March 1995

Efforts In The United States Laser Fusion

Roy R. Johnson

Research activities in laser fusion over the past 30 years have evolved into the next generation of Inertial Confinement Fusion—the National Ignition Facility.

Quantum Cryptography

Franson describes several quantum methods for encoding information. Cost and size reduction are needed to make the systems practical.

Colors From the Eye of a Needle

On Labor Day weekend I was sitting on my front steps on Sunday morning reading in the bright, warm sunlight. As I read I noticed circles of light appearing at the side of my field of vision and I was surprised at what I saw in them. I wasn't surprised to see the circles themselves. I had often seen them when reading in bed with a night light. They are caused by light scattered from specks of seemingly ever present dust on my glasses. These point sources of light, close to the anterior focal point of the eye, cause blur circles to form on the retina.

The Fascinating Behavior of Light in Photorefractive Media

Send a laser beam through a photorefractive crystal of barium titanate and something remarkable happens: the beam spreads out in a complicated and curving fan, then collapses into a corner of the crystal, often forming back on itself into a loop. When the light inside the crystal has settled into a stationary pattern, lo and behold, the laser beam is traveling back exactly along the way it came! The crystal acts as a mirror not an ordinary mirror, but a phase-conjugating one. You perhaps already know about phase conjugation, though in case you don't, we will return to it in a moment. What makes this phenomenon so stunning is the fact that the crystal wants to phase conjugate the incoming light.

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