Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-01-03
2002-11-19
Allen, Stephone (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S214100, C310S303000
Reexamination Certificate
active
06483094
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to optoelectronic devices, and specifically to high-speed shutters for image and optical data modulation.
BACKGROUND OF THE INVENTION
Optoelectronic shutters are well known in the art. Such shutters open and shut in response to an electrical waveform or pulse applied thereto, generally without moving mechanical parts. They are used, inter alia, in high-speed image capture applications, for which mechanical shutters are typically too slow. Optoelectronic shutters known in the art include liquid crystal shutters, electrooptical crystal shutters and gated image intensifiers.
Liquid crystal shutters are simple and inexpensive to manufacture. Their speed, however, is inherently limited to about 20 &mgr;sec switching time. Moreover, in their open state, liquid crystal shutters typically transmit only about 40% of the light incident thereon, whereas in their closed state, they still transmit at least 0.1% of the incident light.
Electrooptical crystal shutters can be switched quickly, on the order of 0.1 nanosecond. They require a collimated light input, however, and have only a narrow acceptance angle within which they can shutter incident light efficiently. The crystals themselves are expensive, and costly, high-speed, high-voltage electronics are also needed to switch the shutters on and off at the rated speed.
Image intensifiers generally comprise an electron tube or microchannel plate, with a photoelectric photocathode input and a light-emitting phosphor-coated anode at the output. Gated intensifiers further include high-speed switching circuitry, which enables them to be gated on and off quickly, with typical switching times as fast as 1 nanosecond. For light to be effectively shuttered or amplified by the intensifier, it must be focused on the photocathode. Although intensifiers are manufactured in large quantities, the manufacturing process involves metal-to-glass vacuum sealing, which is complex, labor intensive and therefore costly. Partly as a result of this complexity, gated intensifiers tend to be large compared to their active area and are available in a very limited range of shapes and sizes.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a compact, high-speed, solid-state optoelectronic shutter, which may be manufactured at relatively low cost in large quantities.
In some aspects of the present invention, the shutter is used for modulating light that is received by an image capture device, such as a high-speed CCD camera.
In other aspects of the present invention, the shutter is used in modulating an image or an array of optically-encoded data, for example, in the framework of a system for optical data processing.
It is a further object of the present invention to provide a method for manufacturing the shutter.
In preferred embodiments of the present invention, an optoelectronic shutter comprises a generally planar substrate made of semiconductor material, having mutually substantially parallel input and output surfaces. A planar photodiode layer is formed on the input surface of the substrate, and a planar light-emitting diode (LED) layer is formed on the output surface, opposite the photodiode layer. A gate layer is formed intermediate the photodiode and LED layers, preferably adjacent the photodiode layer. Preferably, transparent, electrically conductive coatings, for example, indium tin oxide (ITO), are applied to at least a portion of each of the input and output surfaces. An additional biasing layer is preferably formed intermediate the gate and LED layers, for back-biasing the LED.
When light strikes the photodiode layer, photoelectrons are created. Ordinarily, when there is no voltage or only a relatively small voltage applied between the input and output surfaces, the electrons remain in the photodiode layer and recombine, as they are unable to pass the gate. Under these conditions, the shutter is closed.
To open the shutter, a control voltage, preferably in the range of 5 to 15 volts, is applied between the surfaces, to bias the LED positively with respect to the photodiode. In some embodiments of the invention, higher or lower voltages may be used. Preferably the voltage is applied to the conductive coating on the surfaces. This voltage creates a potential difference across the substrate, between the photodiode and the LED. In this state, photoelectrons that are produced in the photodiode pass through the gate and substrate to the LED layer, which emits light in response to the incident photoelectrons. This process continues until the control voltage is removed, whereupon the shutter closes.
Preferably, the substrate comprises silicon, GaAs, InP or other semiconductor material known in the art, preferably in the range of 0.5 to 2 mm thick and 1 to 40 mm across. More preferably, the substrate comprises a high-electron mobility, substantially single crystal of one of the above-mentioned materials, wherein the crystal is oriented so that one of the crystal axes is substantially perpendicular to the input and output surfaces. In this way, when the control voltage is on, photoelectrons emitted by the photodiode travel ballistically along the crystal axis perpendicular to the surfaces, generally without substantial scattering and without significant photoelectron divergence in directions other than perpendicular to the surface. Hence, a photon striking at any point on the input surface of the shutter and generating a photoelectron in the photodiode layer there will cause a photon to be emitted by the LED layer at a corresponding point on the output surface. As a result, if an image is focused onto the input surface, it will be reproduced at the output surface with minimal blurring or distortion.
The active aperture of the shutter, defined by the areas of the photodiode and LED, may be as large as 40 mm across and may be made circular, square or rectangular, depending on the application. Thus, shutters in accordance with the present invention are more compact and may have a substantially greater ratio of active aperture to thickness than high-speed shutters known in the art, such as gated intensifiers and electrooptical crystal shutters.
In some preferred embodiments of the present invention, the photodiode layer comprises an avalanche photodiode. Preferably, an additional transparent conductive layer is interposed between the photodiode layer and the gate, and a voltage preferably of between 20 and 100 volts is applied to this conductive layer so as to reverse-bias the avalanche diode. Each photon incident on the input surface that is absorbed by the photodiode layer will cause an “avalanche” of electrons, to be generated, as is known in the art. When the control voltage is applied, these electrons pass through the gate to the LED layer. Thus, shutters according to these preferred embodiments transmit images with enhanced efficiency and can even provide a modicum of image intensification.
In preferred embodiments of the present invention, the shutter is produced using methods of semiconductor device fabrication known in the art. After the substrate has been suitably cut and polished, the gate, photodiode and LED layers are preferably formed thereon by means of epitaxy, MOCVD and/or ion implantation. Electrical leads are then bonded to appropriate locations on the shutter, specifically to the input and output surfaces thereof, and the shutter is suitably packaged for its application.
It will be appreciated that shutters may be mass-produced in accordance with the principles of the present invention at substantially lower cost than high-speed shutters known in the art. Shutters in accordance with preferred embodiments of the present invention generally include only a single, solid-state component, largely comprising low-cost, readily-available materials. Fabrication of such shutters may be substantially automated. Shutters in accordance with preferred embodiments of the present invention may be made to operate at relatively low voltage: typically 5-10 volts, or at most 100
Iddan Gavriel J.
Yahav Giora
3DV Systems Ltd.
Allen Stephone
Fenster & Company Patent Attorneys Ltd.
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