Micro Sagnac Loop Beat Wave Interferometer Based on High-Order Modes Coupling in Tightly Twisted Micro Fiber Tapers

We experimentally demonstrate a micro Sagnac loop beat wave interferometer by twisting a micro-tapered fiber with a diameter of the wavelength scale. The polarization-dependent coupling for the guided lights occurs at the interknitted region repeatedly while the loop serves as a mirror to reflect the guided lights. A longer interaction length of the interknitted region can lead to a narrower free spectral range. The high-order modes can be excited under a tightly twisted condition and the wavelength beating phenomenon can be more explicitly obtained after the multi-cycle coupling.