Dane J. Phillips

Stretched Lens Array (SLA) for Collection and Conversion of Infrared Laser Light: 45% Efficiency Demonstrated for Near-Term 800 W/kg Space Power System

For the past 2frac12 years, our team has been developing a unique photovoltaic concentrator array for collection and conversion of infrared laser light. This laser-receiving array has evolved from the solar-receiving Stretched Lens Array (SLA). The laser-receiving version of SLA is being developed for space power applications when or where sunlight is not available (e.g., the eternally dark lunar polar craters). The laser-receiving SLA can efficiently collect and convert beamed laser power from orbiting spacecraft or other sources (e.g., solar-powered lasers on the permanently illuminated ridges of lunar polar craters). A dual-use version of SLA can produce power from sunlight during sunlit portions of the mission, and from beamed laser light during dark portions of the mission. SLA minimizes the cost and mass of photovoltaic cells by using gossamer-like Fresnel lenses to capture and focus incoming light (solar or laser) by a factor of 8.5X, thereby providing a cost-effective, ultra-light space power system

Spatio-temporal theory of lasing action in optically-pumped rotationally excited molecular gases

We investigate laser emission from optically-pumped rotationally excited molecular gases confined in a metallic cavity. To this end, we have developed a theoretical framework able to accurately describe, both in the spatial and temporal domains, the molecular collisional and diffusion processes characterizing the operation of this class of lasers. The effect on the main lasing features of the spatial variation of the electric field intensity and the ohmic losses associated to each cavity mode are also included in our analysis. Our simulations show that, for the exemplary case of methyl fluoride gas confined in a cylindrical copper cavity, the region of maximum population inversion is located near the cavity walls. Based on this fact, our calculations show that the lowest lasing threshold intensity corresponds to the cavity mode that, while maximizing the spatial overlap between the corresponding population inversion and electric-field intensity distributions, simultaneously minimizes the absorption losses occurring at the cavity walls. The dependence of the lasing threshold intensity on both the gas pressure and the cavity radius is also analyzed and compared with experiment. We find that as the cavity size is varied, the interplay between the overall gain of the system and the corresponding ohmic losses allows for the existence of an optimal cavity radius which minimizes the intensity threshold for a large range of gas pressures. The theoretical analysis presented in this work expands the current understanding of lasing action in optically-pumped far-infrared lasers and, thus, could contribute to the development of a new class of compact far-infrared and terahertz sources able to operate efficiently at room temperature.

A high-efficiency regime for gas-phase terahertz lasers

Significance Optically pumped far-infrared (OPFIR) lasers are one of the most powerful continuous-wave terahertz sources. However, such lasers have long been thought to have intrinsically low efficiency and large sizes. Moreover, all previous theoretical models failed to predict even qualitatively the experimental performance at high pressures. Here, we have developed an innovative model that captures the full physics of the lasing process and correctly predicts the behavior in the high-pressure regime. Validated against experiments, our model shows that nearly all previous OPFIR lasers were operating in the wrong regime and that 10× greater efficiency is possible by redesigning the terahertz cavity. Our results reintroduce the use of OPFIR lasers as a powerful and compact source of terahertz radiation. We present both an innovative theoretical model and an experimental validation of a molecular gas optically pumped far-infrared (OPFIR) laser at 0.25 THz that exhibits 10× greater efficiency (39% of the Manley–Rowe limit) and 1,000× smaller volume than comparable commercial lasers. Unlike previous OPFIR-laser models involving only a few energy levels that failed even qualitatively to match experiments at high pressures, our ab initio theory matches experiments quantitatively, within experimental uncertainties with no free parameters, by accurately capturing the interplay of millions of degrees of freedom in the laser. We show that previous OPFIR lasers were inefficient simply by being too large and that high powers favor high pressures and small cavities. We believe that these results will revive interest in OPFIR laser as a powerful and compact source of terahertz radiation.

Performance of a microgrid polarizer array employing a micro-optic registration element

IERUS Technologies investigated the feasibility of developing a high resolution, passive MWIR polarimetric imaging system for both day and night operation at short (1 – 5 meters) and long (1 – 2 km) range operation. The sensor system used a micro-polarizer array (MPA) over the focal plane array (FPA) in order to capture four channels of polarimetric information simultaneously. It also used an optical registration array (ORA) over the MPA in order to spatially register the polarimetric information. The MPA-ORA device is integral to the FPA, forming a drop-in-replacement, saving system size and weight relative to other polarimetric imaging technologies. A system was designed for a prototype that mitigates risk and demonstrates the utility of the ORA. The FPA employed is a MWIR array with a reticulated detector array which reduces electrical pixel-to-pixel crosstalk to zero. Polarization and radiometric performance predictions of the design will be presented.

Infrared/Terahertz Double Resonance for Chemical Remote Sensing: Signatures and Performance Predictions

Single resonance chemical remote sensing, such as Fourier-transform infrared spectroscopy, has limited recognition specificity because of atmospheric pressure broadening. Active interrogation techniques promise much greater chemical recognition that can overcome the limits imposed by atmospheric pressure broadening. Here we introduce infrared - terahertz (IR/THz) double resonance spectroscopy as an active means of chemical remote sensing that retains recognition specificity through rare, molecule-unique coincidences between IR molecular absorption and a line-tunable CO2 excitation laser. The laser-induced double resonance is observed as a modulated THz spectrum monitored by a THz transceiver. As an example, our analysis indicates that a 1 ppm cloud of CH3F 100 m thick can be detected at distances up to 1 km using this technique.

The potential of wide band-gap semiconductor materials in laser-induced semiconductor switches

Laser induced Semiconductor Switches (LSS), comprised of a gap antenna deposited on a semiconductor substrate and photoexcited by a pulsed laser, are the primary source of THz radiation utilized in time-domain spectroscopy (TDS). THz-TDS applications such as standoff detection and imaging would greatly benefit from greater amounts of power coupled into free space radiation from these sources. The most common LSS device is based on low temperature-grown (LT) GaAs photoexcited by Ti:sapphire lasers, but its power performance is fundamentally limited by low breakdown voltage. By contrast, wide band-gap semiconductor-based LSS devices have much higher breakdown voltage and could provide higher radiant power efficiency but must be photoexcited blue or ultraviolet pulsed lasers. Here we report an experimental and theoretical study of 10 wide band-gap semiconductor LSS host materials: traditional semiconductors GaN, SiC, and ZnO, both pristine and with various dopants and alloys, including ternary and quaternary materials MgZnO and InGaZnO. The objective of this study was to identify the wide bandgap hosts with the greatest promise for LSS devices and compare their performance with LT-GaAs. From this effort three materials, Fe:GaN, MgZnO and Te:ZnO, were identified as having great potential as LSS devices because of their band-gap coincidence with frequency multiplied Ti:Sapphire lasers, increased thermal conductivity and higher breakdown voltage compared to LT-GaAs, as well as picoseconds scale recombination times.

Optical registration array for imaging polarimeters

Deployable polarimetric imaging systems often use 2×2 arrays of linear polarizers at the pixel level to measure the polarimetric signature. This architecture is referred to as a micro-grid polarizer array (MPA). MPAs are either bonded to or fabricated directly upon focal plane arrays. A key challenge to obtaining polarimetric measurements of sub-pixel targets using MPAs is registering the signals from each of the independent channels. Digital Fusion Solutions, Inc has developed a micro-optic approach to register the fields of view of 2x2 subarrays of pixels and incorporated the device into the design of a polarimetric imager. Results of the design will be presented.