Application of efficient ultraviolet excimer lasers such as the 248 nm KrF laser to laser fusion requires that long laser pulses be efficiently converted to short pulses at high intensity. The backward Raman amplifier is shown to be a promising candidate for this application. Gain, saturation, and limits to amplifier performance are described. It is shown that pump beams of poor spatial quality may be converted to output beams of high spatial quality. Several common gaseous vibrational Raman scatterers are discussed, and it is shown that a simple KrF-pumped backward Raman amplifier using methane at atmospheric pressure will have a saturation fluence near 1 J/cm2and can produce an output five times as intense as the pump in a ten times shorter pulse with an efficiency of about 50 percent. Design tradeoffs and possible techniques for further improving the performance of such amplifiers are discussed.

Raman pulse compression of excimer lasers for application to laser fusion

We have studied experimentally stimulated Raman scattering and 4-wave-interaction processes occuring in hydrogen gas, which is excited by a Nd:YAG-pumped dye laser. The tuning ranges of the various Stokes and anti-Stokes lines, which are generated with high efficiency, cover the spectrum from 185 to 880 nm without a gap.

Tunable coherent radiation source covering a spectral range from 185 to 880 nm

We present results on stimulated Raman scattering in hydrogen excited by 200-fs-duration 1-kHz repetition-rate and 0.6-W average-power Ti:sapphire (780-nm) and 0.2-W average-power second-harmonic (390-nm) laser pulses with linear and circular polarizations. Spectral and temporal measurements of the vibrational and rotational components at Stokes and anti-Stokes frequencies in the wavelength range of 289–1154 nm are reported. We observe threefold shortening of the first vibrational Stokes component pulses, excited by 400-fs-duration second-harmonic pulses.

Stimulated Raman scattering in hydrogen by frequency-doubled amplified femtosecond Ti:sapphire laser pulses.

We have examined the utility of a gas-filled, Nd:YAG-laser-pumped Raman shifter as a possible broad-spectrum light source. Six to nine new output frequencies with pulse energies above 1 µJ are produced when a pure-hydrogen or pure-methane Raman shifter is pumped with 40 mJ of secondharmonic, 20 mJ of third-harmonic, or 11 mJ of fourth-harmonic pump pulse energy. Optimum output occurs at pressures of approximately 10 atm for the pure-gas experiments. We also report the output frequencies and pulse energies of a mixed hydrogen-methane Raman shifter pumped by 20 mJ of the third harmonic of the laser for various proportions of the two gases at pressures up to nearly 20 atm. Depending on composition and pressure, over a dozen new output lines with pulse energies over 1 µJ can be produced. We discuss the nonlinear processes involved, the optimum operating conditions, and the suitability of the source for our application of groundwater monitoring.

Evaluation of Nd:YAG-pumped Raman shifter as a broad-spectrum light source

Tuned narrow-bandwidth UV radiation at 193 nm is generated by SRS of dye laser radiation and is amplified in ArF sections. In a subsequent second hydrogen Raman cell anti-Stokes radiation up to the sixth order (around 130 nm) is generated. Power measurements and calibration are described.

Amplification of 193 nm radiation in argon-fluoride and generation of tunable VUV radiation by high-order anti-Stokes Raman scattering

The results are given of an experimental investigation of parametric processes which govern the structure of the angular distribution of the intensity of the second Stokes and two anti-Stokes components of the stimulated Raman scattering in liquid nitrogen. The investigation was made using a collimated pump beam with a large transverse cross section. The angular structure of the first Stokes component included a highly directional peak against the background of diffuse radiation. These conditions made it possible to observe simultaneously for the first time three types of parametric generation at the second Stokes frequency. The observed angular spectra of the Raman components were explained on the basis of the theory of parametric interactions between plane waves. The reliable interpretation of the angular spectra and the high experimental precision made it possible to correct and extend the long-wavelength edge of the dispersion curve of liquid nitrogen at T = 77 °K.

L L Kulyuk

Papers

Experimental investigation of mechanisms of parametric generation of stimulated Raman scattering components