Sensitivity Enhanced and Temperature-Compensated Fiber Bragg Grating Axial Strain Sensor Based on Dual-Loop Optoelectronic Oscillator Assisted With Harmonic Vernier Effect

In this article, a highly sensitive fiber Bragg grating (FBG) axial strain sensing system based on dual loop optoelectronic oscillator (OEO) enhanced by harmonic Vernier effect is proposed and experimental verified. The reflected signals from two cascaded FBGs serve as the optical carrier in the OEO, which is separated into two distinct beams using a wavelength division multiplexer (WDM). Thanks to the dispersion-compensated fiber (DCF) with different lengths, the strain can be measured by detecting the frequency change of the oscillating signal after the dual-loop superposition with different time delays. In addition, by setting the length of the two loops reasonably, the high-order harmonic Vernier effect can be obtained in the generated microwave signals of OEO. In this case, the sensing sensitivity can be greatly improved. The experimental results show that the sensitivity of single-loop OEO based on DCF is about 34.01 Hz/μϵ. By using the dual-loop OEO with harmonic Vernier effect, the axial strain sensitivity can reach −387.56 Hz/μϵ, which is about 12 times higher than that of single-loop OEO. Furthermore, through fine adjustment of the path length difference, harmonic Vernier effect with different orders including the 1st, 2nd, and 3rd harmonic order is experimentally realized. The sensing sensitivities can reach −780.56 Hz/μϵ, −1115.12 Hz/μϵ and −3155.33 Hz/μϵ, which are 24, 32 and 95 times of the single-loop OEO structure, respectively. To the best of our knowledge, harmonic Vernier effect is applied for the first time to the OEO-based sensors to achieve high sensing performance with ultrahigh-sensitivity and amplification factor. Additionally, by cascading the reference FBG to the sensing FBG, the strain error induced by temperature in sensing probe is reduced from 4.91 μϵ to 0.07 μϵ. The proposed sensor features high sensitivity, high resolution and high repeatability, which may provide a versatile and efficient solution for high performance axial strain measurement.