# Modulated Noncollinear Optical Parametric Amplifier Output Induced by Stimulated Raman Scattering
## 4 Results and Discussion
### Stimulated Raman Processes in Optical Parametric Amplification
#### 4.1.2 Further Discussion  
Comparison of the results and discussion in Sections 4.1.1.1 and 4.1.1.2 above reveals that the difference in wavenumbers is random and thus the structures in the spectra cannot originate from Raman scattering. However, the pattern in Figure 7 gives improved consistency between the experimental results and the Raman shift signals of BBO in the literature. The reason for this difference is explained as follows.  
The periodic structure only appears when the multichannel lock-in amplifier is used. During data collection, the pump light is modulated by a chopper of which the frequency is sent to the lock-in amplifier as the reference signal. Therefore, the periodic structure must be introduced by the pump pulse. Furthermore, the structure and signal intensity modulation must be enhanced by the BBO crystal.  
The results in part B support the conclusion of the structure formation is due to a Raman scattering process by the pump light. There are many Raman signals for BBO listed in Table 1. Thus, the spectral shape and intensity change are due to mixed contributions of stimulated Raman shifts. The contribution of different Raman shift signals can be more clearly obtained by taking the Fourier transform of the traces in Figure 5 than by plotting frequency differences between adjacent spikes. This process is the same as that used in the analysis of real-time traces of time-resolved spectroscopy [37,38]. In a pump-probe experiment, the vibration signal from molecules is excited by the pump light. A possible origin of the complicated structures is the competition and interference among the processes. The pump and seed spectra both have complex structures and further complication takes place in their spectra due to the Raman amplification and deamplification, as discussed above. This Raman amplification and deamplification corresponds to Raman gain and loss processes, respectively. These phenomena appear as alternating absorbance changes \(\Delta A\) with vibrational frequency in the pump-probe experiment. Therefore, the frequency shift  
Figure 7: Distribution of peaks in the Fourier power spectra of Figure 6. The Raman shift signals are plotted as dashed lines. The numbers in the legend refer to the caption of Figure 2. The marks indicate the vibrational modes found in Figure 6 and the ordinate indicates the frequency in wavenumbers.  
of the spectral component in the pump range to the Stokes range is independent of the spectral shape in the original Stokes region. This can be explained using Equation (1), in which the amplification or deamplification coefficient of the Stokes field \(E_{S}\), which depends on the phase of the molecular vibration, is a function of the pump field \(E_{P}\). This differs from the situation in conventional pump-probe experiments in which the pump field spectral shape does not affect the signal when the system is homogeneous. In the same way, the amplification or deamplification coefficient of the anti-Stokes field \(E_{AS}\) at some instant of the molecular vibration phase is given as a function of the pump field \(E_{P}\).  
Another possible origin of the complication is the overlap between the laser spectrum and the absorption spectrum of the sample. However, there is no absorption of the 400-nm pump light in parametric amplification, which implies that the excitation of the BBO is only due to non-electronic resonant stimulated Raman processes.