Opto-electronic shutter

Details Opto-electronic shutter Opto-electronic shutter

1999-09-03

2001-12-04

Epps, Georgia (Department: 2873)

Optical: systems and elements

Optical modulator

Having particular chemical composition or structure

C250S207000, C250S2140VT, C313S528000, C313S529000, C359S241000, C359S244000

Reexamination Certificate

active

06327073

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to optoelectronic devices, and specifically to high-speed shutters for image 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 microsecond 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. However, shutters using microchannel plates are generally non-linear at high frequencies.
Image intensifiers generally comprise an electron tube and 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 attachment of high-voltage feed-through electrode and metal-to-glass sealing, which is complex, labor intensive and therefore costly. Partly as a result of this complexity, gated intensifiers tend to be large and are available in a very limited range of shapes and sizes.
GB 2 082 830 describes an electron beam shutter device that forms an image of a luminous event changing at high speed.
FIGS. 1 and 2
of the reference show devices with electrostatic focusing.
FIG. 3
shows a device in which the image resolution is low (i.e., the image is defocused and
FIGS. 4-8
show a device utilizing a micro-channel plate. With respect to
FIG. 3
, it is believed that the defocusing is caused by the distance required between the cathode and anode due to the construction of the device.
U.S. Pat. No. 4,220,975 shows a shutter device in which a microchannel plate is used.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a compact, high-speed optoelectronic shutter, which may be manufactured at relatively low cost in large quantities.
In some aspects of the present invention, the shutter is used in modulating light that is received by an image capture device, such as a high-speed CCD camera.
It is a further object of some aspects of the present invention to provide a method for manufacturing the shutter.
In preferred embodiments of the present invention, an optoelectronic shutter comprises an input plate and an output plate, both made of transparent, preferably non-conducting material. Each of the plates has an inner surface and an outer surface, and a recess is formed on the inner surface of one or both of the plates. The non-recessed portions of the inner surfaces of the two plates are bonded or fused together to form a vacuum seal along a periphery thereof, so that the recess forms a vacuum-tight vacuum chamber therebetween. A photocathode is formed on the inner surface of the input plate, adjacent the chamber, and a photo-luminescent anode is formed on the inner surface of the output plate, opposite the photocathode.
Preferably, a transparent, electrically conductive coating, for example, indium tin oxide (ITO) is applied to at least a portion of the outer surfaces of the plates and to at least a portion of the inner surface of one of the plates, most preferably over the cathode on the input plate. Alternatively, instead of applying the electrically conductive coatings to the outer surfaces, one or both of these coatings may be applied to the inner surfaces of the respective plates, preferably with the addition of an insulating overlay layer.
Preferably, the plates are made of fused quartz or, alternatively, of glass or of silicon or GaAs material. In some preferred embodiments of the invention the output plate may be a fiber optic face plate. The plates are preferably in the range of 0.5 to 5 mm thick. The actual thickness of the plate is chosen to be thick enough so that, when the chamber is evacuated, the substrate does not bow inward from the pressure, to any substantial extent. The active aperture of the shutter, defined by the areas of the photocathode and anode, may be as large as 40 mm across and may be made circular, square or rectangular, depending on the application. 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, which are generally circular. Furthermore, unlike gated intensifiers, shutters in accordance with the present invention can easily be made in a rectangular shape and size that are similar to the shape and size of an image detector device, such as a CCD detector array.
To operate the shutter, a biasing voltage is applied between the plates, preferably by applying the voltage to the conductive coating on the outer surface of one of the plates. This voltage, preferably in the range of several hundred volts, creates a potential difference across the gap in the chamber between the photocathode and the anode, without breaking down the gap. In this state, the shutter remains substantially non-transmitting to incident light.
To open the shutter, a control voltage, preferably in the range of 10-20 volts, is applied, preferably to increase the potential difference across the gap. In some embodiments of the invention, even lower control voltages may be used. In this state, photons incident on the photocathode cause photoelectrons to be emitted by the photocathode and accelerated across the gap. These electrons strike the anode, which emits light in response to the incident electrons. This process continues until the control voltage is removed, whereupon the shutter closes. Preferably, the shutter takes no more than 2 nanosecond to open or to close.
Alternatively, the shutter may be biased in an open state, in which electrons are normally accelerated across the gap, and the control voltage may be applied to decrease the potential difference and close the shutter.
In preferred embodiments of the present invention, the shutter is produced using micro-electromechanical systems (MEMS) technology, based largely on techniques of photolithography. Such techniques are well known in the art of microelectronics manufacturing. The recess in one or both of the plates is produced by etching the plate, which is initially substantially flat. The photocathode, anode and conductive layers are chemically deposited on the appropriate plate surfaces. The two plates are then sealed together under vacuum, preferably using an indium seal or, alternatively, by brazing them, as is known in the art. Finally, electrical leads are connected to the conductive layers, and the device is potted, preferably in insulating plastic, while leaving the active aperture clear, and packaged for use.
Preferably, the plates are degassed before sealing, as is known in the art. Additionally or alternatively, a getter, such as palladium, may be placed in the chamber before sealing. It will be appreciated that shutte

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