Abstract

Noncritical Type I birefringent phase matching for infrared-to-visible nonlinear frequency conversion in the blue-green region of the spectrum, requires heating LiNbO₃ congruent crystals very frequently to temperatures in excess of 100 °C. We have obtained accurate control of the room temperature fundamental wavelength in noncritical Type I second harmonic generation (SHG) in LiNbO₃ crystals by adding a controlled concentration of K₂O to the congruent melt in a Czochralski LiNbO₃ crystal growth process. A variable concentration of K₂O (0 - 9 wt. %) controls the final Li/Nb ratio in the crystal¹ between congruent and stoichiometric compositions, and thus the amount of birefrigency². We have experimentally obtained room temperature noncritical Type I phase matched second harmonic generated for fundamental wavelengths extending from 1074 nm (congruent, green SHG) to 972 nm (stoichiometric, blue SHG) in a set of LiNbO₃ crystals grown with different K₂O concentrations. The linear variation of fundamental wavelength with K₂O concentration permits accurate tuning of the noncritical fundamental wavelength for SHG. This principle is extensible to other noncritical frequency conversion processes.

© 2000 IEEE