The global defense community requires new approaches for standoff detection of chemical, biological, radiological, nuclear and explosive (CBRNE) threats. Such standoff detection methods must be capable of discriminating the target hazardous materials from the environmental background. Therefore these sensors must exhibit high selectivity. High selectivity detection of CBRNE threats can be accomplished using infrared (IR) spectroscopy, which produces a unique spectral “fingerprint” of the target chemical, enabling discrimination of the target chemical from other chemicals in the background. Standoff detection using IR spectroscopy however requires that enough of the incident source light may be collected at the detector; therefore a high-power source is needed. Commercially available quantum cascade laser (QCL) sources are capable of projecting high power, coherent laser light at targets down range from the source. In order to collect complete IR spectra throughout the entire fingerprint region, the output of multiple QCL modules are combined into a single exit aperture. This is typically achieved using mirrors and other optics which are susceptible to vibrational and temperature misalignments in field systems. In order to provide a more ruggedized solution to combining the beam output of multiple QCL modules, we developed a unique chalcogenide optical fiber beam combiner which combines the output of four commercial QCL modules. This allows for scanning across a spectral range from 6.01 – 11.20 μm encompassing parts of both the IR functional groups and fingerprint regions. We demonstrate the ability of this QCL system to generate high quality IR spectra of hazardous materials.
The development of negative curvature fibers is an exciting advance in optical fiber technology that combines relatively low loss over a broad bandwidth with relatively high tolerance for fabrication imperfections. Tolerance of fabrication imperfections is particularly important for chalcogenide fibers, and negative curvature geometries have made it possible to fabricate hollow-core chalcogenide fibers that can transmit light at 10 μm with a loss of 2.1 dB/m. We review theoretical and experimental work that we have carried out to determine the performance limits and to design and fabricate chalcogenide negative curvature fibers.
The bending loss is a critical parameter for packaging, representing a limiting parameter in the minimization of fiber-based devices. For applications in the mid-infrared spectral band, chalcogenide glass optical fibers are one of the few alternatives for high-power beam delivery. We present experimental results for the bending loss of a sulfide-based multimode chalcogenide fiber for a broad range of infrared wavelengths as well demonstrating >5.8 W power handling for a 6.25-mm radius bend.
The need for small, robust, and highly selective sensors, for both military and homeland security
requirements, is driving the development of portable detectors for hazardous materials. Infrared
spectroscopy exhibits high selectivity because the infrared vibrational transitions correlate to the
molecular structure and functional groups within the molecule. Small FTIR systems exist as
COTS items; however, these systems still require precise moving components to generate the
interferogram. A more desirable approach is to build a solid state system with no precision
moving parts as required by a typical moving mirror interferometer. This work will describe the
design aspects of an optical fiber based mid-infrared FTIR, and focus specifically on the stateof-
the-art mid-infrared transmitting optical fibers and the use of an optical fiber supercontinuum
source for efficient coupling of light into the system.
We report on development and characterization of square registered infrared imaging bundles fabricated from As<sub>2</sub>S<sub>3</sub>fiber for HWIL applications. Bundle properties and cross-talk measurements are presented.
An all-fiber supercontinuum source extending from 1.5 to 5 μm has been demonstrated in single-mode step-index As2S3
fiber using a Raman shifted erbium doped mode-locked silica fiber laser pump source. 140 mW broadband power was
demonstrated with a spectral intensity variation of 10 dB from 1.9 to 4.4 μm and 20 dB from 1.65 to 4.78 μm.
Based on the exact solutions of Maxwell’s equations, we have studied the basic theoretical properties of submicron and nano-diameter air-cladding silica-wire waveguides. The single-mode condition and the modal field of the fundamental modes have been obtained. Silica wires with diameters of 100-1000nm and lengths ranging from hundreds of micrometer to over 1 millimeter have been fabricated. SEM examination shows that these wires have uniform diameters and smooth surfaces, which are favorable for optical wave guiding. Light has been sent into these wires by optical coupling, and guiding light through a bent wire has also been demonstrated. These wires are promising for assembling photonic devices on a micron or submicron scale.