The first-ever all-solid-state Optical Refrigerator created
Optical refrigeration: The concept behind the Los Alamos Solid-State Optical Refrigerator (LASSOR) is that a diode laser beam pumps a cooling element which then radiates its heat through anti-Stokes fluorescence. That is, the cooling element absorbs photons at one wavelength and then emits photons at shorter wavelengths. The energy difference between the absorbed and emitted radiation is supplied by annihilating thermal phonons and thereby cooling the solid. The complete refrigerator also requires some way of disposing of the fluorescence and a means of attaching the load to the cooling element. These devices are vibrationless, compact, and free of electromagnetic interference.
LASSOR design
A design for a 1-watt LASSOR. Source: http://lassor.lanl.gov/pub5.pdf
The figure illustrates a 1-watt version of the Los Alamos Solid-State Optical Refrigerator (LASSOR) that might be used for cooling a spaceborne infrared detector or a germanium gamma-ray spectrometer to 77K.
This design is based on ytterbium doped ZBLAN glass. A diode laser coupled with an optical fiber delivers ~1020 nm pump radiation to the cooling element. The fiber carrying the pump radiation passes through the wall of the cryocooler and to a cylindrical block of Yb doped ZBLAN. Dielectric mirrors that reflect the pump radiation are deposited on both ends of the cooling element. The pump radiation enters the cooling element through a small hole in one of the mirrors. This radiation is directed parallel to the axis of the cylinder so that it is repeatedly reflected from the dielectric mirrors and from the sides of the glass cylinder (by total internal reflection). Ultimately, the pump radiation is absorbed by Yb ions, and the doped glass cools as it fluoresces. Most of the isotropic fluorescent radiation escapes from the cooling element and is absorbed by the warm walls of the cooling chamber. A metal mirror is attached on the back of the upper dielectric mirror creating a completely shadowed region where the cold finger can be mounted. The inner surfaces of the chamber walls contain a coating that readily absorbs 1 micron radiation while having a very high reflectivity for ~10 micron thermal emission. This coating would lessen the radiative heat load from the warm chamber walls.
According to the scientists at Los Alamos, optical refrigerator of the type could operate near 77 K with almost 0.5% efficiency (DC power to cooling power), weigh less than 2 kg/W and have a lifetime of 10 years of continuous operation.
Source: http://lassor.lanl.gov/pub5.pdf
The first-ever all-solid-state Optical Refrigerator created
A team led by University of New Mexico professor, Dr. Mansoor Sheik-Bahae have created the first-ever all-solid-state cryocooler that can be applied to airborne and spaceborne sensors. “Optical refrigeration or solid state optical refrigeration technology offers many advantages over currently used, bulky mechanical coolers because it is vibration free (no moving parts), compact, lightweight and agile (fast turn-on and off),” said Sheik-Bahae.
Graduate students Denis Seletskiy and Seth Melgaard designed and performed the experiments at UNM’s department of Physics and Astronomy in collaboration with researchers from Los Alamos National Laboratory and the University of Pisa, Italy.
Sheik-Bahae said, “We obtained cooling down to 155K using optical refrigeration. We expect that material research may lead to temperatures dipping below 77K (boiling point of liquid nitrogen) and in the future as low as 10K may be possible.”
Previously, only solid-state coolers based on standard thermoelectric devices were able to reach temperatures as low as 170K, and even so, only with minimal efficiency.
In order to increase the cooling efficiency of the device, the scientists besides growing pure crystals, using thin optical fibers and keeping their sample in thermal isolation inside a vacuum, also trapped laser light in a resonant space.
Now they plan to research product purity and new materials for cryocoolers. Sheik-Bahae said, “We are also pursuing optical pump sources that can further enhance cooling efficiency of the devices.”
Source: http://www.wpafb.af.mil/news/story.asp?id=123187569
January 29, 2010