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

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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.