Kannan Krishnaswami

Single-mode low-loss chalcogenide glass waveguides for the mid-infrared

We demonstrate the design, fabrication, and characterization of single-mode low-loss waveguides for mid-infrared (MIR) wavelengths. Planar waveguide structures were fabricated from multilayer thin films of arsenic-based chalcogenide glasses followed by the creation of channel waveguides by using the photodarkening effect. Propagation losses as low as 0.5 dB/cm were measured for a quantum cascade laser end-fire coupled into the waveguides. This is a first step toward the design and fabrication of integrated optical components for MIR applications.

GaSb substrates with extended IR wavelength for advanced space based applications

GaSb substrates have advantages that make them attractive for implementation of a wide range of infrared (IR) detectors with higher operating temperatures for stealth and space based applications. A significant aspect that would enable widespread commercial application of GaSb wafers for very long wavelength IR (VLWIR) applications is the capability for transmissivity beyond 15 μm. Due largely to the GaSb (antisite) defect and other point defects in undoped GaSb substrates, intrinsic GaSb is still slightly p-type and strongly absorbs in the VLWIR. This requires backside thinning of the GaSb substrate for IR transmissivity. An extremely low n-type GaSb substrate is preferred to eliminate thinning and provide a substrate solution for backside illuminated VLWIR devices. By providing a more homogeneous radial distribution of the melt solute to suppress GaSb formation and controlling the cooling rate, ultra low doped n:GaSb has been achieved. This study examines the surface properties and IR transmission spectra of ultra low doped GaSb substrates at both room and low temperatures. Atomic force microscopy (AFM), homoepitaxy by MBE, and infrared Fourier transform (FTIR) analysis was implemented to examine material quality. As compared with standard low doped GaSb, the ultra low doped substrates show over 50% transmission and consistent wavelength transparency past 23 μm with improved %T at low temperature. Homoepitaxy and AFM results indicate the ultra low doped GaSb has a low thermal desorbtion character and qualified morphology. In summary, improvements in room temperature IR transmission and extended wavelength characteristics have been shown consistently for ultra low doped n:GaSb substrates.

Research towards a systematic signature discovery process

In its most general form, a signature is a unique or distinguishing measurement, pattern, or collection of data that identifies a phenomenon (object, action, or behavior) of interest. The discovery of signatures is an important aspect of a wide range of disciplines from basic science to national security for the rapid and efficient detection and/or prediction of phenomena. Current practice in signature discovery is typically accomplished by asking domain experts to characterize and/or model individual phenomena to identify what might compose a useful signature. What is lacking is an approach that can be applied across a broad spectrum of domains and information sources to efficiently and robustly construct candidate signatures, validate their reliability, measure their quality, and overcome the challenge of detection - all in the face of dynamic conditions, measurement obfuscation, and noisy data environments. Our research has focused on the identification of common elements of signature discovery across application domains and the synthesis of those elements into a systematic process for more robust and efficient signature development. In this way, a systematic signature discovery process lays the groundwork for leveraging knowledge obtained from signatures to a particular domain or problem area, and, more generally, to problems outside that domain. This paper presents the initial results of this research by discussing a mathematical framework for representing signatures and placing that framework in the context of a systematic signature discovery process. Additionally, the basic steps of this process are described with details about the methods available to support the different stages of signature discovery, development, and deployment.

Emission and Propagation Properties of Midinfrared Quantum Cascade Lasers

We report divergence, astigmatism, and beam propagation factor (M2) measurements of quantum cascade lasers (QCLs) with emission wavelengths of 8.77 mum. Emission profiles from the facet showed full-width at half-maximum divergence angles of 62deg and 32degplusmn2deg for the fast and slow axes, respectively. Diffraction-limited Ge aspheric microlenses were designed and fabricated to efficiently collect, collimate, and focus QCL emission. A confocal system comprised of these lenses was used to measure M2 yielding 1.8 and 1.2 for the fast and slow axes, respectively. Astigmatism at the exit facet was calculated to be about 3.4 mum, or less than half a wave. To the best of our knowledge, this is the first experimental measurement of astigmatism and M2 reported for midinfrared QCLs.

Compact quantum cascade laser transmitter

In this paper we present design considerations, thermal and optical modeling results, and device performance for a ruggedized, compact laser transmitter that utilizes a room temperature quantum cascade (QC) laser source. The QC laser transmitter is intended for portable mid-infrared spectroscopy applications, where the 3 to 5 μm and 8 to 12 μm atmospheric transmission window is relatively free of water vapor interference and where the molecular rotational vibration absorption features can be used to detect and uniquely identify chemical compounds of interest. Initial QC laser-based sensor development efforts were constrained by the complications of cryogenic operation. However, improvements in both QC laser designs and fabrication processes have provided room-temperature devices that now enable significant miniaturization and integration potential for national security, environmental monitoring, atmospheric science, and industrial safety applications.

