Conductively cooled laser pumping assembly

An improved conductively cooled laser pumping assembly comprises a metallic housing having an elliptically shaped bore in which a pump lamp and laser rod are supported along the respective focal axes of the bore. The laser rod is mounted on the tapered end of a conductively cooled metal body by resilient strap means at opposite ends of the rod and with a relatively low melting temperature solder between the body and the rod to enhance thermal contact with the rod and to accommodate the different thermal expansions of the rod and body, thereby minimizing rod strain. The elliptical bore is coated with a heat absorbing substance and contains a similarly shaped one-piece snugly fitting removable glass liner or shell. The shell has removable glass or quartz end plates backed by heat absorbing shims and both the shell and end plates have a dielectric reflective-transmissive coating on their external surfaces for reflecting pump light toward the laser rod while transmitting radiation having unwanted wavelengths to the adjacent conductively cooled housing. The shell and end plates are resiliently axially biased as a unit within the housing to limit relative movement of these parts.

RELATED APPLICATION 
Serial No. 866,511 filed Jan. 3, 1978 by Taynai et al, assigned to the 
assignee of this invention. 
BACKGROUND OF THE INVENTION 
This invention relates to lasers and more particularly to a conductively 
cooled lamp pumped rod-type laser. 
There has been a continuing problem with conductively cooled solid state 
lasers in the removal of heat from the optically pumped laser rod. Heat 
should be transferred efficiently to the supporting rod heat sink by a low 
resistance thermal path in such a manner as to minimize or eliminate 
mechanical strain in the rod due to differences in the thermal expansion 
of the rod and heat sink. At the same time, a mechanically stable rod 
mount that will withstand vibration and other movements is required. This 
invention is directed to a structure which meets these requirements. 
In addition to the above, there is a need in such lasers for efficient 
coupling of pump lamp radiation at the desired wavelength to the rod as 
well as removal of lamp radiation at undesired wavelengths to the adjacent 
parts of the laser housing. Prior art lasers of this type have utilized a 
dielectric elliptically shaped shell with a coating that reflects to the 
rod only the desired pump wavelengths, such technique being described in 
an article entitled "Dielectric Cavity Raises YAG Cavity Efficiency", by 
Y. H. Hahn et al published in Electro-Optical Systems Design, February 
1975 (Milton S. Kiver Publications). Such lasers, however, have depended 
upon convection cooling of the shell; that is, the shell is simply exposed 
to the air or other similar medium. There is no known technique in the 
prior art for enclosing such a shell in a conductively cooled housing and 
to do so in such a manner as to preserve the efficient transfer of heat 
from the shell to the body. 
Another aspect of the above type of conductively cooled lasers is the need 
for accessibility of parts for maintenance and repair without compromising 
the capability of the assembly to withstand vibration and shock forces. In 
one prior art pump assembly, a plurality of glass inserts were cemented to 
the inner surface of the elliptical bore of the housing. Replacement of 
these inserts required complete disassembly of the structure and 
reprocessing of the bore which is not only costly and time-consuming but 
also is difficult to accomplish in the field. This invention is directed 
to a solution to this problem. 
OBJECTS AND SUMMARY OF THE INVENTION 
A general object of this invention is the provision of a conductively 
cooled laser pumping assembly which efficiently directs pump lamp 
radiation to the laser rod. 
A further object is the provision of a laser pumping assembly which is 
resistant to shock and vibration while having high heat conductivity 
between the rod/shell and the external conductively cooled parts of the 
assembly. 
Still another object is the provision of a laser pumping assembly with a 
laser rod mounted to provide an efficient heat conduction path to the 
external part of the structure without inducing undesired strain in the 
rod. 
These and other objects of the invention are achieved with a laser pumping 
assembly in which an elliptically shaped one-piece, glass reflecting shell 
is removably mounted within the bore of the conductively cooled metal 
housing under a resilient biasing force while maintaining efficient heat 
transfer between these parts. The laser rod is supported on one of the 
focal axes of the shell by a conductively cooled body to which the rod is 
connected by a combination of a low melting point solder and resilient 
mechanical means. The solder permits high heat transfer between contacted 
parts of the rod and body and the mechanical connection secures the parts 
together while permitting different thermal expansions between the body 
and rod so as not to stress the latter.

