We report a new scheme for coherent optical frequency domain reflectometry (C-OFDR) where the coherence length of the lightwaves does not limit the measurable fiber length. In this scheme, we use the beat spectrum which results when we mix reflected and reference lightwaves whose phases are not correlated. We demonstrated this scheme using a narrow-linewidth-lightwave source and an external electro-optical phase modulator. We measured Rayleigh backscattering and Fresnel reflections from a 30-km optical fiber, and achieved a spatial resolution of 5 m for two neighboring Fresnel reflectors located at the far end of the fiber. We estimated the expected spatial resolution and single-way dynamic range for our new scheme and show that it is capable of measuring long optical fibers with high-spatial resolution.
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