In optical transport networks, for flexibility of lightpath establishment and efficiency of wavelength utilization, we need the flexibility of converting an optical signal from one wavelength to another wavelength. Such a conversion process is called wavelength conversion. In general, there are two approaches to realize wavelength conversion, namely (1) OEO-based wavelength conversion and (2) all-optical wavelength conversion. 

OEO wavelength conversion employs an optical to electronic and then to optical process to convert an optical signal from one wavelength to another wavelength. Specifically, the optical signal is first converted into electronic format, then the electronic signal is used to modulate a tunable laser to convert to an optical signal. Here tunable laser will be tuned to the wavelength that we want to convert to. OEO wavelength conversion is a quite mature technique. The tunability of tunable laser can be the unique key limitation of the technique. In addition, because the conversion process needs to go though an electronic processing, such a processing could become a bottleneck of optical transmission system as electronic signal processing is generally slower than its optical counterpart.

All-optical wavelength conversion is a more advanced conversion technique. It does not need to convert an signal into electronic format then to optical format. All optical conversion in general utilizes some nonlinear optical effects in optical components to realize wavelength conversion. These effects include Four Wavelength Mixing, SOA saturation, cross-phase modulation (XPM), etc. There is no so-called electronic bottleneck in optical wavelength conversion. However, viewing the immaturity, all-optical wavelength conversion is normally much more expensive than OEO wavelength conversion. Moreover, all-optical wavelength converters are not commercial available yet. Most of them are only experimentally realized in university or institute labs.