Compact in-line optical notch filter based on an asymmetric microfiber coupler.

Novel compact in-line optical filters with narrow rejection bandwidths are proposed, and one is experimentally demonstrated based on asymmetric microfiber (MF) couplers. It is composed of silica and bismuth-oxide-glass MFs and less than 500 μm long. Single transmission dips in a wavelength range of up to 300 nm are obtained, and the transmission extinction ratio is more than 30 dB. Additionally, a -20  dB bandwidth of 0.88 nm is achieved. This narrow bandwidth of the transmission notch benefits from the huge refractive index difference between the two MFs. These experimental results agree well with the theoretical predictions. With the advantages of having a simple structure, being fiberized, and having a small footprint, this device can be an attractive element for micro/nanophotonics, optical sensing, optical signal processing, and optical fiber communications.