Solid state lasers developed rapidly during the year since the first ruby laser in the 1950's. (T. H. Maiman--Nature 187, 493 (1960)) In 1964 the Nd doped YAG was discovered (J. E. Geusic, H. M. Marcos and L. G. Van Uitert (App. Phys. Lett. 4, 182 [1964]). This Nd-YAG technology is in wide use today and is being continuously improved. By the 1970's solid state lasers entered a period of mass commercialization. Today, many new solid state lasers are based on rare earth and other elements such as uranium, chromium, samarium, ytterbium, thulium, holmium, erbium, and many others are being developed (A. A. Kaminskii "Crystalline Lasers: Physical Processes and operating Schemes", CRC Press 1996).
Many lasers are used by aircraft, tanks, vehicles as well as foot soldiers. Most of the these lasers are either pumped by flash lamps or laser diodes which are bulky. These processes are inefficient and expensive as well. Therefore, there is a need for a low cost, lighter weight, efficient laser. This patent describes lasers which use a thermally stimulated photon emitting light pipe as the pumping device, which may be heated by any heat source such as combustion, nuclear, radioisotopic, chemical, solar or the like.
A variety of different combustion systems useful as a heat source for the thermally stimulated photon emitting light pipe or "light pipe" can be designed depending on specific application. Some combustion systems can employ an emissive matrix combustor having the beneficial characteristics of producing a high power density and providing a high radiant output heat source. Emissive matrix combustion systems provide an extremely high power density (up to 30 W/cm.sup.2) and have a radiant output of about 50% of total chemical energy that is released in a combustion process.
Another approach can be to design a vortex combustion system, having high power density and extended residence time of hot combustion products, in a light pipe containing section that increases the thermal energy transfer from the exhaust to the light pipe.
It is also possible to fabricate a microchannel combustor having a high power specific density (up to 100 W/cm.sup.3) and having a highly extended light pipe area, or combination of these or other combustion technologies such as open flame combustors, surface combustors, and the like.
Superemissive insulation that covers internal shell of the combustion chamber is an additional source of the narrow band photons which can transfer energy through the light pipe to the laser. Superemissive materials can be described as materials that, when heated to a threshold temperature, includes one or more electrons that jump to a different electron energy level in quantum increments which causes the emission of ultraviolet, visible or infrared radiation in a wavelength band related to the atom's inner electron shell vacancy. Emitted radiation produced as a result of such electron transition is often within a narrow band and can, therefore, be absorbed efficiently by a laser system, such as Nd doped YVO.sub.4 or YAG.
Thermally-stimulated superemissive materials produce radiation in relatively concentrated narrow special bands compared to blackbody or greybody emitters which typically exhibit a broadband thermal emission such as from a flash lamp. As a result of the concentrated, narrow spectral band, photons emitted from the superemitter and focused to the laser system one would expect greater efficiency than that generated by a blackbody emitter operating at the same heat flux. However, a blackbody emitter may be constructed to emit visible or infrared radiation in a narrow spectral band by using one or more band-pass filters interposed between the superemitter and the target laser or other device. Photon filtering, however, is not energy efficient. On the other hand, laser diode can emit in a very narrow band, but must be powered by power conditioned electric systems which are expensive, inefficient, heavy and bulky.
Photon generators that use superemitters to emit radiation and, therefore, generate photons are well-known and are disclosed in U.S. Pat. Nos. 4,776,895, 4,793,799, 4,906,178. For example, such a photon generator includes a porous ceramic burner in the shape of a cylinder having an annular passage extending therethrough. A superemitting fiber layer is disposed along the inside wall of the ceramic burner and is made from a high temperature fiber or coating comprising, for example, pure or doped oxides of uranium, thorium, ytterbium, aluminum, gallium, yttrium, erbium, holmium, zirconium, chromium or other high temperature oxides. When subjected to thermal energy, the fiber layer emits radiation that is directed to a central axis running along the annular passage of the cylindrical burner.
As the ceramic burner is heated, the superemissive fiber layer emits radiation that can be filtered, by use of a cylindrical hollow filter disposed within the ceramic burner, to emit radiation at a selected bandwidth. The radiation passing through the filter is directed to the surface of an optical cable that is disposed centrally within the annular passage of the ceramic burner and filter. The photons generated by the fiber layer are, therefore, directed onto the optical cable and are channeled through the cable to each cable end, which is directed to a target comprising a photovoltaic cell. Accordingly, the photons generated by the superemissive fiber layer within the ceramic burner are directed through the optical cable into the photovoltaic cell, and converted to electricity.
The embodiments of the photon generator disclosed in earlier identified U.S. patents and U.S. patent applications do not promote the most efficient generation and collection of photons to pump and power laser systems. In the above-discussed embodiments, the emission of photons is effected through application of thermal energy to the body, which passes through the body of thermal conduction to the superemissive material. Accordingly, a large amount of thermal energy is wasted through the mechanism of thermal conduction. Additionally, the photons emitted by the superemissive material are not collected efficiently, as relatively weaker photons, or photons that are emitted from a distance further away from the optical waveguide than other emitted photons, are not collected within the waveguide. Accordingly, many of the photons generated by the superemitter are not collected by the optical waveguide and, therefore, are not directed to the target(s).
It is, therefore, desirable that a device be constructed to facilitate the economic generation and collection of selected photons, and be capable of directing them to a laser to optically pump the laser. It is desired that the device be simple to construct and be easily adaptable for use with a number of different thermal generation sources. It is desirable that the device be configured to accommodate use with one or more target laser systems and also to generate electricity by conversion of photons to electricity by means of one or more photovoltaic cells. It is desirable that the device be constructed in such a manner as to eliminate or minimize the effects of thermal shock on the cell. It is desirable that the device be capable of being manufactured from conventional materials using current manufacturing techniques. It is also desirable for this unit to be low cost, light weight and compact as well as energy efficient. The ability to operate directly from the available heat source without the need for electric power and power conditioning is very desirable.