Objective  Brillouin scattering is a third-order nonlinear light scattering phenomenon resulting from the interaction of a pump beam incident on a medium with elastic acoustic phonons within the medium. The stimulated Brillouin scattering (SBS) technique is widely used for its high-energy conversion efficiency, small Brillouin frequency shift, and phase conjugation. Currently, the SBS technique has been widely used in Brillouin spectroscopy, pulse width compression, and beam combination, in which the frequency shift and linewidth are two essential parameters of Brillouin scattering. The frequency shift and linewidth have been successfully applied in Brillouin spectroscopy to differentiate the information about medium type, concentration, and temperature characteristics. Previous studies mainly focus on measuring time-domain pulse width in SBS pulse-width compression. The study of frequency-domain linewidth variation is less involved. In contrast, the linewidth variation characteristics are closely related to the information about medium viscosity, temperature, and refractive index, so it is of great significance to investigate the linewidth variation characteristics during SBS compression.

  Methods  The experiment probes the linewidth change of the medium FC-770 during SBS compression by focusing a single-cell setup, which is mainly composed of a generator and a long focusing lens and is characterized by controllable incident energy and a simple structure. The linewidth change incident to the second-stage generator is controlled by the compact two-cell setup, which has high-energy conversion efficiency, and the time-domain waveform of the incident pulse generated by the compact two-cell setup can maintain a better shape to avoid the effect of the time-domain waveform on the linewidth change. Due to the low laser repetition frequency during SBS compression, the Fabry-Pérot (F-P) combined with the COMS beam profilers (CBP) is selected to measure Stokes linewidth at low repetition frequency. The interference scattering plot is obtained by processing the interference circle acquired by CBP, and the Stokes intrinsic linewidth value is obtained after non-linear fitting.

  Results and Discussion   During the SBS compression, pump energy, lens focusing parameters, and incident laser linewidth influence the Stokes linewidth output. With the increase of the pump energy, the energy reflectivity increases rapidly, and the Stokes linewidth first increases rapidly and then gradually narrows. When the pump energy is 35 mJ, the Stokes linewidth can be narrowed to about 400 MHz (Fig.5). As the focal length of the lens increases, the energy density at the focal point decreases, but the increase in focal length of the lens increases the effective interaction length of the pump light with the back-transported Stokes light, and the Stokes linewidth increases rapidly (Fig.6). By taking a compact two-cell setup to control the linewidth incident to the generator, it can be seen that as the value of the linewidth incident to the second-stage generator gradually increases, the value of the output Stokes linewidth gradually increases, and when the value of the linewidth incident to the second-stage generator varies from 280 MHz to about 450 MHz, the value of the output Stokes linewidth varies from 500 MHz to about 680 MHz (Fig.8).

  Conclusions  During the SBS compression process, the variation of the Stokes linewidth of the medium FC-770 output shows a tendency of first increasing and then rapidly narrowing with the increase of the pump power density. As the lens's focal length in front of the generator decreases, the Stokes linewidth becomes narrower. When the focal length of the lens is small, the Stokes linewidth variation is less effective by the pump energy. The input laser linewidth is controlled by secondary compression, and the Stokes output linewidth gradually broadens as the incident linewidth value increases. From the experiments, it can be seen that the linewidth variation in the SBS compression process has a specific law, and the frequency domain linewidth variation contains rich information about the medium properties, so the study of Stokes linewidth variation in the SBS compression process is of great significance for the study of medium properties.