Side pumping arrangement

The use of rectangular cross-section gain material with two or more cylindrical lenses placed in close proximity or attached to opposite sides of the gain material allows for an efficient pumping of the gain material in an optical gain component. The optical gain component can be used in a laser device. Laser diode bars are arranged so as to pump the gain material through the lenses. A cooling apparatus can cool a rectangular cross-section gain material through two opposite flat sides so that no thermal birefringence is created. An optical gain component with two gain materials can orient these two gain materials such that the small-signal gain is almost circular and gaussian in profile with a large center peak tapering off to a much lower gain at the edges.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical gain component having a gain 
material which is side pumped by laser diodes. The optical gain component 
can be used in a laser device. In side pumping a laser gain medium, it is 
desired to achieve both a high average gain value and a uniform gain 
distribution. The large divergence angles typical of the emission from 
laser diodes make these goals difficult to achieve. 
A prior art side pumping arrangement is described in Welford et al. 
"Efficient TEM.sub.00 -mode Operation Of A Laser-Diode Side Pumped Nd: YAG 
Laser", Optics Letters, Vol. 16, No. 23, (Dec. 1, 1991), pp. 850-852. This 
article describes the side pumping arrangement shown in FIG. 1. FIG. 1 is 
a cross-sectional view of a prior art gain material 20. This figure shows 
a cross-section through the optical axis of the gain material 20. The gain 
material 20 has a semicircular curvature 20a used for collecting and 
roughly collimating the emission from the pump diodes 22. The pump energy 
is reflected back through the gain material by a reflective coating on 
side 20b. The gain material 20 is also cooled along this side 20b. A 
five-bar stack of laser diodes 22 is located 0.5 mm from the gain material 
20 along the side 20a. In this prior art arrangement, the gain is 
effectively peaked at the location that the diode energy enters the gain 
material 20. 
This geometry has several disadvantages. First, the rod curvature is used 
for collimating the pump diodes but also restricts the thickness of the 
laser medium seen by the diode. For this reason, the flexibility of this 
technique is limited. Second, this geometry allows for laser diode stacks 
along side 20a only. If additional laser diode stacks are to be used to 
pump the gain material from different directions, another rod must be 
included. Finally, since the cooling of the rod is done along the surface 
20b, the cooling is one dimensional and non-symmetrical. This results in a 
thermal wedge as the rod is pumped harder, so that the laser alignment 
changes with the average pump power. The laser alignment may also change 
over time as the diode ages. For the same reason, the gain center 
(centroid of the gain distribution) does not coincide with the thermal 
lens center. 
It is desired to have an improved arrangement for side pumping a gain 
material in an optical gain component. 
SUMMARY OF THE INVENTION 
An advantage of the present invention is the use of a rectangular 
cross-section gain material with two cylindrical lenses placed in close 
proximity or attached to opposite sides of the gain medium. The lenses can 
collect and roughly collimate pump energy from two stacks of laser diode 
bars. These diode bars do not need to be individually collimated and for 
that reason the less expensive non-collimated laser diodes can be used. 
Additionally, the cylindrical lenses can be attached to the opposite sides 
of the gain material with an optical cement. 
The use of a rectangular cross-section gain material allows for the gain 
material to be pumped from more than one side. 
The gain material, cylindrical lenses and laser diodes form an optical gain 
component. This optical gain component can be used in a laser device. 
Alternatively, the optical gain component could be used in an optical 
signal amplifier. 
Another aspect of the present invention regards a cooling apparatus for use 
with a rectangular cross-sectional gain material. This cooling apparatus 
cools two opposite sides of the gain material. A benefit of using the 
cooling apparatus of the present invention which cools the two opposite 
sides of a rectangular cross-sectional gain material is that when the gain 
material is pumped from its two other sides, a temperature gradient is 
formed wherein the temperature primarily depends on the distance from the 
cooled sides. In effect, the cooling apparatus produces a cylindrical 
thermal lens which can be compensated for by a simple cylindrical lens 
placed on the optical axis of the optical gain component. A benefit of the 
cooling apparatus of the present invention is that thermal birefringence 
is minimized. Thermal birefringence can disturb the polarizations of the 
laser beams in a laser device. 
