Highly Sensitive Detection Ammonia in Water by a Fabry–Perot Interferometer Based on Sparse Sampling and Vernier Effect

For interferometric fiber optic sensors, a method based on sparse sampling to control the sensitivity is proposed and applied to the ammonia measurement in water detected by a Fabry–Perot interferometer (FPI). The designed ammonia sensing interferometer (S-FPI) is fabricated by offset splicing of single-mode optical fibers (SMFs) to form an open cavity, which is filled with ammonia-sensitive oxazine perchlorate (O17). Based on the reaction of O17 with ammonia solution, the refractive index in the cavity is changed, which causes a shift in the interference spectrum of the S-FPI to measure the ammonia concentration. In order to improve the sensitivity of ammonia measurement, the sampling interval of the laser source is controlled to generate the Vernier effect, and the sensitivity of ammonia detection was enhanced by measuring the shift of the sampling-fitted Vernier envelope. Experiments achieved multiple controllable ammonia sensitivities with a maximum sensitivity enhancement of about 16.1 times, verifying the feasibility of this enhanced-sensitivity method. The method does not require the introduction of additional reference interferometer and spectral analyzers, greatly simplifying the sensing system structure. In addition, the sensitivity can be controlled and improved, which has a better application prospect.