Long period waveguide grating filters on lithium niobate

Steric acid is used to fabricate cladding and core layers on z-cut lithium niobate (LiNbO3) substrate by maintaining the solution melt at 280°C for 4 and 2 hours, respectively. After completing the two-step proton exchange (PE), the refractive indices of the core and cladding layers are ascertained by using the prism coupling technique, and with this information at hand, the grating period Λ of 50 μm is deduced by solving a system of transcendental waveguide equations with MATLAB. There are three methods adopted to fabricate the gratings. The first one is to utilize the proton-exchange method by directly diffusing ions into LiNbO₃ to realize phase grating while keeping the solution melt at 280°C for 0.5 hours. The second one relies on using a Shipley S1813 photoresist as the corrugation grating via standard lithography. The third approach is to deposit and subsequently pattern silver metal as corrugation grating. A series of measurements would show that the maximum dip contrast of the phase grating could reach up to 31.188 dB, and the corresponding full width at half maximum (FWHM) is about 0.77 nm. In comparison, the maximum dip contrast of the photoresist corrugation grating attains up to 28.44 dB with an FWHM of approximately 1.18 nm. On the other hand, the maximum dip contrast ratio of the silver corrugation grating is determined to be around 8.15 dB with an FWHM of about 0.6 nm. The thermal dependency of the phase grating is also probed by increasing the temperature from 40 to 60°C and the corresponding dips have appeared to be blue-shifted. All of these devices have managed to demonstrate the multiple rejection bands, which is believably due to the multimode interference (MMI) phenomenon.