Synchronously pumped H2 Raman laser
Introduction
The Raman effect is a widely used tool to frequency shift laser radiation. Energy conversion efficiencies of 60% and higher have been observed in gases, liquids and solid state materials 1, 2, 3. Depending on the laser pulse duration and response time of the nonlinear optical material one distinguishes between transient and stationary stimulated Raman scattering (SRS). In the stationary limit the coherence time of the Raman mode, T2, is much smaller than the pump pulse duration τp. SRS is a threshold process that, in the stationary limit, requires the Raman gain eG=egℓIp≥e30 [4], where g is the Raman gain coefficient, ℓ is the interaction length and Ip is the pump intensity. Transient effects in SRS become important if τp<GT2, see for example Ref. [4]. Transient SRS usually requires a greater pump intensity.
The threshold for SRS can be reduced considerably by placing the Raman material inside a resonator for the Stokes radiation. In stationary scattering, the ratio of pump intensities required for extracavity SRS and lasing can be estimated as −30/ln R, where R is the cavity feedback (mirror reflectivity) for the Stokes radiation. Various Raman lasers have been realized successfully, among them fiber lasers [5], solid state Raman lasers [6], and Raman lasers with a gas as the nonlinear medium [7]. Raman conversion in gases is of particular interest for the frequency shifting of high-energy radiation. Raman laser configurations are necessary to convert long pulses or pulse trains whose power is below that required for extracavity SRS, or to convert cw laser radiation [8]. Raman lasers pumped by a train of pulses (synchronous pumping) have been developed using liquids, solids, optical fibers, hollow waveguides filled with gases, and gas-in-glass fibers as nonlinear material 9, 10, 11, 12, 13. We report on the, to best of our knowledge, first synchronously pumped Raman laser based on rotational scattering in bulk gas H2 [Δ(1/λ)≈587 cm−1] in the transient regime and the generation of various Stokes components. The experimental results are compared with a new theoretical model of a synchronously pumped laser based on the transient Raman equations that takes into account multiple higher order Stokes waves. Particular emphasis is devoted to the features arising from the transient material response and the general impact on synchronously pumped Raman lasers.
Section snippets
Experimental
The experimental layout of the Raman laser is sketched in Fig. 1. A Q-switched Nd:YAG (1.06 μm) laser was modelocked by an acousto-optic modulator driven at 65 MHz. The resulting short pulse train consists of about 15 individual pulses. The pulse spacing is 7.5 ns and the pulse duration was about 1.5 ns. These pulse parameters allowed us to study the shape of individual pulses and the relative timing of pump and Stokes pulse trains with a GHz digital oscilloscope in combination with a 300 ps
Theoretical model and comparison with experiment
The computer simulation traces the circulating Stokes pulses in the cavity and their interaction with the pump pulse train. The Raman interaction in the transient regime is described by the following set of equations for the complex amplitudes of the pump (index p) field, the Raman fields [index iS (iAS) where i is the order of the Stokes (anti-Stokes) component] and the material excitation Q, see for example [15].
Conclusion
Raman lasers, even when pumped with short pulse trains, exhibit thresholds that are substantially below the threshold for extracavity conversion. We have demonstrated a synchronously pumped Raman laser in H2 with a threshold of 10 mJ, as compared to the threshold for extracavity scattering, 40 mJ, using a pump train consisting of 15 pulses. Even though the computer model does not take into account many effects, such as the gaussian beam profile, these numbers compare favorably to those obtained
Acknowledgements
The authors thank Dr. G. Hager and Dr. J. McIver for helpful discussions. This project was supported by AFRL (#98K0038).
References (15)
- et al.
Solid-state barium nitrate Raman laser in the visible region
Opt. Commun.
(1997) - et al.
A synchronously pumped waveguide CH4 Raman laser at 1.54 μm
Opt. Commun.
(1988) - et al.
Stimulated Rotational Raman Conversion in H2, D2, and HD
IEEE J. Quantum Electron.
(1993) - et al.
Efficient Raman frequency conversion in liquid nitrogen
IEEE J. Quantum Electron.
(1982) - et al.
Conversion of tunable radiation from a laser utilizing an LiF crystal containing F2- color centres by stimulated Raman scattering in Ba(NO3)2 and KGd (WO4)2 crystals
Sov. J. Quantum Electron.
(1987) - J.C. White, Stimulated Raman Scattering, in: L.F. Mollemauer, J.C. White (Eds.), Tunable Lasers, Springer, Heidelberg...
- et al.
Laser-diode-pumped phosphosilicate-fiber Raman laser with an output power of 1 W at 1.48 μm
Opt. Lett.
(1999)
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Present address: Department of Gas Lasers, Institute of Physics, Academy of Sciences, 182 21 Praha, Czech Republic.