Mid- and long-wave infrared point spectrometer (MLPS): a miniature space-borne science instrument.

A long standing debate has existed on the utility of Mid Wave Infrared (MWIR) vs Long Wave Infrared (LWIR) remote-sensing imagery for Precision Agriculture applications. We have conducted a combined qualitative and quantitative, multi-field, aircraft remote-sensing study to show that LWIR measurements often result in more actionable for day-to-day farm management applications. LWIR imagery when processed with SaraniaSat's nonlinear, weak-signal detection algorithms is an effective predictor of crops stress and ultimate harvest yield. It is also shown that both processed LWIR and MWIR imagery contain actionable, crop-stress information that is absent in the conventional Normalized Difference Vegetation Index (NDVI) that is currently, widely used for Precision Agriculture. Finally, at a simulated 10m ground resolution (satellite remote sensing) it is shown, once again, that LWIR is superior and more actionable.

Current stand-off hyperspectral imaging detection solutions that operate in the mid-wave infrared (MWIR), nominally 2.5 – 5 μm spectral region, are limited by the number of absorption bands that can be addressed. This issue is most apparent when evaluating a scene with multiple absorbers with overlapping spectral features making accurate material identification challenging. This limitation can be overcome by moving to the long wave IR (LWIR) region, which is rich in characteristic absorption features, which can provide ample molecular information in order to perform presumptive identification relative to a spectral library. This work utilises an instrument platform to perform negative contrast imaging using a novel LWIR optical parametric oscillator (OPO) as the source. The OPO offers continuous tuning in the region 5.5 – 9.5 μm, which includes a number of molecular vibrations associated with the target material compositions. Scanning the scene of interest whilst sweeping the wavelength of the OPO emission will highlight the presence of a suspect material and by analysing the resulting absorption spectrum, presumptive identification is possible. This work presents a selection of initial results using the LWIR hyperspectral imaging platform on a range of white powder materials to highlight the benefit operating in the LWIR region compared to the MWIR.

The increasing availability of complex multivariate data yielded by sensor technologies permits qualitative and quantitative data analysis for material characterization. Multivariate data are hard to understand by visual inspection and intuition. Thus, data-driven models are required to derive study-specific insights from large datasets. In the present study, a partial least squares regression (PLSR) model was used for the prediction of elemental concentrations using the mineralogical techniques mid-wave infrared (MWIR) and long-wave infrared (LWIR) combined with data fusion approaches. In achieving the study objectives, the usability of the individual MWIR and LWIR datasets for the prediction of the concentration of elements in a polymetallic sulphide deposit was assessed, and the results were compared with the outputs of low- and mid-level data fusion methods. Prior to low-level data fusion implementation, data filtering techniques were applied to the MWIR and LWIR datasets. The pre-processed data were concatenated and a PLSR model was developed using the fused data. The mid-level data fusion was implemented by extracting features using principal component analysis (PCA) scores. As the models were applied to the MWIR, LWIR, and fused datasets, an improved prediction was achieved using the low-level data fusion approach. Overall, the acquired results indicate that the MWIR data can be used to reliably predict a combined Pb–Zn concentration, whereas LWIR data has a good correlation with the Fe concentration. The proposed approach could be extended for generating indicative element concentrations in polymetallic sulphide deposits in real-time using infrared reflectance data. Thus, it is beneficial in providing elemental concentration insights in mining operations.

Multispectral infrared imaging systems that simultaneously operate in the long-wave infrared (LWIR) and mid-wave infrared (MWIR) bands offer significant advantages for target detection and recognition. However, conventional infrared optical systems rely on bulky multi-element lens assemblies to accommodate incident wavelengths of LWIR and MWIR bands, making it challenging for compact thermal optics design. Here, we propose and experimentally demonstrate an inverse designed infrared metalens capable of simultaneously focusing LWIR and MWIR radiation at wavelengths of 9.5 μm and 4.75 μm with a focal length variation of 1%. In the experiment, the proposed metalens with detector enables a dual-wavelength thermal imaging with a compact size (26 × 26 × 18 mm3) and a light weight (19 g). This work establishes a robust and scalable inverse design strategy for dual-wavelength infrared metalenses and provides a promising route toward compact, integrated, and multifunctional infrared imaging lens.

Biomimetic staring infrared imaging omnidirectional detection technology

The trade-off between spectral resolution and instrument throughput is analyzed for a compact, uncooled, longwave infrared (LWIR) channeled spectropolarimeter (IRCSP). The IRCSP was developed as a part of the Sub-mm Wave and InfraRed Polarimeters (SWIRP) project out of NASA's Goddard Spaceflight Center. The IRCSP scientific objective targets measurements of AOLP and DOLP with 1-µm spectral resolution from 8.5 - 12.5µm in a single snapshot. The geometry of the field stop determines the field of view (FOV) of the IRCSP. This work relates the spectral resolution, instrument throughput, and polarimetric accuracy of a spectro-polarimeter to the FOV. The accuracy of linear Stokes retrievals for low temperature thermal targets are predicted for varying FOV and measurement noise conditions. This work presents a method to quantify the achievable accuracy in AOLP and DOLP as a function of field stop dimensions and signal-to-noise ratio (SNR). While smaller field stops are shown to improve accuracy as the spectral resolution is increased, low SNR is the dominant source of error for the IRCSP prototype. For the IRCSP, a SNR of at least 80 is required to produce DOLP measurements with < 5% error for targets with DOLP < 0.2.

