Thermally tunable lithium-niobate long-period waveguide grating filter fabricated by reactive ion etching.

We fabricate a long-period grating band-rejection filter with a lithium niobate (LiNbO(3)) channel waveguide, where the grating is formed on the waveguide surface by plasma reactive ion etching. The filter shows three distinct thermal tuning regimes. Below approximately 55 degrees C, the center wavelength of the rejection band decreases linearly with an increase in the temperature at a high sensitivity (-1.4 nm/ degrees C). The temperature sensitivity drops abruptly to a negligible value at approximately 55 degrees C and stays low up to approximately 70 degrees C, above which it jumps abruptly to a large positive value (1.6 nm/ degrees C). We attribute these peculiar tuning characteristics to the formation of different crystal phases in the core and cladding areas of the LiNbO(3) waveguide and the abrupt transitions of the thermo-optic coefficients of some crystal phases. The grating could be developed into various kinds of integrated-optic filters and switches for application in optical signal processing.