Analysis of tunable codirectional vertical couplers with liquid crystal overlays for integrated dye lasers

We propose a guided-wave dye laser using a grating-assisted codirectional vertical coupler as an intracavity tunable filter. A liquid crystal overlay is designed to attain a phase-matching wavelength shift in the vertical coupler and a tunable range of over 60 nm is calculated at a center wavelength of 590 nm with a coupling efficiency of more than –1 dB. The tunable range is limited by the coupling length mismatch rather than by the refractive index variation in the liquid crystal overlay. Sidelobe level in the filter response is increased when the phase-matching wavelength is shifted to the long-wavelength side; nevertheless, the weighted coupling keeps the sidelobe-suppression ratio at more than 30 dB over a spectral span of 100 nm.