Allowable received OTDR light power for in-service measurement in lightwave SCM systems

To measure in-service optical transmission lines in optical video distribution systems using subcarrier multiplexing (SCM) techniques, we clarify how measurement light from an optical time domain reflectometer (OTDR) affects the signal transmission performance when the light enters an optical network unit (ONU). We describe the additional noises induced when OTDR light is injected into the ONU and analyze the relationship between received OTDR light power and carrier-to-noise ratio. Moreover, we use the analysis to provide a technique for estimating the allowable received light power in lightwave SCM systems and show the dominant factors determining this allowable power. Finally, by using the estimation technique, we clarify the required rejection ratio for a filter designed to prevent OTDR light from entering ONU´s in wavelength division multiplexing systems.