Nonlinear Fiber Optics - 4 ed. Agrawal

284 Chapter 8. Stimulated Raman Scattering
Figure 8.4: Five Stokes lines S 1 to S 5 generated simultaneously using 1.06-μm pump pulses.
Vertical line corresponds to residual pump. Powers were measured through a monochromator
with 1.5-nm resolution. (After Ref. [37]; c○1978 IEEE.)
line shape of the first-order Stokes line, generated by launching in a 100-m-long fiber
pump pulses of about 1-ns duration, obtained from a mode-locked argon laser (λ p =
514.5 nm). Figure 8.5 shows the observed spectra at three pump powers. The spectra
exhibit a broad peak at 440 cm −1 (13.2 THz) and a narrow peak at 490 cm −1
(14.7 THz). As pump power is increased, the peak power of the broad peak saturates
while that of the narrow peak keeps increasing.
The appearance of the double-peak Stokes spectrum can be understood by noting
from Figure 8.2 that the dominant peak in the Raman-gain spectrum actually consists
of two peaks whose locations exactly coincide with the two peaks in the Stokes spectra
of Figure 8.5. A detailed numerical model, in which the shape of the Raman-gain spectrum
is included and each spectral component of the Stokes line is propagated along
the fiber including both the Raman gain and spontaneous Raman scattering, predicts
line shapes in agreement with the experimentally observed spectra [45].
The features seen in Figure 8.5 can be understood qualitatively as follows. Spontaneous
Raman scattering generates Stokes light across the entire frequency range of
the Raman-gain spectrum. After a short length of fiber, these weak signals are amplified
with the appropriate gain coefficients while more spontaneous light is added. At
low pump powers, the observed Stokes spectrum looks like exp[g R (Ω)] because of the
exponential amplification process. As pump power is increased, the high-frequency
peak at 440 cm −1 can pump the low-frequency peak at 490 cm −1 through the Ramanamplification
process. This is precisely what is seen in Figure 8.5. Eventually, the
Stokes power becomes high enough to generate a second-order Stokes line. Even
though this model is based on the CW theory of SRS, it is able to explain the qual-