Abstract

Proton exchanged (PE) and annealed proton exchanged (APE) LiNbO₃ waveguides have recently been used for frequency doubling in the form of Cerenkov radiation or by quasiphase matching.¹ The efficiency observed was much larger than predicted in the Cerenkov case but much lower with quasiphase matching. However, both these results can be explained by a strongly reduced nonlinearity in the waveguide.¹ Areduction of the nonlinearity after PE has been reported,² but it is not sufficiently large to explain the efficiencies obtained in Cerenkov radiation and quasiphase matching. We have used surface SHG with a frequency doubled Nd:YAG laser running at 532 nm to measure the nonlinearity of PE LiNbO₃ and, in contrast to the previous reports, found an almost complete reduction of the nonlinearity. This is in much better agreement with conversion efficiencies reported.¹ The short absorption length of the SH (266 nm) guarantees a surface selective measurement in our case, which the previous measurements did not ensure. The successive restoration of the nonlinearity with annealing has also been measured and the implications on nonlinear optical devices using PE and APE waveguides are discussed.

© 1991 Optical Society of America