DESCRIPTION OF PREFERRED EMBODIMENT 
Referring now to the drawings, FIG. 1 is a schematic representation of a 
laser system of the type in which the invention is embodied. The system 
comprises a laser pump lamp 10 having an axis 11 and energized by a power 
supply 12. Lamp 10, which may be of the alkali metal arc type, illuminates 
and optically pumps a rod 14 of lasing material, such as neodymium doped 
yttrium-aluminum-garnet (Nd:YAG), having an axis 15 along which the 
coherent light 16 is generated. Lamp 10 and laser rod 14 are mounted 
within a pumping assembly indicated in broken lines at 17 and described in 
greater detail below. 
Coherent light generated by rod 14 typically has a wavelength of 1.064 
.mu.m and in the system shown by way of example in FIG. 1 is reflected by 
end mirror 19 back through rod 14, passes through a lens 20 and is again 
reflected by folding mirror 22 through a frequency doubling crystal 23 
such as barium sodium niobate (Ba.sub.2 NaNb.sub.5 O.sub.15). This crystal 
doubles the frequency of a portion of the light beam so as to produce a 
beam of 0.532 .mu.m wavelength along with the remaining undoubled 
fundamental beam which are totally reflected by end mirror 25 back through 
crystal 23 where a second portion of the fundamental is also converted to 
the second harmonic, then to mirror 23 which is coated to transmit the 
0.532 .mu.m wavelength light while reflecting the 1.064 .mu.m light. The 
output 26 from mirror 22 is therefore green light having a wavelength of 
0.532. 
Pumping assembly 17 is shown in detail in FIGS. 2 and 3 and comprises a 
split housing 28 with two sections 28a and 28b, a unitary elliptically 
shaped transparent shell 30 such as quartz or heat resistant glass, 
disposed within the housing, laser rod 14' supported on one end of a heat 
conducting body or heat sink 32 within shell 30, transparent end plates 34 
and 35 of quartz or the like over opposite ends of shell 30, and support 
plates 36 36 and 37 secured to opposite ends of the housing and over end 
plates 34 and 35. 
Pump lamp 10' extends through end plates 34, 35 and support plates 36, 37 
for connection at opposite ends to a cathode mount 39 and an anode mount 
40, respectively, both mounts being connected to power source 12. 
Housing 28 has an elliptically shaped bore 45, see FIG. 3, with focal axes 
which are coincident with the axes 11 and 15 of the pump lamp and laser 
rod, respectively, when the pumping assembly is fully assembled. The 
interior of bore 45 is coated with a radiation absorbing substance such as 
black chrome. 
Shell 30 is also elliptically shaped and dimensioned so as to fit snugly 
within bore 45 so that the bore and shell have coincident focal axes. The 
external surface of shell 30 has a multilayer dielectric coating which is 
designed to transmit lamp wavelengths not used to pump the laser rod and 
to reflect lamp wavelengths useful in pumping the laser rod. The unwanted 
lamp radiation which passes through this coating is absorbed by the black 
chrome layer on the housing bore surface and the resulting heat is 
transmitted by conduction through the housing to heat pipes 47 inserted in 
appropriate passages in the housing and connected to external 
refrigeration means, not shown. The elliptical shell with its optically 
reflective-transmissive multilayer dielectric coating is part of the prior 
art and does not per se constitute this invention. 
Housing sections 28a and 28b are connected together by flanges 49 and 50, 
respectively, along a plane which contains the focal axes of the bore and 
shell. The opposite side wall of the housing has an elongated opening 51 
which overlies and registers with a similar opening or slot 52 in shell 
30. These openings provide access to the interior of the housing and shell 
through which the heat sink 32 is inserted, which in turn supports laser 
rod 14' on the focal axis of the cavity. 
Heat sink 32 for laser rod 14' comprises a conductive body having recesses 
for receiving cooling pipes 54 for conductively cooling the structure in 
the manner similar to that for housing 28. In accordance with this 
invention, rod 14' is mounted on the tapered end 32a of the heat sink 
body, see FIG. 4, the rod being engaged substantially tangentially at 
arcuately spaced points 56 and 57 and being separated from the 
intermediate parts of the body by a small gap 58. The surface of rod 14' 
between contact points 56 and 57 has an arcuate extent of .theta.. 
