Formation of Hollow Beams Through Coherent Fiberoptical Bundles.     Dhruv Bansal, The Wheatley School, Harold Metcalf and John Noé, Laser Teaching Center, University at Stony Brook   11794-3800.

Fiberoptical cables are microscopically thin flexible glass rods which transmit light through total internal reflection. When a group of these microscopic cables are fastened together, the result is a fiberoptic bundle. This bundle passes light unchanged through its length.

Most lasers produce a small highly focused dot of light. It is possible, through the use of a special Axicon lens, to refocus this light into a ring without losing too much intensity. The new beam will be a hollow cylinder. Hollow beams are used to trap atoms when one does not want the light interactions that might result using a conventional beam.

In the Laser Teaching Center, it was recently discovered that a diode laser, shined parallel to the axis of the fiberoptical bundle, will produce a wide dot, just as expected. If the laser is tilted off axis, however, the dot becomes a ring. This ring has much the same intensity as the original dot. As a result of this phenomenon, it may be possible to use simple fiberoptic cables as a means of producing hollow beams as opposed to using a more expensive and less versatile Axicon lens.

My project involved determining the optics of a fiberoptical bundle. By varying entrance angles and the aperture of the bundle, I was able to determine what optical devices were at work inside the cable.

Through the use of a simple photodetector, I will soon be able to measure the Gaussian profile of the ring.

Hopefully, this new phenomenon will be an alternative to more expensive Axicon lenses in producing hollow beams.

Dhruv Bansal July 2000