Neil Goldstein

Measurement of molecular concentrations and line parameters using line-locked second harmonic spectroscopy with an AlGaAs diode laser

The technique of line-locked wavelength modulation with 2f detection is applied to the measurement of water vapor concentration and absorption line parameters by using an 820-nm AlGaAs communications diode laser. Measurements of the 2f signal as a function of the modulation amplitude yield accurate concentrations and line parameters over a pressure range of an order of magnitude and half-widths from 0.02 to 0.15 cm(-1). Usingtwo different spectral lines, we determined concentrations and line parameters with 1% precision, and the absolute accuracy of the line parameters is 3% or better. The results have been used to calculate calibration curves for a diode laser humidity monitor.

Laser Raman sensor for measurement of trace-hydrogen gas

A new optical hydrogen sensor based on spontaneous Raman scattering of laser light has been designed and constructed for rugged field use. It provides good sensitivity (better than 100 parts in 10(6)), rapid response (several seconds), and the inherent Raman characteristics of linearity and background gas independence of the signal. Efficient light collection and discrimination by using fast optics and a bandpass interference filter compensate for the inefficiency of the Raman-scattering process. A multipass optical cavity with a Herriott-type configuration provides intense illumination from an air-cooled cw gas laser. The observed performance is in good agreement with the theoretical signal and noise level predictions.

DMD-based adaptive spectral imagers for hyperspectral imagery and direct detection of spectral signatures

Dispersive transform spectral imagers with both one- and two-dimensional spatial coverage have been demonstrated and characterized for applications in remote sensing, target classification and process monitoring. Programmable spatial light modulators make it possible to adjust spectral, temporal and spatial resolution in real time, as well as implement detection algorithms directly in the digitally controlled sensor hardware. Operating parameters can be optimized in real time, in order to capture changing background and target evolution. Preliminary results are presented for short wave, mid-wave, and long-wave infrared sensors that demonstrate the spatial and spectral versatility and rapid adaptability of this new sensor technology.

Diode laser measurements of temperature-dependent line parameters for water vapor near 820 nm

Abstract Air- and self-broadening coefficients and relative line strenghts for 8–10 low-J lines in the (2 1 1) vibrational band of water vapor have been accurately measured over the T = 330–540 K range using an 817–824 nm GaAlAs diode laser. The technique of wavelength modulation with third-harmonic line locking and second-harmonic detection was used to derive the line parameters with accuracies of several percent from measurements with absorptivities as low as 2 x 10 -3 . The linewidths and temperature coefficients are in good agreement with theoretical calculations and with recent measurements in the (2 2 1) and (3 0 1) bands. Absolute line strenghts have been derived from measurements on two of the lines and are found to be 21 ±4% higher than the values listed in the 1991 HITRAN compilation.

Adaptive Spectral Imager for Space-Based Sensing

Optical sensors aboard space vehicles designated to perform seeker functions need to generate multispectral images in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) spectral regions in order to investigate and classify man-made space objects, and to distinguish them relative to the interfering scene clutter. The spectral imager part of the sensor collects spectral signatures of the observed objects in order to extract information on surface emissivity and target temperature, both important parameters for object-discrimination algorithms. The Adaptive Spectral Imager described in this paper fulfills two functions simultaneously: one output produces instantaneous two-dimensional polychromatic imagery for object acquisition and tracking, while the other output produces multispectral images for object discrimination and classification. The spectral and temporal resolution of the data produced by the spectral imager are adjustable in real time, making it possible to achieve optimum tradeoff between different sensing functions to match dynamic monitoring requirements during a mission. The system has high optical collection efficiency, with output data rates limited only by the readout speed of the detector array. The instrument has no macro-scale moving parts, and can be built in a robust, small-volume and lightweight package, suitable for integration with space vehicles. The technology is also applicable to multispectral imaging applications in diverse areas such as surveillance, agriculture, process control, and biomedical imaging, and can be adapted for use in any spectral domain from the ultraviolet (UV) to the LWIR region.