An additional aspect of the present invention concerns the use of two gain 
materials. The two gain materials individually have a saddle shaped gain 
profile. By pumping the second gain material at a different orientation 
about the optical axis from the pumping of the first gain material, the 
superposition of the two gain profiles can produce a more desirable gain 
distribution. For example, the case of a 90.degree. rotation between two 
saddle shaped gain profiles can produce a substantially circular 
bell-shaped gain profile. This substantially circular bell-shaped gain 
profile can help produce a TEM.sub.00 lasing mode. Additionally, the 
resulting combined thermal lens for the two gain materials can produce a 
spherical thermal lens which is simpler to compensate for than a 
cylindrical lens.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 is a cross-sectional view of the gain material 24 of the present 
invention. The gain material 24 can be pumped from sides 24a and 24b. Pump 
light rays are absorbed during the propagation through the gain material 
so that their intensity decreases exponentially. After a distance R, the 
intensity is reduced by a factor EXP(-R/R.sub.0), where R.sub.0 is the 
absorption depth, a constant which is characteristic of the specific 
choice of solid-state laser material, pump wavelength, and pump bandwidth 
(wavelength spread). The deposited pump energy and therefore, the optical 
gain also follow that law, decreasing exponentially with distance from the 
pump source. 
The gain medium 24 has a rectangular cross-section and is pumped through 
one side face 24a or through two opposing side faces 24a and 24b. The 
optical depth considerations imply that the rod width should be around 1-2 
times R.sub.0. If the width is much shorter than R.sub.0, the average gain 
value will be close to the peak value but the net amount of deposited 
energy and the laser conversion efficiency will be low. Much of the pump 
energy in this situation would not be absorbed by the gain material. If 
the width of the gain material is much longer than R.sub.0, then nearly 
all the pump energy will be absorbed in the laser medium but the gain 
distribution will be very non-uniform. 
In the configuration shown in FIG. 2, the height of the gain material 
remains a free parameter. Only the width is fixed by the absorption 
characteristics in the laser medium. In the present invention, the gain is 
maximized by minimizing the height of the pumped region in the gain 
material 24 thereby minimizing the cross-sectional area as viewed along 
the optical axis. Using the embodiment of the present invention shown in 
FIG. 12 which has two gain materials one oriented substantially 90.degree. 
about the optical axis from the other may constrain the height of the gain 
regions, however. 
Looking again at FIG. 2, the gain material 24 in the present invention is 
preferably made of Nd:YAG. Nd:YAG is a well known laser material which has 
an absorption depth, R.sub.0, of about 2.5 mm at the peak absorption 
frequency of 808 nm, assuming a pump band width of around 4 nm full-width 
half-maximum (FWHM). Other gain materials can be used in the present 
invention, such as Nd:YLF, Nd:YVO.sub.4, Nd:Doped Glass, or any other 
lasant material. 
FIG. 3A is a perspective view of the gain material 26 of the present 
invention with attached cylindrical lenses 28 and 30. The optical gain 
component 33 has its optical axis arranged through gain material 26 which 
is a rectangular cross-section lasant rod. The gain material 26 has four 
sides and two ends. The gain material 26 is pumped by stacks 32 and 34 of 
laser diode bars positioned along opposite sides 26aand 26b of the gain 
material 26. The cylindrical lenses 28 and 30 may be made of materials 
with different optical and/or physical properties than that of the gain 
material 26. For example, the cylindrical lenses 28 and 30 can have a 
different index of refraction or a different thermal conductivity from the 
gain material 26. Preferably, the cylindrical lenses 28 and 30 are 
non-absorbent of the pumping energy and have a high index of refraction. 
In the preferred embodiment, the cylindrical lenses 28 and 30 are made of 
undoped YAG. The curvature of the lenses' surfaces can be engineered to 
accommodate different pumping requirements and to shape the distribution 
of the deposited pump energy as can be shown in FIGS. 8A- I discussed 
below. 
Looking again at FIG. 3A, the lenses 28 and 30 can also be attached 
directly to the gain material 26 with an optical adhesive to create a 
monolithic, compact and rugged rod assembly. Flat surfaces of the 
cylindrical lenses 28 and 30 are attached to the gain material 26. The 
optical adhesive is chosen to be non-absorbent of the pumping energy. 
