All-Optical Delay of Images using Slow Light

Two-dimensional images carried by optical pulses (2 ns) are delayed by up to 10 ns in a 10 cm cesium vapor cell. By interfering the delayed images with a local oscillator, the transverse phase and amplitude profiles of the images are shown to be preserved. It is further shown that delayed images can be well preserved even at very low light levels, where each pulse contains on average less than one photon.

Figure 1

Experimental setup for the delay of transverse images. Light pulses of 2 ns duration are incident on a $50:50$ beam splitter. The transmitted pulses then pass through an amplitude mask and a $4f$ imaging system. The transmitted and reflected pulses are recombined at another $50:50$ beam splitter. The transmitted part traverses a path approximately 5 ft shorter than the reflected path, and arrives at the second beam splitter about 5 ns sooner than the reflected pulse, preventing interference between the two pulses. The temperature of cesium vapor can then be adjusted to give 5 ns of delay, resulting in interference. In the low-light-level experiment, the pulses are attenuated such that each pulse contains on average less than one photon and the reflected path is blocked. A scanning optical fiber is used to collect the photons in the image plane and the photon arrival times recorded using a photon counter with time-to-digital converter.

Figure 2

Interference of a delayed image with a slightly diverging local oscillator. (a) An image (a black pattern of bars and a numeral) delayed by 5 ns interferes with a reference beam and produces a ring pattern superimposed with the image. In the central dark spot, the two beams destructively interfere and cancel one another except in the image, which remains relatively bright. In the ring surrounding the central spot, the two beams constructively interfere and add to create a bright ring except in the image, which remains relatively dim. The succeeding rings alternate between constructive and destructive interference. (b) and (c) show the same superposition of the two beams, but in the absence of slow light. In (b), the wavelength of the laser is tuned outside of the dispersive region and in (c) the cesium cell is removed. In both cases, no interference between the beams can be seen.

Figure 3

(a) Delayed and (b) nondelayed one-dimensional low-light-level image with (c) accompanying histograms of photon arrival times. Each pulse contains, on average, 0.5 photons before striking the image mask.

Figure 4

False color representation of a (a) Delayed and (b) nondelayed two-dimensional low-light-level image. An optical fiber was raster scanned across a two-dimensional image consisting of the letters UR. Though attenuated, the delayed imaged shows similar image fidelity and resolution to the nondelayed image. Each pulse contains, on average, 0.8 photons before striking the image mask.

Figure 5

Histogram of photon arrival times showing the delayed (right) and nondelayed (left) two-dimensional image shown in Fig. 5.