Compact multimode Fabry–Perot filters on thin-film lithium niobate at 850 nm

Compact wavelength-selective devices operating at 850 nm are essential for VCSEL-driven short-reach optical links and silicon-photodetector-based receivers. Here, we demonstrate 850-nm thin-film lithium niobate (TFLN) multimode Fabry–Perot (FP) filters based on mode-conversion multimode waveguide gratings (MWGs) and mode (de)multiplexers. The narrowband filter with a single-cavity shows an experimental insertion loss of 0.55 dB, a 3-dB bandwidth of 83 GHz (0.2 nm), and a compact footprint of 1.8 μm × 37 μm with a temperature-dependent wavelength-shift coefficient of 6.9 GHz/K (16.5 pm/K). Furthermore, broadband flat-top spectral shaping is achieved based on the dual-cavity FP filter, which exhibits a 3-dB bandwidth of 747 GHz (1.8 nm), an insertion loss of 0.51 dB, an extinction ratio of 31.3 dB, a free spectral range of 12.1 nm, and a temperature-dependent wavelength-shift coefficient of 7.1 GHz/K (17.1 pm/K), within a footprint of 1.55 μm × 46.5 μm. By integrating narrowband filtering and flat-top broadband spectral shaping, the demonstrated bend-free filters provide a compact and versatile wavelength-selective solution for 850-nm photonic circuits.