Abstract

The detailed experimental investigation indicates that the characteristic feature for all unannealed proton-exchanged (PE) waveguides consists in the absence of the essential photovoltaic (PV) current at light intensities J ≤ 10² W/cm² which is due to the suppression of the PV-effect up to more than two orders of magnitude. At the same time, based on the data of direct measurements it follows that for this intensity level a marked photorefraction (PR) Δn_s ~ 0.7 - 2.0.10^–5 takes place. Moreover, it is establi­shed that for PE LiNbO₃ waveguides electro-optic coefficient r₃₃ is 9 times smaller than for pure LiNbO₃, but photoconductivity increases at the PE reaction by from 3 up to 10 times depending on the PE degree -x (Li_1-xH_xNbO₃). Consequently the usual PV-model predicts only Δn_s < 0.3 10^-6 that indicates clearly noneffectiveness of the PV-model for PR description in the PE LiNbO₃ waveguides. This disagreement may be explained by the existence of a polarization field contribution in PR because of the photoexcitation of the impurity dipoles appearing in considerable numbers at PE-reaction [1]. In present investigation it is established that the polarization field contribution becomes dominant in PR-response only against the background of the suppressed PV-effect in waveguides with high degree of PE. Therefore, these waveguides are used for formation of dynamic holographic gratings [2]. Note that relatively small value of the Δn_s in undoped PE waveguides made its application in real devices ineffective. However, the combined proton and copper exchange at low temperature enables to increase the holographic sensitivity by more than three order of magnitude, as concentration of the photoexcitable centres reaches extremely high values (C_cu ≤ 4 at.%) that represent the necessary condition for appearance under light influence of the reversible polarization field with essential value.

© 1996 IEEE