The high thermal conductivity of silicon has limited its use in thermal radiative devices where minimizing conductive cooling is a priority. Recent advancements in phononic MEMS technology enable a significant reduction in the thermal conductivity of electrically conductive nanowires, achieving extreme thermal isolation for semiconductor applications. This technology can lower thermal conductivity to a nanoWatt/K level approaching the dielectric limit. The nanowires are configured as phononic crystals which increase Bragg scattering of phonons, providing an extreme level of thermal isolation for application devices.&amp;#xD;This paper reports for the first time a highly efficient silicon broadband infrared light source based on phononic MEMS technology with emission in the near infrared (NIR) - midwave infrared (MWIR) – longwave infrared (LWIR) wavelength range. The thermal light source is fabricated from a starting silicon-on-insulator (SOI) wafer wherein a heated micro-platform and supporting nanowires are released from the surrounding substrate by removing the underlying SiO2 layer. The nanowires are configured with phononic crystal structure.&amp;#xD;The infrared source is an array of silicon pixels packaged within a vacuum cavity providing broadband infrared NIR-MWIR-LWIR radiation from micro-platforms. The device is called a light-emitting platform (LEP), which differentiates it from a narrowband LED emitter.&amp;#xD;LEP pixel radiative power efficiency within the vacuum cavity is over 90% with micro-platform temperature as high as 800 ˚C for silicon. The LEP efficiency compares with an Edison incandescent lightbulb at less than 10% for RGB light and 2.5% for uncooled LEDs emitting in the 2.3 µm wavelength range. Applications include MWIR floodlights, spotlights, vehicle headlights, biomedical stimulators, instrumentation light sources, etc.&amp;#xD;

We present a new optical system for infrared (IR) image-spectrum integration remote sensing. The purpose to develop this instrument is to find the key spectral characteristics of typical hot target and to explore a new intelligence fusion method for the recognition. When mounted on a two-dimensional rotation stage, it can track the suspected target by image processing, and then get its spectrum to do recognition. It is a dual-band system with long-wave infrared (LWIR) imaging and mid-wave infrared (MWIR) spectrum. An IR dichroic beamsplitter is used to divide wideband incident infrared into LWIR and MWIR. Compared to traditional infrared combined imaging and spectral-analysis instruments, it yields higher sensitivity for measuring the IR spectrum. The sensors for imaging and spectrum detection are separate, so high spatial resolution, frame rate, and spectrum resolution can all be obtained simultaneously.

Optical design of athermalized infrared dual-band annular folded lens with multilayer imaging diffractive optical elements

The development of a scalable, low cost, low power, and room temperature operating detector technology capable of high spatial resolution imaging over IR bands of interest can advance and extend space and satellite sensing capabilities such as remote sensing and earth observation. Conventional infrared (IR) band photodetectors based on HgCdTe material operating over short-wave infrared (SWIR), mid-wave infrared (MWIR), and/or long-wave infrared (LWIR) bands often require external cooling to achieve high performance IR sensing and imaging. By integrating bilayers of doped graphene to function as a high mobility channel to aid the recombination of photogenerated carriers, a graphene-enhanced IR photodetector with HgCdTe absorbing layer can be developed providing high performance uncooled detection over SWIR, MWIR, and LWIR (~1.3-14 &mu;m) bands. Development of the high-performance IR photodetector technology involves ptype doping of graphene bilayers deposited on Si/SiO₂ with boron using a spin-on dopant (SOD) process, and then transfer of the doped bilayer graphene onto HgCdTe substrates. The graphene-enhanced HgCdTe SWIR/MWIR/LWIR band detectors were analyzed and characterized material and optoelectronically to demonstrate high performance IR detection at room temperature for advanced NASA Earth Science, defense, and commercial applications.

An infrared (IR) sensor model is validated using experimentally derived peak pixel signal-to-noise ratio (SNR) versus range for detection of either an unresolved or a resolved unmanned aerial vehicle (UAV) target. The model provided estimates the time-averaged peak SNR values for the ranges used in the field collection. A mid-wave infrared (MWIR) camera and a long-wave infrared (LWIR) camera provided the measured data. Commercially available UAVs are flown along a line from the cameras to a clear sky region for background. A laser range finder measures the range at seven stopping points along the path. The data result in five ranges of unresolved target information for the MWIR camera and in four ranges for the LWIR camera. We provide details for using the data collected from the model to match the cameras used in the field collection. Also, the processing used to extract peak SNR versus range from imagery is presented.

