This PDF file contains the front matter associated with SPIE Proceedings Volume 7951, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Infrared (IR) space sensing missions of the future depend on low mass, highly capable imaging technologies. Limitations in visible imaging due to the earth's shadow drive the use of IR surveillance methods for a wide variety of applications. Utilization of IR sensors greatly improves mission capabilities including target behavioral discrimination. Background IR emissions and electronic noise that is inherently present in Focal Plane Arrays (FPAs) and surveillance optics bench designs obviates their use unless they are cooled to cryogenic temperatures. The interaction between cryogenic refrigeration component performance and the IR sensor optics and FPA can be seen as not only mission enabling but also as mission performance enhancing when the refrigeration system is considered as part of an overall optimization problem.

In this paper we review our recent progress in development of a ytterbium doped yttrium-lithium fluoride cryocooler. Preliminary modeling results are presented that indicate requirements on the material parameters to reach sub-100 K temperatures.

We utilize highly sensitive spectroscopic local temperature probe to ascertain cooling performance of Yb:YLF crystal as a function of wavelength and temperature. A minimum achievable temperature of 120 K is measured at pump wavelength corresponding to E4-E5 Stark manifold transition. Results verify current model for laser cooling cycle as well as demonstrate the lowest temperature achievable by means of optical refrigeration to date.

In the present work, we report on infrared thermography measurements in Nd-doped KPb₂Cl₅ crystal and powder above and below the barycentre of the ⁴F_3/2 level that were performed in order to assess the relative weights of both the direct anti-Stokes absorption processes and those assisted by either excited state absorption or energy transfer upconversion when cooling takes place in the material. As the laser induced temperature changes are usually small, we used a special configuration of the samples that allowed us to obtain differential measurements where an undoped sample acted as a temperature baseline. This method allows us to ascertain whether the recorded temperature changes are optically induced or they are due to some other effect.

Optical cooling in an all fiber system using fiber laser pumps and cooling fibers doped with rare earth ions has been investigated both theoretically and experimentally. A 2% Tm doped germanate glass was selected from glasses with different Tm concentrations 0.5, 1, 2, 3, 4, 5, 6, 8 and 10% wt for fabrication of the cooling fiber. A high efficiency, single mode Tm-doped fiber laser has been built to pump a Tm-doped fiber cooler. The cooling experiments done in a vacuum chamber show indications that cooling has occurred in the fiber. A theoretical framework to understand the nature of cooling in this laser cooling system has been developed which highlights the cooling power available as a function of various material and fiber parameters including background loss and absorption saturation effects in the cooling fiber. Cooling characteristics, with special emphasis on the fiber's saturation behavior, have been studied using theoretical models of Tm³⁺-doped glass (4-level models) and Tm³⁺ doped KLa(WO₄)₂ crystals (20-level model).

We present a scattering model which enables us to describe the mechanical force, including the velocity dependent component, exerted by light on polarizable massive objects in a general one-dimensional optical system. We show that the light field in an interferometer can be very sensitive to the velocity of a moving scatterer. We construct a new efficient cooling scheme, 'external cavity cooling', in which the scatterer, that can be an atom or a moving micromirror, is spatially separated from the cavity.

External quantum efficiency of semiconductor photonic devices is directly measured by wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent photoluminescence. Differential power-dependent photoluminescence measurement is used to quickly screen the sample quality before processing.

Laser cooling of solids has many potential applications in communications, surveillance and medical science, etc. This paper reviews our recent work on laser refrigeration of solids in stoichiometric hexa-chloroelpasolite crystals with high concentrations of erbium. Our results are one to two orders of magnitude improvements over any previously reported cooling in erbium based materials. We have seen bulk crystal cooling by as low as 6 degrees Celsius below room temperature in elpasolite crystals. The high efficiency and low temperatures of cooling have been attributed to a high concentration of erbium, as high as 80% by formula, in the crystal. This cooling has been achieved by pumping near the IR transition at 870 nm which is the weakest transition of erbium. We report cooling in the 1.5 micron transition as well, using a diode laser to pump this transition. The result is cooling by only a fraction of a degree from room temperature.

A new approach to cool rare earth doped solids with optical super-radiance (SR) is presented. SR is the coherent, sharply directed spontaneous emission of photons by a system excited with a pulsed laser. We consider an Yb3+ doped ZBLAN sample pumped at the wavelength 1015nm with a rectangular pulsed source with a power of ~700W and duration of 20ns. The intensity of the SR is proportional to the square of the number of excited ions. This unique feature of SR permits an increase in the rate of the cooling process in comparison with the traditional laser cooling of the rare earth doped solids with anti-Stokes spontaneous incoherent radiation (fluorescence). This scheme overcomes the limitation of using only low phonon energy glasses for laser cooling.

In thermophotonic cooling (TPC) the requirement of efficient photon extraction out of the semiconductor material is removed by the absorption of the light within the semiconductor structure. TPC cooling currently offers one of the most viable approaches to observing electroluminescent cooling of semiconductors in practice. We discuss a detailed numerical model that accounts for the current and photon transport as well as various loss mechanisms like the mirror losses and the nonradiative losses in the structure. The model essentially consists of the semiconductor equations for current transport and the radiative transfer equation for the photon transport.

A 915 nm T:Sapphire laser was used to excite luminescence from Yb³⁺ doped YAG crystal. The excited ions undergo nonradiative relaxation followed by strong emission at 1030 nm. The heating produced by the nonradiative relaxation increases the sample temperature. A Mach-Zehnder interferometer was setup with 514.5 nm Ar⁺ laser beam. Laser heating causes the interferometer fringes to move. A mathematical formula was developed to estimate the change in sample temperature from the fringe count. When 300mW Ti:Sapphire laser beam was focused through the YAG:Yb³⁺crystal its temperature increased by 6.9°C. This technique works equally well to measure the solid sample temperature changes in laser cooling studies.

We demonstrate application of the thermal reflectance measurement in a balanced detector arrangement to resolve laser induced temperature shifts in ytterbium-doped yttrium-lithium-fluoride (Yb:YLF) during optical refrigeration experiments. Definite signature of cooling versus heating allows for rapid screening of the performance of the laser cooling material.