Chapter 9 - Optical performance monitoring based on RF pilot tones

Abstract

This chapter reviews various optical performance monitoring techniques based on the pilot tone. These techniques utilize a small sinusoidal signal, operating at a unique frequency, imposed on each WDM channel. As a result, various optical parameters, such as optical channel powers, wavelengths, and optical paths of multiple WDM channels can be monitored simultaneously without the need to demultiplex them. However, in the case of using the AM pilot tone, the receiver sensitivity in on-off keying (OOK) system can be degraded as the pilot tone interferes with the data signal. If necessary, one can avoid this AM-tone-induced sensitivity degradation simply by utilizing a high-pass filter within the optical receiver. On the other hand, when the frequency of the AM pilot tone is lower than 10 MHz, the performance of this monitoring technique can be seriously deteriorated by the ghost tones originating from XGM and SRS. In fact, these effects limit the maximum network size that this monitoring technique can support. Thus, it appears that the monitoring technique based on the low-frequency AM pilot tone is most useful for metro network applications. For the use of this monitoring technique in the long-haul network, one needs to either restrict the number of channels that can be monitored at a time (e.g., by using an optical band pass filter) or increase the tone frequency to be higher than 300 MHz. It is also possible to mitigate this limitation by utilizing the PM and FM pilot tones.