Passive polarization splitters in Ti:LiNbO₃ using crossed waveguides

Abstract

Passive polarization splitters with crossed single-mode waveguides configuration for 1.55 [u]m application have been fabricated in x-cut y-propagating LiNbO3 using Ti-indiffusion technique. The operation principle is based on two-mode interference at the intersection region, which is induced by establishing high index wide waveguide geometry at the crossing section. The influence of the angle between crossed waveguides, and fabrication parameters on the polarization splitter performance have been investigated experimentally. Transmission losses were found to be higher for the larger crossing angles. Device performance was characterized by measuring the polarization extinction ratio at the output for each of TE and TM input polarization individually using a DFB laser diode. Best results were obtained from a device structure (sample X11) with 8 [u]m wide waveguides and 1.28° crossing angle that was produced by the thermal diffusion at 1025°C for 12 hours of 1130 [A} thick Ti film ([t]1) for the single mode waveguides simultaneously with an additional 220 [A] thick Ti film ([t]2) for the two-mode crossing section, which provided extinction ratios of 17.92 dB for TE input and 17.11 dB for TM input. Extinction ratios as high as 22.46 dB for TE (sample X12), and 20.83 dB for TM (sample X10) have been observed. The polarization splitting behavior for arbitrary input polarization has also been investigated.