Inclinometers are commonly used in boreholes to monitor the profile of land/soil displacement, which is vital in geotechnical engineering. A new in-place fiber-optic inclinometer (FOI) is developed based on a vertical canti-lever beam and dual fiber Bragg gratings (FBGs). This FOI is characterized by its robust design, bidirectional large range, multiplexing capability, and push-pull configuration with temperature insensitivity. A pair of pre-stretched FBGs with different reflection wavelengths are respectively glued on the opposite surfaces of a verti-cal cantilever beam with tip mass to form a push-pull structure, which is further encapsulated by a solid cylinder made of aluminum alloy. Calibration tests of the proposed inclinometer are performed in the laboratory to examine its measurement performance, revealing a measurement sensitivity of similar to 0.1 nm/degrees, resolution of similar to 0.01 degrees, and a maximum bidirectional measurement range of +/- 30 degrees. The numerically calculated sensitivity based on actual parameters agrees well with the experimental result. Furthermore, three inclinometer units are con-nected by spacers to conduct an assembled inclinometer test in the laboratory, four typical profile patterns are validated with a sensing error of <6.73 %.

We proposed a simple and compact Fabry-Perot (FP) sensor for simultaneous measurement of water pressure and temperature, composed of a standard fiber ferrule connector/flat contact (FC/FC) connector and aluminum foil attached to a thin metal plate. Two dips in the interference spectrum are selected to illustrate the response of the FP sensor to the water pressure and temperature, we can obtain the simultaneous measurement via a method of sensitivity coefficient matrix. Experimental results show the maximum sensitivity of −2.948 nm/kPa for water pressure sensing in 0~9 kPa and 1.119nm/°C for temperature sensing in 33~45 °C. In addition, the dual-parameter sensor has the advantages of compactness, stability, simple production, small size, and high sensitivity, making it suitable for a wide range of practical applications.

In this work, a new method for measuring slight tilt based on the dynamic Vernier effect is proposed and verified. A pair of the Fabry-Perot interferometers (FPIs) are fixed on the surfaces of a cantilever beam horizontally, and a mass block at the tail end of the beam supplies longitudinal strain on the surfaces, resulting in opposite variations in the cavity lengths (CLs) of FPIs. According to the principle of dynamic Vernier effect at the superposition of reflected light, the shift of the Vernier envelope would be proportional to the tilt angle of the entire structure. The experimental results revealed a strain sensitivity of as high as 6.733 nm/mu epsilon with a magnification factor of 41. Furthermore, the theoretical simulated results show that the maximum sensitivity would achieve from 204.408 nm/degrees to 613.25 nm/degrees within the tilt range from 0 degrees to 15 degrees. In addition, the merits of the proposed tilt sensor such as simple in structure, low cost, and temperature insensitivity in normal environments, all of which promise great application prospects in monitoring suddenly major structural deformation such as mountains and buildings

In this work, we have experimentally demonstrated the refractive index (RI) sensing characteristics of a S-shape taper refractometer (STR) based on modal interference theory. Our preliminary theoretical analysis reveals that there exists a critical cladding mode, which is essential for understanding the sensing characteristics. When the dominant cladding mode involved in a core-cladding interference is close to the critical cladding mode, the resulting RI sensitivity tends to reach a maximum value. Moreover, both the critical and the dominant cladding mode are dependent on ambient RI. Our sensor achieves a high RI sensitivity of 2109.7 nm/RIU for a transmission dip at around 1505 nm with a measurement range of 1.36 to 1.39.

A systematic comparison of fiber-based gas pressure sensors using different types of hollow-core photonic crystal fibers (PCFs) is conducted. The sensor was fabricated by splicing a segment of hollow-core silica capillary between a single mode fiber (SMF) and a hollow-core PCF. A Fabry-Perot (FP) cavity configuration is thus formed with the capillary tube as the sensing cavity, while the PCF acts as gas passage to the external environment. External pressure change would lead to the variation in the refractive index of air in the sensing cavity, resulting in a wavelength shift of the interference dips. The pressure sensitivity is measured to be around 4 nm/MPa, with a high linearity of 99.7% or above, regardless of the type of the PCFs we used as gas inlet. Among the four different PCFs, the large-mode-area (LMA) PCF shows the highest fringe contrast in the reflection spectrum. Modal analysis reveals that this is due to the high reflectivity caused by the solid core of LMA-PCF. Our experimental results also indicate that the length of the sensing cavity, as well as the offset fusion splice will influence the fringe contrast. The sensor has potential application in gas pressure sensing for advantages of high sensitivity, compact size and ease of fabrication.