A STUDY OF SCATTERED COHERENT LIGHT: INTERFERENCE OF DOUBLE IMPOSED LASER SPECKLE. Salvatore Barra, Harold Metcalf and John Noe, Laser Teaching Center, Department of Physics and Astronomy, SUNY at Stony Brook.

Laser speckle patterns have been observed since the invention of the Helium-Neon laser in 1960. This phenomenon is created by a random combination of constructive and destructive interference on a rough surface, which results in bright and dark regions respectively. The purpose of my study is to observe the interference pattern of two overlapping speckle patterns. The resulting image will be affected by the added interference of the colliding beams.

My setup consists of a 632.8 nm He-Ne laser, four one-inch diameter plano-convex lenses, a beam splitter, a front surface mirror, a neutral density filter, and a phototdiode on a motorized rail carrier. The laser beam is first expanded from 1 mm to 1 cm diameter by two lenses; the expanded beam is then separated into two perpendicular beams by the beam splitter. The two perpendicular beams are further expanded and then recombined over an area about 25 cm in diameter using the remaining two lenses and the mirror. The neutral density filter is used to match the final intensity of the two beams. The photodiode scans across the speckle pattern and converts the fluctuating light intensity pattern to 2500 voltage readings. These are stored in a computer and analyzed using the Quattro Pro spreadsheet program and a QBASIC autocorrelation program.

The result of an overlapped speckle scan is expected to be more complex than simply the sum of the individual speckle images, due to the interference between the two light beams. Currently the setup is complete and aligned, and the first scans about to be made.

This study was supported by a Symbol Technologies Fellowship and ONR and ARO grants.

Sal Barra 3-August-2000