Long Measurement Range OFDR Beyond Laser Coherence Length

We present a novel method to significantly extend the measurement range of an optical frequency domain reflectometry (OFDR) beyond the laser coherence length for both discrete reflection (such as Fresnel reflection) and Rayleigh backscattering measurements. A key to the method is to increase the laser frequency tuning speed while measuring the phase noise of the reflected optical signal that carries the location and reflectivity information. Using this technique in a conventional OFDR with a noise floor of -120 dB and a dynamic range of 53 dB, we achieved a measurable distance of 170 km for Fresnel reflection and 120 km for Rayleigh backscattering using a laser with a coherence length of only 13.6 km.