Phase-decorrelated FMCW reflectometry for long optical fiber characterization by using a laser diode with modulated external-cavity

We describe FMCW reflectometry for characterization of long optical fibers by using an external-cavity laser diode as a light source. Since the optica l path difference between the reference beam and the reflected beam from th e optical fiber under test is much longer than the coherence length of the light source, the reference and the reflected beams are phase-decorrelated. As a result, the beat spectrum between the reference and the reflected bea ms is measured. In the phase-decorrelated FMCW reflectomety, the spatial re solution is enhanced by narrowing the spectral linewidth of the light sourc e and increasing the repetition frequency of the optical frequency sweep as well as increasing the chirping range of the optical frequency sweep. In t he experiments, an external-cavity DFB laser is used as a narrow linewidth light source, and the optical frequency is swept by minute modulation of th e external cavity length. Long single mode optical fibers are characterized , and the maximum measurement range of 80 km is achieved, and the spatial r esolutions of 46 m, 100 m and 2 km are achieved at 5 km, 11 km and 80 km di stant, respectively. The Rayleigh backscattering is clearly measured and th e propagation loss of optical fiber is also measured. The optical gain of a n erbium-doped optical fiber amplifier (EDFA) is also estimated from the ch ange in the Rayleigh backscattering level in the optical fiber followed aft er the EDFA.