Instabilities and Their Characterization in Mach‐Zehnder Ti:LiNbO₃ Optical Modulators

For the Ti‐diffused LiNbO 3 optical modulator, which is expected to be applied to optical communication systems and optical measurement instruments, quantization of various unstable phenomena degrading the practical operations is attempted. The characteristic parameters related to each phenomenon and their temperature dependence were measured under the condition of a constant optical output. The following observations were made: (1) The polarity and the relaxation time of the short‐term dc drift (namely, the variation of the driving point immediately after the application of the dc bias) depend on the configuration of the buffer layer; (2) The temperature drift (namely, the variation of the driving point due to that of the ambient temperature) is a short‐term drift phenomenon and its relaxation time is related to that of the short term dc drift; (3) The long‐term dc drift (namely, the variation of the driving point over a long time) depends on the magnitude and polarity of the electric field applied on the waveguide region. Its relaxation time is much larger than those of other phenomena; and (4) From the temperature dependence of the relaxation time of each phenomenon, it is found that their activation energies are almost equal and are about 0.7 eV. Further, based on the equivalent circuit model newly proposed, the relationship among the parameters was evaluated. It is found that each unstable phenomenon can be explained by the equivalent circuit model in which the formation of the space charge layer is taken into account.