DIFFRACTIVE OPTICS: Optical elements are electrically switchable

Faced with designing a free-space optical system that requires adjustments to its light path during operation—for example, changes in focus, beam position, or spectral bandpass—an optical engineer typically incorporates a mechanical motion into the design.

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Faced with designing a free-space optical system that requires adjustments to its light path during operation—for example, changes in focus, beam position, or spectral bandpass—an optical engineer typically incorporates a mechanical motion into the design. Although a few nonmechanical alternatives exist—such as acousto-optic deflectors or a combination of electro-optic modulators and polarization optics—they are useful only in special cases. And so, in vast numbers, filter wheels spin, stages slide, lenses zoom, and mechanical parts wear and break down. In what may be a way out,

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DigiLens Inc. (Sunnyvale, CA) has announced that it has developed electrically switchable diffractive optics that could in many cases eliminate mechanical motions—especially in view of the 35-µs switching time the company is claiming (see figure).

RIGHT. Liquid-crystal-based diffractive lens can be electrically switched on and off within 35 µs (left). Similar switchable holograms are capable of light deflection, spectral filtering, and distortion and aberration correction. The fastest electrically switchable hologram constructed to date has a switching speed of 10 µs. Associated software calculates many device characteristics, including diffraction efficiency over a range of input angles and wavelengths (right). The software is freely downloadable as an aid to optical designers.

A form of phase volume (or Bragg) hologram, the switchable diffractive element replaces the usual holographic medium with a mixture of dye-sensitized polymer and liquid crystal, which solidifies when exposed to light. The resulting layer contains regions populated with liquid crystal to varying degrees depending on exposure intensity. In an electric field, the liquid crystal droplets reversibly change orientation, reducing the refractive-index modulation and switching off the hologram. When switched on, the hologram has a diffraction efficiency of 70%-80%, with close to 100% expected on further development; when switched off, efficiency falls to less than 0.5%. The material can be used at wavelengths between 400 and 1600 nm, support grating pitches of 150 nm, and be cycled indefinitely. The elements can be fabricated using standard holographic contact replication techniques. Uses include switchable lenses, prisms, optical filters, and mirrors.

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DigiLens has been pursuing the technology for years in secret, according to Jonathan Waldren, the company's founder and chief executive officer. The switchable liquid crystal material was originally developed for laser defense systems to be used by the US Air Force. Along with the material itself, DigiLens has developed associated software to interface with popular optical-design programs such as Code V (Optical Research Associates; Pasadena, CA), as well as to model the optical exposure process used to create the holograms—a model that is critical to successful fabrication, says Waldren.

Early applications of the switchable holograms include rear-projection monitors in which monitor depth is reduced, resolution is multiplied by sequential projection of images on different portions of the screen (or "tiling"), and color is achieved by sequential switching of red, green, and blue filters. DigiLens has also developed a small wearable near-eye virtual display in which color filters are switched in sequence with red, green, and blue illumination sources. Resolution is increased due to the use of monochrome projection, as well as tiling. "With near-eye displays, we are stitching [aligning adjacent images] with subpixel resolution," notes Waldren.

John Wallace

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