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Physical Review Letters

Beam-pointing fluctuations in gain-guided amplifiers

S. J. Kuo, D. T. Smithey, M. G. Raymer

Published May 20, 1991• 20 citations
Publisher

Abstract

We report a measurement of the quantum-mechanical beam-pointing fluctuations in the output of a gain-guided, single-pass amplifier using stimulated Raman scattering. The experimental result is in reasonable agreement with a theoretical model that incorporates the effects of excess spontaneous noise associated with the transverse modes of a gain-guided amplifier.

DOI
10.1103/physrevlett.66.2605

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Quantum-mechanical pointing fluctuations in stimulated Raman scattering

S. I. Kuo, D. T. Smithey, M. G. Raymer +1 more

We have observed fluctuations in the direction of propagation of light pulses generated by stimulated Raman scattering (SRS). This fluctuation is analogous to the pointing fluctuation of laser beams,1 and is due to quantum mechanical uncertainties in the occupation numbers of different spatial modes. When SRS is generated in an interaction region with Fresnel number less than unity, the lowest order spatial mode dominates the generation process, but higher order spatial modes are also weakly and randomly excited. This results in small fluctuations in the wavefront of the light field. The direction of propagation is characterized by the transverse component of an average wave-vector determined by the gradient of the wave-front. We report experimental results by measuring the "center of mass" of the far-field intensity pattern of a single Stokes beam, and the fringe separation of an interference pattern formed by superposing two independently generated Stokes pulses. The experimental results are compared to theory.2

Annual Meeting Optical Society of America•Jan 1, 1990•0 citations
We report a measurement of the quantum-mechanical beam-pointing fluctuations in the output of a gain-guided, single-pass amplifier using stimulated Raman scattering. The experimental result is in reasonable agreement with a theoretical model that incorporates the effects of excess spontaneous noise associated with the transverse modes of a gain-guided amplifier.
We present a measurement of the beam-pointing fluctuations of a transient Raman-amplifier output using an interferometric method and a theoretical treatment of the three-dimensional, spatial propagation of transient stimulated Raman scattering (SRS) including the fluctuations of the Stokes field giving rise to the beam-pointing fluctuations. The theory is based on an effective steady-state model for the transient SRS process. By using nonorthogonal transverse modes to expand the field, we incorporate the effects of excess spontaneous-emission noise associated with gain-guided amplifiers. The agreement of the measurements with the theoretical treatment demonstrates that the beam-pointing jitter is caused by quantum noise. The beam-pointing fluctuations increase as the Fresnel number of the interaction volume increases because more spatial modes are excited.
The gain-guided nature of an amplifier can lead to more noise in the output than would be predicted based on the quantum noise limit of 1 photon per mode. We report the results of an experiment which demonstrate that the gain-guided nature of a focused-gain Raman amplifier can lead to excess noise in the amplified field. The results are compared with a theoretical analysis of the output of an amplifier which incorporates the effects of gain guiding as well as the focused nature of the gain profile.
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
Gain-guiding in an optical amplifier can have a significant effect on the quantum noise properties of the amplified signal. It has been demonstrated, both theoretically and experimentally, that gain-guiding in a steady state Raman amplifier can lead to excess noise in the output signal. Experiments using a Raman amplifier operating in the extreme transient regime also indicate that gain-guiding can have a significant effect on the output Stokes field. In this paper, a theoretical description of transient stimulated Raman scattering in a focused gain geometry is developed. The theory accounts for diffraction, gain-guiding, and quantum initiation.
The non-Hermitian character of a gain-guided amplifier leads naturally to a description of the amplified field in terms of a nonorthogonal mode basis. We present a theoretical analysis, based on a nonorthogonal mode expansion, for the output of an amplifier with focused gain. The theory predicts that the gain-guided nature of the amplification process can lead to excess noise in the output of the amplifier. We then report the results of two experiments in which the output from a focused-gain Raman amplifier was measured. The experimental results are found to be in reasonable agreement with the nonorthogonal mode theory.
We have observed fluctuations in the direction of propagation of light pulses generated by stimulated Raman scattering (SRS). This fluctuation is analogous to the pointing fluctuation of laser beams,1 and is due to quantum mechanical uncertainties in the occupation numbers of different spatial modes. When SRS is generated in an interaction region with Fresnel number less than unity, the lowest order spatial mode dominates the generation process, but higher order spatial modes are also weakly and randomly excited. This results in small fluctuations in the wavefront of the light field. The direction of propagation is characterized by the transverse component of an average wave-vector determined by the gradient of the wave-front. We report experimental results by measuring the "center of mass" of the far-field intensity pattern of a single Stokes beam, and the fringe separation of an interference pattern formed by superposing two independently generated Stokes pulses. The experimental results are compared to theory.2