Propellants commonly used in military and civilian applications consist of hydrazine (HZ) and/or its derivatives. These hypergolic propellants (HZ and/or its derivatives) are toxic to humans at lower concentrations and are highly flammable and explosive at higher concentrations. Hence, the development of a simple, low-cost and, of course, a reversible sensor for detection of HZ is necessary.

This paper reports the development of a HZ fiber optic reversible sensor operating in the lowest attenuation wavelength range (1310-1550 nm) of commercial silica fibers. This makes it a suitable sensor for wide area fiber optic HZ detection and reporting systems.

The developed sensor contains pentacenediquinone (PDQ) organic compound mix as an active sensing element in a 5 cm long modified cladding fiber sensor. The refractive index of this modified cladding is adjusted to 1.492 to closely match that of the silica fiber core. When the PDQ reacts with HZ, oxygen atoms from the PDQ are replaced by a molecule of HZ resulting in water as a by-product. This replacement results in significant increase in the absorption of the mix, specifically at wavelengths between 1331 and 1430 nm as well as at 1550 nm. However, this reaction is self reversible, i.e. in the presence of water; the by-products of the reverse reaction would be PDQ and hydrazine.