Multimode Interference Based Narrow Spectral Width Bandpass Fiber Optic Filter Using NCF-FMF Combinations

In this research, we present the conceptualization and implementation of a multimode interference filter using novel specialty fiber combinations to achieve a narrow transmission bandwidth with minimal insertion loss. The filter structure comprises no-core, few-mode (NCF-FMF) fibers sandwiched between two single-mode fibers to realize the multimode interference effect. The proposed filter exhibits an insertion loss of about 0.37 dB and a narrow 3-dB spectral width of 3.6 nm at the center wavelength of 1029.56 nm. To mitigate the interference between wavelength-division multiplexing channels, isolation of about 40 dB is achieved over a wavelength span of about 45 nm. In addition to its significant application in spectroscopy and sensing, the filter has the potential to be employed in the domain of fiber-optic communications and LIDARs.