For over two decades, Physical Sciences Inc. (PSI) has contributed technology innovations for defense against chemical and biological threats. In this newsletter, we highlight the latest generation of our advances. PSI has recently won two programs that address a similar standoff detection application. Although these programs differ in their approach to the laser source and spectral discrimination, they both operate in the long-wave infrared (LWIR) and share many of the same technical challenges, such as speckle mitigation and ambient clutter suppression.
Dual Optical Comb LWIR Source
In the first contract award, PSI is developing a dual optical LWIR source and sensor as part of the DARPA Spectral Combs from UV and THz (SCOUT) program.
Standoff detection of chemical films using dual optical frequency combs in the long-wave infrared
PSI, in collaboration with the research group of Professor Jérôme Faist at the Swiss Federal Institute of Technlogy (ETH) in Zürich, Switzerland and IRsweep, are developing a chip-scale dual Optical Frequency Comb (OFC) source in the LWIR, and will integrate this source into a compact sensor system, and demonstrate its application to the detection of chemical films, as shown in the above concept. The OFC's will be generated by two Quantum Cascade Lasers (QCLs) optimized for broadband operation, high output power, and stable comb operation. The dual OFC will be grown and processed on a single epitaxial substrate. Each OFC will be electrically driven and free-running requiring no optical locking mechanisms. Key innovations that will be demonstrated include OFC dispersion management, dual-OFC on-a-chip packaging, real-time multi-heterodyne signal processing, adaptive comb control, and statistical spectral algorithm optimization for clutter suppression.
The figures show a scanning electron micrograph (SEM) of the front facet of a fabricated dual OFC on a single chip along with the output of the dual OFC chip incident on liquid crystal thermal paper and both OFCs operating simultaneously.
Dual comb on a chip concept and front facet of actual fabricated device
Simultaneous dual comb output incident on thermal paper
Active Standoff Chemical Identification Detector
A team led by PSI has been awarded a contract to develop an Active Standoff Chemical Identification Detector (ASCID) sensor under IARPA's Standoff ILluminator for Measuring Absorbance and Reflectance Infrared Light Signatures (SILMARILS) program. This sensor employs broadband target surface illumination using quantum cascade laser (QCL) technology, a high-throughput spatial heterodyne spectrometer (SHS) functioning as the receiver, and an Adaptive Cosine Estimator (ACE) automated detection algorithm. The combined capability enables standoff (5-30 m) detection of liquid and solid surface contaminants at 0.1 mg/cm2 surface coverage with a probability of detection and false alarm of 95% and 0.01%, respectively. This technology also enables gas phase detection with 0.1 ppm·m concentration-pathlength sensitivity. The figure below shows the proposed system layout.
ASCID Concept. An array of broadband, spectrally continuous QCLs are coaligned to the receiver field of view. A scanner is used to rapidly scan the laser illumination/transmitter/receiver field of view on the target at a 5-30 m standoff. Scattered/reflected energy from the target surface is collected and spectrally resolved by the SHS. The spectral image is converted to a detection map by the ACE algorithm
The concept of employment is based on a flying spot model in which coaligned transmitter illumination and receiver collection fields of view are rapidly swept across the required detection area. The resulting spectral image is then converted to a detection map. The key innovations are:
• An array of broadband, coaligned QCLs in a compact form factor. This array covers the 900-1400 cm-1 spectral range with greater than 10 mW/cm-1 of spectral power density.
• A no-moving parts and high-throughput, SHS interferometric spectrometer that spectrally resolves the reflected energy from the target with 8 cm-1 resolution. The full spectrum of a spot on the target is captured by every frame of its fast-framing mercury cadmium telluride focal plane array.
• An automated target detection algorithm employing a variant of ACE. It has been used by PSI for active and passive hyperspectral surface contamination and chemical vapor detection. This algorithm utilizes robust background characterization and class-based target set selection to provide target detection and identification from low signal to noise (clutter limited) reflectivity data.
• Coverage of the required target area accomplished using a scanner positioned at a pupil plane to rapidly scan the transmitter illumination and receiver field of view.
• A 30-cm diameter transmitter/receiver optic providing eye-safe illumination at all points between transmitter and target while also collecting the reflected energy from the surface.
The sensor concept enables breakthrough standoff contaminant detection capability relative to existing hyperspectral imaging and lidar systems. Key features include:
• Active illumination that, when coupled with the SHS, supports sufficient signal to noise ratio to detect 0.1 mg/cm2 surface concentration at a range of 30 m. The spectrally bright broadband illuminator and the high throughput SHS receiver reduce intensity requirements and development risk on the transmitter.
• The ACE algorithm variant enables contaminant detection and identification in the signal to noise regime achievable by the transmitter and receiver combination. Moreover, the novel approach to class-based target screening enables the achievement of the Pd/Pfa requirements for the large library of targets and backgrounds (50/1500).
This material is based upon work supported by the United States Air Force under Contract Number FA8650-16-C-9109. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of United States Air Force.
This research was funded by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA), through the AFRL Contract FA8650-16-C- 9109. All statements of fact, opinion or conclusions contained herein are those of the authors and should not be construed as representing the official views or policies of IARPA, the ODNI, or the U.S. Government. The Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright annotation thereon.
PSI recently received the following research contracts:
"Hyperspectral Compressive Imager Based on an Electronically Tunable Tandem SWIR Fabry-Perot Filter" and " High Performance Airborne Laser Scanner for Routine Mapping of Terrain from UAS's" from the Department of Energy
"Fractal Heat Exchanger for Gas Turbine Pre-Cooling" from the Air Force Wright-Patterson
"High Performance, Electrically-Assisted Ionic Monopropellant Thrusters" from the Navy, Office of Naval Research
For more PSI contract news, visit www.psicorp.com/news.