Optical waveguides – With optical coupler – Switch
Reexamination Certificate
1999-05-04
2001-07-17
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Switch
C250S2140LS, C313S484000
Reexamination Certificate
active
06263124
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a photoconductive switch comprising at least a first layer and two contact layers arranged on said first layer and connectable to different potentials for applying a voltage thereacross, said first layer being adapted to be conducting upon applying a voltage across said contact layers when exposed to light from an illumination source of an energy high enough for creating charge carriers in the material of said first layer.
A switch of this type finds many applications, and it may for instance be used in equipment for handling high electric power for switching high voltages (which may be 2-400 kV) and currents, for example in surge diverters, current limiters and the like. An advantage of a switch of this type is that illumination control provides for a very fast switching, which is of particular importance in high power applications for protection of the equipment when faults occur.
One problem of such switches already known is that the illumination sources thereof have poor efficiency, i.e. an unproportionally high power input is required for obtaining the degree of illumination necessary for appropriately controlling the switch. The reason for this is that only certain wavelengths of the light produced by such an illumination source may be utilized for creating said charge carriers in the material of said first layer and accordingly close the switch, namely the wavelengths being shorter than a threshold wavelength corresponding to the energy gap between said valence band and conduction band, and the illumination sources used in such switches generate light distributed over a comparatively wide spectrum, so that a large part thereof may not be utilized for the generation of free charge carriers in said layer. Accordingly, much of the power consumed in producing illumination required to trigger the switch such that when conductive, low on-state losses are obtained, is wasted, Activating the switch thusly is not cost-effective.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a photoconductive switch of the type defined in the introduction being in at least some aspect improved with respect to such switches already known and which at least partially solves the problems discussed above of the switches already known.
This object is according to the present invention obtained by providing such a photoconductive switch, in which the illumination source is a device comprising a volume filled with a gas or gas mixture having gas atoms able to form unstable electronically excited dimers when such a voltage is supplied over a gap in said volume that free electrons are created and accelerated to come into collision with said gas atoms for excitation thereof, said dimers decomposing into two gas atoms while emitting a photon of an energy suitable for creating charge carriers in said material of the first layer, the pressure of the gas and the dimension of said gap being selected for allowing the formation of electronically excited dimers to take place.
Accordingly, a so-called excimer lamp is used as an illumination source in the photoconductive switch according to the invention, and such an illumination source is known to emit intense radiation of a certain wavelength with very low background, i.e. a very low intensity of light with other wavelengths. The wavelength at which the light is emitted is determined by the choice of gas atoms for said gas or gas mixture, since the “excimers” (excited dimers) created emit a photon of a well-defined energy when they decompose. This means that the power efficiency of the illumination source will be very high, so that it will be easy to quickly and reliably control the switch without any powerful energy source. The fact that said dimers only exist in the excited state, i.e. under normal conditions they do not possess a stable ground state, explains a very important property of these illumination sources, namely that they are completely transparent to the radiation generated by the discharge in the gas creating said free electrons. Reabsorption of their own radiation cannot occur, as there are no stable molecules which could absorb the radiation, allowing unhindered passage of the photons formed through the gas. This is one of the main reasons for the high intensities and high efficiency of these illumination sources.
According to a preferred embodiment of the invention said illumination device comprises at least one blind hole structure with at least a bottom-near part forming a first electrode and an upper part forming a second electrode and means for applying said voltage for creating said free electrons across said electrodes forming a cathode and an anode. Such a blind hole structure is known through for example “Emission of excimer radiation from direct current, high-pressure hollow cathode discharges”, Appl. Phys. Lett. 72 (1), Jan. 5, 1998, and it is called “hollow cathode” although it is not necessarily a cathode that is hollow, but the cathode and anode being a part of a hollow structure. It has been found that discharges generating the emission of photons when said unstable electronically excited dimers decompose are stable at much higher currents in such blind hole structures of small hole diameters than when the mechanism of creating unstable electronically excited dimers for a generation of light is utilized by applying a voltage across an anode and a cathode separated by said gas or gas mixture without any blind hole structure. Accordingly, such an illumination device may efficiently create a high power for the illumination of the first layer of the switch making it very fast and closing with a low on-state resistance.
According to another preferred embodiment of the invention the first and second electrode of the blind hole structure are separated by wall parts made of a dielectric material, and the arrangement of such a dielectric material between the two electrodes prevents the hot, high-velocity electrons created through the voltage applied across the electrodes from causing an arc discharge.
According to another preferred embodiment of the invention the switch comprises a means for generating said voltage high enough for creating said free electrons and adapted to generate a voltage of 20 V-2 kV across said electrodes. A voltage within this range will be enough for creating a high intensity light for advantageously controlling a switch of this type.
According to another very preferred embodiment of the invention said device comprises an array of said blind hole structures. By arranging an array of said blind hole structures light of a very high intensity with very little background radiation may be created, especially if the largest cross-section dimension of each blind hole structure, i.e. normally the diameter of the blind hole, is small, such as smaller than 500 &mgr;m and preferably smaller than 200 &mgr;m, which enables the provision of more than 10
4
, especially more than 10
5
, and preferable more than 5×10
5
blind hole structures in said array. This means that power in the order of megawatts may be emitted by such an illumination device.
According to another preferred embodiment of the invention constituting a further development of the embodiment last mentioned said illumination device comprises means for focusing light emitted from the respective blind hole structure towards said first layer of the switch, so that the light emitted from each individual blind hole structure may be added to the total light applied on said first layer of the switch, and said focusing means is according to another preferred embodiment of the invention formed by arranging said array of blind hole structures along a circle arc with the axis of the respective blind hole pointing substantially towards the center of said circle and arranging said first layer of the switch in the region of said center.
According to another preferred embodiment of the invention said first layer is made of an intrinsic material, and the switch is adap
Bernhoff Hans
Ekberg Mats
Irwin Mark
Isberg Jan
Isberg Peter
ABB AB
Connolly Bove Lodge & Hutz
Lee John D.
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