Compact, lightweight, sensitive correlation spectrometers for detecting gaseous byproducts of the onset of fire are under development. These spectrometers would be installed in aircraft, where early detection of fire could enable crews to respond with timely fire-suppression actions. Correlation spectroscopy involves measurements of absorption spectra of chemical species of interest but is not the same as classic absorption spectroscopy, which has been used for decades for detecting airborne chemicals. Classic absorption spectroscopy involves steadystate techniques that are not suited for rapid detection of compounds of immediate interest that may be present along with other compounds that are not of immediate interest.

Fiber Bragg Gratings Are Stretched Periodically to modulate the wavelengths of light reflected from them. The wavelength- modulated light is sent through a test cell, where the light is used for correlation spectroscopy to measure the concentration of a gas (in this case, HCl).
In classical absorption spectroscopy, one measures the spectrum of light transmitted through an atmospheric region or a gas cell that is suspected of containing a compound of interest (hereafter denoted the target compound). The absorption spectrum is then computed from the transmission spectrum. In correlation spectroscopy, one uses a photodetector to measure the amount of light transmitted while illuminating the atmospheric region or sample cell by use of a phase- or wavelength-modulated, narrow-band optical source, the steadystate or nominal wavelengths of which coincide with known absorption spectral lines of the target compound.

The modulation causes the spectral lines of the illumination to move periodically into and out of registry with the absorbance bands of the target compound. The modulation appears in the output of the photodetector, with an amplitude related to the concentration of the target compound. The modulation in the photodetector output is measured with the help of a lock-in amplifier. Because the manifold of absorption spectral lines for each compound is unique and the use of phase or wavelength modulation in conjunction with a lock-in amplifier offers high sensitivity, correlation spectroscopy makes it possible to detect trace amounts of target compounds while discriminating against other compounds that might also be present in complex gas mixtures. Hence, correlation spectroscopy is well suited for detecting compounds typical of the early stages of fire while preventing the triggering of false alarms by other compounds.

The figure depicts a laboratory apparatus used to demonstrate the feasibility of a correlation spectrometer for detecting hydrogen chloride, which is one of the gases most commonly emitted at the onset of burning of paneling materials in typical aircraft. (Carbon dioxide, which is a product of complete combustion, is not useful for the sensitive detection of the onset of fire.) The source of light was a light-emitting diode (LED) with an emission spectrum spanning the wavelength band from 1,200 to 1,400 nm and a peak at about 1,300 nm. The light from the LED was coupled into an optical-fiber spectral reflector comprising four fiber Bragg gratings spliced in series. A fiber Bragg grating is an optical fiber, the index of refraction of the core of which is perturbed with a longitudinal spatial period chosen to obtain reflection at a desired wavelength. In this case, the spatial periods of the fiber Bragg gratings were chosen to obtain reflection peaks at wavelengths near 1,220 nm — wavelengths slightly less than those of a set of HCl absorption lines.

Light reflected from the fiber Bragg gratings was coupled into a test cell containing either atmospheric-pressure air or an atmospheric-pressure mixture of air with 500 parts per million (ppm) of HCl. After passing through the cell, the light was detected by use of an avalanche photodiode. The output of the photodiode was sent through a transimpedance amplifier and a lock-in amplifier to a data-acquisition system. The reference (synchronizing) signal for the lock-in amplifier was the same one used to drive a power amplifier to effect wavelength modulation as described below.

The fiber Bragg gratings were mechanically clamped to an electromechanical stretcher, which was used to stretch the gratings in order to increase the wavelengths of their reflection peaks, thereby effecting wavelength modulation. The output of the power amplifier was used to drive an electromagnet that actuated the stretcher. The frequency of the reference signal, and thus of the modulation, was 60 Hz. In operation, this apparatus was found to provide indication of the concentration of the HCl gas in the cell, with a signal-to-noise ratio of 350 at 500 ppm. Further development efforts are expected to yield increases in sensitivity.

This work was done by Kisholoy Goswami of Intelligent Optical Systems, Inc., for Glenn Research Center.

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
Ohio 44135.

Refer to LEW-16897.