We report detection and identification of trace quantities of explosives at standoff distances up to 150 m with high sensitivity (signal-to-noise ratio of approximately 70) and high selectivity. The technique involves illuminating the target object with laser radiation at a wavelength that is strongly absorbed by the target. The resulting temperature rise is observed by remotely monitoring the increased blackbody radiation from the sample. An unambiguous determination of the target, TNT, in soil samples collected from an explosives test site in China Lake Naval Air Weapons Station is achieved through the use of a tunable CO(2) laser that scans over the absorption fingerprint of the target explosives. The theoretical analysis supports the observation and indicates that, with optimized detectors and data processing algorithms, the measurement capability can be improved significantly, permitting rapid standoff detection of explosives at distances approaching 1 km. The detection sensitivity varies as R(-2) and, thus, with the availability of high power, room-temperature, tunable mid-wave infrared and long-wave infrared quantum cascade lasers, this technology may play an important role in screening personnel and their belongings at short distances, such as in airports, for detecting and identifying explosives material residue on persons.

A strain-balanced, AlInAs/InGaAs/InP quantum cascade laser structure, designed for light emission at 4.0 μm using nonresonant extraction design approach, was grown by molecular beam epitaxy. Laser devices were processed in buried heterostructure geometry. An air-cooled laser system incorporating a 10-mm × 11.5-μm laser with antireflection-coated front facet and high-reflection-coated back facet delivered over 2 W of single-ended optical power in a collimated beam. Maximum continuous-wave room temperature wall plug efficiency of 5.0% was demonstrated for a high-reflection-coated 3.65-mm × 8.7-μm laser mounted on an aluminum nitride submount.high power | midinfrared Q uantum cascade lasers (QCLs) are important infrared light sources with various applications in defense and civilian fields. Low atmospheric absorption in the first atmospheric window spanning from 3.5 to 4.8 μm gives rise to a number of applications based on free light propagation, such as light detection and ranging sensors and beacons. Strong carbon dioxide absorption for wavelengths from 4.2 to 4.4 μm splits the window into two subbands: 3.5-4.2 and 4.4-4.8 μm. Light propagation at wavelengths in either of the two subbands does not experience any significant atmospheric losses. Currently, most of the systems for commercial and defense applications in the wavelength regions rely on expensive and often unreliable optical parametric oscillators (OPOs) or flash lamps as the optical radiation sources. Recent breakthrough developments in continuous-wave (CW) QCL performance at 4.6 μm (1, 2) make them ideal for applications in these systems as light sources in the longer wavelength region of the first atmospheric window. Availability of highperformance QCLs emitting in the shorter wavelength region covering 3.5-4.2 μm, in addition to the high-performance 4.6-μm QCLs, would allow replacing OPOs and flash lamps with compact, reliable, more energy-efficient, and less-expensive QCL systems. However, QCL performance in the shorter wavelength region has lagged significantly behind that of their longer wavelength counterparts. The highest CW room-temperature wall plug efficiency (WPE) and optical power reported, until now, are ∼3% and 500 mW, respectively, for lasers mounted on diamond submounts (3). In the present work, we report significant improvement in 4.0-μm QCL performance and realization of reliable and compact air-cooled 4.0-μm QCL systems capable of delivering over 2 W of optical power in a collimated beam.One of the reasons for the poorer performance of QCLs at wavelengths shorter than 4.5 μm is carrier leakage from the upper laser level through closely located indirect states. As laser transition energy increases, the upper laser level moves up, approaching the bottom of the indirect band profile corresponding to indirect X or L valleys. Reduced energy spacing between the upper laser level and bottom of indirect valleys increases carrier scattering from the upper laser level to indirect states leading to a reduction in population inversion. Another ...

Dielectric mirror leakage at large angles of incidence limits the effectiveness of solid state optical refrigerators due to reheating caused by photon absorption in an attached load. In this paper, we present several thermally conductive link solutions to greatly reduce the net photon absorption. The Los Alamos Solid State Optical Refrigerator(LASSOR) has demonstrated cooling of a Yb 3 + doped ZBLANP glass to 208 K. We have designed optically isolating thermal link geometries capable of extending cooling to a typical heat load with minimal absorptivereheating, and we have tested the optical performance ofthese designs. A surrogate source operatingat 625 nm was used to mimicthe angular distribution of light from the LASSOR cooling element. While total link performance is dependent on additional factors, we have found that the best thermal link reduced the net transmission of photons to 0.04%, which includes the trapping mirrors 8.1% transmission. Our measurements of the optical performance of the various link geometries are supported by computer simulations of the designs using Code V, a commercially available optical modelingsoftwarepackage.

Optical refrigeration has been demonstrated by several groups of researchers, but the cooling elements have not been thermally linked to realistic heat loads in ways that achieve the desired temperatures. The ideal thermal link will have minimal surface area, provide complete optical isolation for the load, and possess high thermal conductivity. We have designed thermal links that minimize the absorption of fluoresced photons by the heat load using multiple mirrors and geometric shapes including a hemisphere, a kinked waveguide, and a tapered waveguide. While total link performance is dependent on additional factors, we have observed net transmission of photons with the tapered link as low as 0.04%. Our optical tests have been performed with a surrogate source that operates at 625 nm and mimics the angular distribution of light emitted from the cooling element of the Los Alamos solid state optical refrigerator. We have confirmed the optical performance of our various link geometries with computer simulations using CODE V optical modeling software. In addition we have used the thermal modeling tool in COMSOL MULTIPHYSICS to investigate other heating factors that affect the thermal performance of the optical refrigerator. Assuming an ideal cooling element and a nonabsorptive dielectric trapping mirror, the three dominant heating factors are ͑1͒ absorption of fluoresced photons transmitted through the thermal link, ͑2͒ blackbody radiation from the surrounding environment, and ͑3͒ conductive heat transfer through mechanical supports. Modeling results show that a 1 cm 3 load can be chilled to 107 K with a 100 W pump laser. We have used the simulated steady-state cooling temperatures of the heat load to compare link designs and system configurations.

We have used the thermal modeling tool in COMSOL Multiphysics to investigate factors that affect the thermal performance of the optical refrigerator. Assuming an ideal cooling element and a non-absorptive dielectric trapping mirror, the three dominant heating factors are blackbody radiation from the surrounding environment, conductive heat transfer through mechanical supports, and the absorption of fluoresced photons transmitted through the thermal link. Laboratory experimentation coupled with computer modeling using Code V optical software have resulted in link designs capable of reducing the transmission to 0.04% of the fluoresced photons emitted toward the thermal link. The ideal thermal link will have minimal surface area, provide complete optical isolation for the load, and possess high thermal conductivity. Modeling results imply that a lcm' load can be chilled to 102 K with currently available cooling efficiencies using a 100 W pump laser on a YB:ZBLANP system, and using an ideal link that has minimal surface area and no optical transmission. We review the simulated steady-state cooling temperatures reached by the heat load for several link designs and system configurations as a comparative measure of how well particular configurations perform.