Fiber optic strain sensor based on enhanced harmonic Vernier effect

In sensors utilizing the traditional harmonic Vernier effect, it is imperative to isolate the reference interferometer to mitigate the impact of external environmental fluctuations on the reference spectrum, thereby averting measurement inaccuracies. However, achieving such isolation between interferometers in compact fiber optic sensors poses significant challenges. We propose a fiber optic strain sensor based on the enhanced harmonic Vernier effect. In the enhanced harmonic Vernier effect, two interferometers are simultaneously involved in sensing. Hence, the isolation of interferometers is no longer a concern. Furthermore, the two interferometers exhibit responses with opposite signs, leading to a greater displacement of the envelope. Consequently, the sensitivity of the sensor is further enhanced without altering the harmonic order. Firstly, we conducted numerical simulations to analyze the enhanced harmonic Vernier effect. Secondly, we fabricated a Fabry–Perot interferometer (FPI) and a Mach–Zehnder interferometer (MZI) with strain sensitivity of opposite signs, generating a first-order harmonic. The experimental results indicate that the sensor’s sensitivity is 15.1 pm/ μ ϵ, which is 1.41 and 2.80 times higher than the traditional harmonic Vernier effect using FPI and MZI as sensing interferometers, respectively. The proposed sensor and enhancement method hold promising potential for applications in high-sensitivity measurement fields.