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Physical Review Letters

Time-resolved measurements of stimulated-Brillouin-scattering phase jumps

Metin S. Mangir, John J. Ottusch, D. Crispin Jones, David A. Rockwell

Published Mar 16, 1992• 18 citations
Publisher

Abstract

We report time-resolved measurements of the phase of the Stokes beam produced by stimulated Brillouin scattering in gases. We observed phase jumps, slow phase drifts, and sudden frequency shifts during the temporal evolution of individual Stokes pulses. Phase jumps, and the intensity variations that accompany them, are macroscopic manifestations of the noise that initiates and sustains stimulated scattering.

DOI
10.1103/physrevlett.68.1702

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We report time-resolved measurements of the phase of the Stokes beam produced by stimulated Brillouin scattering in gases. We observed phase jumps, slow phase drifts, and sudden frequency shifts during the temporal evolution of individual Stokes pulses. Phase jumps, and the intensity variations that accompany them, are macroscopic manifestations of the noise that initiates and sustains stimulated scattering.
The stochastic nature of initial stimulated Brillouin scattering (SBS) can lead to a deep modulation of Stokes intensity in fibers, even under conditions of strong pump depletion. It is known that at a moderate pump depletion, when parameter T1/T2 ≫ 1 (T1 is the light propagation time through the effective amplification length and T2 is the hypersound relaxation time), fluctuations correspond to the Gaussian statistics of temporal field distribution. As T1/T2 → 1 the statistics of fluctuations can change. Gaeta and Boyd1 were the first to show that at a sufficient pump depletion (by our estimate, parameter T1/T2 in Ref. 1 was ~4) the correlation function of the Stokes intensity c(τ) = (〈I(t)I(t + τ)〉 - 〈I(t))2)/(/(t)〉2 differs from the Gaussian by a more rapid initial fall and the presence of the region in which it is negative.
Manifestations of quantum fluctuations in stimulated Raman scattering ( SRS ) have been intensively studied lately1. Quantum noise present in spontaneous scattering stimulates fluctuations both in phase and in amplitude characteristics of output Stokes emission. At present statistical properties of amplitude fluctuations are studied in detail and it is shown that in linear transient regime of SRS without pump depletion statistical scatter of Stokes energy Es has negative exponential distribution2. In saturation regime of SRS the amplitude fluctuations are almost suppressed simultaneously with pump depletion. The phase fluctuations manifest both in linear and in nonlinear SRS and produce beam pointing fluctuations3, a change in Stokes emission spectra from pulse to pulse4 and cause soliton appearance in Rayleigh component of SRS5. Phase noise is observed in saturation regime due to relation between phases of Stokes and phonon waves which is preserved in processes of stationary and quasistationary SRS amplification. Spectrum phase fluctuations manifest as fine structure of SRS spectrum4, fluctuations of average Stokes frequency and Stokes linewidth fluctuations. The evolution of phase fluctuations in time is determined by the nature of spontaneous scattering and polarization of Raman media and is of great interest as one can follow its development from vacuum modes level to saturation regime. However up to now this problem has not been studied yet.
A pencil shaped region of a Raman-active medium forms a source for Stokes-shifted light via stimulated Raman Scattering (SRS) when driven by an intense laser pulse. Stokes photons are scattered spontaneously at first, and then amplified during propagation through the medium. Since the scattering is initiated by quantum noise, the actual number of photons produced in any given pulse will be highly uncertain, even if the pulse energy of the driving laser is constant1. As the Stokes light propagates through such an extended region, it's coherence properties will be changed. The spatial coherence is determined by the size and shape of the region, which may be characterized by its Fresnel number F, defined as F= A/λSL, where A is the cross-sectional area of the interaction region, which has length L, and λS is the Stokes wavelength. The rate at which the microscopic scatterers (atoms or molecules) are dephased by collisions affects the temporal coherence of the Stokes light, characterised by the temporal coherence parameter Γτ /gL, where Γ is the collisional dephasing rate, τ is the laser pulse duration and g is the steady-state Raman gain coefficient.
Stimulated Raman scattering (SRS) produces Stokes light pulses that originate from quantum noise and exhibit macroscopic amplitude and phase fluctuations from one shot to another.1 By using transient stimulated Raman scattering, we generated light pulses with macroscopic energy and temporal coherence times that were long compared to their duration. Stokes pulses were generated independently in each arm of an interferometer and were interfered together at the output. The position, modulation depth, and brightness of the interference fringes fluctuated randomly from one shot to the next. We obtained both the ensemble-averaged intensity and the fourth-order intensity correlation function of the interfered Stokes light. The average has a smooth envelope and shows no modulation, whereas the intensity correlation function has a depth of modulation of 0.35 for Stokes generation in the unsaturated (thermal-like) regime and 0.46 in the saturated (laser-like) regime. This is in agreement with predicted values of 0.33 and 0.5.2
Results from a study of quantum noise and its macroscopic manifestations in stimulated Raman scattering are presented.Shot to shot fluctuations in quantum noise initiated Stokes spectra are considered.The Stokes spectra were recorded experimentally using a Fabry-Perot interferometer.For comparison, numerically generated Stokes spectra were produced using the semi-classical model of stimulated Raman scattering with a numerically generated "vacuum" input field.The amount of structure on each spectrum was quantified by calculating the spectrum's standard deviation from its mean frequency.It was found that ensembles with the noisy spectra removed had narrower distributions of mean frequencies than ensembles that included noisy spectra.