Optical fiber in-line Mach-Zehnder interferometer based on down-up tapers for refractive index measurement

•The device is fabricated in a fusion splicer by initially creating an up fiber taper using heating the pushing technique and followed by producing a down taper using heating and pulling method. The down taper and up taper are connected with separation, which makes the whole device ultra-compact, only 3 mm in length.