The Future of Nuclear Archaeology: Reducing Legacy Risks of Weapons Fissile Material

This report describes the value proposition for a “nuclear archaeological” technical capability and applications program, targeted at resolving uncertainties regarding weapons fissile materials production and use. Central to this proposition is the notion that one can never be sure that all fissile material is adequately secure without a clear idea of what “all” means, and that uncertainty in this matter carries risk. We argue that this proposition is as valid today, under emerging state and possible non-state nuclear threats, as it was in an immediate post-Cold-War context, and describe how nuclear archaeological methods can be used to verify fissile materials declarations, or estimate and characterize historical fissile materials production independent of declarations. Methods for accurately estimating plutonium production from graphite reactors have been demonstrated and could be extended to other reactor types. Proposed techniques for estimating HEU production have shown promise and are under development.

Characterization of Single-mode Chalcogenide Optical Fiber for Mid-Infrared Applications

Chalcogenide fibers display a wide transmission window ranging from 2-10.6 μm, ideally suited to the development of passive and active mid-infrared (MIR) sensors. They are essential building blocks for the integration and miniaturization of laser-based MIR optical systems for terrestrial, airborne and space-based sensing platforms. Single-mode chalcogenide fibers have only recently become commercially available and therefore performance data and standard reproducible processing techniques have not been widely reported. In this paper we present a method for producing high quality cleaved facets on commercial single-mode As-Se fibers with core and cladding diameters of 28μm and 170μm respectively. The emitted beam profile from these fibers, using the 9.4μm line of a tunable CO2 laser, showed the presence of leaky cladding modes due to waveguiding conditions created by the protective acrylate jacket. These undesirable cladding modes were easily suppressed by applying a gallium coating on the cladding near both input and output facets. We provide experimental data showing efficient mode suppression and the emission of a circular nearperfect Gaussian beam profile from the fiber. Furthermore, analyses of the beam, acquired by scanning an HgCdTe detector, yielded a 1/e2 numerical aperture of 0.11 with a full width half maximum divergence of 11° for these fibers. The availability of single-mode MIR fibers, in conjunction with recent advances in room temperature quantum cascade lasers (QCL), could provide compact and light-weight transmitter solutions for several critical defense and nuclear nonproliferation needs.

Lateral Shearing Interferometer for Measuring Photoinduced Refractive Index Change in As2S3

We have built and demonstrated a lateral shearing interferometer as a process engineering and control tool for the fabrication and characterization of direct-laser-written waveguide structures in chalcogenide glasses. Photoinduced change in refractive index of 0.154+/-0.002 was measured for as-deposited amorphous As(2)S(3) thin films at 633 nm with an estimated measurement uncertainty of 1.3% for this air-gap interferometer configuration. The simple design of this interferometer can easily be adapted to other wavelengths including mid- and long-wave infrared regions to measure changes in refractive index or material inhomogeneities in transmissive materials.

Single-Mode Low-Loss Chalcogenide Glass Photonic Components for Mid-Infrared Operations

We report the fabrication of photonics components designed for mid-infrared quantum cascade lasers based on photodarkening of thin-film chalcogenide glasses. We measure propagation losses of 0.5 dB/cm for single-mode waveguides and demonstrate evanescent wave couplers.

Design and fabrication of efficient collimation and focusing optics for mid-IR quantum cascade lasers

Quantum cascade lasers (QCL) are a new class of solid-state lasers capable of delivering mid-infrared (mid-IR) radiation wavelengths from 3.5 μm to 25 μm. QCLs are finding extensive use in chemical sensing applications due to the abundance of absorption features in the molecular fingerprint region spanned by these sources. They are also being exploited in the field of electro-optical infrared countermeasures. QCL devices exhibit an elliptical emission profile that is highly divergent in the fast axis of the laser waveguide. The far-field profile of the QCL emission, 62° and 32° ± 2° for the fast and slow axes, respectively, places stringent demands on the design of efficient collimation lenses. Because of the current lack of commercially available mid-IR optical components, QCL users must design and fabricate custom micro-optics to efficiently collect, collimate, and focus the QCL emission. In this paper, we report the design, fabrication, and characterization of germanium aspheric collimating and focusing optics designed for mid-IR Fabry-Perot QCLs with an emission wavelength of 8.77 μm. Custom aspheric collimating and focusing lenses with a numerical aperture (NA) of 0.85 and 0.20, respectively, were designed and fabricated using single-point diamond turning. Measurements of the transmitted wavefront error at mid-IR wavelengths showed diffraction-limited performance with Strehl ratios >0.94 and >0.99 for the collimation and focusing lenses, respectively. A beam propagation figure of merit (M2) of 1.8 and 1.2 was measured for the fast and slow axes, respectively, of a Fabry-Perot QCL using a confocal optical system comprised of these lenses.