Title of Invention

OPTICAL CRYOCOOLER

Abstract

Certain substances such as Yb3+ ZBLANP glass emits anti-Stokes fluorescence when a suitable tuned laser beam is pumped in and undergoes multiple reflections. As the energy of emitted radiation is greater than that of pump laser, it results in cooling of the substance. An optical cryocooler with an auxiliary thermo-electric cooler that is driven by the electricity generated from the waste fluorescence emitted from the working material is claimed. Further, it is claimed that by suitably selecting the working materials and the pump in radiation, it is possible to get the anti-Stokes florescence in an optimum range of frequency to get maximum conversion efficiency with an appropriate photocell.

Full Text

This invention relates to Optical Cryocoolers. More particularly this invention relates to Optical Cryocooler with thermal shields cooled by waste fluorescence energy driven thermo-electric cooler. A cryocooler is a low temperature refrigerator used to cool, for example, infrared detectors, scientific/ biological samples, and superconducting devices. One type of cryocooler such as Stirling/ Pulse tube coolers uses oscillating, compressor driven gas flow for producing cold. Another type is a solid- state cryocooler, which uses laser-induced optical refrigeration. DESCRIPTION OF PRIOR ART: The basic principles of optical refrigeration namely cooling by anti-Stokes fluorescence were first suggested by Pringsheim L. P. [Z.Physik, 57, 739 (1929)]. The basic mechanism of a fluorescent cooler requires a working material to absorb monochromatic radiation at one frequency and to emit fluorescent radiation at higher frequency. The increase in the energy of the fluorescent radiation is due to the removal of thermal energy from the working material thereby resulting cooling of the working material. R. I. Epstein et al. [Nature, 377, 500 (1995)] were first to demonstrate a fluorescent cooler with Yb3+:ZBLANP glass as the working material. When the laser photon pump-in wavelength is tuned to be greater than the mean-fluorescent radiation wavelength of 995 nm, the host material exhibited cooling. Gosnell (Opt letters, 24, 1041 (1999) has demonstrated cooling of Ytterbium doped fiber by 65K from ambient. An U. S. Patent No.6041610 has been obtained by Bradley C. Edwards et al. in March 2000, for an "Optical refrigerator using reflectively tuned dielectric mirrors". They m have discussed a thin disc of work material, with the two end faces dielectric coated such that one of them will allow transmission of shorter wave lengths of fluorescent radiation but reflects the longer wavelengths of fluorescent radiation as well as the monochromatic pump radiation. The broadband dielectric coating on the opposite face connected to the cooled device, reflects the pump radiation and fluorescent radiation of all waves lengths. The transmitted fluorescent radiation of shorter wavelengths are absorbed and dissipated by the chamber walls. In another variation of the device, the radiation is absorbed by an array of photo-diodes, which converts the fluorescent radiation into electricity, augmenting the input power to the laser. LIMITATIONS OF THE PRIOR ART: The following deficiencies are observed from the published literature and patent information, in the present state of art of the optical coolers: (a) There is the absence of thermal shields at intermediate temperature between the cooled components (the cooling material, thermal coupler and cold finger on which the cooled device is mounted) and the ambient temperature chamber wall. Therefore, the cooled components are subjected to thermal radiation, limiting the lowest temperature achieved by the device. The U. S. Patent No. 6,041,610 of March 2000 mentions only the means of removing waste heat from chamber walls but neither the description nor the drawings cover the application of any thermal shields. (b) The above-referred U. S. patent 6,041,610 of March 2000 discusses an array of photo diodes to convert the waste fluorescent radiation to electricity, which can partially augment the input power to the laser unit. However, the patent does not cover the use of the generated electricity to drive auxiliary cooling devices such as thermo-electric coolers. OBJECTS OF THE PRESENT INVENTION: It is the primary object of invention to invent an Optical Cryocooler with thermal shields cooled by waste fluorescence energy driven thermo-electric cooler. It is another object of the invention to invent an Optical Cryocooler whose optimally cooled thermal shields are positioned in the vacuum chamber enclosing the boundaries of the cooled components of the optical cooler except the face of the cooling material transmitting fluorescent radiation. Therefore, the cooled components are not subjected to thermal radiation, and hence the limitation of achieving the lowest temperature by the device no longer remains. It is another object of the invention is to use thermo-electric coolers to cool the thermal shields. It is another object of invention to use the generated electric energy from transmitted fluorescent radiation to drive the thermo-electric coolers. It is another object of the invention is to optimally convert the frequency of the transmitted fluorescent radiation suitable for high efficiency photocells. PROPOSED SOLUTION: This invention, will now be described with reference to the accompanying drawings wherein the Optical Cooler with thermal shield cooled by thermo-electric cooler is illustrated. Generally, an optical cryocooler (Ref. Figl), consists