Fiber-Coupling Loss and Drive Voltage Simultaneous Reductions by Tapered Titanium-Thickness Diffusion in LiNbO3 Waveguide Switches

Titanium-diffused LiNbO3 waveguide switches and modulators are foreseen as elements in optical switching networks1) or external modulators2) in high bit-rate optical communication systems, because of their broadband and low loss capability. In practical applications, both low insertion loss, when coupled to optical fibers, and low operating voltage for high speed driving are required in these devices. Recently, low Ti:LiNbO3 waveguide to single-mode fiber coupling loss has been reported.3) The relation between drive voltage and waveguide structure has also been discussed.4) However, a low loss and low drive voltage switch or modulator has not been realized, because the conditions for achieving the above two purposes are different. That is, mode size matching between waveguide and fiber is required for low loss coupling. On the other hand, small mode size is preferable for low drive voltage.