filtering technique was used to enhance the monitoring sensitivity of degree-of-
polarization-based OSNR monitors [60]. Fiber nonlinear effects such as FWM and
optical parametric amplification (OPA) have also been employed to monitor
OSNR [61–64]. The techniques based on fiber nonlinearities are all-optical and
fast, though they generally require high power light or special fiber as nonlinea r
medium.
Electronic techniques such as histogram analysis are also powerful tools for
OSNR moni toring, which will be discussed in more details in Section 7.6.
Note that OSNR monit oring is an excellent measure of optical amplifier
performance and is frequently used for troubleshooting optical power induced or
amplifier induced network faults.
7.4 DISPERSIVE IMPAIRMENTS ...