Nonlinear Fiber Optics - 4 ed. Agrawal

8.3. SRS with Short Pump Pulses 303
Figure 8.15: Spectra of pump and Raman pulses showing spectral sidelobes as a result of XPMinduced
modulation instability. (After Ref. [131]; c○1988 Springer.)
So far, only first-order SRS has been considered. When the input peak power of
pump pulses exceeds the Raman threshold considerably, the Raman pulse may become
intense enough to act as a pump to generate the second-order Stokes pulse. Such a cascaded
SRS was seen in a 615-nm experiment using 5-ps pump pulses with a peak power
of 1.5 kW [118]. In the near-infrared region multiple orders of Stokes can be generated
using 1.06-μm pump pulses. The efficiency of the SRS process can be improved
considerably by using silica fibers whose core has been doped with P 2 O 5 because of a
relatively large Raman gain associated with P 2 O 5 glasses [137]–[139].
From a practical standpoint ultrafast SRS limits the performance of fiber-grating
compressors [140]; the peak power of input pulses must be kept below the Raman
threshold to ensure optimum performance. In this case SRS not only acts as a loss
mechanism but also limits the quality of pulse compression by distorting the linear
nature of the frequency chirp resulting from the XPM interaction between pump and
Raman pulses [141]. A spectral-filtering technique had been used to improve the quality
of compressed pulses even in the presence of SRS [129]. In this approach a portion
of the pulse spectrum is selected by using an asymmetric spectral window so that the
filtered pulse has nearly linear chirp across its entire width. Good-quality compressed
pulses can be obtained in the strong-SRS regime but only at the expense of substantial
energy loss [133].
Figure 8.16: Cross-correlation traces of pump and Raman pulses at the output of a 12-m-long
fiber. The intensity scale is arbitrary. (After Ref. [118]; c○1985 Elsevier.)