Induced-Modulation-Instabilities Limit
to “Fast” Light Propagation
Michael D. Stenner and Daniel J. Gauthier
Duke University, Department of Physics, Box 90305, Durham, NC 27708
Voice: (919) 660-2511, FAX: (919) 660-2525, gauthier@phy.duke.edu
Abstract: We observe a fantastic spectral broadening of a bichromatic field propagating
through a potassium vapor due to a modulation instability, which places severe restrictions
on gain-assisted “fast-light” pulse propagation.
c 2001 Optical Society of America
°
OCIS codes: (190.5530) Pulse propagation and solitons; (030.1670) Coherent optical effects
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. It does this by creating two gain features with similar frequencies. Between these gain
features, the phase velocity dispersion is anomalous and the group velocity dispersion can be minimized. As
a result, a pulse traveling through the medium will experience so-called “fast-light” propagation with little
distortion if its frequency falls between these two gain features. To obtain large group advancement using
this technique, the gain must be very large.
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 [2]. This effect can be interpreted as a phase-matched four-wave-mixing
process. The instability is enhanced when the field is bichromatic, and is then often referred to as the induced
modulation instability.
In the present context, the induced modulation instability leads to extreme frequency broadening of the
Raman pumping beams. As a result, the pump field evolves from a purely bichromatic field at the entrance
face of the medium to a very broad-band multi-frequency field at the exit. Therefore, the atom-field dispersion
properties experienced by the pulse vary spatially and are very different from the ideal picture.
To observe the effects of the modulation instability on the pump field, we inject an intense bichromatic laser
beam into a potassium vapor and characterize the light that emerges. Figure 1(a) shows the experimental
apparatus.
The input light is both spectrally pure, with two components of roughly 1 MHz bandwidth separated by
25 MHz, and pure in polarization, with less than 10−5 of its power in the orthogonal polarization. We find
that the outgoing beam is defocused, depolarized (with as much as 40% of its power in the orthogonal
polarization) and spectrally broadened. Figure 1(b) shows the electronic spectrum (i.e., the spectrum of the
output of the photoreceiver) of the light that emerges from the medium in the orthogonal polarization. The
light emanating from the cell has significant spectral components over more than 2.5 GHz.
We will discuss the induced modulation instability in the context of high-gain bichromatically-driven atomic
systems and specifically in the context of fast-light pulse propagation. We will further discuss the practical
limitations that the induced modulation instability imposes on this method for creating faster-than-c pulses.
We gratefully acknowledge the financial support of the National Science Foundation.
References
1. L.J. Wang, A. Kuzmich, and A. Dogariu, ‘Gain-assisted superluminal light propogation,’ Nature 406, 277-279 (2000).
2. G.P. Agrawal, Nonlinear Fiber Optics second edition (Academic Press, Boston, 1995), Sec. 5.1.
(a)
polarizers
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K vapor
cell
bichromatic
Raman pump field
high-speed
photoreceiver
detector power (dBm)
Stenner et al., Induced-Modulation-Instabilities Limit to “Fast” Light Propagation
QELS2002/2001 Page
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(b)
-20
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-60
-80
-100
0
0.5
1
1.5
frequency (GHz)
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Fig. 1. (a) Experimental setup used to analyze the effects of the modulation instability on a bichromatic
Raman pumping field. (b) Power spectrum of the photocurrent fluctuations of modulation instability. The
lower curve shows the electronic noise inherent in the measurement apparatus.
2.5
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