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Phase-matched sum frequency generation in KNbO3 crystals for efficient upconversion of Nd:YAG lasers into the blue wavelength range

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Abstract

In KNbO3 the refractive indices and their dispersion allow noncritical 90° phase-matched sum frequency generation of Nd:YAG and laser light in the red spectral range (λ = 685 nm) at room temperature, Efficient infrared upconversion of Nd:YAG laser radiation with cw krypton (λp = 676 nm) and pulsed ruby (λp = 694 nm) lasers into the blue spectral range (λsum = 415 nm in the first case and λsum = 420 nm in the second case) is demonstrated at medium pump powers. For the sum frequency generation the nonlinear optical coefficient d32sumpumpsignal) = 20.4 pm/V in orthorhombic KNbO3 (point group symmetry mm2) crystals was employed. The measured phase-matching temperatures are Tpm = +27.2°C for wavelengths λsignal = 1064 nm and λpump = 694 nm and Tpm = −4°C for λsignal = 1064 nm and λpump = 676 nm. Using a signal power of Psignal = 5 W (cw) and a peak pump power of Ppump = 20 kW (pulsed in 50 ns), a peak power at the sum frequency Psum = 59.4 W (in 50 ns) was obtained. Both the pump and the signal beam propagate collinearly along the crystallographic a axis and were focused down to a spot size w0 100 m in the center of a KNbO3 crystal which was 8.7 mm long. No optical damage of the crystal was observed at power densities up to 60 MW/cm2. Continuously tunable sum frequencies between 400 nm < λsum < 450 nm can be generated in KNbO3 by mixing a signal wave of λsignal = 1300 nm with a tunable dye laser (rhodamine, DCM) with 520 nm < λpump < 670 nm. Noncritical 90° phase matching is possible over the whole temperature range by crystal temperature tuning between T= 20°C and T = 180°C.

© 1986 Optical Society of America

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