On the validity of using CW tones to test the linearity of multichannel M-QAM subcarrier multiplexed lightwave systems

We investigate the validity of using multiple continuous-wave (CW) tones as the signal source to test the linearity of a multichannel M-ary quadrature-amplitude-modulation (M-QAM) subcarrier multiplexed lightwave system. We consider the following representative optical fiber system nonlinearities: 1) laser clipping and 2) the combined effect of laser frequency chirp and fiber dispersion. Our results show that if all orders of nonlinear distortions (NLDs) in a signal bandwidth are included in the total NLD power, the error caused by replacing M-QAM signals with CW tones can be within measurement uncertainty.