Bradley Bobbs

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)].

Microwave phase conjugation via degenerate four-wave mixing in an artificial Kerr medium

Collinear microwave phase conjugation was observed in an artificial Kerr medium consisting of short graphite fibers suspended in a binary liquid mixture. Using an 18 GHz pump beam with up to 20 W continuous power, characterization of the changes in the 94 GHz refractive index were made by interferometry. A nonperturbative method for describing the response of the medium was used to analyze the phase-shift measurements for the static berefringence and the time response as functions of microwave intensity.

Angular Compensation For Multiline Dispersion In Raman Amplifiers

In a Raman amplifier pumped by a multiline laser, it is well-known that dispersion in the Raman medium may decouple the Raman gains of the lines from each other. The small-signal gain of each line is then reduced by the fraction of the total pump power in each line. If these gains are very different from each other, then the overall conversion efficiency to the Stokes-shifted output may be limited by the difficulty in optimizing the amplifier cell length for all lines simultaneously. We describe here how the gains may be completely coupled together to solve these problems by adjusting the relative angles between the input beams so as to compensate for medium dispersion, and thereby eliminate the four-wave mixing phase mismatch. The theoretical basis for the effect, different geometries for achieving coupling, and computer simulation results for sample cases are presented.

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.

The zigzag broadband Raman amplifier

The use of angle-tuned phase matching can greatly reduce limitations on the pump laser bandwidth in crossed-beam Raman amplifiers. Potential advantages over collinear broadband amplifiers predicted to be attainable with this technique include improved intensity homogeneity, reduction of intensity-phase print-through, simplified beam combining, and avoidance of competing nonlinear processes. Expressions for allowed bandwidth are given.

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.