Integrated-optic wavelength multiplexers on lithium niobate based on two-mode interference

The principle of the wavelength-dependent two-mode interference (TMI) in guided wave devices is utilized to build up integrated-optic dual-channel wavelength-division multi-/demultiplexers (WDM) for single-mode systems. The device characteristics are analyzed by analytical and numerical methods where special emphasis is given to the wavelength dependence of the channel spacing. These devices are fabricated by titanium indiffusion into X-cut and Y- and Z-propagating LiNbO₃, where the Z-propagating configuration is preferred for polarization independent operation. In the wavelength range between 1.3 and 1.6 μm, insertion losses (fiber-waveguide-fiber) below 3 dB, channel spacings of 30-40 nm, and far-end crosstalk values of 26-40 dB are achieved. The measured low temperature sensitivity of the wavelength shift of 5.6×10^-2 nm/°C and the possibility of electrooptic tuning at a rate of 2.5 nm/V allow the realization of high performance devices at reduced fabrication tolerances