**6.2 Cross phase modulation (XPM)**

In XPM, phase change of the nonlinear element is used rather than the gain change. The optical input signal power controls the phase difference acquired by a pump along the two arms through the refractive index of the nonlinear element [68]. There are number of interferometric configurations that have been used for XPM, some of these are shown in **Figure 8**. A very promising method is XPM in delayed interference signal wavelength convertor configuration (DISC) [62, 69]. 3R regeneration with DISCs has been demonstrated in [70] and a Michelson's interferometer (MI) [71], Mach-Zehnder (MZ) [72]. Some of the first implementations to appear using the interferometric technique is Terahertz Optical Asymmetrical Demultiplexer (TOAD) [73] configuration for the MZI where high-speed operation has been achieved [75].

These converters can perform ultra-high bit rate operations, e.g., demultiplexing from 250 Gb/s [76]. Configurations like the TOAD or the Ultra-fast Nonlinear Interferometer (UNI) [77] have been used mainly for optical processing functions other than wavelength conversion. For instance, UNI configurations have successfully been used in [78] for optical packet switching and in [79] for clock recovery.

**Figure 7.** *Schematic of wavelength conversion by XGM in nonlinear medium [27].*

**Figure 8.**

*Different interferometric configurations that have been used as optical gates (a) MZ, (b) TOAD, (c) NOLM, (d) MI, (e) DISC or (f) UNI and (g) push-pull [74].*

The use of SOAs is favored over the other elements mentioned above by virtue of compactness and the potential for amplification in suitable all active configurations [56].

The most promising implementation is based on Mach-Zehnder (MZ) structures, their compactness and broad range of functionalities give a very versatile, and thereby a very cost effective device. Nevertheless, owing to the carrier recovery time, the operating bit-rate of the MZ is limited, like the XGM gates. To tackle this problem push-pull configurations have been suggested where, by applying phase changes in both arms [56], the performance can be significantly enhanced [80, 81]. This configuration has been used in a number of different signal processing applications such as demultiplexing [81], regeneration [82], add-and-drop multiplexing [83], regenerative add-and-drop multiplexing [84] and format and wavelength conversion [85]. In fiber-based devices, the nonlinear loop mirror (NOLM) is of particular interest [86, 87]. Due to the inherently ultra-fast response of the Kerr nonlinearity, the NOLM is capable of performing a number of fast bit-level processes e.g., 640 to 10 Gb/s demultiplexing [88], regeneration [89], simultaneous 10 Gb/s wavelength conversion and regeneration [90], and clock extraction [91]. Other than in interferometric configurations, phase modulation can be translated into power modulation by a simple notch filter like that in [92] and XPM can be achieved. XPM in fibers has been used in other configurations to provide 160 Gb/s conversion and 3R regeneration [93] and low penalty wavelength conversion [94]. Recently Raman enhanced selfphase modulation in fibers attracted a lot of attention as an ultra fast technique with noise suppression capabilities [95], however the method is still immature. Generally, nonintegrated devices suffer from stability problems and encounter control issues.
