A prototype laser ignition system has been developed for use in a hydrogen/oxygen-burning spacecraft thruster. The system could readily be adapted to terrestrial applications in which spark, catalytic, or pyrotechnic igniters are now used.

The operations of spark igniters and some previously investigated pulsed laser igniters involve high-voltage transients that give rise to electromagnetic interference. In addition, the previously investigated laser igniters project their beams into combustion chambers via windows, which are susceptible to obscuration by deposition of byproducts of combustion. Moreover, neither spark igniters nor the previously investigated laser igniters are designed to test themselves. In contrast, the present laser ignition system operates at low voltage, does not depend on a transparent window, and incorporates self-testing features.

Laser Energy Heats the Ceramic Target, and black-body radiation from the target is measured to determine its temperature.

In the present laser ignition system, the ignition device is basically an optically heated glow plug: The output of a diode laser is delivered, via an optical fiber, to a ceramic target in a combustion chamber (see figure). The laser beam heats the target above the hydrogen/oxygen autoignition temperature. Some of the black-body radiation from the heated target travels back along the optical fiber, is separated from the laser beam by a beam splitter and a laser-blocking filter, and impinges on a photodiode. The output of the photodiode is an indication of the temperature of the target; as such, it provides complete verification of the functionality of the entire optical train from the laser to the target. This self-testing feature can be used alone or in combination for verification of combustion by measurement of pressure in the combustion chamber.

In one version of this system, aspherical lenses are used to couple light (1) between the laser and the beam splitter and (2) between the optical fiber and the beam splitter. In another version, the aspherical lenses and the beam splitter are replaced by a unitary coupling/beam-splitting optic that consists of a hemispherical lens bonded to a 45° polarizing prism. The advantages of the latter version are that there are fewer

optical components that must be aligned with each other and efficiency is increased because the number of optical surfaces through which light must pass (and thus the amount of light lost in Fresnel surface reflections) is reduced.

This patent-pending device is available for license and can be used as a coupling optic for medical and industrial sensors, nearly lossless power summation and insertion of two diode lasers into a single fiber, read/write optics for optical disk drives, bidirectional fiber-optic communications, and wavelength division multiplexed fiber-optic communication.

In a typical application, this laser ignition system would be part of a closed-loop ignition-control system. A controller would command a laser power supply to operate at a set-point voltage that would correspond to a requested target temperature. The temperature signal from the photodiode would be used as a feedback signal to adjust the power-supply output to reduce any deviation from the requested target temperature.

The system is particularly attractive for use in applications in which there are requirements for one or more of the following characteristics: (1) ignition without contamination, (2) verification of operation of igniters prior to ignition, (3) compactness, (4) low power consumption, (5) low-voltage operation, (5) no accidental ignition, and (6) no electromagnetic interference. Examples of potential applications include jet engines, gas water heaters, furnaces, and industrial processing equipment that exploits high-purity combustion.

This work was done by David B. Duncan of Duncan Technologies, Inc., for Johnson Space Center.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Judy Duncan, President
DuncanTech
11824 Kemper Road
Auburn, CA 95603

Refer to MSC-22872

Photonics Tech Briefs Magazine

This article first appeared in the January, 2001 issue of Photonics Tech Briefs Magazine.

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