Abstract:
A laser pumping assembly with a housing having a removable heat resistant glass liner or shell enclosing a pump and a laser rod comprises removable glass or quartz plates over opposite ends of the shell together with a heat absorbing shim between each end plate and an adjacent heat conductive support plate attached to the housing. The outside surface of each end plate is coated with a dielectric layer which reflects the desired wavelength of pump lamp radiation inwardly of the shell toward the laser rod and transmits undesired wavelengths to the adjacent shim. The shims are held tightly against the end plates and pass heat from the unreflected pump lamp radiation to the respective support plates for dissipation in the cooling system.

Description:
This invention was made under a contract with the Department of the Air Force. 
    
    
     RELATED APPLICATION 
     Ser. No. 866,512 filed Jan. 3, 1978 by Radecki et al, assigned to the assignee of this invention. 
     BACKGROUND OF THE INVENTION 
     This invention relates to an improved 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 optical pump cavity to adjacent parts of the housing while maintaining efficient coupling of pump lamp radiation at the desired wavelength to the rod. 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 &#34;Dielectric Cavity Raises YAG Cavity Efficiency&#34;, 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, the interior surfaces of the metallic end support plates defined part of the interior of the pump cavity and were gold plated. The difficulty with this construction is that unavoidable impurities in the gold plating were &#34;cooked out&#34; in the high cavity temperature and contaminated the interior of the cavity. Furthermore, it was difficult to insulate metallic support plates from the high voltage lamp energizing leads so as to prevent arcing. 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 pump assembly with inert end plates which are heat resistant and free of contaminants. 
     Still another object is the provision of such end plates which are easy to clean and which prevent arcing between the housing and high voltage lamp circuits. 
     These and other objects of the invention are achieved with a laser pumping assembly in which opposite ends of the glass cavity liner are closed by removable glass plates, respectively, held in position by the respective support plates. The exterior of each of these end plates has a dielectric coating which reflects pump lamp radiation at the desired wavelength back into the cavity but transmits undesired wavelengths for removal as heat through the adjacent support plate. Each end plate has a contaminant free inner surface and supports and electrically insulates the lamp parts to prevent arcing. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic drawing of a laser system of the type which emobdies this invention; 
     FIG. 2 is an exploded view of a laser pumping assembly embodying this invention; 
     FIG. 3 is a transverse section of the assembly showing the relative positions of the laser rod, pump lamp and elliptical shell; 
     FIG. 4 is a greatly enlarged view of the laser rod portion of FIG. 3; 
     FIG. 5 is an enlarged elevation showing the connection of the laser rod to the heat sink by a spring loaded strap; 
     FIG. 6 is a view taken on line 6--6 of FIG. 5; 
     FIG. 7 is an elevation of the pumping assembly; and 
     FIG. 8 is a greatly enlarged view of a portion of FIG. 7 showing details of the end plate connection. 
    
    
     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 μm 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 2  NaNb 5  O 15 ). This crystal doubles the frequency of a portion of the light beam so as to produce a beam of 0.532 μm 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 22 which is coated to transmit the 0.532 μm wavelength light while reflecting the 1.064 μm 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&#39; 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 and 37 secured to opposite ends of the housing and over end plates 34 and 35. 
     Pump lamp 10&#39; 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 and connected to external refrigeration means, not shown. The elliptical shell with its optically reflective-transmissive multi-layer 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&#39; on the focal axis of the cavity. 
     Heat sink 32 for laser rod 14&#39; 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&#39; 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&#39; between contact points 56 and 57 has an arcuate extent of θ. 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° 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° 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° 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&#39; 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 on 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&#39;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 glass 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. 
     Glass end plates 34 and 35 have openings 34a and 35a, respectively, (see FIG. 2) in which opposite end portions of lamp 35 are supported for extension through larger openings 36a and 37a in support plates 36 and 37, respectively. Thus end plates 34 and 35 electrically insulate the lamp from the metallic housing parts and prevent possible arcing. In addition, the inner surface of each end plate may readily be cleaned during maintenance of the assembly. 
     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. Small axial dimensional differentials between the 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.