of a vacuum chamber (1), in which the device to be cooled (2) is placed above the cold finger (3). A metal mirror (4) is placed below the cold finger. A thermal coupler (5) is placed in between the cold finger (3) and the cooling device which consists of a cooling materials 6 placed in between two dielectric mirrors (7) & (8). Mirror (7) is a broad band dielectric mirror and mirror (8) is a tuned dielectric mirror. Fluorescent photons required for operation are supplied by the laser pump (9). The cryocooler of the present invention provides a thermal shield (10) around the object to be cooled (2), the cold finger (3) and the cooling device (Ref Fig.l). The said thermal shield is placed enclosing all the boundaries of the cooled components of the optical cooler except the tuned dielectric coated end face of the working material, emitting fluorescent radiation (14) of shorter wavelengths, which falls on the photocell (11). In order to cool the thermal shields (10), thermo-electric coolers (12) are used which are positioned as shown in the figure and which are driven by the electricity generated by the photo-cells (11) and suitably converted by DC/DC converter (13) or by external AC/DC converter (15) or a combination of the two. This proposed concept of a solid-state cooler comprising a combination of fluorescent cooler and thermo-electric cooler will enable more efficient cool down from ambient to low temperatures than the fluorescent cooler alone as it would not allow thermal radiation of the cooled components which is the limitation of the present fluorescent coolers. The Thermal shield used in the present invention, is made of high conducting copper material of about 0.3 mm thickness, gold plated to reduce emissivity and cooled to an optimum temperature in the range of -50°C to -100°C. The Thermo-electric coolers of the present invention are single/ multi modules of about 0.25 W heat pump capacity and about 1.75 W heat pump capacity thermally anchored to the thermal shield, the said thermoelectric coolers are of multistage to provide the above heat pump capacities from 25°C to -50°C or 25°C to -100 °C. The electric energy generated from the transmitted fluorescent radiation drives the 0.25 W thermo-electric cooler, by employing photodiode arrays suitable for fluorescent radiation at about 995-1000nm wavelength and a converter to provide an operating current of about 1.75 amperes and operating voltage of about 4.5 volts. The optimal conversion of wavelength of the 995- lOOOnm fluorescent radiation from the optical cooler to 850nm wavelength radiation so that high conversion efficiency photocells such as GaAs are employed to absorb the radiation and convert it to electric power to drive the thermo electric cooler(s) or partially augment the input power to the thermoelectric cooler(s). The proposed solutions envisages also modifications of the wavelengths of transmitted fluorescent radiation before they impinge upon the photocells such that the wavelengths are suited for highly efficient photocells such as GaAs, which can result in better conversion efficiency of fluorescent radiation to electrical energy. WE CLAIM: 1. An optical cryocooler comprising a chamber containing the object to be cooled placed above a cold finger and a cooling device characterized in that thermal shields are provided enclosing all the boundaries of the cooled components of the optical cooler except the face of the cooling material transmitting fluorescent radiation. 2. An optical cryocooler as claimed in claim 1, wherein the thermal shield is made of high conducting copper material of about 0.3 mm thickness, gold plated to reduce emissivity and cooled to an optimum temperature in the range of -50°Cto -100°C. 3. An optical cryocooler as claimed in claim 1, wherein thermo-electric coolers are single/ multi modules of about 0.25 W heat pump capacity and about 1.75 W heat pump capacity thermally anchored to the thermal shield, the said thermoelectric coolers are of multistage to provide the above heat pxmip capacities from 25°C to -50°Cor25°Cto-100°C. 4. An optical cryocooler as claimed m any one of claims 1 to 3, wherein the electric energy generated from the transmitted fluorescent radiation drives the 0.25 W thermo-electric cooler, by employing photodiode arrays suitable for fluorescent radiation at about 995-1 OOOnm wavelength and a converter to provide an operating current of about 1.75 amperes and operating voltage of about 4.5 volts. 5. An optical cryocooler as claimed in any one of claims 1 to 4, wherein optimal conversion of wavelength of the 995- lOOOnm fluorescent radiation from the optical cooler to 850nm wavelength radiation so that high conversion efficiency photocells such as GaAs are employed to absorb the radiation and convert it to electric power to drive the thermo electric cooler(s) or partially augment the input power to the thermoelectric cooler(s).

Documents:

625-mas-2002 abstract (provisional).pdf

625-mas-2002 abstract duplicate.pdf

625-mas-2002 abstract.pdf

625-mas-2002 claims duplicate.pdf

625-mas-2002 claims.pdf

625-mas-2002 correspondence others.pdf

625-mas-2002 correspondence po.pdf

625-mas-2002 description (complete) duplicate.pdf

625-mas-2002 description (complete).pdf

625-mas-2002 description (provisional).pdf

625-mas-2002 drawing (provisional).pdf

625-mas-2002 drawing duplicate.pdf

625-mas-2002 drawing.pdf

625-mas-2002 form-1.pdf

625-mas-2002 form-13.pdf

625-mas-2002 form-19.pdf

625-mas-2002 form-26.pdf

625-mas-2002 form-3.pdf

625-mas-2002 form-4.pdf

625-mas-2002 form-5.pdf