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A Raman OTDR (ROTDR) fault location for a 100-km-long single-mode optical fiber has been tested. A Ge p-i-n photodiode cooled by liquid nitrogen has been used as a backscattered light receiver. The signal-to-noise ration (SNR) improvement of the cooled Ge p-i-n photodiode from a Ge avalanche photodiode (Ge APD), which operates at room temperature, is 14 dB. Spectral Stokes light power distributions, which have been emitted from a long single-mode fiber, have been measured to estimate the maximum locatable length for a single-mode fiber. It has been shown, from the measurements and calculations, that a 100- km-long and a 30-dB total attenuation at a 1.55-μm wavelength single-mode fiber fault location can be achieved with the aid of an averaging technique. On the basis of these results, a 102-km-long single-mode fiber fault location experiment has been successfully carried out.