Conduction Cooling Of Nd:Glass Slab Lasers

Abstract

Isolation cooling of slab geometry solid state lasers potentially solves a number of problems with direct cooling. We have tested a static gas cooled Nd:Glass slab laser to determine the advantages and limitations of this type of cooling system. This paper reports on these tests and discusses approaches to improve the average power of such a laser. The zig-zag slab geometry laser is an established method of reducing thermo-optical effects in solid state lasers. Eggleston et.al.,1 built and tested a small test-bed Nd:Glass slab laser that confirmed computer model performance predictions but demonstrated the need to carefully control the heat distribution within the slab. Eggleston et.al.'s work also indicated problems with direct fluid cooling the slab. In addition to coolant sealing difficulties a degradation of the total internal reflection faces due to contamination and etching was apparent. Since the zig-zag geometry involves many reflections off this surface any increase in loss is extremely undesirable. This under-standing led to the design and construction of a second generation of slab lasers that avoided the difficulties of direct fluid cooling by using cooling across a thin static aas gap.2 Joseph Chernoch was awarded the patent for conduction cooled slab lasers in July 19723 but to our knowledge a slab geometry laser using this cooling approach was never constructed. The laser built to examine the merits of static gas conduction cooling is shown in cross-section in Fig. 1. A (150 x 45 x 6.3) mm slab of Hoya LHG 5 phosphate glass with 8% Nd doping was used for these tests. Pumping was provided by two 15 cm long xenon flashlamps surrounded by second surface silver reflectors. The inner cooling pane was a 1 mm thick pyrex window supported and sealed by flexible elastomer such that it was free to be pressed against teflon spacers over the slab by the water flow. This allowed a variable gas gap thickness and easy mounting for the glass slab. This sealed static environment avoids slab surface contamination and provides a 'soft' thermal boundary against sudden changes in coolant temperature. One disadvantage of the gas conduction cooling design is the reflection loss of pump radiation at the solid to gas interfaces. We measured a slope efficiency increase of 33% when the gas gap was filled with water. Antireflection coatings on the window and the slab would improve flashlamp coupling but cannot prevent the rejection of radiation outside the critical angle for this solid to gas boundary. It can be noted that a permanent conduction layer of static fluid would be acceptable if a seal around the slab face could be made and convection of the thin static fluid layer in the flat plane of the slab could be effectively baffled.