Ross, G.W., Pollnau, M., Smith, P.G.R., Britton, P.E., Clarkson, W.A. and Hanna, D.C.
High-power blue light generation in periodically-poled lithium niobate
At Quantum Electronics Conference (QE13), United Kingdom.
08 - 11 Sep 1997.
Full text not available from this repository.
Frequency doubling of the 946nm line of Nd:YAG offers a possible route to the generation of high average power in the blue with an all-solid-state technology. Recent results include the generation of 410mW at 473nm by intracavity frequency doubling using KNbO3. Periodically-poled lithium niobate (PPLN), with its combination of high nonlinearity and non-critical phase-matching offers the prospect of high conversion in a simple, extra-cavity single-pass arrangement. A previous demonstration with PPLN gave 49mW of 473nm power, although the blue beam quality was significantly degraded at this power level by the photorefractive effect. Here we report an order of magnitude increase in mean blue power, without any sign of beam degradation. We prepared the PPLN sample, of 15mm length, 0.5mm thickness, grating period 4.5µm, by electric-field poling through a photolithographically patterned photoresist on the -z face. The period was chosen for phase-matching at an elevated temperature of 140°C, thereby greatly reducing the photorefractive effect. The phase-matching temperature bandwidth, measured as 1.2K, is consistent with the 15mm crystal length.
The fundamental laser source was a Nd:YAG laser pumped by a single, beam-shaped 20W diode-bar. The Nd:YAG resonator was chosen to provide compensation for the strong lensing that occurs as a result of intense pumping. This laser has produced a cw power of 2.1W in a linearly polarised fundamental mode (M2 < 1.1). However, in these harmonic generation experiments average powers in the 1 - 1.5W range were used, with the laser operating with a regular pulsed output at the relaxation oscillation frequency (160kHz, 300nsec pulses). This behaviour was induced via feedback from the uncoated PPLN surface, together with a piezoelectrically driven dither of the output coupler.
Under these conditions, with an average fundamental power of 1.13W inside the PPLN, focussed to a spot radius of 30µm, a generated blue power of 450mW, corresponding to 40% conversion efficiency, was obtained. The output blue beam had a circular profile with measured M2 values of Mx 2 = My 2 ~ 1.25, indicating that photorefractive effects were not significant at this average power/intensity level for the 140°C temperature of operation. There is also no sign of roll-off from the quadratic relation between harmonic power and fundamental power. These results suggest that with attention paid to optimisation of the laser, and with provision of coatings for the PPLN crystal, blue powers exceeding 1W should be achievable in this way.
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