In the preferred embodiment, the optical adhesive is "Norland 61" UV-curing 
photopolymer available from Norland Products Incorporated which is located 
in New Brunswick, N.J. The "Norland 61" UV-curing photopolymer has a 
refractive index of about 1.56 and the undoped YAG lenses 28 and 30 and 
Nd:YAG gain material 26 have refractive indexes of 1.82. For this reason, 
there is a partial index match between the adhesive and the undoped YAG 
lenses 28 and 30 and Nd:YAG gain material 26 which reduces the effective 
reflectance of the optical surface and improves pump energy transfer. More 
importantly, the optical adhesive should have a low viscosity before 
curing and form a thin adhesive layer during assembly. A thin adhesive 
layer minimizes thermal problems. 
Additionally, the attachment of the cylindrical lenses 28 and 30 to the 
gain material 26 should be done in a very clean environment. If dust 
particles are trapped in the adhesive layer, these particles may absorb 
pumping energy when the optical gain component is operating. This 
absorption will cause the dust particles to heat up and may damage the 
optical gain component. 
The cylindrical lenses 28 and 30 can be used to optically compress the 
height of the pumped region. Furthermore, the pumped beams can be 
effectively focused and become progressively more intense as they 
propagate through the gain material 26 so that the focusing offsets the 
accompanying intensity loss due to absorption of the pump radiation by the 
rod. This allows for the resulting gain to remain substantially flat 
across the rod width. In the orthogonal direction along height of the gain 
material 26, the small signal gain will mimic the pump laser intensity 
profile and look somewhat gaussian, tapering off to zero on each side of a 
well defined central peak. The small-signal gain can be seen best with 
respect to FIG. 9B which is discussed below. 
Looking again at FIG. 3A, note that the stack of laser diodes 34 is 
arranged such that its long axis 35 is parallel to the long axis 29 of the 
cylindrical lens 28. This allows the curvature of the cylindrical lens 28 
to focus the pump beams perpendicular to the laser diode bar. The pump 
beams perpendicular to the laser diode bar are of good quality as 
discussed below with respect to FIGS. 4A-B and FIG. 5. 
Looking again at FIG. 3A, the width of the gain material 26 is dictated by 
the absorption characteristics of the gain material. Under some 
circumstances, this width may be greater than is considered desirable. If 
angled end faces are fabricated onto the rod, the laser beam within the 
rod can be expanded along the width of the gain material so as to 
compensate for the undesirable width of the pumped region. This refractive 
expansion is equivalent to apparent external image compression. For 
example, in 3 mm-wide rod made from Nd:YAG a gain region 3 
mm-wide.times.1.6 mm-high would "look" like only 1.65 mm.times.1.6 mm to 
an external observer if its faces 37 and 39 were tilted at a Brewster 
angle. The Brewster angled ends 37 and 39 also define the preferred 
polarization of the laser beam. 
The gain material 26 can be configured in such a way that the stacks 32 and 
34 of laser diode bars do not directly face each other but are offset 
along the length of the gain material 26. A highly reflective coating 36 
reflects the unabsorbed pump light from the stack 32 back through the gain 
material 26 and a highly reflective coating 38 reflects the unabsorbed 
pump light from the stack 34 back through the gain material 26. This 
effectively doubles the optical thickness of the rod since the pump light 
can be absorbed as it moves towards the highly reflective coatings 36 and 
38 and as it is reflected back from the highly reflective coatings 36 and 
38. The use of the highly reflective coatings 36 and 38 allows for 
narrower rod designs or for enhanced absorption efficiency and higher gain 
for a given rod dimension. 
FIG. 3B is a perspective view of an alternate embodiment of the gain 
material 40, cylindrical lenses 42 and 44 and stacks 46 and 48 of laser 
pumping diode bars. Note that in this alternate embodiment, the stacks 46 
and 48 of laser pumping diode bars can be arranged so that they directly 
face each other across the gain material 40. 
