# Modulated Noncollinear Optical Parametric Amplifier Output Induced by Stimulated Raman Scattering
## 4 Results and Discussion
### Effect of Stimulated Raman Processes on Parametric Amplification  
Next, we consider the effect of pulse compression of the amplified signal in the parametric amplification by utilizing pulse characterization based on the frequency-resolved optical gating (FROG) technique. This characterization was performed for different types of experimental configurations. Figure 8 shows an example of FROG results together with the amplified signal spectrum. In this experiment, the signal spectrum was adjusted to have relatively intense peaks under the action of the stimulated Raman process. The signal pulse was measured after passing through air from the BBO crystal in the OPA to the BBO crystal for the FROG measurement without compression by a prism pair and/or chirped mirrors. This measurement provides timing information about the signal pulse since the air imparts only a linear chirp to the pulse. The two-dimensional pattern of the sum-frequency distribution shown in Figure 8b indicates that the chirp in the signal pulse is nonlinear. For example, components around 320 nm, corresponding to a fundamental signal wavelength of 640 nm, appear at delay times around \(\pm\)100 and 0 fs. Since the parametric amplification processes do not change the chirp of the seed pulse and the continuum seed generated by focusing the 800 nm light onto the sapphire plate gives rise to a linear chirp, the nonlinear chirp in the amplified signal is concluded to be due to the stimulated Raman process. In fact, the pattern in Figure 8b shows a periodic distribution of the same sum-frequency components at different delay times, which deteriorates the pulse characteristics much more than the nonlinear chirp for further compression of the amplified signal pulse. In the present technique of chirping compensation, rearrangement of periodically distributed spectral components hinders pulse compression. Because stimulated Raman amplification and deamplification are third-order nonlinear optical processes, the pump intensity needs to be lower than the threshold intensity to avoid obtaining spectra with prominent peaks and valleys. It is thus critical for the seed pulse not to have such a structure to prevent the structure from being amplified. Since the processes of stimulated Raman process including Raman gain and Raman loss processes, continuum generation, spatial self-focusing processes and others are third-order or higher, they are sensitive the intensity of the NOPA pulse and the spectral shape. Some spectral components are more effectively contributing to some of the processes and pairs of some partial spectral bands are more efficient in some other processes. These phenomena may result in the instability of measurement. The instability of the spectrum may even change the temporal shape of the NOPA output which may deteriorate the results obtained by the experiments. In the application of ultrashort pulse frequently nonlinear processes, such as SHG and SFG, are utilized since linear measurement using conventional detectors which do not have femtosecond time resolution is not possible. Therefore, the spectral pattern in the amplified signal may give serious errors in the spectral information of the experimental results. To avoid such problems, if any, it is recommended to investigate the dynamic range of the signal in the experiment. For example, before starting a type of pump/probe experiment, which includes optical pump/optical probe to study time-resolved induced absorption/ photoconductivity and others, it is recommended to study the linearity of signal intensity with respect to pump intensity. This is for the linearity of the phenomenon to be studied. Of course, the linearity of the detector with respect to the signal light intensity must be verified beforehand.