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Journal of the Optical Society of America B

Manipulation of fast light using photorefractive beam fanning

A. A. Grabar, P. Mathey, Grégory Gadret

Published Apr 4, 2014• 14 citations

Abstract

Light pulse group velocity manipulations due to the specific dispersion of a medium (so-called “slow” and “fast” light phenomena) can be obtained on the basis of several mechanisms. One of these techniques is two-wave mixing in a photorefractive crystal. This work presents a modification of this method, exploiting the strong beam fanning in Sb-doped Sn2P2S6 crystals. Our experimental results demonstrate a “fast light” behavior of Gaussian pulses transmitted through a Sn2P2S6:Sb sample. The phenomenon is due to the beam fanning (i.e., the self-diffraction of the incident beam on self-induced noisy photorefractive gratings) that ensures a significant depletion of the input beam. Due to the relatively fast photorefractive response of the Sn2P2S6:Sb crystals this depletion occurs with times in the range of 10–100 ms, depending on the beam intensity, and the “fast light” feature is observed. The temporal and amplitude characteristics of the output beam are measured in function of the intensity and polarization azimuth of the incident beam. Besides, a negative phase shift of the periodical output beam relative to a sinusoidal intensity-modulated input beam is also obtained experimentally. It is shown that the phase and amplitude relation between the input and output periodic signals are described by a simple analytical expression that takes into account the beam fanning strength (depletion factor) and its dynamics (depletion time constant). It is also demonstrated that the pulse advance (or phase shift of the modulated signal) can be regulated by the light polarization azimuth. The advantages of the proposed method are discussed.

DOI: 10.1364/josab.31.000980

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Slow and fast light are achieved in two experiments using SPS photorefractive crystal at room temperature. We report that the photorefractive gain achieved by the two wave mixing (TWM) method can control the group velocity of the transmitted light pulses at visible wavelength. It is shown theoretically and experimentally that the time delay and the shape of the output pulse change as a function of the photorefractive gain. The beam fanning has also been used to control the velocity propagation of a single light pulse in the same crystal at λ = 1064 nm. Depending on the orientation of the polar axis, it is possible to accelerate or decelerate a short pulse with duration of order of μs in the crystal.

Slow-down of the light velocity has potential applications in information processing [1] and interferometers. Recently, several works have experimentally [2] demonstrated the deceleration of light in photorefractive crystals using a two wave mixing (TWM). A low velocity value of the order of 0.02 cm/s has been reported in BaTiO3crystal for Gaussian pulse profiles of long duration of 1.3 s [3]. It was also reported that the time delay depends mainly on the coupling photorefractive gain. However, the photorefractive grating build-up time in that crystal does not allow to slow down pulses shorter than ms, which limits its interest for telecoms applications.

In this work, we experimentally demonstrate the capacity of the photorefractive (PR) crystal to control the group velocity of light pulses, specifically at the telecommunication wavelength, by using the so-called beam fanning at room temperature. Significantly, we show for the first time that this method can effectively decelerate a modulated signal propagating through this material at 1310 nm. Furthermore, we illustrate that the performance of this slow light system can be adjusted by changing the intensity, duration, and angle of polarization of the input pulse. The results obtained through this technique can potentially improve the performance of specific components in telecommunication networks.

A slowing down of light pulses using backward-wave four-wave mixing is achieved in photorefractive crystals with different coupling strength. The delay and width of the output pulse are studied as a function of the input pulse width and pump intensity ratio for the amplified transmitted beam and for the phase-conjugated beam. The delay characteristics are compared with those of the two-beam coupling. It is demonstrated that the four-wave mixing process ensures a larger slowing down of short pulses (pulses with width shorter than the photorefractive response time) as compared to the photorefractive two-beam coupling scheme and guarantees the elimination of forerunners. The delay of long pulses in the four-wave mixing configuration is almost the same as the one that can be reached with two-beam coupling. The effect of absorption on the nonlinear shape transformation of the output pulses is discussed qualitatively.

Recently, several works have shown that the light group velocity can be reduced using the recording of dynamic refractive index gratings in photorefractive (PR) crystals [1] , [2] , [3] . The slowdown of light is performed using optical beams wave mixing: it yields the output pulse amplification and increases the light dispersion inside the PR crystal. For example, the TWM method demonstrated that the delay time ∆ τ can be varied as a function of the TWM gain values Γ and the input pulse widths T 0 . However, several works have shown that the results achieved by this method are limited in terms of pulse duration. Indeed, only long pulses with time duration of ms−s can be slowdown in a PR crystal both at the visible and the infrared wavelengths [4] .

We experimentally observe an ultralow effective group velocity of 0.9 cm/s of light pulses using the two-wave mixing process in an Sn2P2S6 (SPS):Te crystal at a visible wavelength. The time delay can be controlled through the nonlinear photorefractive gain and the input pulse duration. By optimizing the nonlinearity strength, we achieve a maximum fractional delay of 0.79 for a pulse duration of 100 ms. Our photorefractive slow light system allows combining low group velocity with large delay-bandwidth product for pulse durations spanning three orders of magnitude.