Efficient Second-Harmonic Generation with Diffraction-Limited and High-Spectral-Radiance Nd-Glass Lasers

Abstract

Efficient second-harmonic generation (SHG) at 5300 Å has been achieved with KDP and LiNbO3 crystals when Nd-glass lasers of high radiance and narrow bandwidth were used. With KDP and a diffraction-limited laser system having a 19 Å bandwidth, 15 J of radiant energy at 5300 Å were obtained with a 51% energy-conversion efficiency. Since the harmonic light pulse was narrower than the fundamental pulse, the peak-power-conversion efficiency was 70%. The peak power in the green was 1.0 GW. The ratio of harmonic-fundamental pulse duration increased as SHG increased into the saturation region as expected. For LiNbO3, the relatively large dependence of phase-matching angle on wavelength limits the maximum SHG efficiency to several percent when broad-band lasers are used. With LiNbO3, therefore, a laser system was used having mode selectors which limited the bandwidth to less than 0.5 Å while the beam divergence was 1.5 mrad. In this case 21% energy conversion and 33% peak-power conversion were obtained with a fundamental flux density of only 2 MW/cm2 inside the crystal. The values found for the elements of the nonlinear dielectric tensor, corrected for the random multimode nature of the laser, are d36(KDP) = (1.1±0.1)×10−9 esu and d31(LiNbO3) = (17±6)×10−9 esu. At low conversions a laser beam with fluctuations due to random multimoding is expected to give as much as twice the harmonic produced by a single-mode laser. At high conversions in the saturation region this ``doubling'' does not occur as verified by our measurements.