Mark D. Skeldon

Effect of laser mode structure on stimulated Brillouin scattering

The gain and reflectivity of the stimulated Brillouin scattering (SBS) process used as a Stokes-wave generator are shown theoretically to be independent of the mode structure of the pump laser provided that the pump-laser mode spacing exceeds the Brillouin linewidth and that the laser coherence length exceeds the characteristic gain length of the SBS process. Under the same set of conditions, the gain of an SBS amplifier is found to depend on the degree of correlation between the laser and Stokes fields. However, due to nonlinear coupling, these two fields become correlated within several characteristic gain lengths, and the subsequent propagation of the two fields is governed by the same set of equations that apply for the case of a single-mode pump laser. These theoretical predictions are tested experimentally for an SBS generator using acetone, carbon disulfide, and methanol as the Brillouin-active media, and the results are in full agreement with the theoretical predictions.

Non-frequency-shifted, high-fidelity phase conjugation with aberrated pump waves by Brillouin-enhanced four-wave mixing

The results of an experimental investigation of a new geometry for producing phase conjugation by Brillouin-enhanced four-wave mixing are presented. In this geometry, the four-wave mixing medium is carbon disulfide, and the backward-going pump wave is created from the transmitted forward-going pump wave by stimulated Brillouin scattering (SBS) in glycerol. The two pump waves are hence phase conjugates of each other, and the quality of the phase-conjugation process is not degraded even by the use of an aberrated pump wave. The probe wave is created by SBS in carbon disulfide, which has a Brillouin frequency half that of glycerol, and the conjugate wave is therefore generated at the same frequency as the probe. Since the pump and signal waves differ in frequency by the Brillouin frequency of the carbon disulfide four-wave mixing medium, high reflectivities (approximately 2000%) are obtained as a result of Brillouin resonance enhancement.

TEM 00 -mode and single-longitudinal-mode laser operation with a cholesteric liquid-crystal laser end mirror

We describe the unique properties of a cholesteric liquid crystal as a laser end mirror. We show how it gives rise to TEM(00)-mode operation as well as single-longitudinal-mode operation of a solid-state laser resonator.

Optical Pulse-Shaping System Based on an Electro-optic Modulator Driven by an Aperture-Coupled-Stripline Electrical-Waveform Generator

An optical pulse-shaping system using an electro-optic modulator driven by an aperture-coupled-stripline (ACSL) electrical-waveform generator has been developed. The ACSL electrical-waveform generator consists of two electrical striplines stacked and coupled through an aperture layer. By changing the aperture layer, the pulse-shaping system is capable of producing arbitrarily shaped optical pulses with 100-ps structure over a 1- to 5-ns pulse envelope.

Efficient harmonic generation with a broad-band laser

Harmonic generation using a broadband input beam is studied theoretically and experimentally. Angular spectral dispersion is imposed on the input beam to improve the phase matching and thereby increase the conversion efficiency. The coupled amplitude equations are derived for a grating-dispersed input beam, and it is shown that, in the parameter range of interest, a simple theoretical model based on the intuitive concept of instantaneous frequency suffices to describe the nonlinear interaction. A laser having a frequency-chirped output with a 25-AA bandwidth is used to demonstrate the technique for third-harmonic generation in the small-signal regime. >

Stimulated rotational Raman scattering in air with a high-power broadband laser.

We study the growth of stimulated rotational Raman scattering (SRRS) from noise by propagating one beam of the OMEGA fusion laser over a 35-m path in air. The spectrum at the end of the path is measured and calibrated to give the energy scattered into all the Stokes lines. Pure phase-modulated bandwidth as well as the bandwidth known as smoothing by spectral dispersion (SSD) are installed on the laser. The experimental results show that the threshold for SRRS is unchanged when one uses a pure phase-modulated bandwidth and increases when one uses a SSD bandwidth.

Pulse-shaping system for the 60-beam, 30-kJ (UV) OMEGA laser

The optical pulse-shaping system for the 60-beam 30-kJ (UV) OMEGA fusion laser is capable of producing complex temporally shaped optical pulses for amplification and delivery to fusion targets. The pulse-shaping system consists of optical modulators driven by an optically activated electrical waveform generator. The electrical waveform generator consists of Si photoconductive switches, and variable impedance microstrip lines. Complex optical pulse shapes with 50 to 100 ps structure have been produced.

Modeling the temporal-pulse-shape dynamics of an actively stabilized regenerative amplifier [OMEGA Nd:YLF laser rod]

We have modeled the temporal evolution of a shaped optical pulse injected into a feedback-stabilized regenerative amplifier (regen). We solve the coupled-rate and energy-transport equations including upper- and lower-laser-level lifetimes explicitly. The regen dynamics including the losses due to the feedback stabilizer are included in the model. We provide a prescription for determining the injection-pulse shape required to produce a given output-pulse shape from this regen. The model shows excellent agreement with the measured regen output.

Transverse-mode structure of a phase-conjugate oscillator based on Brillouin-enhanced four-wave mixing

A phase-conjugate oscillator consisting of a phase-conjugate mirror and a conventional mirror is discussed. The phase-conjugate mirror is based on a Brillouin-enhanced four-wave mixing process and produces a conjugate wave with no frequency shift and with a reflectivity greater than 100%. The beam divergence and near-field spot size of this oscillator have been measured for various cavity lengths and conventional mirror radii of curvature. A theoretical analysis of the mode structure of this oscillator has been performed assuming a Gaussian reflectivity profile for the phase-conjugate mirror. The measurements are in good agreement with the predictions of this model. >

Spatial evolution of laser beam profiles in an SBS amplifier

We have performed an experimental and theoretical analysis of the modification of the transverse structure of a probe beam on propagation through an SBS amplifier. The theoretical analysis is based on performing a mode decomposition of the incident fields and determining how each mode amplitude is modified as a result of the nonlinear coupling. For the case of input beams with Gaussian intensity profiles and in the limit of negligible pump depletion, we find that the input probe profile is preserved during amplification only when the probe spot size is smaller than that of the pump in the focal region.