Direct-current bias stable Ti:LiNbO(3) TE-TM mode converters produced by magnesium postdiffusion

Depression of the surface index of Ti:LiNbO(3) channel waveguides by secondary diffusion of MgO forms a buffer layer that optically isolates the guided modes from attenuation caused by electrodes placed over the guides. For x-cut, z-propagating mode-conversion devices, we find high stability to optical drift when a dc voltage as high as 60 V is applied to electrodes (typical gap of 5 microm) to establish bias operating points. Photoconductivity is found to limit the long-term stability. Devices operating at 0.633-microm wavelength generally suffer significant drift in their mode-conversion efficiency when 100 microW or more of throughput power is present and a bias of 14 V is used. 10-microW throughput at the same wavelength is found to be stable over an 8-h period. At 0.8-microm wavelength, bias voltages as high as 60 V may be used for stable operation even at 100-microW optical throughput.