6.2.1 Introduction

In all-optical "wavelength continuous" networks, the light-path between two stations is set on a given wavelength that cannot be changed along the route. A new connection requiring a wavelength already allocated to another connection on a given portion of light-path is blocked: Two connections cannot use the same wavelength on the same fiber. This constraint is known as the "wavelength-continuity constraint." Wavelength conversion at the nodes of the network enables the system to avoid this constraint, improving flexibility and efficiency.

If optoelectronic conversion is accepted, the optical signal can be translated in the electronic domain and a tunable laser is set to the new wavelength [24]. For all-optical wavelength conversion, the main technological options are:

• Optical-gating with cross-gain or XPM in semiconductor amplifiers and lasers;

• Wave mixing in a nonlinear medium such as FWM in a SOA or frequency shift in dispersion-shifted doped fibers.

The XPM in an MZ structure appears very promising [25, 26].

6.2.2 Influence of Wavelength Conversion on Network Performances

It is very important to evaluate the potential benefit of wavelength conversion or wavelength interchange within the optical networks through wavelength interchange optical cross-connects (WIXC) [27 35]. No consensus has been reached so far.

Several groups studied the influence of a limited wavelength conversion on WDM network performance under dynamic traffic condition. It was shown that the influence on network blocking limitation could be relatively small above a few wavelength converters per fiber, for Poisson or non Poisson traffic characteristics, on national-scale long-distance...