Nonlinear Fiber Optics - 4 ed. Agrawal

10.4. Parametric Amplification 393
Figure 10.13: Measured signal gain as a function of signal wavelength at several pump powers.
Solid curves show the theoretical prediction. (After Ref. [72]; c○2001 IEEE.)
γ > 10 W −1 /km are used to make modern FOPAs. A 200-nm gain bandwidth was
realized as early as 2001 by using a 20-m-long fiber with γ = 18 W −1 /km [70]. The
required pump power (∼10 W) was large enough that the signal was also amplified by
SRS when its wavelength exceeded the pump wavelength.
Another scheme for enhancing the bandwidth of FOPAs manages fiber dispersion
along the fiber either through periodic dispersion compensation [65] or by using multiple
fiber sections with different dispersive properties [71]. Figure 10.13 shows the
gain spectra at several pump-power levels for such a single-pump FOPA [72]. At a
pump power of 31.8 dBm (about 1.5 W) at 1563 nm, the FOPA provided a peak gain
of 39 dB. The 500-m-long FOPA was designed by cascading three fiber sections with
zero-dispersion wavelengths at 1556.8, 1560.3, 1561.2 nm. Each fiber section had γ
close to 11 W −1 /km and a dispersion slope around 0.03 ps/(nm 2 /km). Such devices
can be used to amplify multiple WDM channels simultaneously [73].
As discussed earlier, FOPAs can act as wavelength converters because the idler
wave generated internally contains the same information as the signal launched at the
input end but at a different wavelength [74]. As early as 1998, peak conversion efficiency
of 28 dB was realized over a 40-nm bandwidth using a pulsed pump source [64].
More recently, transparent wavelength conversion over a 24-nm bandwidth (the entire
pump tuning range) was realized using a single-pump FOPA made with just 115 m of
a highly nonlinear fiber [75].
The performance of FOPAs is affected by several factors that must be considered
during the design of such devices [76]–[83]. For example, although FOPAs benefit
from an ultrafast nonlinear response of silica, they also suffer from it because fluctuations
in the pump power are transferred to the signal and idler fields almost instantaneously.
Indeed, noise in FOPAs is often dominated by the transfer of pump-power
fluctuations to the amplified signal [76]–[79]. Moreover, as the pump beam is typically
amplified using one or two erbium-doped fiber amplifiers (EDFAs) before it is
launched inside the FOPA, spontaneous emission added by EDFAs can degrade the
FOPA performance considerably. In practice, an optical filter is used to block such
noise. Another source of noise is spontaneous Raman scattering that occurs whenever