Fiber-optic hydrogen sensor based on polarization-diversity loop interferometer

In this study, we have demonstrated a fiber-optic hydrogen sensor using a polarization-diversity loop interferometer composed of a polarization beam splitter, two quarter-wave plates, and a polarization-maintaining fiber coated with palladium whose thickness was ∼400 nm. One dip in the output interference spectrum of the proposed sensor, chosen as a sensor indicator, was observed to spectrally shift with increasing hydrogen concentration. At a hydrogen concentration of 4%, the sensing indicator showed a wavelength shift of ∼2.48 nm. The response time of the proposed sensor was measured as 10–12.5 s and did not show significant dependence on the hydrogen concentration except for a hydrogen concentration of 4%. In particular, compared with other hydrogen sensors based on side-polished fibers or fiber gratings, the proposed sensor is much more durable because UV illumination or physical/chemical etching process is not necessary for the optical fiber and thus is highly resistant to external stress applied on a transverse axis of an optical fiber.