The optical Vernier effect magnifies the sensing capabilities of an interferometer, allowing for unprecedented sensitivities and resolutions to be achieved. Just like a caliper uses two different scales to achieve higher resolution measurements, the optical Vernier effect is based on the overlap in the responses of two interferometers with slightly detuned interference signals. Here, we present a novel approach in detail, which introduces optical harmonics to the Vernier effect through Fabry–Perot interferometers, where the two interferometers can have very different frequencies in the interferometric pattern. We demonstrate not only a considerable enhancement compared to current methods, but also better control of the sensitivity magnification factor, which scales up with the order of the harmonics, allowing us to surpass the limits of the conventional Vernier effect as used today. In addition, this novel concept opens also new ways of dimensioning the sensing structures, together with improved fabrication tolerances.

A Fabry–Perot (FP) cavity of simple design and based on a pure silica diaphragm-free hollow tube is proposed. Its operation is based on a first reflection of light at the end of the single-mode fiber that illuminates the silica rod and in a second reflection that takes place at the end of the rod. The FP cavity is characterized for high temperature, pressure and refractive index sensing, showing useful characteristics for the measurement of these three parameters. The diaphragm-free configuration simplifies the measurement of the refractive index of fluids.

A high sensitivity Fabry-Pérot (FP) strain sensor based on hollow-core ring photonic crystal fiber was investigated. A low-finesse FP cavity was fabricated by splicing a section of hollow-core ring photonic crystal fiber between two standard single mode fibers. The geometry presents a low cross section area of silica enabling to achieve high strain sensitivity. Strain measurements were performed by considering the FP cavity length in a range of 1000 μm. The total length of the strain gauge at which strain was applied was also studied for a range of 900 mm. The FP cavity length variation highly influenced the strain sensitivity, and for a length of 13 μm a sensitivity of 15.4 pm/με was attained. Relatively to the strain gauge length, its dependence to strain sensitivity is low. Finally, the FP cavity presented residual temperature sensitivity (~0.81 pm/°C).