In FIGS. 4A-B and 5 a ray tracing representation of the "beam" emitted by 
the typical bars is shown. FIG. 4A is a top view of a stack of laser diode 
bars used with the present invention. The ray density is shown as 
proportional to the observed intensity of emission. Diode bar 50 has a 
long axis 51. The aperture width of the typical diode bar is about 1 
centimeter along this long axis 51. In a plane parallel to the long axis 
51, the emitted light has a relatively low spreading angle (beam 
divergence) of about 10.degree. full-width half-maximum FWHM but the beam 
quality is very poor. The beam quality can be defined as a product of the 
beam size at the focus multiplied by the beam divergence. For a 
diffraction-limited beam, this product is directly proportional to a 
lasing wavelength .lambda.: 
EQU size.times.divergence=0.44.lambda. 
where the size is the beam FWHM at the location of the minimum spot size 
along the propagation path and the divergence is the FWHM of the beam 
spread expressed in radians. For a typical diode bar, the resultant 
product is around 4,900 times larger than a diffraction-limited or 
"perfect" beam and the emitted light can be considered very incoherent 
spatially. The minimum beam size that can be obtained using a fast 
focusing lens will therefore be of the order of around 1 centimeter. 
FIG. 4B is a side view of a laser diode bar 50 used with the present 
invention. Looking at the plane perpendicular to the long axis 51 of the 
diode bar 50, in the typical commercial laser diode bar the emitting 
regions are around 1 micron or 0.000040 inches thick. The emitting regions 
have a relatively large divergence of around 30.degree. to 45.degree. FWHM 
but the beam quality is excellent, with a nearly gaussian intensity 
profile. The light emitted in this plane is spatially highly coherent, and 
the minimum beam size that can be obtained by using a fast focusing lens 
would be of the order of 1 micron, approaching the same size as the 
emitting aperture. Note that as seen in FIG. 3A, aligning the long axis 35 
of the stack 34 of diode bars with the long axis of the cylindrical lens 
allows for the highly coherent light in the plane perpendicular to the 
long axis 35 of the stack 34 to be focused by the curvature of the 
cylindrical lens 28. The cylindrical lenses used with the present 
invention collect and roughly collimate the emitted pump energy. 
FIG. 5 shows a side view of a stack 52 of five laser diode bars. A typical 
laser-diode side-pumped solid state laser may use several bars put 
together in assembly called a laser diode bar stack. In the preferred 
embodiment, the non-collimated laser diode bars are available from SDL, 
Inc., San Jose, Calif. 95134. A stack of non-collimated laser diode bars 
is defined to be a stack of laser diode bars which are not individually 
collimated by a short focal length lens attached to each bar. Note that in 
the present invention, the stack 52 of laser diode bars need not be 
collimated laser diode bars. Collimated laser diodes use very short focal 
length collimating lenses attached to each bar of the laser diode stack. 
These collimated laser diode bars are much more expensive than 
non-collimated laser diode bars. Additionally, because the collimating 
lenses have very short focal lengths, they must be positioned very 
precisely with respect to the laser diode bar's aperture. An example of 
collimated laser diode bars is shown in the 1993 Laser Diode Product Guide 
for Spectra Diode Labs on page 83. 
FIG. 6 is a cross-sectional view of a cooling apparatus 54 of the present 
invention. This cooling apparatus contacts two opposite flat surfaces of 
the gain material 26'. These two opposite flat surfaces are the surfaces 
of the gain material 26' which are not pumped by the stacks of laser 
diodes (not shown). The cooling apparatus 54 is comprised of metal support 
blocks 54a, pads 54b made of a deformable, compliant, thermally-conductive 
material and coolant passages 54c. The pads 54b made of a deformable, 
compliant, thermally-conductive material provide an intimate contact 
between the gain material 26' and the metal support blocks 54a for good 
heat transfer. The pads 54b could be made of indium foil or alternately 
made of a T-pli material, a highly-conformable, thermally-conductive 
elastomer available from Thermagon, Inc., located in Cleveland, Ohio. The 
T-Pli material is less thermally-conductive than indium foil but has the 
advantage that less pressure is required to form a good thermal contact 
with the gain material. Too much pressure on the gain material may cause 
undesirable stresses within the gain material. 
