The figure shows a first-phase prototype of a miniature, rugged long-wavelength infrared (LWIR) spectrometer that incorporates recent advances in the design and fabrication of microelectronic and integrated optical devices. Initial development efforts have been directed toward the intended use of the instrument in measuring the concentrations of certain chemical constituents (e.g., CO2, hydrocarbons, NO2, N2O, and HCl) in aircraft turbine exhaust streams. The instrument would be small and rugged enough to be mounted aboard an aircraft for diagnostic engine monitoring or even for feedback engine control. The basic instrument design could be varied to obtain automotive engine monitors, chemical-composition monitors for hot industrial processes, hand-held meters for identifying unknown chemicals or for measuring deviations from the nominal composition or purity of known chemicals, and mounted or hand-held instruments for detecting toxic or otherwise hazardous gases in outdoor or indoor air.

One especially notable component of the instrument is a unitary silicon chip that performs all of the functions of a conventional infrared analyzer. Optical elements of the analyzer - a diffraction grating, mirrors, apertures, and beam dumps - are micromachined directly onto the chip. The main body of the chip is also an important optical element in that it has a high index of refraction and acts as a slab waveguide; the use of a high-index slab waveguide reduces (in comparison with the use of simple propagation of light through air) the needed optical path length and makes it possible to design a smaller instrument. By virtue of its unitary construction, the micromachined infrared analyzer is rugged and permanently aligned. It is insensitive to vibration and to thermal transients. It is also opaque to visible light and other interference.

This Prototype of a Miniature, Rugged LWIR Spectrometer was tested and found to satisfy signal-to-noise and spectral-resolution requirements to justify further development into an on-board aircraft engine exhaust-gas monitor.

Another especially notable component of the instrument is a micromachined, silicon-based linear array of 64 thermopile-type photodetectors - one for each of 64 channels that span the wavelength range from 2.8 to 5.6 µm with a resolution (bandwidth per channel) of about 0.04 µm. Each photodetector is 1.5 mm long with a pixel pitch of 75 µm. Each photodetector comprises a 0.6-µm-thick silicon nitride membrane with eleven (Bi-Te)/(Bi-Sb-Te) thermocouples.

Thermopiles typically operate over a broad temperature range (including room temperature) and are insensitive to drifts in substrate temperature, so that it is not necessary to provide for either cooling or stabilization of temperature. Thermopiles are passive devices that generate voltage outputs, without need to supply bias voltage. Thus, in comparison with other infrared detectors in the same class (bolometers, pyroelectric devices, and ferroelectric devices), thermopiles consume less power and can be supported by simpler readout circuits. Moreover, if thermopiles are read out with high-input-impedance amplifiers, they exhibit negligible 1/frequency noise because there is negligible readout current. Moreover, thermopiles typically exhibit highly linear response over many orders of magnitude of incident infrared power.

It is necessary to process the thermo-pile readouts to extract chemical-composition information from overlapping spectral peaks. In the case of the first-phase prototype of the instrument, the outputs of the thermopiles are amplified, multiplexed, and digitized, then processed in an external laptop computer. A planned second-phase prototype would incorporate a digital signal processor that would perform neural-network processing to extract the required information.

This work was done by Edward A. Johnson and James Daly of Ion Optics, Inc., for Glenn Research Center. No further documentation is available.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4 8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16828.


Photonics Tech Briefs Magazine

This article first appeared in the November, 1999 issue of Photonics Tech Briefs Magazine.

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