An ultra-high sensitivity open-cavity Fabry–Perot interferometer (FPI) gas refractive index (RI) sensor based on the photonic crystal fiber (PCF) and Vernier effect is proposed and demonstrated. The sensor is prepared by splicing a section of PCF to a section of fiber tube fused with a section of single mode fiber. The air holes running along the cladding of the PCF enable the gas to enter or leave the cavity freely. The reflection beam from the last end face of the PCF is used to generate the Vernier effect, which significantly improves the sensitivity of the sensor. Experimental results show that the proposed sensor can provide an ultra-high RI sensitivity of 30899 nm/RIU. This sensor has potential applications in fields such as gas concentration analyzing and humidity monitoring.

A high-sensitivity fiber-optic strain sensor, based on the Vernier effect and separated Fabry-Perot interferometers (FPIs), is proposed and experimentally demonstrated. One aircavity FPI is used as a sensing FPI (SFPI) and another is used as a matched FPI (MFPI) to generate the Vernier effect. The two FPIs are connected by a fiber link but separated by a long section of single-mode fiber (SMF). The SFPI is fabricated by splicing a section of microfiber between two SMFs with a large lateral offset, and the MFPI is formed by a section of hollow-core fiber sandwiched between two SMFs. By using the Vernier effect, the strain sensitivity of the proposed sensor reaches 1.15 nm/με, which is the highest strain sensitivity of an FPI-based sensor reported so far. Owing to the separated structure of the proposed sensor, the MFPI can be isolated from the SFPI and the detection environment. Therefore, the MFPI is not affected by external physical quantities (such as strain and temperature) and thus has a very low temperature cross-sensitivity. The experimental results show that a low-temperature cross-sensitivity of 0.056 με/°C can be obtained with the proposed sensor. With its advantages of simple fabrication, high strain sensitivity, and low-temperature cross-sensitivity, the proposed sensor has great application prospects in several fields.

This study experimentally demonstrates a high-sensitivity fiber-optic relative humidity (RH) sensor based on sensitivity amplification and a reduction mechanism, employing an internal–external Fabry–Perot cavity (IEFPC) Vernier effect and a chitosan film as a Fabry–Perot (FP)-sensing cavity. The proposed sensor is constructed using cascaded FP interferometers comprised of an air cavity formed by a hollow-core fiber (HCF), a chitosan cavity, and an air–chitosan hybrid cavity. The chitosan cavity is fabricated by dipping the HCF into a chitosan solution to form a thin chitosan film. Thus, the thickness of the chitosan film could be controlled precisely based on dipping time and capillary effect. As the optical path lengths of an air–chitosan hybrid cavity and an air cavity are similar, the IEFPC Vernier effect is generated, amplifying the air–chitosan hybrid cavity’s low sensitivity to the chitosan cavity’s high sensitivity. The experimental results agree with the theoretical analysis, supporting the fact that the sensor’s sensitivity is related only to the thickness of the chitosan film. The sensitivity of the sensor reaches up to 7.15 nm/% RH, ranging 40%–92% RH at 25°C. Fabrication of the proposed sensor is cost-effective. The proposed sensor also exhibits superior stability performance, a low-temperature cross-sensitivity of 0.0068% RH/°C, and repeatable fabrication. The proposed IEFPC Vernier effect model functions well for cascaded cavities, which plays a guiding role in the sensitivity improvement of such a structure within a fiber-optic sensing context.

A high-sensitivity Fabry-Perot interferometer (FPI) strain sensor based on a concave-core photonic crystal fiber (CPCF) was proposed and demonstrated. The FPI was formed by directly splicing a CPCF to a standard single-mode fiber. The CPCF was fabricated by cleaving a high-numerical-aperture solid-core photonic crystal fiber under axial tension using a traditional fiber cleaver. An FPI strain sensor with a cavity length of 4.85 μ m and a strain sensitivity of 31.58 pm/με was successfully produced. The sensor also demonstrated a low temperature sensitivity of 0.653 pm/°C, reducing the cross-sensitivity between tensile strain and temperature. The benefits of the proposed sensor include simple fabrication, high strain sensitivity, and low temperature cross-sensitivity, making it attractive for practical applications.

We report a fast response microfluidic Fabry–Perot (FP) interferometer refractive index (RI) fiber sensor based on a concave-core photonic crystal fiber (CPCF), which is formed by directly splicing a section CPCF with a section of single mode fiber. The CPCF is made by cleaving a section of multimode photonic crystal fiber with an axial tension. The shallow concave-core of CPCF naturally forms the FP cavity with a very short cavity length. The inherent large air holes in the cladding of CPCF are used as the open channels to let liquid sample come in and out of FP cavity. In order to shorten the liquid channel length and eliminate the harmful reflection from the outside end face of the CPCF, the CPCF is cleaved with a tilted tensile force. Due to the very small cavity capacity, the short length and the large sectional area of the microfluidic channels, the proposed sensor provides an easy-in and easy-out structure for liquids, leading to great decrement of the measuring time. The proposed sensor exhibits fast measuring speed, the measuring time is less than 359 and 23 ms for distilled water and pure ethanol, respectively. We also experimentally study and demonstrate the superior performances of the sensor in terms of high RI sensitivity, good linear response, low temperature cross-sensitivity and easy fabrication.

A high sensitivity humidity fiber sensor based on all-agar Fabry-Perot (FP) interferometer is proposed and experimentally demonstrated. The sensor is fabricated by filming a thin layer of moisture-sensitive agar to single mode fiber with end dipping method. Due to the all-agar FP cavity structure, both the size and the RI changes of agar film in the swelling effect are fully utilized to induce the frequency shift, the sensor exhibits a high sensitivity of 4.20 nm/%RI!, with relative humidity (RI!) ranging from 25%RI! to 95%RI! at 25 °C. Moreover, owing to the short FP cavity, 7.10 μm, the proposed sensor provides a short response time less than 340 ms with the humidity level changed from 64.0%RI! to 74.2%RI!. The proposed sensor also exhibits other superior performances in terms of good stability, low cost, and easy fabrication. Such good characteristics make the proposed sensor a good candidate for practical applications.