IMAGE PROCESSING: All-optical image delay uses slow-light medium

An all-optical image-delay technique that preserves the phase and amplitude of the stored image has great potential in image processing, holography, and quantum-information applications. To date, analog-to-digital electronic conversion of optical images sometimes results in lost information, and long free-space delay lines cause diffraction and impose physical space restrictions on data-storage systems. Using slow light, researchers in the Department of Physics and Astronomy at the University of Rochester (Rochester, NY) have overcome many of these obstacles and successfully delayed 2-D images carried by 2- to 3-ns-long optical pulses by up to 10 ns.¹

The delayed images have low background noise and high signal-to-noise ratio-even at very low light levels of less than one photon on average per pulse-because the slow-light medium used in the experiment is prepared without additional laser beams.

Hot vapor

Other slow-light techniques such as electromagnetically induced transparency, coherent population oscillation, or spectral hole-burning require additional light fields that result in transverse spatial inhomogeneities in the group velocity of the medium. But by using a hot cesium (Cs) vapor as the slow-light medium, delay-bandwidth products (the delay of the light signal in the medium multiplied by the bandwidth of the signal) in excess of 50 can be achieved by spectrally tuning the signal between the two D2 hyperfine resonances of Cs at 852 nm. Because the group velocity is the same in all directions in this Cs system, each part of the image is delayed by the same amount, even in the presence of significant diffraction. Because the spectral region between the resonances is very transparent and flat, losses are low and pulse broadening is minimal.

In one experiment, light pulses with a 2 ns (full width at half maximum) duration that repeat every 7 ns are generated by passing a continuous-wave laser beam through a fiber-coupled electro-optic modulator that can operate from 0 to 16 Gbit/s. These pulses then enter an unbalanced Mach-Zehnder interferometer with a free-space path mismatch of 5 ns. The long-path reference pulses interfere with the pulses exiting the slow-light medium in the short path. These short-path pulses are directed onto a 4.5-lines/mm image mask in front of the Cs slow-light cell and are called image pulses. The group delay in the Cs cell is varied by changing the vapor pressure of the cell using temperature control-in effect, controlling the pulse delay of the image. The long-path and short-path beams are recombined with additional optics and interfere at the image plane where they are captured by a CCD camera.

In another experiment, the light level in each pulse is attenuated so that there exists on average only one photon per pulse and the long arm of the interferometer is blocked. The weak pulses are passed through an image mask (containing the letters “U R” for the University of Rochester) and are recorded, one photon at a time, by a scanning optical fiber and a photon counter.

In both experiments, the research team compared the quality of delayed and nondelayed pulses for the 2-D images. In the low-light-level experiment, each pulse contained on average 0.8 photons before arriving at the mask, and was delayed by approximately 3 ns. The delayed image was reconstructed by raster-scanning a fiber across the image plane for a total time of approximately 48 seconds and a time-binned filtering technique was used to remove background counts from the 2-D image. Despite the 3 ns delay for every photon used to construct the delayed image, it was preserved with high fidelity and with a resolution comparable to the nondelayed image (see figure).

“While we’ve extended optical buffering to two spatial dimensions, we are a long way from any practical storage applications,” says researcher Ryan Camacho. “It’s a hard thing to teach photons to sit still and remember what they’re told.” The next phase of the research work involves increasing the delay time and showing that the quantum properties of light can also be preserved while being delayed.

REFERENCE

1. R.M. Camacho et al., Physical Rev. Lett. 98, 043902 (January 2007).