Birefringence Free Spectral Tuning of Bragg Grating Based Fabry-Pérot Resonator in LiNbO₃ Waveguide Employing Counter- Propagating Cladding Modes

We propose and analyse a novel, birefringence free, wavelength filter based on counter-propagating cladding mode assisted integrated-optic Fabry-Pérot (FP) resonator. The FP cavity is formed by suitably designed counter-propagating cladding-mode exciting Bragg gratings in LiNbO3 ridge waveguide. Through rigorous vectorial modeling, we show that appropriate waveguide dimensions and crystal orientation enable birefringence-free operation for both the fundamental core mode as well as the excited cladding mode. We also show that the zero birefringence point of the filter remains unchanged regardless of the applied voltage across the crystal; the reasons behind this behaviour are also explained by the linear electro-optic effect of LiNbO3. Using this we demonstrate a linear electro-optic tuning of the individual Bragg grating spectrum as well as the FP resonances with a typical tuning coefficient of 4 pm/V. Our results may find application in designing high-speed tunable wavelength filters free from input polarization state with linear spectral tuning capabilities.