Refractive index sensor based on inline Mach-Zehnder interferometer coated with hafnium oxide by atomic layer deposition

A fiber-optic interferometer based on a Mach-Zehnder configuration is coated with hafnium oxide by atomic layer deposition (ALD) to increase the sensitivity to the refractive index of the surrounding medium (SRI). The sensor is based on the intermodal interference between the core and a cladding mode of a photonic crystal fiber (i.e. PCF interferometer). Prior to coating of PCF interferometers, the purge time, a process parameter of ALD is investigated to achieve precise control over coating thickness. Three different depositions with purge times of 10 s, 20 s, and 30 s were selected. The coating thickness and uniformity were measured by scanning electron microscopy on standard single-mode optical fibers. As a reference, the film thickness was measured by ellipsometry on a silicon wafer. The results show that control of coating thickness on optical fibers is achieved by selecting longer purge times, compared to conventional depositions on flat substrates. The suitable process parameters to coat standard single-mode fibers were selected for the coating of the PCF interferometers. The effect of coating thicknesses on the sensitivity of these sensors was investigated by depositions targeting coating thicknesses of 30 nm, 40 nm, 50 nm, 60 nm, 70 nm and 90 nm. Sensitivity of 1307 nm/SRI at an SRI range in the vicinity of water was achieved for an interferometer coated by a deposition targeting a coating thickness of 60 nm. This sensitivity value is the highest reported in the literature for a PCF interferometer.