SBS pulse compression to 200 ps in a compact single-cell setup

Temporal compression of nanosecond pulses from a commercial Q-switched Nd:YAG laser at second and third harmonic into the sub-nanosecond regime is demonstrated. In a geometry consisting of a single cell only, compression in several liquid media is investigated. For both 532 and 355 nm smooth and reliable operation is obtained with output pulses as short as 200 ps. A new concept is proposed to separate SBS pulses from the pump beam at low optical losses. PACS: 42.65.Es; 42.65.Hw; 42.65.Re Stimulated Brillouin scattering (SBS) is a nonlinear optical process, discovered in the first decade of laser physics, i.e. in the 1960s. For an introduction to the physics underlying SBS we refer to the book of Boyd [1]. Since its discovery the main application of SBS has been in optical phase conjugation. In the past two decades the use of SBS for the compression of laser pulses has been investigated theoretically and experimentally [2–6] for various types of lasers and active media. Dane et al. [7] demonstrated pulse compression at very high energies, and Fedosjevs and Offenberger [8] achieved compressibility by a factor of 60. As a third application of SBS we mention its use to filter spontaneous emission from a laser beam [9]. Various geometries of SBS laser-pulse compressors have been proposed, usually showing the features of a generator for a SBS-backscattered wave followed by amplification in one or more stages. Damzen and Hutchinson [10] investigated the special design of a tapered waveguide. Recently Schiemann et al. [11] reported on a compact generator–amplifier setup (CGAS) in which efficient compression of 532-nm pulses from a Nd:YAG laser was achieved. Following up on the CGAS design we present a simplified SBS scheme consisting of a single cell and using a limited number of optical components only. In this setup we demonstrate efficient compres∗ Present address: Sofia University, Faculty of Physics, J. Bourchier blvd. 5, BG-1164 Sofia, Bulgaria sion of pulses of 4–5 ns duration from an injection-seeded Nd:YAG laser at its second and third harmonics. The SBS behaviour in water, methanol, ethanol, and CCl4 is investigated. Particularly the relation between the phonon lifetime, i.e. the relaxation damping time of hypersound in the liquid, and optimum compressibility has been studied. The phonon lifetime τp is a material property of the medium, which scales with wavelength as [1, 5]: τp = τ ′ p(ν/ν). (1) Hence at shorter wavelengths as well as in CCl4 with the shortest phonon lifetime, the highest compression is expected. At both wavelengths, 532 and 355 nm, we provide a prescription to achieve pulse durations of 200 ps, which may turn Nd:YAG lasers, available in many laboratories, into more versatile tools for dynamical and nonlinear optics studies. Also we propose an alternative method for separating the compressed Stokes pulses from the incident pump beam. Usually the beams are separated by polarization; we suggest taking advantage of the frequency shift of the SBS output. 1 Experimental setup and measurement methods The geometries of relevance for the present experiments are schematically depicted in Fig. 1. In our previous work SBSpulse compression was demonstrated in a compact generator– amplifier setup (CGAS) [11], depicted in Fig. 1a. The CGAS consists of two separate cells filled with liquid: one for the generation of the SBS-Stokes pulse and the other for amplification, without use of attenuators. The underlying concept of this approach is that the necessary attenuation of the pump beam propagating towards the oscillator results from pump depletion induced by the SBS amplification. In Fig. 1b we introduce a simplified geometry based on a single cell involving a concave mirror, coated for high reflectivity at 532 or 355 nm, that redirects the generated SBS-pulse through