Characteristics of Ti-diffused lithium niobate optical directional couplers

The dependence of the coupling strength of titanium-diffused lithium niobate directional couplers upon fabrication parameters and guided wavelength has been investigated. Measurements of the coupling strength as a function of interguide separation have been made at He-Ne, GaAs, and Nd:YAG wavelengths. To good approximation, the coupling strength depends exponentially upon guide separation. Rather strong dependence upon the metal thickness diffused and the guided wavelength were observed. Good comparison was obtained with theoretical results calculated using an effective index method. Transfer lengths as short as 200 microm have been obtained. Fabrication parameters for which the coupling strength for the TE and TM modes are equal were determined.