7.7. OPTICAL PHASE CONJUGATION 303
Figure 7.14: Experimental setup for dispersion compensation through midspan spectral inver-
sion in a 21-km-long dispersion-shifted fiber. (After Ref. [83];©c1993 IEEE; reprinted with
permission.)
experimental setup. A bandpass filter (BPF) is used to separate the phase-conjugated
signal from the pump.
Several factors need to be considered while implementing the midspan OPC tech-
nique in practice. First, since the signal wavelength changes fromωstoωc= 2 ωp−ωs
at the phase conjugator, the GVD parameterβ 2 becomes different in the second-half
section. As a result, perfect compensation occurs only if the phase conjugator is slightly
offset from the midpoint of the fiber link. The exact locationLpcan be determined by
using the conditionβ 2 (ωs)Lp=β 2 (ωc)(L−Lp), whereLis the total link length. By
expandingβ 2 (ωc)in a Taylor series around the signal frequencyωs,Lpis found to be
Lp
L=
β 2 +δcβ 3
2 β 2 +δcβ 3, (7.7.9)
whereδc=ωc−ωsis the frequency shift of the signal induced by the OPC technique.
For a typical wavelength shift of 6 nm, the phase-conjugator location changes by about
1%. The effect of residual dispersion and SPM in the phase-conjugation fiber itself can
also affect the placement of phase conjugator [94].
A second factor that needs to be addressed is that the FWM process in optical
fibers is polarization sensitive. As signal polarization is not controlled in optical fibers,
it varies at the OPC in a random fashion. Such random variations affect the FWM effi-
ciency and make the standard FWM technique unsuitable for practical purposes. For-
tunately, the FWM scheme can be modified to make it polarization insensitive. In one
approach, two orthogonally polarized pump beams at different wavelengths, located
symmetrically on the opposite sides of the zero-dispersion wavelengthλZDof the fiber,
are used [85]. This scheme has another advantage: the phase-conjugate wave can be
generated at the frequency of the signal itself by choosingλZDsuch that it coincides