# Modulated Noncollinear Optical Parametric Amplifier Output Induced by Stimulated Raman Scattering
## 4 Results and Discussion
### Stimulated Raman Processes in Optical Parametric Amplification
#### 4.1.1 Intensity Modulation Structure in the Amplified Signals  
The amplified signal intensities at wavelengths of 552, 612, 660, and 720 nm are plotted as a function of the pump-seed delay time in Figure 5. These curves have the following two characteristic features. First, there is a longer time range for the pump pulse because its stretching extends the amplification range of the seed. Second, the modulation structure of the curve is more evident during the stretched pump pulse, possibly due to elongation of the amplification range resulting in a stronger signal intensity change due to Raman scattering in the BBO. This phenomenon can be explained in terms of molecular vibrations of the stimulated Raman scattering excited by the pump pulse. The BBO crystal can be excited by the pump light and two subsequent processes can occur when the seed pulse arrives: optical parametric amplification satisfying the phase-matching condition or \begin{table}
\begin{tabular}{c c c c c c} \hline
**No.** & **RS (cm\({}^{-1}\))** & **No.** & **RS (cm\({}^{-1}\))** & **No.** & **RS (cm\({}^{-1}\))** \\ \hline
1 & 58 & 6 & 480 & 11 & 790 \\
2 & 124 & 7 & 599 & 12 & 1526 \\
3 & 180 & 8 & 622 & 13 & 1547 \\
4 & 247 & 9 & 639 & 14 & 1559 \\
5 & 385 & 10 & 665 & 15 & \\ \hline \end{tabular}
\end{table} 
Table 1: Raman frequency shift (RS) in BBO crystal from Ref. (Chin. Phys. Lett. 19 (2002) 205). Figure 4: The distribution of the difference between the spectral peak and valleys in the amplified signal spectrum shown in Figure 2. The experimental condition of the pre-compensation and stretching of the pump pulse are the same as in Figure 1. The marks represent the difference of the peak-peak, valley–valley, and peak–valley in Figure 2 and the differences in wave number are given by the ordinate. The positions of the marks in the abscissa are arbitrarily set for the plotting to be easily observed with moderate density of the dots without any specific meaning. Raman scattering for which the intensity of the amplified signal varies with the phase of the molecular vibrations. For certain phases, some spectral ranges are amplified by the stimulated Raman scattering and others are deamplified in accordance with the phase relations between the coherent molecular vibrational amplitude and the laser field. The two spectral components can serve as pump and Stokes fields whose behaviors are in anti-phase; in other words, the pump is amplified, and the Stokes field is deamplified or vice versa depending on the molecular vibrational phase described by Equation (1). The same phenomenon can occur between the spectral components that satisfy the relation between the pump and anti-Stokes pulses described by Equation (3). In each case, the amplified and deamplified spectral ranges are exchanged after a half period. In the central part of the seed spectrum, there is a spectral range where frequency components can function as both pump and Stokes pulses. There is another spectral range where the frequency component can function as both pump and anti-Stokes pulses. The former range extends from the highest frequency (\(\
u_{\text{max}}\)) edge to the frequency equal to the difference of \(\
u_{\text{max}}\) and the frequency (\(\
u_{\text{r}}\)) of the relevant mode. The latter range is from the lowest frequency (\(\
u_{\text{min}}\)) edge to the frequency equal to the sum of \(\
u_{\text{min}}\) and the frequency (\(\
u_{\text{r}}\)) of the relevant mode. Therefore, all frequency components in the spectral range of the seed between \(\
u_{\text{max}}-\
u_{\text{r}}\) and \(\
u_{\text{min}}+\
u_{\text{r}}\) function as pump, Stokes, and anti-Stokes pulses. Competition and interference among these processes are expected to make the observed phenomena extremely complicated. A Fourier transform analysis was performed on the traces in Figure 5 to obtain the wavenumber of the periodic features. The results are plotted in Figure 6, which shows some peaks at a lower frequency.