Electrically switchable optical elements using wavelength...

Details Electrically switchable optical elements using wavelength... Electrically switchable optical elements using wavelength...

2001-08-31

2003-09-02

Sikder, Mohammad (Department: 2874)

Optical waveguides

Directional optical modulation within an optical waveguide

Electro-optic

C385S008000, C385S009000, C385S016000, C385S017000, C385S018000

Reexamination Certificate

active

06614948

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electrically switchable optical elements, such as application specific integrated elements including filters, lenses and switches, using wavelength locked feedback loops, and more particularly pertains to a combination of Electrically Switchable Bragg Grating (ESBG) technology with a wavelength locked feedback loop to provide variable focal length optical systems which automatically adjust the focal length of incident light.
2. Discussion of the Prior Art
Electrically Switchable Bragg Grating (ESBG—pronounced “S-Bug”) technology has recently become available from companies such as DigiLens Inc. These optical components possess electrically switchable diffractive optical elements or waveguides in a single solid-state device. The resulting device is capable of providing a unique blend of complex optical functionality within a tiny integrated package. The availability of this technology has opened up many new potential applications.
The ESBG technology has been presented in three basic variations of Application Specific Integrated (ASI) technology, namely Application Specific Integrated Filters (ASIF), Lenses (ASIL) and Switches (ASIS). The ASIL is discussed as a specific example below, with the understanding that other forms of ESBG technology can be substituted and used with dither wavelength locked feedback loops pursuant to the present invention.
An ASIL has different computer holograms imaged onto each layer. Each hologram corresponds to a different diffraction grating. For example, a diffractive lens with a variable focal length is capable of switching light at 35 &mgr;sec, 10× faster than electrical switching, and provides wavelength insensitive focusing in a compact package. A key element of this technology is a holographic, polymer-dispersed liquid crystal. 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 different. A monomer and polymer liquid crystal are combined in such a way that upon exposure to a laser light fringe pattern, an area of pure polymer is created in the light fringes, and a mixture of monomer and polymer liquid crystal remains in the dark fringes. 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 microdroplets' optical axes oscillate 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 applied, 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 “diffractive lens,” independent of wavelength.
These hologram layers, typically 5 to 30 &mgr;m thick, are deposited on a 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 microseconds occurring within each layer. In this manner, the resultant optical switch can replace large refractive components, magnifying up to a factor of 20 to 40× and providing full-color capabilities. Ultimately switching speeds may become as fast as 10 microseconds or less.
Instead of transmitting only a very narrow band of wavelengths, ASIL allows diffraction gratings that transmit red, blue or green light to be layered on top of each other. These layers can be switched on and off in turn at frequencies greater than 85 hertz, giving the full spectrum of color with no apparent flicker. Any optical effect that can be produced with conventional lenses can be written onto ASILs. ASIL devices are typically mounted on glass or plastic thin film substrates about 0.2 mms thick; the resulting devices are very lightweight and thus suitable for near-eye and handheld applications such as cell phone displays and mobile computing. As another example, combined with other technologies, ASILs can be used for high definition television (HDTV) projections.
There are many possible applications for this technology, including an electro/optical wavelength filter for wavelength multiplexing combined with a dynamic optical add/drop multiplexer. This area enables a new generation of optical switch components, including for example a multi-channel Dynamic Spectral Equalizer (DSE), which allows a real-time adjustment of power distribution within a wavelength multiplexing system. This ensures spectral flatness across all wavelength channels, which would otherwise be distorted by the highly non-uniform and dynamically varying gain profiles induced by cascaded erbium doped optical amplifiers (EDFA's) and active Add/Drop functionality within the optical network. The resulting DSE is polarization-insensitive, eliminates the need for a multiplexing or demultiplexing layer within the network, and can potentially exhibit switching speeds as fast as 50 microseconds in either a free space or a guided wave optical design.
In telecommunications, all optical interconnects are currently converted to electricity where switching functions are performed and then converted back into light for transmission through the fiber. Optical switches can maintain transmission speed through high-bandwidth, fiber-based systems. With this technology, in addition to providing the ability to switch light and handle very complex routing systems, an asynchronous digital subscriber loop (ADSL) can switch between different frequencies of light. For example, in wavelength division multiplexing, the ADSL can selectively switch any particular frequency by effectively adding filters in each layer and switching among the layers.
For optical filtering, a selectable wavelength filter can be implemented for applications such as rear projection televisions and computer monitors. This has applications in compact disk and optical storage media, including volume holography. Optical designers have recognized the benefits of using holographic lenses in microdisplay applications because of their small size and lightweight. Holographic elements, however, only diffract a narrow bandwidth of light, typically 20 to 30 nm wide, thus limiting them to a single color, typically green. This is often seen in head-mounted displays for the military and in high-end industrial and medical applications. The ADSL technology is capable of switching the lens from clear to a red, blue or green holographic lens in stacked layers quickly enough to visually blend a flicker-free miniature display. The result is a full-color holographic display suitable for wearable computer displays and portable internet devices in general. This technology is used at the heart of several new devices that can “electrically switch” diffractive optical Bragg gratings on or off. This unique functionality opens up a broad range of components and subsystems used to control light, especially at the very high speeds required for optical telecommunications applications. This technology allows for recording of complex Bragg gratings which can encapsulate binary optical features, thereby reducing size, improving efficiency and lowering cost.
For wireless devices, an integrated sequential holographic lens is possible for enabling visual displays on compact devices such as cell phones. This has potential applications to handheld internet devices, CMOS device imaging, and wearable display technology. The long-term viability of portable and handheld devices, including smart phones, is dependent upon their providing an easy to use tool with which people can access information. Microdisplays (tiny high resolution displays on a chip) are part of the solution for these applications; ASIL can holographically encapsulate the prescriptions of multiple lenses within a slim glass (laminated) solid state device, saving space and decreasing weight, complexity and cost.

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