Frequency-shifted Mach-Zehnder interferometer for locating multiple weak reflections along a fiber link

We propose and demonstrate a novel technique for measuring fiber lengths and locating multiple weak reflections along the fiber using a low-cost continuous wave source, a folded Mach-Zehnder interferometer (MZI), and a frequency shifter. By sweeping the frequency of the shifter, which is located in one path of the MZI, the optical length of the test fiber can be determined precisely from the interference pattern constructed by the reflected signals from the fiber end. Experimentally, for fibers shorter than 10 km, the standard deviations are less than 1 mm. Furthermore, by incorporating lock-in detection and fast Fourier transform techniques, both the locations and the reflectance of multiple reflection sites along one fiber can be resolved with a spatial resolution of about 5 m and a sensitivity better than -67 dB. This could be a powerful interrogating technique for optical fiber sensor arrays.