*DOI: http://dx.doi.org/10.5772/intechopen.88354 All Optical Signal Processing Technologies in Optical Fiber Communication*

when light of two or more unlike wavelengths is launched in a fiber, offering ascend to another wave. When two pump photons are formed, two photons are created: the first one at the signal frequency, the other one at a complementary frequency called idler as shown in **Figure 9** [96, 97]. A schematic of the FWM process, that takes place in nonlinear media, is shown in **Figure 10**. The beating of two distinctive frequency waves modulates the medium's polarization and generates a grating. The input wave interaction with the gratings leads to new components of the frequency. The cause of FWM in SOAs [98–102], is linked to interband and intraband carrier dynamics, while in passive devices and fibers it is because of the induced polarization of the medium under an electric field. The major disadvantage of FWM is its low efficiency, which results in low power FWM products. The main parameter that affects both the efficiency and optical signal-to-noise ratio (OSNR) is the unsaturated gain [68, 103], which can be enhanced by utilizing either longer SOAs, with a smaller active layer [104] or different structures such as multiquantum well devices [103]. The use of an assisted beam has been suggested for the enhancement of the FWM efficiency in [105, 106].

Another disadvantage is that FWM is normally polarization sensitive. The problem has been tackled by using dual-pump configurations [107, 108]. In [109] a 80 nm conversion bandwidth was achieved by dual pump configuration at 2.5 Gb/s. In [110] an almost constant efficiency (<3 dB variation) over a 36 nm wavelength conversion of a 10 Gb/s channel, was achieved.

Due to the nature of the nonlinearities, four wave mixing is very fast. In [111] a multiplexed channel of 100 Gb/s was converted over a range of 3.2 nm by a 2 mm SOA. In addition to converting high bit rate pulses [112–114], the method has been utilized to convert modulation formats [115, 116]. Very fast FWM conversion has been applied as in the demonstration of a 6.3 GHz clock extraction from a 400 Gb/s signal [117], 100 to 10 Gb/s demultipler based on photonic downconversion for a stable add/drop operation [118]. Regenerative properties of wavelength converters based on FWM in a semiconductor optical amplifier have also been demonstrated in [119].

Another advantage of FWM is that it supports the simultaneous conversion of multiple wavelengths. This has been demonstrated in [48] where 26 WDM channels were simultaneously converted in a highly nonlinear, specially designed fiber. In [101, 120] multichannel SOA based FWM is investigated. In [65, 121] it has been used for multicasting. In a highly nonlinear fiber for FWM was used to demonstrate simultaneous conversion of 200 Gb/s [122] and 32 × 10 Gb/s channels [123].

FWM is a very promising technique, but due to the complexity of the configuration for polarization insensitive operation or wide tunability it will probably be used only for converters operating at 100 Gb/s and beyond.
