This paper proposes a new technique for measuring in-service optical fibers, that uses an optical time domain reflectometer (OTDR). The feature of the proposed technique is that the OTDR light is in the same wavelength band as the video signal, which is distributed by using the subcarrier multiplexing (SCM) technique. In a 40-channel SCM system operating at a signal wavelength of 1.558 μm, we show that the required video quality can be maintained, by using the proposed OTDR operating in the 1.55 μm band, even though the measured fiber is in service and the OTDR light enters an optical receiver. Moreover, we clarify the conditions for undertaking measurements, without the need for optical filters designed to prevent OTDR light from degrading the SCM signal quality.
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