Stimulated Brillouin Scatterring(SBS) and Brillouin-enhanced four-wave mixing(BEFWM) have been investigated using $CS_2$ as a nonlinear medium to compress an output pulse of a photodissociation iodine laser. Since the frequency shift induced by a simple SBS in $CS_2$ is comparable with gain bandwidth of the iodine laser, it is impractical to use it in a laser amplifier system. A new BEFWM scheme without frequency shift is proposed and investigated theoretically and experimentally.
A pulse compression by simple SBS is, first, investigated theoretically and experimentally. A coupled electrodynamic equation for optical wave and acoustic wave is derived, and is transformed to a simple time derivative by their propagation characteristics, which give relatively simple method for computer simulation to analyze the dynamics of pulse compression by SBS.
To obtain the condition for high compression ratio with high conversion efficiency, some parameters (pulse width, cell shape, cell length, etc.) are investigated using this method. For an explanation of the role of a secondary pulses shown in SBS signal and transmitted pump laser, a transient anti-Stokes scattering is discussed.
Experimentally the pulse compression by simple SBS is investigated. A laser with 50-ns duration is focussed into a $CS_2$ cell of 1.2 m long by a lens of 1-m focal length, and a back-reflected SBS signal with 5-ns duration and a power reflectivity more than 1 is obtained.
To achieve the frequency recovery of an SBS signal, the characteristics of frequency up-shifting (anti-Stokes) BEFWM is investigated. Four waves consisting BEFWM are two pump, probe, and output signal waves. The interference between one pump wave with laser frequency and the probe wave with Brillouin down-shifted frequency makes a moving grating. Another pump wave with Brilloun down-shifted frequency is reflected by this grating and as a result, its frequency is up-shifted to the laser frequency.
The same simulation method as simple SBS is applied to analyze the dynamics of BEFWM interaction. Some parameters affecting to the pulse shape (powers of four waves, cell length) are investigated. The modulation shown in the anti-Stokes signal is explained by the competition between two different (anti-phase) kinds of the acoustic waves.
For an experimental investigation, two cells are prepared; one is used to generate a Brillouin down-shifted signal and the other for the interaction cell. The SBS signal generated from one cell is used as both probe and backward pump waves, distinguished by their polarization. As a simple arrangement to observe the frequency up-shifting BEFWM, the characteristics of a combined SBS generator-amplifier system is investigated, and it is compared with the case of an independent interaction cell. The latter method, compared with the former, gives a more efficient and more compressed output signal. The best compressed pulse is obtained when the pump 2 and the probe beam crosses with pump 1 with a small angle ($\approx$1 mrad).
To ascertain that the output signal is the result of BEFWM interaction, the beam property is investigated and the frequency recovery is checked by double pass amplification of the output signal.
The power conversion efficiency of 30% through BEFWM, that is 5% of incident laser, and pulse compression from 50 ns to 3 ns between incident laser and output signal are obtained.
