Cascaded Mach–Zehnder Interferometers With Vernier Effect for Gas Pressure Sensing

We propose a compact ultrasensitive gas pressure sensor based on two cascaded fiber Mach-Zehnder interferometers (MZIs) with Vernier effect. Each MZI is composed of a hollow core fiber (HCF) sandwiched in between two short multimode fibers. The lengths of the HCFs in the two MZIs are set a little different in order to generate an envelope in the transmission spectrum of the cascaded structure for Vernier effect sensing. The HCF in one MZI is opened to make the air hole connect to the external environment and act as a sensing element. The HCF in the other MZI is kept closed serving as a reference element. The two MZIs have quite different responses to changes in gas pressure but have almost the same response to temperature variation. Therefore, the Vernier effect occurs when the gas pressure changes but not when the temperature varies. As a result, the proposed structure has an ultra-high gas pressure sensitivity with a quite low-temperature cross sensitivity. The experimental results show that the gas pressure sensitivity referring to the envelope of the proposed sensor is -73.32 nm/MPa in the range of 0-0.8 MPa, with a temperature cross sensitivity of only 0.72 kPa/°C.