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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Simultaneous generation of slow and fast light for Raman coupled beams

G. S. Pati, M. Salit, K. Salit, S. Tseng, M. S. Shahriar

Published Feb 7, 2008• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.772236

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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.
We have recently proposed [9], the use of fast-light media to obtain ultrahigh precision rotation sensing capabilities. The scheme relies on producing a critically anomalous dispersion, in a suitable dispersive medium, which is introduced in the arms of a Sagnac interferometer. We present here an experimental investigation of the anomalous dispersion properties of bi-frequency Raman gain in rubidium vapor, with the goal of using this medium for producing the critically anomalous dispersion condition. A heterodyne phase measurement technique is used to measure accurately the index variation associated with the dispersion. The slope of the negative linear dispersion (or group index) is experimentally varied by more than two orders of magnitude while changing the frequency separation between pump fields, responsible for producing gain. Using this result, we have identified the experimental parameters for achieving a null value of the group index, corresponding to the critically anomalous dispersion condition necessary for enhanced rotational sensitivity.
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.