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
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 4F3/2 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