The pulse compression and phase conjugation characteristics of stimulated Brillouin scattering (SBS) in $CS_2$ was investigated using an iodine laser of 40-ns duration as pumping source. SBS was generated with the focusing lenses of f=20 cm, 50 cm, and 100 cm. SBS peak-power reflectivity approached a saturation at about 60% and 70% in the case of f=20 cm and 50 cm, respectively. When the longer focusing lens of f=100 cm was used, a meaningful pulse compression upto 2-ns pulsewidth was observed and the peak-power reflectivity was about 2 times of that of the pumping pulse. The compressed Stokes pulse consisted of a highly compressed front part and an uncompressed tail part.
The phase conjugation fidelity was measured through the Energy-in-bucket method which tells the time-integrated fidelity of whole Stokes pulse. The maximum fidelity at the focusing lenses of f=20 cm, 50 cm reached about 85% and that at the focusing lens of f=100 cm was about 80%. However, the Stokes pulse with pulse compression had poor fidelity of about 45%. Power-in-bucket method relating to the time-resolved fidelity was performed for the case of f=100 cm only. The highly compressed part of Stokes pulse possessed higher fidelity of about 90%, and the uncompressed tail part had lower fidelity. It confirmed that the compressed part by SBS is a best candidate for application with high peak-power reflectivity and high fidelity.
The temporal change of fidelity was explained in terms of the SBS interaction length. The highly compressed part results from the long interaction length through which a phase conjugated mode can take a large gain overwhelming a nonconjugated mode, keeping a good amplification discrimination against the nonconjugated mode. The interaction length for the compressed part is from near focal point to the limit of SBS interaction. On the otherhand, the uncompressed tail part is reflected from the strong acoustic wave formed by leading pumping pulse in front of the focal point. So the interaction length reduced, which means the amplification discrimination between the conjugated and the nonconjugated modes is weaker. It results in a high fidelity at the highly compressed front part and a low fidelity at the uncompressed tail part of Stokes pulse.
The multipulse effect on transient stimulated Brillouin scattering in $CS_2 (t_{ph}=9.8 ns)$ and 20-atm $SF_6$ gas ($t_{ph}=35$ ns) using an iodine laser as pumping source was investigated. A single pulse of 4-ns duration from the Kerr cell shutter was modified into several pulses through a pulse-divider composed of one partial mirror and one full mirror. The enhancement of SBS reflectivity by multipulse employment was observed in the two media among which $SF_6$ gas of longer phonon-lifetime showed more distinguished response than $CS_2$. The double-pulse condition for the higher SBS reflectivity in $CS_2$ was searched. When the power ratio of preceding two pumping pulses was 1:1.6 and the pulse separation between the peaks of the two pulses was 8 ns, the maximum SBS peak-power reflectivity increased about 2 times and its absolute reflected power was improved up to 50% compared to those of an original single pulse which suffers optical breakdown at high pumping intensity and small transient SBS gain.
A computer simulation was performed to understand the multipulse effect by observing the change of acoustic-wave intensity in the medium. The simulation results confirmed that the acoustic wave generated in the medium at the second pumping pulse has stronger intensity than that of the single pulse with same power. It also proved the multipulse effect is significant in the medium of long phonon-lifetime.
