Nanomaterial-based robust oxygen sensor

Abstract

The development of a sensitive oxygen sensor suitable for deployment in the ullage of an aircraft fuel tank is discussed here. Instead of using plain optical fibers, fibers with a long period grating are utilized to produce lightweight optical sensors that strongly couple the core guided light into the cladding and vice versa. The detection principle is based on the quenching of the luminescence of a ruthenium complex, which is incorporated into the optical fiber using sol-gel techniques. Data are shown on sensitivity enhancement and performance of the sensor splashed with jet fuel. I. Introduction HE growing concern of aircraft fuel tank safety has taken an added dimension in the post 9/11 world where both commercial and military aircrafts are vulnerable to terrorist attacks utilizing MANPADS (MAN-Portable Air Defense Systems), explosives in shoes/socks, and small firearms. Fuel tanks also need protection from explosions caused by ballistic impact, lightning, and other sources of ignition. To prevent undesired ignition and explosion, it is critical to maintain the oxygen concentration in the fuel tank headspace (also called ullage) to about 9% by volume, the ambient oxygen level being 21%. The Federal Aviation Administration (FAA) has focused research to support two primary methods of fuel tank protection: ground-based and on-board fuel tank inerting. Ground-based fuel tank inerting involves a combination of fuel scrubbing and ullage washing with nitrogen enriched air (NEA). On-board fuel tank inerting involves ullage washing with OBIGGS (on-board inert gas generating system), a system that generates NEA during aircraft operations. Essential to the utilization of OBIGGS is an oxygen sensor that can operate in the aircraft’s ullage environment. OBIGGS can function economically by precisely determining when to start and when to stop. Without an oxygen sensor it is difficult to know when to replace the air separation module (ASM). A highly sensitive sensor with a large dynamic range is a necessary component in an OBIGGS system. II. Oxygen Sensor Detection Principle Optical fiber gratings were utilized to construct the oxygen sensor. A fiber optic long period grating (LPG) is a periodic refractive index perturbation in the core of an optical fiber that resonantly couples core-guided light into the cladding at one or more wavelength bands. When light is launched into the grating, a narrow band of light is reflected (see Figs. 2-1 and 2-2) within the grating by successive, coherent scattering from the index variations. An LPG has a period typically in the range of 100 Pm to 1 mm. With proper design, LPGs can be fabricated so that 100% of the excitation light is coupled from the core to the cladding where the oxygen indicator exists. This strong interaction results in more efficient excitation of the oxygen indicator, producing a highly sensitive sensor. The same boundary conditions allow 100% coupling of the cladding-bound modes of the luminescence signal back