“…There are many OPM methods [10][11][12][13]. However, the techniques are usually based on direct measurement of optical parameters such as OSNR [14][15][16], pilot tones [17], Q-factor measurement [18,19], in/out-of-service BER measurement/estimation [20][21][22], subcarrier multiplexing [23], data correlation [24], non-linear detection [25]. They all require some degree of signal tapping [26], resulting in an intrusive approach, which will be introducing at least minor performance degradation due to either increased attenuation or monitoring signal injection.…”