Hyperspectral imaging spectrometers are useful in numerous applications including remote sensing, environmental monitoring, surveillance, minerology and precision agriculture. Historically, high cost and complexity has limited the number of fielded hyperspectral imagers. The Computational Reconfigurable Imaging Spectrometer (CRISP) sensor is a novel hyperspectral imaging spectrometer suitable for high-resolution air or space-based missions. CRISP uses a computational imaging approach to reduce the system’s overall size and complexity. It exploits platform motion and a spectrally coded focal-plane mask to temporally modulate the optical spectrum, enabling simultaneous measurement of multiple spectral bins (i.e. multiplexing). The novel design enables high performance from smaller and less-expensive components (e.g. uncooled microbolometers), and is thus suitable for small space and air platforms. This talk discusses our demonstrator system (including recent flight results) and compares it to theory. Our flights demonstrate plume detection using an uncooled, airborne, longwave infrared CRISP imaging spectrometer. We discuss progress developing algorithms to enable spectral recovery in the presence of motion blur, utilizing the CRISP architecture to advantage. These algorithms enable a fast scanning mode, trading off computational complexity and reconstruction quality for fast area coverage rate.
Short optical pulses emitted from a tunable Q-switched laser (800 to 2000 nm) generate laser ultrasound (LUS) signals at the surface of biological tissue. The LUS signal’s acoustic frequency content, dependence on sample type, and optical wavelength are observed in the far field. The experiments yield a reference dataset for the design of noncontact LUS imaging systems. Measurements show that the majority of LUS signal energy in biological tissues is within the 0.5 and 3 MHz frequency bands and the total acoustic energy generated increases with the optical absorption coefficient of water, which governs tissue optical absorption in the infrared range. The experimental results also link tissue surface roughness and acoustic attenuation with limited LUS signal bandwidth in biological tissue. Images constructed using 810-, 1064-, 1550-, and 2000-nm generation laser wavelengths and a contact piezoelectric receiver demonstrates the impact of the generation laser wavelength on image quality. A noncontact LUS-based medical imaging system has the potential to be an effective medical imaging device. Such a system may mitigate interoperator variability associated with current medical ultrasound imaging techniques and expand the scope of imaging applications for ultrasound.
Lincoln Laboratory of Massachusetts Institute of Technology has developed a technique known as dynamic photoacoustic spectroscopy (DPAS) that could enable remote detection of trace gases via a field-portable laser-based system. A fielded DPAS system has the potential to enable rapid, early warning of airborne chemical threats. DPAS is a new form of photoacoustic spectroscopy that relies on a laser beam swept at the speed of sound to amplify an otherwise weak photoacoustic signal. We experimentally determine the sensitivity of this technique using trace quantities of SF 6 gas. A clutter-limited sensitivity of ∼100 ppt is estimated for an integration path of 0.43 m. Additionally, detection at ranges over 5 m using two different detection modalities is demonstrated: a parabolic microphone and a laser vibrometer. Its utility in detecting ammonia emanating from solid samples in an ambient environment is also demonstrated.
MIT Lincoln Laboratory has developed a concept that could enable remote (10s of meters) detection of trace
explosives' residues via a field-portable laser system. The technique relies upon laser-induced photodissociation of
nitro-bearing explosives into vibrationally excited nitric oxide (NO) fragments. Subsequent optical probing of the first
vibrationally excited state at 236 nm yields narrowband fluorescence at the shorter wavelength of 226 nm. With proper
optical filtering, these photons provide a highly sensitive explosives signature that is not susceptible to interference from
traditional optical clutter sources (e.g., red-shifted fluorescence). Quantitative measurements of trace residues of TNT
have been performed demonstrating this technique using a breadboard system, which relies upon a pulsed optical
parametric oscillator (OPO) based laser. Based on these results, performance projections for a fieldable system are made.
To enable development of novel signal processing circuits, a high-speed surface-channel charge
coupled device (CCD) process has been co-integrated with the Lincoln Laboratory 180-nm RF fully depleted
silicon-on-insulator (FDSOI) CMOS technology. The CCDs support charge transfer clock speeds in excess of
1 GHz while maintaining high charge transfer efficiency (CTE). Both the CCD and CMOS gates are formed
using a single-poly process, with CCD gates isolated by a narrow phase-shift-defined gap. CTE is strongly
dependent on tight control of the gap critical dimension (CD).
In this paper we review the tradeoffs encountered in the co-integration of the CCD and CMOS
technologies. The effect of partial coherence on gap resolution and pattern fidelity is discussed. The impact of
asymmetric bias due to phase error and phase shift mask (PSM) sidewall effects is presented, along with
adopted mitigation strategies. Issues relating to CMOS pattern fidelity and CD control in the double patterning
process are also discussed.
Since some signal processing CCD structures involve two-dimensional transfer paths, many required
geometries present phase compliance and trim engineering challenges. Approaches for implementing non-compliant
geometries, such as T shapes, are described, and the impact of various techniques on electrical
performance is discussed.