The arrows in FIG. 6 indicate the path of the heat flow in this 
cross-sectional view. The cooling apparatus 54 exploits the rectangular 
cross-section of the gain material 26' to efficiently extract heat in the 
plane perpendicular to the direction of pumping. This induces a 
temperature gradient confined predominantly to the height of the rod. That 
is, the temperature is dependent upon the distance from the two opposite 
flat surfaces which contact the coolant apparatus 54. This temperature 
gradient produces a simple cylindrical lens with an axis which is 
independent of the pump power level (no prismatic distortion is 
introduced) and can be compensated for by placing one or more cylindrical 
lens beyond one or both ends of the laser rod. Additionally, this 
one-dimensional heat removal technique is well adapted to reducing the 
thermal birefringence in high power applications. Thermal birefringence 
can affect the polarization of the laser beam in a gain material. 
FIG. 7 is a side view of a laser device 56 showing the optical gain 
component 57 of the present invention. Shown is a gain material 26" pumped 
with stacks 34' and 32' of laser diode bars through cylindrical lenses 28' 
and 30'. Since the gain material 26" has Brewster-angled end faces, the 
optical axis 59 of the laser device 56 turns in the gain material 26". The 
laser device 56 includes one concave end mirror 58 and one convex end 
mirror 60. The use of a convex end mirror 60 can expand the laser beam 
mode volume so that it is similar to the pumping mode volume in the gain 
material 26". A cylindrical lens 62 is used to compensate for the thermal 
lens formed by the cooling of the gain material 26". A round aperture 64 
is placed inside the laser cavity so that the laser device 56 is forced 
into a TEM.sub.00 mode. Looking at FIG. 9A and 9B, the small-signal gain 
tends to be slightly saddle shaped which can cause a TEM.sub.01 mode to 
form which has two lobes. The aperture 64 prevents the TEM.sub.01 mode 
from forming. Alternately, a slit could be used rather than the round 
aperture 64. For clarity the cooling apparatus is not shown but the 
cooling apparatus would be attached to the two opposite flat sides not 
pumped by the stacks of the laser diode 32' or 34'. Note that additional 
elements such as mirrors, Q-switches and non-linear materials can be used 
with the laser device 56. 
The optical gain component 57 comprising the gain material 26", the 
cylindrical lenses 28' and 30', the laser diode stacks 32' and 34', and 
the cooling apparatus (not shown) could also be used in a device to 
amplify optical signals. 
FIGS. 8A-I are cross-sectional views of a gain material showing the effect 
of lens curvature and lens thickness upon the pump energy density for a 
given pump configuration. A matrix of cross-sectional views corresponding 
to a 3 mm wide.times.2.5 mm tall gain material pumped from opposite sides 
by a pair of stacks of laser diode bars is shown. Along the bottom is 
given the lens thickness for each column. Along the left hand side is 
given the lens curvature, R, of the cylindrical lenses for each row. Using 
a ray tracing program, one can find the gain in the medium. The best 
shaped cylindrical lens can be found according to several criteria: 
highest gain (as seen by certain size gaussian TEM.sub.00 beam crossing 
the laser medium; and/or best selectivity for the TEM.sub.00 mode, i.e. 
high gain for the TEM.sub.00 mode as opposed to the TEM.sub.01 mode. In 
the preferred embodiment shown in FIG. 8G, the lens thickness 0.67 mm and 
curvature R=1.5 mm would be used for the 3 mm wide.times.2.5 mm tall rod 
pumped by opposite sides pair of 3-bar laser diode stacks with a distance 
to the lens of 0.3 mm. The distance between the laser diode stack and the 
lens is desired to be as small as possible. The details of the preferred 
embodiment discussed above correspond to the embodiment shown in FIG. 3A, 
however, the ray tracings in FIGS. 8A-I do not show reflections off the 
highly reflective coating for the sake of clarity. 
It is desired that the laser diode bars are stacked as close together as 
possible. Due to heat dissipation requirements, however, the duty factor 
(time on/total time) of the laser diode bars is limited by the spacing 
between the laser diode bars. Currently, a laser diode stack with a 6% 
duty factor is available with a spacing between the laser diode bars of 
0.8 mm. 
