8.2. WDM COMPONENTS 359
beams are present simultaneously, all 1 bits are directed toward the bar port because of
the refractive-index change induced by theλ 1 beam. The physical mechanism behind
this behavior is the cross-phase modulation (XPM). Gain saturation induced by theλ 1
beam reduces the carrier density inside one SOA, which in turn increases the refractive
index only in the arm through which theλ 1 beam passes. As a result, an additionalπ
phase shift can be introduced on the CW beam because of cross-phase modulation, and
the CW wave is directed toward the bar port during each 1 bit.
It should be evident from the preceding discussion that the output from the bar port
of the MZ interferometer would consist of an exact replica of the incident signal with
its wavelength converted to the new wavelengthλ 2. An optical filter is placed in front
of the bar port for blocking the originalλ 1 signal. The MZ scheme is preferable over
cross-gain saturation as it does not reverse the bit pattern and results in a higher on–off
contrast simply because nothing exits from the bar port during 0 bits. In fact, the output
from the cross port also has the same bit pattern but its polarity is reversed. Other types
of interferometers (such as Michelson and Sagnac interferometers) can also be used
with similar results. The MZ interferometer is often used in practice because it can be
easily integrated by using SiO 2 /Si or InGaAsP/InP waveguides, resulting in a compact
device [125]. Such a device can operate at high bit rates (up to 80 Gb/s), offers a large
contrast, and degrades the signal relatively little although spontaneous emission does
affect the SNR. Its main disadvantage is a narrow dynamic range of the input power
since the phase induced by the amplifier depends on it.
Another scheme employs the SOA as a nonlinear medium for four-wave mix-
ing (FWM), the same nonlinear phenomenon that is a major source of interchannel
crosstalk in WDM systems (see Section 8.3). The FWM technique has been discussed
in Section 7.7 in the context of optical phase conjugation and dispersion compensation.
As seen in Fig. 8.20(d), its use requires an intense CW pump beam that is launched into
the SOA together with the signal whose wavelength needs to be converted [119]. Ifν 1
andν 2 are the frequencies of the input signal and the converted signal, the pump fre-
quencyνpis chosen such thatνp=(ν 1 +ν 2 )/2. At the amplifier output, a replica of
the input signal appears at the carrier frequencyν 2 because FWM requires the presence
of both the pump and signal. One can understand the process physically as scattering
of two pump photons of energy 2hνpinto two photons of energyhν 1 andhν 2 .The
nonlinearity responsible for the FWM has its origin in fast intraband relaxation pro-
cesses occurring at a time scale of 0.1 ps [130]. As a result, frequency shifts as large as
10 THz, corresponding to wavelength conversion over a range of 80 nm, are possible.
For the same reason, this technique can work at bit rates as high as 100 Gb/s and is
transparent to both the bit rate and the data format. Because of the gain provided by
the amplifier, conversion efficiency can be quite high, resulting even in a net gain. An
added advantage of this technique is the reversal of the frequency chirp since its use
inverts the signal spectrum (see Section 7.7). The performance can also be improved
by using two SOAs in a tandem configuration.
The main disadvantage of any wavelength-conversion technique based on SOAs is
that it requires a tunable laser source whose light should be coupled into the SOA, typ-
ically resulting in large coupling losses. An alternative is to integrate the functionality
of a wavelength converter within a tunable semiconductor laser. Several such devices
have been developed [119]. In the simplest scheme, the signal whose wavelength needs