In-situ adjustable fiber-optic piezometer based on parallelly structured external Fabry-Perot interferometers with Vernier effect and its harmonics

Translating interferometric applications into practical field use with the required flexible precision and measurement range, is a recognized challenge. We report an in-situ adjustable fiber-optic piezometer based on parallelly structured external Fabry-Perot interferometers (EFPIs) with the Vernier effect and its harmonics. By accommodating the EFPI structure with an adjustable and a fixed cavity length, the EFPIs are utilized as a referencing fiber piezometer (RFP), and a sensing fiber piezometer (SFP), respectively. The Vernier effect with amplified sensitivity is formed by connecting the RFP and SFP in parallel via a 3 dB optical coupler. By simply tuning the cavity length of the RFP, the magnification factor M of the Vernier effect is in-situ continuously adjustable. Using the two prototypes SFP1 and SFP2, water level measurement is performed in the fundamental Vernier effect (FVE) mode and the harmonic Vernier effect (HVE) mode. Experimental results demonstrate that in the FVE mode, the sensitivity can be predictably tuned from -0.15 nm/cm to -7.02 nm/cm with M=1∼48 for SFP1, and -0.31 nm/cm to -7.22 nm/cm with M=1∼23 for SFP2. In the HVE mode, the sensitivity can be further enhanced as high as -9.08 nm/cm, while the sensing performance fluctuates during the adjusting process. Benefiting from the merits of in-situ adjustable sensitivity and measurement range, simplicity of composition, robustness, and remote sensing capability, the proposed scheme can be applied to various practical applications.