Hardman, P.J., Clarkson, W.A., Martin, K.I. and Hanna, D.C.
High-power diode-bar-pumped intracavity-frequency-doubled Nd:YAG and Nd:YLF ring lasers
At Quantum Electronics Conference (QE13), United Kingdom.
08 - 11 Sep 1997.
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The use of diode-pumped solid-state lasers as sources of high power visible light is an area which has attracted growing interest in recent years. This interest stems from the increased efficiencies available and compact size compared to Ar+ lasers. A further attractive feature of intracavity-frequency-doubled, single-frequency lasers, on which initial results were recently reported, is that axial-mode-hopping is suppressed. The explanation for this behaviour is based on the fact that adjacent (non-lasing) axial modes are further suppressed by an additional loss due to sum-frequency generation. This is twice the loss experienced by the lasing mode due to second harmonic generation. In a low loss resonator with efficient intracavity-frequency-doubling this extra loss can more than offset any gain advantage of adjacent modes closer to the gain peak. The net result is that continuous (mode-hop-free) tuning is possible over many axial mode spacings. Here we describe an intracavity-frequency-doubled Nd:YAG ring laser end-pumped by a 20W diode bar, and a Nd:YLF ring laser end-pumped by two 20W diode bars. In each case, a simple bow-tie cavity design was employed, with a Brewster-angled LBO crystal. In the case of Nd:YAG for a non-optimised laser mode size in the LBO crystal the laser produced ~1.4W of single-frequency output in the green at 532nm. By varying the cavity length we obtained a single-frequency, continuous (mode-hop- free) tuning range of ~40GHz corresponding to ~80 axial mode spacings. This range is consistent with predictions of a simple model accounting for the effects of nonlinear loss due to sum frequency generation.
Nd:YLF offers the potential of an extended tuning range through its broader linewidth. Furthermore Nd:YLF is attractive for operation at higher powers due to its superior thermo-optical properties on the sigma-polarisation compared to Nd:YAG, providing that appropriate steps are taken to avoid thermally-induced stress-fracture. Results for this laser, end-pumped by two 20W diode-bars, include the generation of ~10.3W of single frequency 1053nm output in a TEMoo beam (M2 < 1.1), and 6.2W of green output at 526.5nm (corresponding to ~8.5W generated internally in the LBO) and a conversion efficiency of ~5% with respect to intracavity power. We have obtained a single frequency, continuous (mode-hop-free) tuning range of 72GHz corresponding to ~150 axial mode spacings. So far, the mode-hop-free tuning range has been limited by etalon effects due to imperfect AR coatings on the TGG Faraday rotator. With the elimination of these etalon effects, mode-hop-free tuning over a considerable fraction of the gain bandwidth should be achievable
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