Nonlinear crosstalk and two countermeasures in SCM-WDM optical communication systems

We investigate, theoretically and experimentally, crosstalk between wavelen gths in subcarrier-multiplexed (SCM) wavelength division multiplexed (WDM) optical communication systems. Crosstalk arises mainly from interactions be tween subcarriers on one wavelength and the optical carrier of another wave length. In a dispersive fiber, crosstalk can be attributed to stimulated Ra man scattering (SRS) and cross-phase modulation (XPM) combined with group v elocity dispersion (GVD). We investigate the phase relationship between SRS -induced and XPM-induced crosstalks. Crosstalks induced by SRS and XPM add in the electrical domain and can interfere constructively or destructively, Experimental results show that the combined crosstalk level can be as high as 40 dBc after 25 km of SMF with two wavelengths and 18 dBm per wavelengt h of transmitted power. We propose two crosstalk countermeasures. The first countermeasure uses parallel fiber transmission. We show theoretically tha t both SRS-induced and XPM-induced crosstalks can be canceled to the first order. We present an experimental demonstration of concept which has achiev ed 15 dB of crosstalk cancellation over 200 MHz. The second countermeasure uses optical carrier suppression. We show theoretically and experimentally, that by suppressing the optical carrier, we can significantly reduce cross talk while maintaining the same link budget and carrier-to-noise ratio (CNR ) at the receiver. 20 dB of crosstalk reduction over 2 GHz has been demonst rated experimentally.