Study of thermal lensing in Nd:YVO₄ under intense diode-end-pumping

Abstract

With its natural birefringence and high στ_f product, Nd:YVO₄ is a popular choice as a lasing material for diode pumped solid-state lasers. The short absorption length for pump light at ~809nm relaxes the constraints on diode beam quality allowing tighter pump beam focusing and hence the use of relatively small laser mode sizes. However, in comparison to Nd:YAG it has poorer thermal properties, resulting in strong thermal-lensing, a knowledge of which is essential for optimum laser design. We present the results of a detailed study, considering the effect on thermal lensing in Nd:YVO₄ of neodymium concentration, heat-sinking arrangement and laser configuration.
The thermal lens powers in end-pumped Nd:YVO₄ were determined by measuring the induced phase difference as a function of transverse position using a Mach-Zehnder interferometer, figure 1. The interferometer was also equipped with a set of mirrors to form a stable non-collinear resonator for the Nd:YVO₄ crystal to allow thermal-lensing to be investigated under lasing and non-lasing conditions.
Figure 1 The interferometer set-up and figure 2 example results.
Figure 2 shows the results obtained for 1% and 0.3% Nd-doped YVO₄ crystals with cooled faces perpendicular to the c-axis and probe beam polarization parallel to the c-axis. The thermal lens power is ~5 times greater under non-lasing than for the lasing conditions for the 1% doped crystal, which we attribute mainly to additional heating from ETU, further compounded by reduction of thermal conductivity at higher temperature. The much smaller corresponding value for the 0.3% doped crystal indicates the potential benefits to (~2) be achieved with lower Nd concentrations. The results for the perpendicular heat-sinking direction and the orthogonal probe beam polarization, which suggest that thermally-induced stresses play a role in thermal lensing behaviour will also be presented. The results provided by this study allow formulation of a design strategy for further power-scaling of Nd:YVO₄ lasers and amplifiers whilst maintaining good output beam quality.

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