Tunable in-line fiber-optic bandpass filter

An in-line fiber-optic bandpass wavelength filter has been demonstrated based on a single-mode fiber, side polished into the core, in evanescent contact with a high-index multimode overlay waveguide. Passband linewidths down to 5.2 nm were observed, while spacing between transmission peaks was seen to range from 45 to 256 nm for variation of the overlay waveguide parameters. Additionally, rejection ratios of >20 dB and insertion losses as low as 0.5 dB were observed.