A cost-effective detector for hydrogen gas leaks will be needed in many hydrogen-fueled technologies of the future. 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 that could provide adequate sensitivity, response speeds and reliability in an automobile application. A new, 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 just above the critical angle for total internal reflection. The evanescent wave stimulates resonant absorption by free electrons in the metal to produce a so-called surface-plasmon (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 coating produces a coupled resonance at the SP wavelength that is very sensitive to the optical constants of the tungsten oxide. When hydrogen reacts with the tungsten oxide the resonance frequency shifts and this shift is detected in the spectrum of the reflected light beam. The facets are angled at 45 degrees to the fiber axis so as to reflect the light back along the fiber with a doubling of the SP absorption from the double reflection. A facet perpendicular to the fiber axis produces a reflected signal that is not affected by hydrogen that is used to produce an internal reference signal for comparison to the resonance, hydrogen-sensitive signal. The ratio of these two signals cancels out noise due to variation in the transmittance of the optical fiber. A patent application has been filed for this new design and a small business partner has formed a CRADA with NREL to develop a commercial detector based upon it.
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