Passive polarization splitter using zero-gap directional coupler in LiNbO3

Low-crosstalk zero-gap directional coupler (ZDC) based TE-TM polarization splitter/combiner was designed at 1.55 µm transmitting wavelength using titanium indiffused LiNbO3 technology and effective-index based matrix method (EIMM). Single mode waveguides and ZDC were designed from its fabrication parameters. The coupling length of ZDC was optimized to 1826 µm to achieve a splitter crosstalk around −60 dB for both polarizations. At input and output ends of ZDC low-loss cosine-generated S-bent waveguides were used. For proof-of-concept, the device was fabricated and tested. Details of fabrication technology of the device are discussed. DC sputtering process was adopted for titanium film deposition, and wet-oxygen ambient was used during high-temperature diffusion of titanium into LiNbO3 to suppress lithium out-diffusion. Our first experiment with the splitter yields crosstalk values −10.79 dB for TE and −10.21 dB for TM mode. The measured fiber-device total insertion losses were 3.13 dB and 5.15 dB for TE and TM polarizations. The higher crosstalk values of the fabricated polarization-splitter can be significantly lowered by using optical detector of higher efficiency, and coupling length compensation during mask-data generation.