Distributed detection of hydrogen and deuterium diffusion into a single-mode optical fiber with chirped-pulse phase-sensitive optical time-domain reflectometry.

For some infrastructures such as oil and gas extraction boreholes or radioactive waste repositories, where distributed optical fiber sensors are employed to grant the safety of the facilities, the presence of gas species such as hydrogen or deuterium is one of the most relevant parameters to monitor. The possibility of employing the same kind of sensors for this purpose is of special interest, reducing the cost by employing a single interrogator, able to measure multiple parameters by simply employing adequate sensing fibers. To meet this goal, we present here a chemical sensor based on chirped-pulse phase-sensitive optical time-domain reflectometry (CP-φOTDR), which is able to detect these species while they diffuse into the silica fiber. The ability of chirped-pulse φOTDR to measure a change in refractive index with sensitivity around 10 has allowed determining hydrogen concentration with accuracy on the order of 10  mol/m and spatial resolution ∼6  m. Another experiment provides an indirect measurement of the solubility of deuterium in a standard telecom-grade optical fiber, which is found to be around 1.47×10  m/bar.