Enhancements to Fiber Optic Bragg Grating Sensors and Demodulation Systems

The Boeing Company, St. Louis, Missouri

The Boeing Company has developed and patented certain enhancements to commercially available Bragg grating sensors and demodulation systems. These patents (Nos. 4,471,659, 4,668,093, 5,380,995, 5,563,967, and 5,627,927) are available for license. Knowhow related to these patents is available through laboratory and engineering services offered by Boeing.

The enhancements cover two areas, the sensors themselves and sensor signal readout. The double-core Bragg grating fiber optic sensor consists of a fiber optic designed to transmit different wavelengths of light. Bragg gratings at two wavelengths are written at the same location on the fiber optic. The response from these gratings can be used to simultaneously determine both the strain and temperature on the fiber optic. The multiple overlapping grating method further enhances the separation of variables by allowing separation of x-axis strain, y-axis strain, z-axis strain, and temperature from one sensing location.

Subscale Hydrogen Tank Compression Test (-220°F)

High-speed sensor signal readout is improved via an additional pending patent that allows efficient integration of multiple strings of sensors and the use of less expensive sources at the same time. This technique could readily be adapted to telecom applications to reduce cost.

Bragg gratings represent a relatively simple, compact, and low-cost approach to measuring strain or temperature in carbon and glass composite embedding applications. The simplicity arises because there is no need to measure optical phase, and hence no need for a coherent light source. Light demodulation instruments are difficult and costly to manufacture; thus the use of Bragg gratings results in a significant cost savings. Since these gratings can be written at different wavelengths, many individual sensors can be wavelength-division-multiplexed and integrated onto a single fiber optic strand. Fiber optic sensors can be embedded into the composite without degrading the mechanical performance of the part.

Bragg gratings provide significant and important enhancements to fiber optic technology, enabling the integration of the grating filters, reflectors, tunable lasers, couplers, and sensing elements directly in the waveguiding core of the fiber optic. The Federal Highway Administration has attached Bragg grating sensors to reinforcing rods and to concrete in numerous roads and bridges.

Bragg gratings may be fabricated on most commercially available fibers. The gratings' reflection and transmission response can also be custom-designed to specifications.

These Bragg grating enhancements are now available for license for use with commercially available Bragg grating and demodulation systems. Bragg grating enhancements are continually being researched and developed further by the Boeing Company, drawing upon the successful implementation of this technology on the Structural Health Monitoring System for the composite Clipper Graham^®LH₂ cryotank (see graph) built for NASA.

The Boeing Company is currently developing business relationships with companies interested in applying Boeing technologies to their products. If actively interested, please contact Dennis Donahue, Marketing Manager, Licensing; MC 306-1285, PO Box 516, St. Louis, MO 63166; (314) 234-7093; fax (314) 232-4313; http://www.boeing.com/assocproducts/mdip/ .