Son et Lumiere

Son et lumiere which in French means “sound and light,” is the name for popular nighttime outdoor shows in which pictures are projected on the walls of famous buildings (in France and other countries) accompanied by stories played out over loudspeakers. Now scientists hope to make a miniature sound-and-light show in fibers with the intention of producing not entertainment but ultrasensitive optical switches or the means of transporting bits in future all-optical computers.

The new scheme being developed by scientists at Ben-Gurion University and Tel Aviv University uses sound waves to help slow light nearly to a halt under conditions (ordinary materials at room temperature) more practicable than for most other slow-light experiments. Richard Tasgal (tasgal@bgu.ac.il) and his colleagues use as their medium a so-called Bragg grating fiber; the UV-sensitive core of a fiber is exposed through a mask to ultraviolet light.

This treatment changes the germanium-doped silica fiber core in periodic way along its length so that the index of refraction varies periodically. Light sent into such a fiber, and encountering a regularly changing index of refraction, will reflect multiply, not just at the ends but all along the fiber. A fiber with this condition is sometimes referred to as a distributed mirror.

If, furthermore, the light beam is intense and the fiber material possesses a nonlinear response to light, the net effect of light wavelets propagating in the forward and backward direction can be a light pulse traveling at speeds much less than the speed of light in vacuum. Here’s where the sound part comes in. Very intense light will cause a slight bunching in the density of the fiber and this can create sound waves.

This process is enhanced when the light pulse is traveling close to the speed of sound (around 5 km/s) in the material, and recent work has shown this could be achieved. But the enhancement can work both ways. A passing sound wave alters very slightly the material’s index of refraction and this in turn can result in a shortening and slowing of a passing light pulse-in this case referred to as opto-acoustic solitons.

Tasgal says that he and his colleagues are the first to recognize the potential of sound waves in slowing and processing light pulses in this way. The first great difficulty in implementing the whole scheme is to getting light pulses to enter and stay in a Bragg fiber in the first place since the fiber looks at first just like one long mirror. This might be achieved by gradually increasing the strength of the grating along the fiber. (Tasgal, Band, Malomed, Physical Review Letters, upcoming article)