Optical Fiber In-Line Mach-Zehnder Interferometer Based On an Inner Air-Cavity With Long Cavity Length

An optical fiber in-line Mach-Zehnder interferometer based on an inner air-cavity with long cavity length is demonstrated. The device is fabricated by using femtosecond laser to inscribe a waveguide structure in the optical fiber core, followed by discharging the waveguide area with a fusion splicer. The inner air-cavity structure is highly robust, and the relatively long cavity length corresponds to a small free spectral range, which makes it possible to implement accurate measurement. Such an inner air-cavity device fabricated in single mode fiber has good high temperature sustainability. The device is also “open” to the external environment when being fabricated in multimode fiber, thus supporting refractive index measurement. The proposed device is attractive in many optical fiber sensing applications.