(a) Field of the Invention
The invention relates to electronic and quantum devices, and more particularly, to laser cathode-ray tubes (CRT), e.g. , used in projection television systems for displaying images on large screens.
(b) Description of the Related Art
Projection television apparatuses based on conventional cathode-ray tubes having a phosphor screen are widely used for displaying images on the projective screens having an area of up to several square meters. However, the size of an image on the projective screens of the television apparatuses is limited because the phosphor screens of the projective apparatuses cannot form light flux of high intensity, thus making it difficult to form television images having required brightness and contrast.
An effective way to improve the parameters of projection television systems is concerned with using laser CRT""s (see, for example, U.S. Pat. No. 3,558,956).
As distinct from the conventional CRT""s, the source of radiation in the laser CRT is a laser target, not a luminophor layer, the laser target being a thin semiconductor monocrystalline plate having both of its parallel surfaces covered by light reflecting coatings. A fully reflecting metal coating mirror is usually applied to the surface on which the electron beam is incident, while the opposed side of the plate is covered with a semitransparent mirror coating. The mirror surfaces constitute an optical resonator, while the semiconductor plate between them acts as an active medium of the laser with electron-beam excitation (pumping). The laser target is fixed to a substrate of a transparent dielectric material, the substrate serving as the optical output window of the laser CRT and also as a heat sink for the laser target. A cooling system is connected to the substrate to cool the laser target on which a great amount of heat is generated under the action of the high energy electron beam. The substrate is usually made of material having a high thermal conductivity at low temperatures, such as sapphire. The laser target and the transparent substrate constitute the laser screen of the laser CRT (laser screen).
The electron beam penetrates the metal coating into the semiconductor plate and induces spontaneous light radiation. When the current density of the laser target surface produced by the electron beam exceeds a threshold value, the power of the induced light radiation will be greater than the losses in the optical resonator and the element of the laser target on which the electron beam is incident will generate laser radiation. When light bounces repeatedly through the resonator, its spectrum narrows, with the result that the emitted light is monochromatic. The laser light is radiated through the semitransparent mirror coating perpendicularly to the surface of the semiconductor plate and leaves the CRT through the sapphire output window.
In U.S. Pat. No. 5,280,360 a laser CRT is described, the CRT including a cathode for generating an electron beam, a laser screen including a laser target of semiconductor plate placed on a face of a transparent dielectric substrate, a focusing system for focusing the electron beam on the laser target, the cooling system being connected to the side surface of the substrate.
The method of exciting the screen of the laser CRT according to the above-mentioned patent comprises generating an electron beam and directing this beam on an element of the laser screen.
For required acceleration of the electron beam, the laser target shall be under a high positive potential (about 50-70 KV) with respect to the cathode. In the known laser CRT the cathode is connected to a source of a high negative potential and the laser target is grounded. Such a scheme of feeding the accelerating voltage enables the laser screen to be easily connected to the grounded system for cooling the laser target.
However, applying the high potential to the cathode extremely complicates the electrical circuits connected to the cathode and to the electrodes located near the cathode. Such circuits include, for example, cathode filament supply circuits, video signal amplifiers, bias voltage sources. The complication of these electrical circuits is caused by the necessity to take measures for electrically isolating these circuits from the ground and results in higher manufacturing expenses and thus higher cost of the apparatuses based on such laser CRT""s.
The principle object of the invention is to provide a laser CRT and a method for exciting its screen, wherein the high accelerating voltage is applied in such a way as to provide the possibility of grounding the electrical circuits connected to the cathode of the CRT and thus permit their simplification and decrease in the cost of the apparatuses based on the laser CRT.
With these principle object in view, there is proposed a laser CRT provided with an electron gun having a cathode for generating an electron beam, a laser screen having a laser target of semiconductor plate, a focusing system for focusing the electron beam on the laser target, a deflection system for deflecting this beam and a cooling system for cooling the laser target, wherein, according to the invention, the cathode is connected substantially to ground potential and the laser target is connected to high positive potential.
The cooling system in the proposed laser CRT is substantially to the ground potential.
From the laser target the cooling system is isolated by an electrical isolator.
The laser screen of the CRT includes a transparent dielectric substrate constituting said isolator and the laser target being fixed to one of the end faces of the substrate, while the cooling system is connected to a peripheral portion of the opposite face of the substrate.
Connection of the cooling system with the substrate end face opposite to the laser target, instead of with its side surface, as in the known laser CRT, allows the dielectric substrate of the laser target to be used as means ensuring reliable electrical isolation of the latter from the cooling system and thus makes it possible to apply high positive potential to the laser target, while the cooling system remains grounded. In this design the side surface of the substrate, unlike known designs, has no metal coating.
The cooling system is preferably connected to said periphery of the substrate face through a metal flange.
The metal flange is preferably connected to the second metal flange soldered in the bulb of the CRT.
The transparent substrate is preferably made as a sapphire disc.
The high potential is preferably applied to the laser target by the high voltage input device connected to the laser target through a conductive cylinder made of non-magnetic material and positioned in longitudinally coaxial line with the CRT.
The conductive cylinder edge connecting the laser target is preferably provided with at least one contact spring.
The proposed laser CRT may further comprise a dielectric cylinder having a greater diameter than the conductive cylinder and enclosing a conductive cylinder portion adjoining the laser target, the dielectric cylinder being fixed to peripheral portion of the transparent substrate face joining the laser target co-axially with respect to the conductive cylinder.
The inner surface of the CRT casing between the high potential input device and the second metal flange is preferably coated with a non-conductive coating for preventing surface discharges.
The outer area of the tube casing around the high potential input device is preferably covered with isolating compound.
With the above principle in view, there is also proposed a method for exciting the screen of a laser CRT, wherein an generated electron beam is directed on an element of the laser target for exciting the laser radiation, in which method, according to the invention, the electron beam is generated with a cathode connected substantially to ground potential, while the laser target is connected to a high positive potential.