The long-wave infrared (LWIR) band holds critical value in applications such as industrial monitoring, biomedical imaging, and spectral analysis. However, frequency conversion for wavelengths above 5 μm, which covers the core LWIR spectral range, remains challenging due to the limited effective nonlinear coefficients of most optical crystals and severe absorption losses in the mid-infrared to LWIR region, restricting the efficiency of upconversion processes. Meanwhile, mainstream commercial mercury cadmium telluride (HgCdTe) detectors typically require cryogenic operation to achieve high signal-to-noise ratios (SNRs), coupled with high fabrication costs, which severely hinders their widespread deployment under room-temperature conditions. This paper presents a room-temperature LWIR detection device based on frequency upconversion technology, which enables high-sensitivity detection across a broad spectral range (7.5-9 μm) without the need for cooling. Experimental results demonstrate that the system achieves a minimum detectable energy at the fJ level under nanosecond-pulse operation, along with excellent wavelength stability and high responsivity. Compared with typical commercial HgCdTe detectors, the proposed device exhibits a sensitivity improvement of up to approximately 2.4 orders of magnitude at room temperature, significantly enhancing detection accuracy and signal-to-noise ratio for LWIR signals under room temperature conditions. This work provides an effective technical pathway toward the development of room-temperature, low-cost, and high-performance LWIR detection systems, showing promising potential for applications in real-time monitoring, portable sensing, and spectroscopic analysis.

In the past five years, significant progress has been accomplished in the reduction of infrared detector pitch and detector size. Recently, longwave infrared (LWIR) detectors in limited quantities have been fabricated with a detector pitch of 5 μm. Detectors with 12-μm pitch are now becoming standard in both midwave infrared (MWIR) and LWIR sensors. Persistent surveillance systems are pursuing 10-μm detector pitch in large format arrays. The fundamental question that most system designers and detector developers desire an answer to is: "How small can you produce an infrared detector and still provide value in performance?" If a system is mostly diffraction-limited, then developing a smaller detector is of limited benefit. If a detector is so small that it does not collect enough photons to produce a good image, then a smaller detector is not much benefit. Resolution and signal-to-noise are the primary characteristics of an imaging system that contribute to targeting, pilotage, search, and other human warfighting task performance. We investigate the task of target discrimination range performance as a function of detector size/pitch. Results for LWIR and MWIR detectors are provided and depend on a large number of assumptions that are reasonable.

A MEMS based tunable Fabry-Perot filter array has been designed and fabricated to provide spectral information in the long-wave infrared (LWIR) band, while remaining transparent in the mid-wave infrared (MWIR) band. Due to the stringent requirements on the optical design, relatively thick optical reflector and anti-reflector coatings are needed. A two-sided, SOI device layer transfer process has been developed to fabricate the thick coating layers and maintain a balanced stress state on the thin moveable membrane of the filter. Fabricated parts have less than 11 nm of deflection across a 200 mum filter. Measured filters show a <120 nm bandwidth peak in the LWIR that is tunable from 11.8 mum to 7.9 mum.

The InAs/InSb/GaSb/AlSb family of III-V alloys and superlattice materials offer unique possibilities for band structure engineering, because they can be grown on GaSb or InSb substrates with high quality and satisfactory control of strain, doping and composition. The band profiles and oscillator strengths are also quite predictable, enabling full simulation of detector performance from a basic knowledge of layer and stack thicknesses. In conventional III-V p-n devices, Shockley-Read-Hall (SRH) traps generate a significant flow of thermal carriers in the device depletion region. At SCD, we have overcome this problem by developing XBn and XBp barrier device architectures that suppress these depletion currents, leading to higher operating temperatures or lower dark currents. Our first barrier detector product was launched in 2013 and operates at 150K. It uses a mid-wave infrared (MWIR) XBn device with an InAsSb absorber well matched to the most transparent of the atmospheric windows, at wavelengths between 3 and 4.2μm. However to span the full MWIR and to sense the long-wave infrared (LWIR) spectrum, we have investigated InAs/GaSb type II superlattices (T2SLs), because they offer full tunability. In this work we show that minority carriers in n-type T2SLs are localized and diffuse by variable range hopping, even when the period is short and the valence miniband has a width of 30-40 meV. Unfortunately, this leads to sub-micron diffusion lengths and a low quantum efficiency (QE) of ~20% in a full MWIR XBn device. On the other hand, p-type layers exhibit “metallic” minority carrier transport with much longer diffusion lengths, typically ~7 μm in our LWIR device layers. The successful development of p-type devices has led to our second barrier detector product, which uses an XBp LWIR T2SL and operates at 77K with a cut-off wavelength of 9.5 μm, a focal plane array (FPA) QE of ~50% and background limited performance up to ~90K at F/3. Moreover, the FPA operability is typically above 99.5%, based on stringent production-line criteria. Together with high spatial uniformity and good temporal stability, these barrier detectors are already a realistic alternative to MCT photodiode arrays, and further products operating at other wavelengths will be launched in due course.