Page 202 - Optical Communications Essentials

Optical Amplifiers

192 Chapter Eleven

reasons. For notational clarification purposes, suppose we examine what is hap-
pening in WDM network 1. First, if there is no coordination of wavelength allo-
cations in different networks, an optical signal coming from an external
network may not have the same wavelength as that of any of those used in net-
work 1. In this case the incoming signal needs to be converted to a wavelength
that network 1 recognizes. Second, suppose that within network 1 a signal com-
ing into a node needs to be sent out on a specific transmission line. If the wave-
length of this signal is already in use by another information channel residing
on the destined outgoing path, then the incoming signal needs to be converted
to a new wavelength to allow both information channels to traverse the same
fiber simultaneously.
Although a number of all-optical techniques have been investigated for achiev-
ing wavelength conversion, none of them are commercially mature yet.
Therefore, currently the most practical method of wavelength conversion is to
change the incoming optical signal to an electrical format and then use this elec-
tric signal to modulate a light source operating at a different appropriate wave-
length. However, for those readers who are curious about all-optical techniques,
the following two subsections briefly discuss two all-optical wavelength conver-
sion methods, one based on optical gating and the other on wave mixing.

11.6.1. Optical-gating wavelength converters
A wide variety of optical-gating techniques using devices such as semiconductor
optical amplifiers, semiconductor lasers, or nonlinear optical-loop mirrors have
been investigated to achieve wavelength conversion. The use of an SOA in a
cross-gain modulation (CGM) mode has been one of the most successful tech-
niques for implementing single-wavelength conversion. The configurations for
implementing this scheme include the Mach-Zehnder or the Michelson inter-
ferometer setups shown in Fig. 11.15.
The CGM scheme relies on the dependency of the refractive index on the car-
rier density in the active region of the SOA. As depicted in Fig. 11.15, the basic
concept is that an incoming information-carrying signal at wavelength λ s and a
continuous-wave (CW) signal at the desired new wavelength λ c (called the probe
beam) are simultaneously coupled into the device. The two waves can be either
copropagating or counterpropagating. However, the noise in the latter case is
higher. The signal beam modulates the gain of the SOA by depleting the carri-
ers, which produces a modulation of the refractive index. When the CW beam
encounters the modulated gain and refractive index, its amplitude and phase are
changed, so that it now carries the same information as the input signal. As
shown in Fig. 11.15, the SOAs are placed in an asymmetric configuration so that
the phase change in the two amplifiers is different. Consequently, the CW light
is modulated according to the phase difference. A typical splitting ratio is 69/31
percent. These types of converters readily handle data rates of at least 10Gbps.
A limitation of CGM architecture is that it only converts one wavelength at a
time. In addition, the refractive index varies as the carrier density changes

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