Radiation shields, not shown, may be provided to cover the portions of the 
heat sink within the elliptical cavity. 
Gap 58 contains a solder having a relatively low melting point, i.e., 
approximately 10.degree. C., and high heat conductivity in the liquid 
state. By way of example, the solder useful for this purpose is an 
indium-gallium-tin composition made by American Indium Corporation and 
designated as Alloy #51. At room temperature a bond is formed between the 
rod and heat sink by the solder. The liquid solder wets both the heat sink 
and the laser rod with excellent adherence and makes a near perfect 
thermal contact. In addition, the solder has a low vapor pressure and high 
reflectivity which are advantageous in the high radiation fields of the 
pump light. Minimal strain is induced in the rod by use of the solder bond 
because the laser rod temperature typically is approximately 10.degree. C. 
or slightly less when the laser is operating. Thus the rod and heat sink 
are capable of expanding and contracting independently down to 10.degree. 
C. and no induced stress is transmitted to the rod by the solder joint at 
higher temperatures. This results in significant improved laser 
performance because stress induced birefringence leading to depolarization 
of the laser beam in passing through the rod is considerably diminished. 
Rod 14' is slightly longer than adjacent parts of heat sink 32 and is 
disposed so that the rod ends project slightly beyond opposite ends of the 
heat sink. This length differential insures against solder covering the 
ends of the rod. Each of these overhanging rod ends is clamped to the heat 
sink by a strap 63, see FIGS. 5 and 6, anchored at one end to the heat 
sink body by pin 64 and at the opposite end by a spring 65 secured to the 
body by a pin 66. The tension of spring 65 is adjusted by selection of the 
location of pin 66 and permits slight relative movement between the rod 
and heat sink due to thermal expansion and contraction while preventing 
undue freedom of movement between these parts that might damage the rod. 
Each clamping mechanism is covered by an end cover 67 secured to the sides 
of the heat sink which protect the anti-reflection coatings in each end of 
the rod. 
Glass shell 30 may lose reflectivity over a period of time due to 
degradation of dielectric coating or due to contamination of the shell's 
inner surface because of the high temperature and radiation environments 
to which it is subjected during operation of the laser. In order to 
provide for convenient and quick replacement of shell 30, support plates 
36 and 37 are removably connected to housing 28. Opposite ends of shell 30 
are closed by abutting end plates 34 and 35 as shown in detail in FIGS. 7 
and 8. Each end plate 34, 35 is dimensioned so its peripheral portion 
extends into a recess 70 in the elliptical surface of the housing. 
In order to further enhance the efficient transmission of pump light to the 
laser rod and to remove undesired radiation from the ends of the cavity, 
the exterior surface of each end plate has the same multilayer dielectric 
coating that is applied to the external surface of shell 30. The coated 
outside surface of end plate 34 is indicated at 72 in FIG. 8. Adjacent to 
and covering these coated end plate surfaces are thin metallic shims, one 
of which is shown at 74 in FIG. 8, having inside surfaces covered with a 
light absorbing substance such as black chrome. Shims 74 absorb the 
undesired lamp radiation transmitted through the dielectric coating on the 
end plates and transmit this as heat to support plates 36 and 37 for 
removal through the cooling system. 
The subassembly consisting of shell 30, end plates 34 and 35, and shims 74 
is removable axially from the housing cavity for maintenance and 
replacement if necessary. This invention is described and claimed in 
copending application Ser. No. 866,511 described above. Small axial 
dimensional differentials between shell 30 and housing 28 due to operating 
temperature differences are accommodated by an axial clearance 75 between 
shim 74 and support plate 36 and by clearance 78 within gap 70. Clearance 
75 is held to a minimum in order to provide efficient heat transfer from 
the shim to the end plate and is preferably 0.001-0.005 inches. In order 
to hold these subassembly parts tightly together, a plurality of retainer 
springs, one of which is shown at 76 in FIGS. 2 and 8, are mounted in 
appropriate axially parallel recesses 77 in support plate 36 and abut 
against shim 74. The shell-end plate-shim subassembly is thus axially 
resiliently held against support plate 37 so as to make it vibration 
resistant while accommodating thermal expansion and manufacturing 
tolerances.