The focusing of the pump energy is used to produce a small pumped 
cross-sectional area which produces a larger small signal gain. 
Additionally, as discussed above, it is desired that the pumping beams 
become focused as they propagate so as to offset the accompanying 
intensity loss due to absorption in the gain material. The emitted pump 
energy from the different diode bars converge towards the centerline of 
the gain materials as they propagate. Another aspect of the present 
invention involves using two rectangular rods each of which has a saddle 
shaped gain profile. These rods can be arranged so that the net gain would 
appear to be almost circularly symmetric and bell-shaped in profile with a 
large center peak tapering off to a much lower gain at the edges. 
Furthermore, the resulting thermal lens will now look like a spherical 
lens which is easier to compensate for than a cylindrical lens. The 
composite structure will still be largely free of thermal birefringence, a 
major advantage over other schemes. FIGS. 9A-B, 10A-B and 11A-B illustrate 
this concept. 
FIG. 9A is a schematic view of a gain material 66 pumped from two opposite 
sides 66a and 66b showing ray tracing representations. Also shown are 
laser diode stacks 68 and 70. The gain material 66 can be cooled along 
sides 66c and 66d. 
FIG. 9B is a 3-dimensional image showing the small-signal gain for the 
apparatus shown in FIG. 9A. Note that in the direction of line 72, the 
small-signal gain is substantially gaussian. Along the direction of line 
74 the small-signal gain is substantially flat. Note that lobes at points 
76 and 78 may tend to produce the TEM.sub.01 mode in the single gain 
material configuration unless an aperture is used as described above. 
FIG. 10A is a schematic view of another gain material 80 which is oriented 
90.degree. about the optical axis from the gain material 66 shown in FIG. 
9A. FIG. 10B is a 3-dimensional image showing the small-signal gain from 
the apparatus of FIG. 10A. 
FIG. 11A is a schematic view superimposing the ray tracing representations 
of the FIGS. 9A and 10A. FIG. 11B is a 3-dimensional image showing the 
combined small-signal gain through the apparatus of FIG. 11A. This figure 
shows the almost circularly symmetric and gaussian profile of the combined 
small-signal gains. For some combinations, the small signal gain 
distribution may be rectangular rather than circular. That is, the 
small-signal gain will be symmetric about two orthogonal lines. The 
combined small signal gain shown in FIG. 11B preferentially supports the 
TEM.sub.00 mode. 
Additionally, looking again at FIGS. 9A and 9B, the gain material 66 is 
cooled at sides 66c and 66d. For that reason, the temperature in the gain 
material 66 will look similar to the graph of the small-signal gain shown 
in FIG. 9b which would correspond to a cylindrical thermal lens. In the 
same manner, the gain material 80 of FIGS. 10A and 10b corresponds to a 
cylindrical thermal lens which is oriented 90.degree. about the optical 
axis from the cylindrical thermal lens of FIGS. 9A and 9B. For this 
reason, the thermal lenses for the combined system will form a 
substantially spherical thermal lens which is easier to compensate for 
than a cylindrical lens. 
FIG. 12 is a schematic representation of one possible embodiment of the 
present invention giving a side view of a laser device 82 showing the 
optical gain component 83 including two gain materials 66' and 80'. The 
two gain materials 66' and 80' do not have Brewster-angled ends. The ends 
of the two gain materials 66' and 80' are near normal with respect to the 
optic axis 90 through the optical gain component 83. The gain material 66' 
is pumped through cylindrical lens 84 and a cylindrical lens (not shown) 
on the other side. The gain material 80' is pumped through cylindrical 
lenses 86 and 88. The gain material 80' is pumped through a side 80a 
oriented at a rotation of substantially 90.degree. about the optical axis 
90 from the side 66b' pumped on gain material 66'. The laser device 
includes a spherical lens 92 for compensating the combined spherical 
thermal lens of both gain materials 66' and 80'. The 90.degree. rotation 
about the optical axis 90 can be visualized by imagining a screw running 
along the optical axis 90. 
In the optical gain component 83, the side 66b' is substantially 
perpendicular to the sides 80a and 80b which are pumped by the laser 
diodes not shown. 
