358 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.20: Four schemes for wavelength conversion: (a) optoelectronic regenerator; (b) gain
saturation in a semiconductor laser amplifier (SLA); (c) phase modulation in a SLA placed in
one arm of a Mach-Zehnder interferometer; (d) four-wave mixing inside a SLA.
emission, and (iii) phase distortion because of frequency chirping that invariably occurs
in SOAs (see Section 3.5). The use of an absorbing medium in place of the SOA solves
the polarity reversal problem. An electroabsorption modulator (see Section 3.6.4) has
been used for wavelength conversion with success [127]. It works on the principle of
cross-absorption saturation. The device blocks the CW signal atλ 2 because of high
absorption except when the arrival of 1 bits atλ 1 saturates the absorption.
The contrast problem can be solved by using the MZ configuration of Fig. 8.20(c) in
which an SOA is inserted in each arm of a MZ interferometer [119]. The pulsed signal
at the wavelengthλ 1 is split at the first coupler such that most power passes through
one arm. At the same time, the CW signal at the wavelengthλ 2 is split equally by this
coupler and propagates simultaneously in the two arms. In the absence of theλ 1 beam,
the CW beam exits from the cross port (upper port in the figure). However, when both