Long-atmospheric-path measurements of near-visible absorption lines of O 2 isotopes and H 2 O with a prototype AlGaAs laser transceiver system

Near-visible absorption lines of ambient H(2)O vapor and normal and heavy isotopes of O(2) have been measured over atmospheric paths of up to 0.46 km by using two wavelength-modulated, line-locked AlGaAs laser sources with a retroreflector-telescope system. The absolute signal levels agree with theoretical calculations for the O(2) isotopes to within 2%, which is similar to the accuracy with which the column densities were known. Measurements of (16)O(2) linewidths and line strengths were made, and they agree with literature values to within experimental error. The detection sensitivity for (16)O(18)O was found to be 0.1 part in 10(6) atm. km, correspondingto an absorbance sensitivity of 1 × 10(-5). It is concluded that atmospheric trace-gas sensing will be feasible with this apparatus over distances of several kilometers and at levels under 1 part in 10(6).

TEMPERATURE AND TEMPERATURE PROFILE MEASUREMENTS IN THE COMBUSTOR FLOWPATH USING STRUCTURED EMISSION THERMOGRAPHY

Structured Emission Thermometry (SET) is a new optical technique for deriving temperature and species concentrations in high-temperature regions of a combustor flow-path from measurements of water and soot emission spectra in the 1 micron region. A prototype instrument has been built for gas- and liquid-fuel combustors with temperatures ranging from 1400–2500K (2100–4000F). This instrument can be used for test stand instrumentation and routine monitoring in fixed installations. With further evolution, the technique may be suitable for an aircraft engine control system. The emissions are measured along narrow lines of sight using small, simple passive fiber probes placed at a short standoff distance from the hot gas flow-path. With proper placement of multiple probes, data may be collected over overlapping lines of sight and inverted to produce a low-resolution spatial map of the temperature and emitter density in the flow-path. In this work, we describe a series of combustor measurements, including cross-validation measurements in a well-controlled laboratory flame and measurements in combustor test stands. Both line-of-sight measurements and reconstructed temperature profiles are presented and discussed.Copyright

Programmable Adaptive Spectral Imagers for Mission-Specific Application in Chemical/Biological Sensing

An innovative passive standoff system for the detection of chemical/biological agents is described. The spectral, temporal and spatial resolution of the data collected are all adjustable in real time, making it possible to keep the tradeoff between the sensor operating parameters at optimum at all times. The instrument contains no macro-scale moving parts and is therefore an excellent candidate for the development of a robust, compact, lightweight and low-power-consumption sensor. The design can also serve as a basis for a wide variety of spectral instruments operating in the visible, NIR, MWIR, and LWIR to be used for surveillance, process control, and biomedical applications.

Innovative Minimally Intrusive Sensor Technology Development for Versatile Affordable Advanced Turbine Engine Combustors

The feasibility of an innovative minimally intrusive sensor for monitoring the hot gas stream at the turbine inlet in high performance aircraft gas turbine engines was demonstrated. The sensor uses passive fiber-optical probes and a remote readout device to collect and analyze the spatially resolved spectral signature of the hot gas in the combustor/turbine flowpaths. Advanced information processing techniques are used to extract the average temperature, temperature pattern factor, and chemical composition on a sub-second time scale. Temperatures and flame composition were measured in a variety of combustion systems including a high pressure, high temperature combustion cell. Algorithms for real-time temperature measurements were developed and demonstrated. This approach should provide a real-time temperature profile, temperature pattern factor, and chemical species sensing capability for multi-point monitoring of high temperature and high pressure flow at the combustor exit with application as an engine development diagnostic tool, and ultimately, as a real-time active control component for high performance gas turbines.Copyright

Infrared adaptive spectral imagers for direct detection of spectral signatures and hyperspectral imagery

Field test results are presented for a prototype long-wave adaptive imager that provides both hyperspectral imagery and contrast imagery based on the direct application of hyperspectral detection algorithms in hardware. Programmable spatial light modulators are used to provide both spectral and spatial resolution using a single element detector. Programmable spectral and spatial detection filters can be used to superimpose any possible analog spectral detection filter on the image. In this work, we demonstrate three modes of operation, including hyperspectral imagery, and one and two-dimensional imagery using a generalized matched filter for detection of a specific target gas within the scene.