Photorefractive Correction of the Coupling Ratio of an Integrated Optical Directional X-Coupler on a Lithium Niobate Substrate

The possibility of correcting the power coupling ratio of an integrated optical directional X-coupler upon excitation of a local photorefractive response in the substrate material (LiNbO₃) has been analyzed. The X-coupler operation has been numerically simulated, and the regions of its maximum sensitivity to photorefraction have been determined. The correspondence between the calculated and experimental data has been obtained. The maximum correction value of the coupling ratio is 1–2%. The photorefractive correction of the coupling ratio has been used to increase the extinction ratio of a Mach–Zehnder modulator to 47 dB.