On-chip multi-mirror Fabry-Pérot filters with discrete FSR reconfiguration via tunable Bragg reflectors

Achieving wide tunability of the free spectral range (FSR) in on-chip optical filters remains challenging. Conventional continuous tuning offers negligible FSR adjustability due to limited refractive-index variation, while alternative architectures often involve increased structural and design complexity. Here, we propose discrete FSR reconfiguration in a cascaded Bragg-grating Fabry-Pérot (FP) microcavity by digitally redefining the effective cavity boundaries through localized thermal detuning. Selective activation of the gratings switches the effective cavity length among multiple states, enabling stepwise control of channel spacing. As a proof of concept, a three-mirror FP microcavity experimentally realizes three discrete FSR states of 7.58, 6.06, and 2.86 nm, with the smallest-FSR state exhibiting a thermal tuning efficiency of ~83 pm/°C. This digital cavity-boundary switching strategy provides a compact and scalable route toward programmable, multi-state integrated photonic filters.