Controlled generation of nanosecond optical pulses by SBS

Abstract

We report experiments describing controlled production of intense optical pulses of variable width in the 2-25-ns range using stimulated Brillouin scattering (SBS) both in transient as well as quasisteady state regimes. It is known that the SBS process is accompanied by a short-lived transient state before becoming stationary. Hence if the experimental conditions are so controlled that the SBS process is not allowed to develop into the stationary state, the accompanying short-lived transient state can be used to generate narrow pulses. This can be accomplished even when the phonons are short-lived relative to the pump pulses. A passively Q-switched ruby laser was used to generate pump pulses of 25-ns width with power in the megawatt range. Experiments were carried out with sample lengths of 7, 12, and 25 cm using acetone and n-hexane. For a fixed cell length L, the pump intensity was varied giving rise to corresponding variations in the exponential gain G = gIL, where g is the stationary gain coefficient and I is the pump intensity, thereby allowing the SBS process to go from stationary to nonstationary state. At pump intensities where G values forced the SBS to be nonstationary, optical pulses with peak power in the kilowatt range are produced with width about an order of magnitude shorter than pump pulse. As cell lengths are reduced, nonstationarity developed faster resulting in narrow pulses.