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Quantum Electronics and Laser Science Conference

Induced-modulation-instabilities limit to "fast" light propagation

Michael D. Stenner, Daniel J. Gauthier

Published Jun 25, 2003• 0 citations
Publisher

Abstract

Summary form only given. A recent technique for creating "fast" light pulses (i.e. pulses of light having group velocity greater than c or less than zero) uses a continuous-wave bichromatic Raman pump laser beam to tailor the dispersion properties of an atomic vapor. We have discovered a number of effects that impose limits on fast-light propagation as the gain increases. Most important is the modulation instability, a nonlinear effect that can lead to temporal modulation of intense continuous-wave laser fields. We discuss the induced modulation instability in the context of high-gain bichromatically driven atomic systems, specifically in the context of fast-light pulse propagation. We further discuss the practical limitations that the induced modulation instability imposes on this method for creating faster-than-c pulses.

DOI
10.1109/qels.2002.1031272

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Summary form only given. A recent technique for creating "fast" light pulses (i.e. pulses of light having group velocity greater than c or less than zero) uses a continuous-wave bichromatic Raman pump laser beam to tailor the dispersion properties of an atomic vapor. We have discovered a number of effects that impose limits on fast-light propagation as the gain increases. Most important is the modulation instability, a nonlinear effect that can lead to temporal modulation of intense continuous-wave laser fields. We discuss the induced modulation instability in the context of high-gain bichromatically driven atomic systems, specifically in the context of fast-light pulse propagation. We further discuss the practical limitations that the induced modulation instability imposes on this method for creating faster-than-c pulses.
We investigate the behavior of a system for generating ``fast-light'' pulses in which a bichromatic Raman pumping beam is used to generate optical gain at two frequencies and a region of anomalous dispersion between them. It is expected that increasing the gain will increase the pulse advancement. However, as the gain increases, the pumping field becomes increasingly distorted, effectively limiting the pulse advancement. We observe as much as 12% of the input pump power converted to orthogonal polarization, broadening of the initially bichromatic pump field (25 MHz initial frequency separation) to more than 2.5 GHz, and a temporal collapse of the pump beam into an erratic train of sub-500-ps pulses. The instability is attributed to the combined effects of the cross modulation instability and stimulated Raman scattering. Extreme distortion of an injected pulse that should (absent the instability) experience an advancement of 21% of its width is observed. We conclude that the fast-light pulse advancement is limited to just a few percent of the pulse width using this pulse advancement technique. The limitation imposed by the instability is important because careful study of the information velocity in fast-light pulses requires that pulse advancement be large enough to distinguish the velocities of different pulse features. Possible methods for achieving pulse advancement by avoiding the distortion caused by the instability are discussed.
There are several nonlinear optical processes that can lead to instabilities in a laser beam as it propagates through a nonlinear optical medium. Self focusing effects, for instance, can lead to the growth of new spatial frequency components in a laser beam. A laser beam can also be unstable to the growth of new spectral components. Stimulated Raman scattering is a well-known example of a process that can lead to this sort of instability. In this paper, we present the results of a series of experiments on the somewhat different instabilities that occur for the case of the propagation of a laser beam through an atomic vapor. We find that these instabilities become particularly pronounced when the frequency of the incident laser is near to that of a one- or two-photon resonance of the atom. In either case, we find that the laser beam is unstable to the growth of new frequency components, and that in general these new frequencies are not emitted collinearly with the incident laser. These instabilities are important because they place a limit on the laser intensity that can be used to drive nonlinear optical processes.
This paper presents an experiment to realize both slow and fast light effects simultaneously using the Raman gain and pump depletion in an atomic vapor. Heterodyne phase measurement shows the opposite dispersion characteristics at pump and probe frequencies. Optical pulse propagations in the vapor medium also confirm the slow and fast light effects due to these dispersions. The method being experimentally simple, and allowing the use of intense pulses experiencing anomalous dispersion via the fast light, can be applied in rotation sensing and broadband detection schemes proposed recently.
Abstract We study light-pulse propagation in a dynamically controllable periodic structure (grating) resulting from Raman interaction of a weak probe pulse with a standing-wave pump and a second control laser field in N-type four-level atomic media. The grating is induced due to periodic spatial modulation of the Raman gain in a standing pump field (Raman gain grating). We show that it is possible to control both the probe pulse amplitude and the group velocity of the pulse from subluminal to superluminal by varying the pump or control field. Such a grating is of interest for all-optical switches and transistors.