Charles Warner

Raman-resonant four-wave mixing and energy transfer

A common assumption that Raman-resonant four-wave mixing does not transfer energy between the light and the Raman medium is shown to be incorrect. The derivation of the correct energy-transfer picture is compared with those for stimulated Raman scattering and for nonresonant mixing. The physical mechanism for Raman-gain suppression due to Stokes/anti-Stokes coupling under phase-matched conditions is clarified by this understanding of the energy transfer. An experimental demonstration of the nature of the energy transfer is proposed.

Effects of off-resonant Raman interactions on multimode Stokes conversion efficiency and output wave front

Interactions involving frequency differences that are near but not equal to the Raman shift are considered in a model of stimulated Raman conversion of a laser beam with many longitudinal modes. A formalism based on expansions in mode space is developed and used to predict the effects on output wave front and conversion efficiency. Closed-form solutions are obtained for the case of detuned inputs with large mode spacing and for low conversion cases. Numerical integration results are shown for low to high conversion examples in which only first-order off-resonant terms are important.

Absence of second Stokes in a Raman generator with no four-wave mixing

A simple one-dimensional model is given for Stokes-beam amplification and expansion in a Raman generator (i.e., a stimulated Raman scattering cell with no seed inputs or feedback) for those cases in which four-wave mixing is suppressed by phase mismatching. Expansion of the first Stokes beam decreases the gain for growth of a second Stokes beam. This model accounts for the absence of second Stokes observed experimentally by Carlsten et al. [Opt. Lett. 9, 353 (1984)].

Simple model for optical extraction from a flowing oxygen-iodine medium using a Fabry-Perot resonator

A loaded-gain model of a CW flowing oxygen-iodine medium is described, and its principal assumptions and approximations are discussed. The model includes pumping of the upper laser level by O(2)(1 Delta), deactivation losses by water, and stimulated emission. The solution of the model in the absence of flux is obtained, from which a small-signal gain is determined. The model is solved for the output power from a Fabry-Perot resonator as a function of the medium and resonator parameters. It is shown that the maximum available power from the medium is determined by the O(2)(1 Delta) concentration at the nozzle exit plane.

Unstable Resonator Studies For A 1-Joule Per Pulse KrF Avalanche Discharge Laser

Experiments on output beam divergence for a 1-Joule KrF laser with M = 4 and M = 10 unstable resonators for 112.5 cm, 187.5 cm and 225 cm separation are presented. Results gave beam divergence no better than some 30 times diffraction limited in the best case.

Closed-Form Solution For Second Stokes Generation In Raman Amplifiers

The efficiency of a first Stokes Raman amplifier may be seriously reduced by the generation of a second Stokes beam. The dominating process for this generation is often a Raman four-wave mixing interaction which couples the pump and second Stokes beams. A one-dimensional model for this effect has a closed-form solution, simply described in terms of an equivalent input, under conditions where high first Stokes conversion efficiency is possible. Excellent agreement with numerical integration results has been obtained.

Raman gain suppression, four-wave mixing, and energy transfer

The physical mechanism for Raman gain suppression due to Stokes/anti-Stokes coupling under phase matched conditions is clarified by a discussion of energy transfer in Raman resonant four-wave mixing. The fallacy of a common misconception that this mixing does not transfer energy between the light and the Raman medium is illustrated.

Raman Generator Modeling

Stokes seed production in a Raman generator has been modeled for a pump of long duration and with many longitudinal modes. The model uses three-dimensional wave optics and includes pump depletion. The modeling is similar to that of Lewenstein in its use of stochastic c-number equations. The model simulates the spontaneous Stokes source with effective Stokes fields supplied in a prescribed manner. The analysis to support this simulation and examples of computer results will be presented.