A taper-in-etch based hybrid fiber Mach-Zehnder interferometer hydrogen sensor

Abstract

A hydrogen (H2) sensing application is proposed and experimentally demonstrated using a multimode fiber (MMF)–tapered-in-etch single-mode fiber (SMF)–MMF (MMF–TiESMF–MMF)-based Mach–Zehnder interferometer (MZI). The MZI is made of an SMF segment placed between two MMF segments. The MMF segments function as mode couplers, splitting and combining light due to a core diameter discrepancy between the MMF and the SMF. The SMF segment undergoes cladding modification by chemically etching to achieve a 50 µm diameter over a 2 cm length, followed by the tapering to obtain a 15 µm waist diameter with a 1 cm length. A layer of palladium–graphite nanofibers is applied to the modified area of the TiESMF to produce H2 adsorption. The sensor is investigated against H2 over the 0.25–1% concentration range at room temperature. When H2 molecules were desorbed on the sensing layer, its index of refraction was changed so that the output spectrum resulting from the MZI is blue-shifted due to H2 molecules desorption. The MMF-TiESMF-MMF MZI exhibits a sensitivity of 0.332 nm/vol% and a linear fit coefficient of 99.72% for H2 concentrations in the range selected. MZI based sensors showed a linear, stable, and repetitive response.