Electro-optic long-period waveguide gratings in lithium niobate

We report electro-optic (EO) long-period waveguide gratings (LPWGs) fabricated in a special lithium-niobate (LiNbO3) waveguide structure. The waveguide consists of a clad core formed with a two-step proton-exchange process and a thin cover layer created by an additional reverse proton-exchange process for the restoration of the EO strength of the waveguide. Using several LPWG samples, we demonstrate experimentally the effects of using different cladding modes and waveguide parameters on the grating performance. One of our 10-mm long samples shows a 27-dB rejection band at a driving voltage of 95 V with a center wavelength tunable thermally at a sensitivity of -0.4 nm/°C. Our analysis of the theoretical limit of the efficiency of such an EO grating suggests room for significant further improvement by optimizing the waveguide and electrode designs. The LiNbO3LPWG provides an EO control of the grating strength and a thermo-optic control of the operating wavelength and thus opens up many new opportunities for high-speed applications, such as dynamic optical filtering and optical intensity modulation.