304 CHAPTER 7. DISPERSION MANAGEMENT
with the signal frequency. This feature follows from the relationωc=ωp 1 +ωp 2 −ωs,
whereωp 1
=ωp 2. Polarization insensitivity of OPC can also be realized by using a
single pump in combination with a fiber grating and anorthoconjugate mirror[90],
but the device works in the reflective mode and requires the separation of the conjugate
wave from the signal by using a 3-dB coupler or an optical circulator.
The relatively low efficiency of the OPC process in optical fibers is also of some
concern. Typically, the conversion efficiencyηcis below 1%, making it necessary
to amplify the phase-conjugated signal [83]. Effectively, the insertion loss of the
phase conjugator exceeds 20 dB. However, the FWM process is not inherently a
low-efficiency process and, in principle, it can even provide net gain [106]. Indeed,
the analysis of the FWM equations shows thatηcincreases considerably by increas-
ing the pump power while decreasing the signal power; it can even exceed 100% by
optimizing the power levels and the pump-signal wavelength difference [92]. High
pump powers are often avoided because of the onset of stimulated Brillouin scatter-
ing (SBS). However, SBS can be suppressed by modulating the pump at a frequency
∼100 MHz. In a 1994 experiment, 35% conversion efficiency was realized by using
this technique [107].
The FWM process in a semiconductor optical amplifier (SOA) has also been used
to generate the phase-conjugated signal for dispersion compensation. This approach
was first used in a 1993 experiment to demonstrate transmission of a 2.5-Gb/s signal,
obtained through direct modulation of a semiconductor laser, over 100 km of standard
fiber [84]. Later, in a 1995 experiment the same approach was used for transmitting
a 40-Gb/s signal over 200 km of standard fiber [93]. The possibility of highly nonde-
generate FWM inside SOAs was suggested in 1987, and this technique is used exten-
sively in the context of wavelength conversion [108]. Its main advantage is that the
phase-conjugated signal can be generated in a device of 1-mm length. The conver-
sion efficiency is also typically higher than that of FWM in an optical fiber because of
amplification, although this advantage is offset by the relatively large coupling losses
resulting from the need to couple the signal back into the fiber. By a proper choice of
the pump-signal detuning, conversion efficiencies of more than 100% (net gain for the
phase-conjugated signal) have been realized for FWM in SOAs [109].
A periodically poled LiNbO 3 waveguide has also been used to make a wideband
spectral inverter [102]. The phase-conjugated signal is generated using cascaded second-
order nonlinear processes, which are quasi-phase-matched through periodic poling of
the crystal. Such an OPC device exhibited only 7-dB insertion losses and was capable
of compensating dispersion of four 10-Gb/s channels simultaneously over 150 km of
standard fiber. The system potential of the OPC technique was demonstrated in a 1999
field trial in which a FWM-based phase conjugator was used to compensate the GVD
of a 40-Gb/s signal over 140 km of standard fiber [100]. In the absence of OPC, the
40-Gb/s signal cannot be transmitted over more than 7 km as deduced from Eq. (7.1.2).
Most of the experimental work on dispersion compensation has considered trans-
mission distances of several hundred kilometers. For long-haul applications, one may
ask whether the OPC technique can compensate the GVD acquired over thousands of
kilometers in fiber links which use amplifiers periodically for loss compensation. This
question has been studied mainly through numerical simulations. In one set of simu-
lations, a 10-Gb/s signal could be transmitted over 6000 km when the average launch