Coherent optical frequency domain reflectometry (C-OFDR) is demonstrated for a long single-mode optical fiber at a wavelength of 1.55 µm by using a highly coherent lightwave source and an electrooptical (EO) external phase modulator. Rayleigh backscattering and Fresnel reflection from a 16.4-km optical fiber are observed when the modulation frequency is swept from 4 GHz to 8 GHz for 40 ms. The insertion of an optical fiber delay line in a probe lightwave path makes it possible to distinguish the Rayleigh backscattering profile due to the first-order modulation sideband in the phase-modulated lightwave from those due to the higher order sidebands. A spatial resolution of better than 100 m is achieved for two neighboring Fresnel reflectors located near the far end of the test fiber.
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