Design and Development of a Low-Cost Fiber-Optic Hydrogen Detector

A cost-effective detector for hydrogen gas leaks will be needed in many hydrogen-fueled technologies of the future. In particular, the hydrogen-fueled automobile may require hydrogen leak sensors in several locations and their cost could be prohibitive if conventional sensor technology is used. This project is directed at the development of low-cost fiber-optic (FO) hydrogen gas detectors thatcould provide adequate sensitivity, response speeds and reliability in an automobile application. The FO sensor relies upon a reversible chemical reaction between gaseous hydrogen in air and a thin film of tungsten oxide. We have studied the kinetics of this hydrogen reaction with the sensor coating systematically. Different design concepts have been evaluated. The initial sensor designconsisted of a thin tungsten oxide film coating on the flat end of a polymer or glass optical fiber. A very thin overcoating of palladium catalyzes the reaction between the hydrogen and the tungsten oxide and reflects a light beam back along the optical fiber to a light intensity monitor. When the hydrogen reacts with the coating, the tungsten oxide becomes optically absorptive and attenuatesthe reflected light. The sensitivity and response speed of this sensor were studied over a range of temperatures and found to be too slow for the intended application. A faster sensor design was invented that relies upon the resonant absorption of light at a beveled facet on the end of the optical fiber. The resonance occurs when the incident light strikes the metal coated facet at an angle justabove the critical angle for total internal reflection. The evanescent wave stimulates resonant absorption of the light at certain wavelengths by free electrons in the metal to produce a so-called surface-plasmons (SP). An overcoat of thin tungsten oxide on top of the metal film is designed to provide an optical wave-guide for light at the surface plasmon resonance. The two layer coatingproduces a coupled resonance at the SP wavelength that is ver sensitive to the optical constants of the tungsten oxide. When hydrogen reacts with the tungsten oxide, the resonance frequency shifts and this shif is detected in the spectrum of the reflected light beam. The intensity of the total reflected light beam is used to produce an internal reference signal for comparison to the intensity ofthe narrow wavelength resonance. The ratio of these two signals cancels out the signal noise that is due to variation in the intensity of the light source and in the transmittance of the optical fiber. A patent application is in process and a small business partner has formed a CRADA with NREL to develop a commercial detector based upon this design.