Integrated Si3N4 Optical Filter Assisted by Phase-Shifted Bragg Grating

Phase-shifted gratings are expected to play a significant role in wavelength division multiplexing, lasers, and optical signal processing due to their exceptional performance, including an ultra-narrow bandwidth, a high extinction ratio, and a sharp line shape. The key to introducing an accurate phase shift is to implement high-precision lithography, which is currently not available. However, this bottleneck can be addressed by employing the method of equivalent phase shift. Compared to implementing a true phase shift, the requirement for an equivalent phase shift can be reduced by three orders of magnitude. Currently, most phase-shifted gratings are fabricated in silicon-on-insulator (SOI) thanks to their compatibility with CMOS technology. Since the grating's periodic sides affect the phase shift, they also impact the full width at half maximum of the transmission peak. Ultra-low-loss waveguides based on silicon nitride are a favorable platform for researching microwave photonics and their applications in a wide variety of fields at present. Nevertheless, to date, a survey of phase-shifted gratings reveals a paucity of those fabricated on silicon nitride platforms. In this paper, we propose an optical filter based on a silicon nitride platform utilizing equivalent phase shift technology. This component under consideration comprises sampled gratings and true phase shift, with a length of 5 mm on a 350-nmthick Si₃N₄ platform. This component has a pitch of 480 nm and a sampling length of 100 μm, utilizing side-wall modulation. For this device, a narrow and sharp passband window of approximately 0.75 nm was observed over a stopband region at the target wavelength of 1566.35 nm. The reflection spectrum also shows good repeatability of the passband response for both the 1595.43 nm and 1598.12 nm wavelengths. Moreover, this scheme has broad potential applications in optical frequency combs, optical communication, and optical sensing.