Optical characterisation of Z-cut proton-exchanged LiNbO3 waveguides fabricated using orthophosphoric and pyrophosphoric acid

Abstract

We carry out a systematic study of proton-exchange waveguides fabricated using orthophosphoric and pyrophosphoric acid. Proton-exchange is carried out using three different processes: surface-coating, spin-coating, and immersion. Waveguide refractive index profiles and effective diffusion coefficients (210°C to 235°C), as determined from optical waveguide prism coupling measurements, have been compared to those of waveguides fabricated by immersion in benzoic acid. In addition to there being no apparent differences in the optical quality and properties of waveguides fabricated using phosphoric and benzoic acids, contrary to previous reports, we do not find any major advantages in using the surface coating or spin coating techniques for waveguide fabrication. The only situation in which we have obtained as high a value of Δne as has been claimed by other workers, e.g. Δne = 0.145, λ = 0.6328 μm, is where the waveguide refractive index profile is calculated using the inverse WKB method for waveguides which support only three or four modes. Under such conditions, the IWKB method is inaccurate.