Abstract

Proton exchanged waveguides are often used in quasiphase-matched nonlinear devices in LiNbO₃. Conversion efficiencies are often significantly smaller than the theoretical values, indicating the need for accurate measurements of the nonlinear susceptibility of proton exchanged LiNbO₃. Reflected second harmonic generation (SHG) from interfaces can be used to measure the nonlinear susceptibility of a film on a nonlinear substrate; however, the air–film and film–substrate interfaces both contribute to the total reflected SH signal. Generation of harmonic wavelengths above the film absorbtion edge will discriminate against the film–substrate interface reflection, so that the reflected harmonic will only be sensitive to the nonlinear properties of the film. We report measurements of the d₃₃ nonlinear coefficient of proton exchanged LiNbO₃ using reflected SHG of 266-nm radiation and find that upon proton exchange d₃₃ is reduced to ≈1% of the bulk LiNbO₃ value. This result disagrees with previous reports of ≈50% reduction for doubling 1.06-µm radiation.¹,² A discussion of the discrepancy among these measurements along with additional measurements of d₃₃ as a function of annealing are presented.

© 1991 Optical Society of America