1 microns
(.mu.m) are realizable. Furthermore, by operating the laser at
short pulse duration and high repetition rate, good dose control
and conversion efficiency to soft x-ray radiation and/or EUV
radiation may be achieved while controlling debris by generating
only clusters of copper that can be easily controlled by, for
example, helium gas inside the radiation generator 360.
[0135] In another exemplary embodiment as illustrated in FIG. 21,
the beam 20 that exits the pre-amplifier 340 is split among a
plurality of amplifiers 350 that are configured in parallel. After
being amplified, each beam enters a corresponding harmonic
generator 390 that generates a harmonic beam (e.g., a second
harmonic beam). The harmonic beams then enter the radiation
generator 360 and impinge upon a target (e.g., metallic target).
The resulting plasma then emits soft x-ray radiation and/or EUV
radiation which is then collimated into a single beam by a
collimator 400. For example, the collimator 400 may collect a large
solid angle of soft x-ray radiation and/or EUV radiation and
reflect the radiation into a collimated beam. The collimated beam
is then used in, for example, a stepper 410 of a lithography
system.
[0136] High power and high intensity lasers according to the
present invention also may find application in many other fields.
For example, such high power and high intensity lasers may find
application in micromachining, target ranging and industrial
applications such as large-scale cutting, drilling and machining of
a broad variety of materials including metals. In an exemplary
embodiment, the present invention provides that the high power and
high intensity laser is integrated with or mounted on a computer
controlled robotic arm for use in an industrial setting.
[0137] Thus, it is seen that systems and methods for amplifying
coherent light are provided. One skilled in the art will appreciate
that the present invention can be practiced by other than the
preferred embodiments which are presented in this description for
the purpose of illustration and not limitation, and that the
present invention is limited only by the claims that follow. It is
noted that equivalents for the particular embodiments discussed in
this description may be employed in order to practice the present
invention as well.
* * * * *