Higher-order mode grating devices in As₂S₃ chalcogenide glass rib waveguides

We report on the design and fabrication of high-quality long-period gratings in chalcogenide glass (As₂S₃) rib waveguides utilizing the strong photosensitivity. Higher-order modes of the rib waveguides are analyzed by inspection of the spectra of Bragg gratings written into these waveguides. Based on these measurements, we infer the effective indices of higher-order modes, which are in good agreement with modeling results using a beam propagation method. High-quality long-period gratings are then designed and written into the rib waveguides using a simple shadow mask technique. Coupling the fundamental to the HE₀₂ mode strong resonances of up to ∼20 dB depths are obtained. The gratings have a length of L=26 mm and a period of ∧ =86 μm. In situ monitoring of the writing process allows the growth dynamics of the grating to be studied. A theoretical fit to the measured transmission curve gives an average index change of ∼10^-3.