MIT heralds fresh approach to holographic video

A new approach to generating holograms could lead to cheaper holographic-video displays being developed and the resolution of conventional 2D displays being increased.
Daniel Smalley, a graduate student at MIT’s Media Lab, is building a prototype color holographic-video display which has a resolution of about half that of a standard-definition TV. It can update video images 30 times a second - fast enough to produce the illusion of motion.

The heart of the display is an optical chip that resembles a microscope slide which was built for about $10. The technology uses acousto-optic modulation, in which precisely engineered sound waves are sent through a piece of transparent material, in this case a crystal of lithium niobate. Just beneath the surface of the crystal microscopic channels known as waveguides are created, which confine the light traveling through them. Onto each waveguide Smalley put a metal electrode which can produce an acoustic wave.

Each waveguide corresponds to one row of pixels in the final image. In Smalley’s chip the waveguides with their individual electrodes can be packed mere micrometers apart from each other.

Beams of red, green and blue light are sent down each waveguide, and the frequencies of the acoustic wave passing through the crystal determine which colors pass through and which are filtered out. Combining red and blue to produce purple doesn’t require a separate waveguide for each color; it just requires a different acoustic-wave pattern.

Michael Bove, Smalley’s thesis advisor, said: “Until now, if you wanted to make a light modulator for a video projector, or an LCD panel for a TV, or something like that, you had to deal with the red light, the green light and the blue light separately.
“If you look closely at an LCD panel, each pixel actually has three little color filters in it. There’s a red subpixel, a green subpixel and a blue subpixel.
“That’s inefficient, because the filters, even if they were perfect, would throw away two-thirds of the light. Second, it reduces either the resolution or the speed at which the modulator can operate.”