KEYWORDS: Atomic clocks, Oscillators, Frequency combs, Rubidium, Chemical species, Fiber lasers, Signal to noise ratio, Solids, Intermodulation, Photodetectors, Clocks
We discuss an optical atomic clock based on a two-photon transition at 778 nm in rubidium. In particular, we discuss
the fundamental limitations to the short-term stability of a system based on a commercial C-band telecom laser as
opposed to a near infrared laser. We show that this system is fundamentally capable of besting a hydrogen MASER in
frequency stability and size.
Europium doped calcium fluoride is a machinable and alkaline-earth resistant crystal that is suitable for constructing a
calcium or strontium vapor cell. However, its heat capacity, emissivity, and high coefficient of thermal expansion make
it challenging to achieve optically dense calcium vapors for laser spectroscopy on narrow linewidth transitions. We
discuss a low size, weight and power heating package that is under development at the Air Force Research Laboratory.
Optical cooling of solids is a promising and innovative method to provide cryogenic cooling to infrared sensors. Currently insulator crystals, specifically ytterbium-doped yttrium- lithium-fluoride (Yb:YLF), have shown the most promise for cooling to low temperatures. This method has demonstrated cooling below the National Institute of Standards and Technology (NIST) cryogenic temperature definition of less than 123 K. Optical refrigeration utilizes a phenomenon called anti-Stokes fluorescence to generate cooling power. Incident laser light is absorbed by the cooling crystal and photons are spontaneously emitted at a higher, and thus more energetic, frequency. The difference in frequency is proportional to the cooling power of the crystal. Anti-Stokes cooling is highly dependent on doping percentages and YLF crystal purity and structure. Space based infrared sensors and their coolers are operated in a radiation environment where protons, gamma, rays, heavy ions, and other radiation species are common and of varying severities depending on operational orbit. To ensure that radiative effects on cooling crystal performance are minimal, we irradiated two samples with 63 MeV protons to a total of ionized dose of 100 Krad (Si) and 1 Mrad (Si), and compared cooling crystal efficiency parameters before and after dosing.
Optical refrigeration is currently the only completely solid state cooling method capable of reaching cryogenic temperatures from room temperature. Optical cooling utilizing Yb:YLF as the refrigerant crystal has resulted in temperatures lower than 123K measured via a fluorescence thermometry technique. However, to be useful as a refrigerator this cooling crystal must be attached to a sensor or other payload. The phenomenology behind laser cooling, known as anti-Stokes fluorescence, has a relatively low efficiency which makes the system level optimization and limitation of parasitic losses imperative. We propose a variety of potential designs for a final optical refrigerator, enclosure and thermal link; calculate conductive and radiative losses, and estimate direct fluorescence reabsorption. Our simulated designs show losses between 60 and 255 mW, depending on geometry and enclosure choice, with a lower bound as low as 23 mW.
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