Study of the physics of the power-scaling of end-pumped solid-state laser sources based on Nd:YVO₄

Abstract

Using a modified Mach-Zehnder interferometer the thermal lensing in Nd:YV0₄ was measured for several different operating conditions. The thermal lens focal length can be determined from the measured transverse phase profile. It was found that the thermal lensing was weakest for p-polarised light and that the bulk expansion plays a part in modifying the power of the thermal lenses. By comparing the thermal lensing with cooling direction it was found that the providing cooling along the a-axis generated the weakest thermal lensing. Comparing the thermal lensing under lasing and non-lasing conditions demonstrated that the heating in the laser crystal under non-lasing conditions is significantly greater than under lasing conditions. The thermal lenses are almost 5 times stronger under non-lasing conditions than lasing condition for the 1% doped crystal. By comparing the effect of dopant concentration on thermal lensing the effect of ETU could be seen, with the thermal lensing for the 0.3% doped crystal being much lower than that of the 1% doped crystal under non-lasing conditions.An amplitude modulated mode-locked laser was built based on Nd:YVO₄ generating 600mW of diffraction limited output and 100ps pulses. Multipass amplification was then investigated as a means to increase the average power of the source. This was achieved with 5W of output achieved, with the beam remaining diffraction limited. The prospects for further power scaling are investigated and it was shown that the limit to power scaling via amplifiers is the eventual beam quality degradation that will be suffered as the signal beam passes through the thermal lenses in the laser crystal. An equation was finally presented that analysed the limitations of scaling via amplifiers, finding that when stress-fracture and beam quality degradation are considered, Nd:YVO₄ represents an excellent choice for further power scaling.

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