Abstract

Quasi-phase matched (QPM) LiNbO₃ (i.e. a kind of optical superlattices) has a usable highest nonlinear coefficient d₃₃ and phase matching ability in its full transparency range.¹ The crystal also has large acceptance angle, a large acceptance bandwidth, and good temperature stability. Thus it is very useful in frequency doubling of a laser diode for constructing a compact short wavelength source.^2-4 In this paper we report the demonstration of green, blue and violet light generation and direct frequency doubling of a cw 810 nm GaAlAs laser diode, both in optical superlattice LiNbO₃. Optical superlattice LiNbO₃ crystals were grown by Czochralski method. By using a tunable optical parametric oscillator (OPO) as a fundamental source, three samples with different modulation periods have been choiced to measure green, blue and violet light generation. The OPO has a pulse rate of 1 Hz, in which the pulse width is 30 ps and the line width is less than 1 nm. Green 565 nm light generation in a 1.5 mm long LiNbO₃ sample, which has a modulation period of 8.3 μm, was shown in Fig. 1. The maximal frequency doubling efficiency is 19.8%. The FWHM of the curve is about 3 nm. Fig. 2 shows blue 435 nm light output which was generated in a 0.52 mm long sample with modulation period of 3.5 μm. The maximal efficiency obtained is about 3.8%. The FWHM of the curve is about 3.5 nm. The violet 385 nm light was generated in a 0.98 nm long sample with a modulation period of 6.8 μm. In this experiment third-order QPM was used. The maximal efficiency is 1.6%. The FWHM here for third-order QPM condition is about 4.4 nm. The sample used for green 565 nm light generation was further used for direct frequency doubling of a cw 810 nm, 1W GaAlAs laser diode. The modulation period, 8.3 μm, nearly corresponds to 6 times of the coherent length of doubling of 810 nm output through third-order QPM. The polarized output light of the laser diode was focused into the sample with a set of N.A. = 1.5 lens. A filter was used to block the transmitted infrared light. No temperature adjustment was used. At 250 mW of infrared power incident upon the sample, 0.35 mW of 4.5 nm output power is produced, with an optical conversion efficiency of 0.14%. Careful studies on frequency doubling of a laser diode and on optical parametric interaction in optical superlattice LiNbO₃ have also been down.

© 1995 IEEE