Application-specific integrated lenses speed microdisplay designs
Developed for the military by DigiLens (Sunnyvale, CA) and SAIC (San Diego, CA), a new class of electrically switchable polymer-dispersed liquid-crystal materials, dubbed application-specific integrated lens (ASIL) technology, has recently been demonstrated within a wearable eyepiece.
Developed for the military by DigiLens (Sunnyvale, CA) and SAIC (San Diego, CA), a new class of electrically switchable polymer-dispersed liquid-crystal materials, dubbed application-specific integrated lens (ASIL) technology, has recently been demonstrated within a wearable eyepiece. In the demonstration, a high-resolution color display is focused directly onto the wearer's retina. Switchable lenses in the ASIL diffract red, green, or blue bands of light in a sequential manner, providing color images.
Conventional head-mounted-display designs position heavy optical elements and display components in front of the eyes. These approaches typically result in discomfort after a relatively short period of use. DigiLens ASIL technology allows off-axis optical designs to be used that can position the center of gravity of the display back toward the ear. The final projection lens, an RGB layered switching holographic optical element, allows see-through operation, which is preferential for single-eye displays to help reduce eye strain.
"In the DigiLens ASIL, each layer is composed of a switchable Bragg (or volume) hologram that performs light deflection, focusing, spectral filtering, distortion, and aberration correction," says Stephen Sagan, vice president of optical engineering at DigiLens. "An electric field, applied to temporarily erase the hologram, allows light to pass undisturbed through the ASIL substrate, which is made up of clear glass or plastic."
Color imaging is achieved by implementing an ASIL as a stack of red, green, and blue sensitive Bragg holograms, switched in sequence with red, green, and blue illumination sources creating three separate monochromatic optical channels. This as similar to the three converging RGB light paths seen in a typical CRT projector used by airlines to show in flight movies.
"An ASIL is created by placing a layer of the stack in the interference pattern created by two intersecting beams of laser light," explains Sagan. After the interference pattern is recorded in the layer, it can be regenerated by illumination from one of the original recording beams. "In the DigiLens ASILs, each layer is a dye-sensitized polymer/liquid crystal mixture, which solidifies upon exposure to light and permanently "freezes" in the holographic interference pattern of the constructing beams," Sagan says.
During recording, the polymer-dispersed liquid-crystal mixture undergoes a phase separation, creating regions populated by liquid-crystal microdroplets, interspersed by regions of clear photopolymer. When an electric field is applied to the hologram with a pair of electrodes, the orientation of liquid crystal droplets is changed, causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels, effectively erasing the hologram. The resulting phase-volume hologram exhibits high diffraction efficiencies and fast switching times.