Electro-optically tunable super-broadband filter based on long period grating in Ti:LiNbO₃ waveguide.

We report an electro-optically tunable optical filter based on a parallel structure of two long period gratings in the two same Ti:LiNbO3 strip waveguides: one 675-μm-pitch grating in one waveguide and another 880-μm-pitch grating in the other waveguide. The stop-band is observed in the 1.1–1.3 (1.4–1.6) μm spectral region for the grating pitch 675 (880) μm. Its contrast increases linearly to ∼30  dB as the voltage is increased to 300 V, and the linearity is similar for the two cases of 675 and 880 μm pitches. Higher than 300 V, the contrast decreases due to photorefractive (PR) effect and/or over-coupling. Accompanying the contrast modulation, the resonant wavelength is simultaneously linearly tuned by making use of the PR effect. For the 675 (880) μm pitch, the tuning range is 160 (200) nm for the 400 (300) V voltage change range. With the two gratings, one can realize >360  nm super-broadband filtering.