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 niobáte (LiNbO8) 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 ∼55°C, the center wavelength of the rejection band decreases linearly with an increase in the temperature at a high sensitivity (-1.4 nm/°C). The temperature sensitivity drops abruptly to a negligible value at ∼ 55 °C and stays low up to ∼70°C, above which it jumps abruptly to a large positive value (1.6 nm/°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.