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Physical Review A

Raman gain suppression with multimode lasers

Richard M. Heinrichs, Irwin C. Winkler

Published Jan 1, 1990• 1 citation
Publisher

Abstract

We have found the gain of a Raman amplifier to be almost completely suppressed when the amplifier is pumped by a multilongitudinal mode laser and seeded with an initially uncorrelated Stokes beam that is nearly as intense as the pump. This effect is predicted by a plane-wave model of Raman amplification that accounts for multimode pump and Stokes fields. Our experiments have also demonstrated that the beam quality of the pump is maintained to the extent that the amplification is suppressed.

DOI
10.1103/physreva.41.395

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We have found the gain of a Raman amplifier to be almost completely suppressed when the amplifier is pumped by a multilongitudinal mode laser and seeded with an initially uncorrelated Stokes beam that is nearly as intense as the pump. This effect is predicted by a plane-wave model of Raman amplification that accounts for multimode pump and Stokes fields. Our experiments have also demonstrated that the beam quality of the pump is maintained to the extent that the amplification is suppressed.
Measurements of the output of a Raman amplifier when there is no input Stokes seed are presented.It is shown that a fully quantum mechanical scaled planewave theory describes the growth from spontaneous emission well in the high gain limit.However, in the low gain regime there is a significant increase in the amount of Stokes output which is not predicted by the plane wave theory.To account for the deviation from planewave growth observed at low gain, a fully quantum mechanical theory of amplification in a focused gain geometry is presented.The theory is based on a non-orthogonal mode expansion of the amplified field and incorporates the effects of both gain-guiding and diffraction.In addition to accounting for the observed Stokes output at low gain the non-orthogonal mode theory predicts that the Stokes field cart experience enhanced gain as well as have excess noise.Results from an experiment are presented which verify the existence of enhanced gain in a multipass Raman amplifier.Though the experiment does not give a direct measure of the excess noise factor the results
The quantum theory-of Stokes generation by stimulated Raman scattering of a multimode laser is developed and analyzed. For large laser-mode spacing, the generated Stokes intensity is found to be independent of the number of laser modes for both high and low gains. The result applies to both free-running and mode-locked lasers and is in contrast to that in the Raman amplifier, in which gain is found to depend on the number of modes. The Stokes light is generated with mode amplitudes and phases in perfect correlation with those of the laser. Also, in steady state the instantaneous Stokes intensity is found to follow exactly the rapid fluctuations of the laser intensity.
We have studied the phase pulling and locking of an injected Stokes pulse in a transient Raman amplifier both experimentally and theoretically. In addition, we have investigated the pulse energy statistics and related spatial modes of a high gain amplified Stokes pulse. Experiments reported here show that in the unsaturated case, the phase of the amplified Stokes is pulled almost instantaneously to that of the pump, as soon as integrated gain levels of slightly greater than unity are reached. For higher input seed-Stokes levels, such that the amplified Stokes energy is saturated, the phase locking is completed early, and the phase of the amplified Stokes then follows the phase excursion of the pump. The phase pulling has been modelled theoretically for the specific parameters of our experiment. We have also used the pulse energy statistics to study the seed-Stokes control of a high gain Raman amplifier. For an input seed-Stokes beam to control a transient, high gain Raman amplifier, operated near threshold, it must dominate the self-generated Raman which builds from quantum noise. For a near-unity-Fresnel-number Raman amplifier we have measured the number of input seed-Stokes photons needed to produce an unchanged statistical signature on the amplified Stokes pulse and have found that a surprisingly large number (109) were needed. For a larger Fresnel number Raman amplifier, a near-field camera was used to study the level at which the seed-Stokes pulse controlled the spatial output of the amplifier. Control of the spatial modes seems to have a sharp threshold with input Stokes energy and was fully established with an input of 2x103 photons per mode.
We have confirmed, for the first time to our knowledge, the nonexponential character of Raman amplification at the phase matching angle ϕ for which 2kL = ks + kA. The experiments reported here used spatially filtered pump and probe beams at 532 and 633 nm, respectively, of 30-ps duration interacting in a one-half meter cell filled with deuterium at a pressure of 600 psi. The pump and probe beams were crossed at various small angles in the cell and the gain experienced by the probe pulse was measured. At angles below 5 mrad the Stokes amplification was exponential in character and reached levels of 107. At an angle of 7 mrad, the phase matching angle for our conditions, there was no observed amplification of the Stokes pulse. At yet larger angles (~9 mrad) the gain was again exponential in character and reached levels of 107. We have also observed a double ring pattern in the anti-Stokes radiation produced in our experiment. This pattern was predicted by Bloembergen1 with the explanation that a single anti-Stokes ring is split by a region of gain suppression at the phase matching angle. We have verified that the diameter of the dark band in the anti-Stokes emission grows as the square root of the pressure, exactly as would be predicted assuming that the dark region is caused by gain suppression at phase matching.
The development of the Stokes spectrum and the dependence of Raman gain upon pump laser bandwidth in a narrow linewidth Raman medium are analyzed in terms of a closed set of multimode equations. The analysis predicts that the Stokes gain coefficient takes on its maximum value, which is independent of bandwidth, when the Stokes spectrum replicates the pump spectrum. It is also shown that in the absence of dispersion the spectrum of a weak Stokes wave is driven to duplicate the pump spectrum, and that for noise input the average power generated in the Stokes wave is independent of the pump bandwidth.
We have studied the transverse modes of a transient stimulated Raman signal generated in unsaturated Raman amplifiers with Fresnel numbers from one to seven, both with and without a seed Stokes pulse. In the absence of a seed Stokes pulse and for a pump Fresnel number greater than 1.5, the spatial intensity pattern of the stimulated Stokes signal is highly variable from shot to shot, reflecting the random nature of the quantum-noise source. However, when a separately generated and sufficiently strong seed Stokes pulse with a Gaussian spatial profile is injected into the amplifier, the spatial mode of the output Stokes beam assumes a smooth Gaussian spatial pattern that is significantly smaller in diameter than either the pump or the seed Stokes beam. This spatial mode control, which is due to the injected Stokes signal, persists down to a level of approximately 300 seed Stokes photons per spatial mode and, surprisingly, is observed to suppress the noise-initiated signal by at least a factor of 10 in the outer regions of the amplified Stokes beam. We have also studied the Stokes pulse-energy statistics over a range of Fresnel numbers. Data taken with systems having Fresnel numbers between 4 and 7 follow the same pattern as the spatial-mode-control results in that the statistical character of the Stokes pulse energies changes rapidly from that characteristic of a quantum-noise-initiated process to one characteristic of a stable source as the level of the injected seed Stokes is increased. However, for interaction regions with Fresnel numbers of 1 or smaller, the Stokes pulse-energy statistics show a gradual change as the level of the seed Stokes signal is increased and do not become stabilized until the seed Stokes is more than 100 times the total level needed to control the spatial character of the output Stokes in the larger-Fresnel-number cases. This slow change in the statistics for the Fresnel 1 system, and the suppression of the quantum-noise-initiated signal in the larger-Fresnel-number systems, is not fully understood at this time.