FSR-free filters with ultra-wide tunability across S+C+L band

Optical filters are essential parts of advanced optical communication and sensing systems. Among them, the ones with an ultrawide free spectral range (FSR) are especially critical. They are promising to provide access to numerous wavelength channels highly desired for large-capacity optical transmission and multipoint multiparameter sensing. Present schemes for wide-FSR filters either suffer from limited cavity length or poor fabrication tolerance or impose an additional active-tuning control requirement. We theoretically and experimentally demonstrate a filter that features FSR-free operation capability, subnanometer optical bandwidth, and acceptable fabrication tolerance. Only one single deep dip within a record-large waveband (S+C+L band) is observed by appropriately designing a side-coupled Bragg-grating-assisted Fabry–Perot filter, which has been applied as the basic sensing unit for both the refractive index and temperature measurement. Five such basic units are also cascaded in series to demonstrate a multichannel filter. This work provides a new insight to design FSR-free filters and opens up a possibility of flexible large-capacity integration using more wavelength channels, which will greatly advance integrated photonics in optical communication and sensing.