**6.1 Cross gain modulation (XGM)**

In its general form the principle of operation of the technique is that an optical input signal to be wavelength converted is used to saturate the gain of an active nonlinear element and thereby modulate a continuous wave (CW) signal (pump) at the desired output wavelength [49, 56–58] as shown in **Figure 7**.

Several approaches have emerged to implement all optical wavelength conversion using XGM in SOA, but its conversion speed is determined by the carrier dynamics [59], which are governed by slow interband carrier recombination [60]. Some indicative examples include the work presented in [61] where a 1.2 mm long SOA is used for 40 Gb/s conversion with 1.5 dB power penalty. In [62] a 2 mm long SOA is used for achieving 100 Gb/s wavelength conversion based on XGM. In [63] 80 Gb/s conversion with reportedly low penalty has been achieved.

XGM WCs are attractive because of their simplicity and high conversion efficiency, and the conversion can be made independent of the polarization of the incoming signal. The XGM converter has a number of shortcomings, such as (bit stream) data polarity inversion and the relatively large chirp of the output signal due to the large gain modulation. Finally other than wavelength conversion, XGM has been used for many functions like: format conversion [64], multicasting [65] and header processing in packet switches [66, 67].
