High-resolution, large dynamic range fiber length measurement based on   frequency-shifted asymmetrical Sagnac interferometer
  We propose and experimentally demonstrate a single-mode fiber length and dispersion measurement system based on a novel frequency-shifted asymmetric Sagnac interferometer incorporating an acousto-optic modulator (AOM). By sweeping the driving frequency of the AOM, which is asymmetrically placed in the Sagnac loop, the optical length of the fiber can be determined by measuring the corresponding variation in the phase delay between the two counter-propagating light beams. Combined with a high-resolution data processing algorithm, this system yields a dynamic range from a few centimeters to 60km (limited only by our availability of long fibers) with a resolution about 1ppm for long fibers. 
Bing Qi, Andrew Tausz, Li Qian, Hoi-Kwong Lo