Lens Performs Full-Color Optical Switching
Ruth DeJule, Associate Editor -- Semiconductor International, 1/1/2000
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For many years, optical designers have recognized the benefits of using holographic lenses in microdisplay applications because of their small size and light weight. Holographic elements, however, only diffract a narrow bandwidth of light, typically 20 to 30 nm, thus limiting them to a single color, typically green. This often is seen in the head-mounted displays for the military or for high-end industrial and medical applications. The DigiLens technology is capable of switching the lens from clear to a red, blue or green holographic lens stack quickly enough to visually blend a flicker-free miniature display. The result is a full-color holographic display suitable for wearable PC displays and portable internet devices in general.
The heart of DigiLens' optical switch is a holographic, polymer-dispersed liquid crystal, developed in partnership with Science Applications International Corp. (SAIC, San Diego). While it contains materials common to liquid crystals used in flat-panel displays, the way in which the actual material builds the optical elements is unique. A monomer and liquid crystal are combined in such a way that upon exposure to laser light, an area of pure polymer is created in the light fringes, and a mixture of monomer and liquid crystal remains in the dark. This plane of differing refractive indices is called a phase volume hologram. In the dark fringes, the liquid crystal is embedded in very small microdroplets. When an AC voltage is applied across the plate, the LC microdroplets' optical axis oscillates to match the refractive index of the monomer/liquid crystal area with that of the pure polymer. Thus the entire field looks like a clear window. With no voltage, the plane is a hologram containing a number of optical elements, essentially, a diffractive lens. The result is the ability to switch the optical elements in and out of a "lens," independent of wavelength (see Figure).
These hologram layers, typically 5 to 30 µm thick, are deposited on glass or plastic substrate. They can be stacked so that red, green and blue hologram optical elements can be contained in three separate layers with switching at 35 µsec occurring among each layer. For microdisplays, the optical switch can replace large refractive components, magnifying up to a factor of 20 to 40X and providing full-color capabilities.
In telecommunications, all optical interconnects are currently converted to electricity where switching functions are performed and converted back into light for transmission through the fiber. Optical switches can maintain transmission speed through high-bandwidth, fiber-based systems. With this new technology, in addition to the ability to switch light and handle very complex routing systems, it also can switch into different frequencies of light. For example, in wavelength division multiplexing, the DigiLens can selectively switch any particular frequency by effectively adding filters in each layer and switching among the layers.
"In due course, we think our material system is capable of going faster than the current 35 µsec," said DigiLens President Jonathan Waldern. "In fact we have already demonstrated speeds as fast as 10 µsec in the laboratory." It is this high-speed capability, combined with optical quality lensing, which presents intriguing design options.
DigiLens www.digilens.comScience Applications
International Corp. www.saic.com
