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Research On Thermal Effect In Conduction Cooled End-Pumped Slab Amplifier

Posted on:2011-03-23Degree:DoctorType:Dissertation
Country:ChinaCandidate:L LiuFull Text:PDF
GTID:1118330332987026Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
High average power with high quality is a common goal that every designer pursues in the domain of solid-state laser. The key to such goal is how to properly manage the waste heat deposited in the active medium.Solid-state lasers with a rod active medium suffer serious thermal effects. Then, zigzag slab lasers were proposed to eliminate stress induced biaxial focusing and birefringence. This dissertation presented a brief analysis of thermal effects in rod lasers, and a detailed theoretical and experimental investigation of thermal effects in conduction cooled end-pumped slab (CCEPS) amplifier.Thermal effects in side-pumped rod lasers were studied, operating in the mode of continuous-wave or heat capacity. The expressions of focal length in side-pumped rod lasers were studied. The results reveal that underestimation of thermal lens accrued from overestimating the material's thermal conduction and without considering the nonuniform heat deposit, when Koechner's theory was used. An analytic model was developed to describe the dynamic average thermal distortion and phase difference between the two principal polarizations in side-pumped Nd:YAG and Nd:Glass heat capacity rod lasers. It can be predicted that the average thermal distortion was proportional to the temperature profile on the cross-section from the analytic expression, and in the case of the same pumping and cooling conditions, there were the same orders of depolarization loss for Nd doped YAG, LG-680, LG-750, LG-760 and LG-770 glass rod lasers.In the part of theory research in slab lasers, a semi-analytical analysis of temperature field was introduced. The errors in the model of thermal stress-induced depolarization in earlier papers were pointed out. A detailed review of how to calculate the depolarization loss and get correct piezo-optic tensor was presented. Based on the corrected theory model, thermal effect in 2D composite slab was studied, and a primary understanding of the ability of eliminating thermal effect by zigzag propagation was obtained. Then, 3D model was established, and thermal effects in CCEPS were studied with different cooling conditions. The results revealed that it was the thermo-optic effect that mainly caused the thermal distortion in Nd:YAG slab. In slab lasers, thermal and stress induced focusing could be eliminated along the zigzag path. However, many factors could aggravate thermal effect in zigzag slabs, such as improper structure of the composite slab, little bounce number of the light in zigzag propagation through the slab, heterogeneous cooling conditions, and so on. In zigzag slab lasers, thermal stress induced depolarization loss was less than 0.5%, thus it could be neglected. The geometric models exchange between software ANSYS and ASAP was carried out, which provided a method of studying the optical distortion due to deformation. In the part of experimental research, four projects were studied, including:temperature distribution in the active medium, absorption spectrum of the undoped YAG, beam quality of the slab lasers and the effect of assemble stress on the slab laser's performance. The temperature measurement revealed an unexpected high temperature existing in the undoped YAG end caps. The absorption spectrum measurement indicated a weak absorption of the pump power in undoped YAG, with a absorption coefficient of 0.015cm-1 at 808nm. Simulation proved that it was the weak absorption in undoped YAG that lead to the unexpected temperature rise in the composite slab. The wavefront measurement showed thermal lens existed in the slab's width direction. Simulation indicated that thermal distortion did exist in the width direction if the heat sinks were wider than the slab at the contact area. The intensity distribution of the transmitted beam was measured under different assemble stresses, and it showed that serious thermal distortion existed if improper assembly was adopted.In 2009, a new design for a bounce geometry slab amplifier, the Compact Face-Folded INternally (COFFIN) amplifier, was presented. Thermal effects in COFFIN slab were studied. The results showed that COFFIN slab was not suitable to be an active medium in high-power and high beam quality solid-state laser. However, thermal distortion in hexagonal slabs with optimal design could be compensated easily, and the output power in hexagonal slab was about two or three times than that in COFFIN slab, in the case of the same beam quality.
Keywords/Search Tags:solid-state lasers, thermal management, thermal effect, slab lasers, heat capacity lasers, stress-induced birefringence, Thermal/structural /optical Integrated Analysis
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