A spherical lens for compensating the thermal lens formed by gain materials 
66' and 80' can be formed by grinding a negative contour on an end of one 
of the gain materials 66' or 80' since neither gain material has 
Brewster-angled ends. 
FIG. 13A is a top schematic view of another embodiment of a laser device 
104 showing an optical gain component 107 of the present invention. The 
optical gain component 107 includes a gain material 106 and gain material 
108. FIG. 13B is a side schematic view of the embodiment of the laser 
device shown in FIG. 13A. Both of these schematics are oriented such that 
the optical axis 105 through the gain material 106 is along the Y axis. 
FIG. 13C is a graph which is used to show the vectors for FIGS. 13A and 
13B. 
These diagrams show a normal N.sub.1 to the face 106a which is pumped by a 
stack of laser diode bars (not shown) and a normal N.sub.2 to the face 
108a of gain material 108 which is pumped by another stack of laser diode 
bars (not shown). The normal N.sub.1 runs along the X axis. 
The optical axis 105 goes through the first gain material 106 and second 
gain material 108. Since the gain materials 106 and 108 have 
Brewster-angled ends, the optical axis 105 changes orientation in the 
laser cavity. The laser device 104 includes a half-wave plate or 
90.degree. quartz rotator 114 which is used to compensate for the 
different orientations of the Brewster angled ends. 
O.sub.1 is a vector that defines the orientation of the optical axis 105 in 
the first gain material 106. O.sub.2 is a vector that defines the 
orientation of the optical axis 105 in the second gain material 108. The 
vectors N.sub.1 and O.sub.1 define a plane, which is shown in FIG. 13C as 
the XY plane. A projection of the vector O.sub.2 onto this plane (the XY 
plane) is shown as the vector P.sub.2. A vector V.sub.1 in the plane (the 
XY plane) is orthogonal to vector P.sub.2. The normal N.sub.2 through the 
first side of the second material is oriented at substantially a 
90.degree. rotation about the optic axis O.sub.2 in said second gain 
material from said first vector V.sub.1. Note that the length or the sign 
of the vectors discussed above does not affect the above analyses. 
Because the first gain material 106 and the second gain material 108 are 
oriented in the above described manner, the total small-signal gain can be 
almost circularly symmetric and gaussian in profile with a large center 
peak tapering off to a much lower gain at the edges. 
The above specified orientation describes the laser device 82 of FIG. 12 in 
a more trivial fashion. FIG. 14 is a graph which is used to show the 
vectors for FIG. 12. The optical axis 90 does not change orientation in 
the laser device 82 so vector O.sub.1 ' giving the orientation of the 
optical axis in gain material 66' and vector O.sub.2 ' giving the 
orientation of the optic axis in gain material 80' both run along the Y 
axis. The normal N.sub.1 ' runs along the X axis. Vectors O.sub.1 ' and 
N.sub.1 ' define the XY plane. The projection P.sub.2 ' of the vector 
O.sub.2 ' into the XY plane is the vector O.sub.2 '. V.sub.1 ' is 
orthogonal to P.sub.2 ' in the XY plane. Rotating V.sub.1 ' substantially 
90.degree. about the vector O.sub.2 ' gives the normal N.sub.2 '. The 
normal N.sub.2 ' to the pumped side 80a is along the Z axis as shown in 
FIG. 12. 
FIG. 15 is a perspective view of an alternate embodiment of a laser device 
94 showing the optical gain component 95 of the present invention. This 
alternate embodiment shows the use of two gain material rods 96 and 98 to 
produce a saddle-shaped small-signal gain that is twice as large as the 
gain of the optical gain component 57 shown in FIG. 7. Looking again at 
FIG. 15, gain material 96 is pumped from side 96a and gain material 98 is 
pumped through side 98a. This orientation has the benefit that the optical 
axis lays in a plane. Any number of gain material rods can be placed in 
the cavity in this manner to get the desired small-signal gain. 
Various details of the implementation and method are merely illustrative of 
the invention. It is to be understood that various changes in such details 
may be within the scope of the invention, which is to be limited only by 
the appended claims.