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In this work, an optical fiber vacuum sensor based on a single-mode multimode single-mode (SMS) structure coated with polydimethylsiloxane (PDMS) is studied. The SMS structure generates an interferometric pattern based on multimode interference. The structure is dip-coated with a layer of PDMS, whose optical properties change when it is subjected to varying vacuum pressure. Different strategies are applied in an attempt to improve the final performance of the sensor, such as decreasing the diameter of the fiber and modifying the properties of the coating by modifying the proportion of solvent. Decreasing the diameter of the optical fiber and using toluene as a solvent are both proved to be successful strategies for increasing the sensitivity of the sensor. The devices are studied in the $1\times 10^{-3}$ –10 mbar range with a maximum wavelength shift of 12 nm, leading to a maximum sensitivity of 35 nm/mbar. The simplicity of the fabrication process, which can be applied to more sensitive structures, suggests that PDMS may be a good choice for the development of optical fiber vacuum sensors. [ABSTRACT FROM AUTHOR]

An optical fiber inclinometer applying modal interference based on an abrupt tapering cascading a peanut-shape configuration is proposed. The abrupt taper splits part of the core mode into cladding modes and the interference occurs when the peanut-shape recombines the cladding modes and core mode together. Four inclinometers with different lengths have been fabricated and characterized, and the high sensitivities of −1.76 and −4.49 nm/° are obtained in the angle range of 0°–3° and 3°–6.66°, respectively. [ABSTRACT FROM PUBLISHER]

An ultrasensitive temperature sensor based on a thin-core fiber modal interferometer (TCFMI) is proposed and demonstrated in this paper. After its middle is etched to a thin residual diameter, the TCFMI is sealed in a capillary tube filled with refractive index (RI) liquid. The thermo-optic effect of RI liquid turns the TCFMI into an ultrasensitive temperature sensor with sensitivity up to -22.23 nm/°C at 3.2 °C. The sensitivity is over 1000-fold that of the bare TCFMI in air. This proposed temperature sensor is also simple, cost-effective, and suitable for a variety of applications. [ABSTRACT FROM PUBLISHER]

An optical in-fiber modal interferometer-based volume strain sensor for earthquake prediction is proposed and experimentally demonstrated. The sensing element is formed by wrapping a multimode-singlemode-multimode fiber structure onto a polyurethane hollow column. Due to the modal interference between the excited guided modes in the fiber, strong interference pattern could be observed in the transmission spectrum. Theoretical analysis verifies that the resonant wavelength shifts as a result of the volume strain variation caused by the column deformation with square root relationship. Sensitivity >3.93~pm/\mu\varepsilon within the volume strain ranging from 0 to 1300 \mu\varepsilon is also experimentally demonstrated. By taking the response of bidirectional change of volume strain and the sluggish character of the employed sensing material into consideration, the sensing system presents good repeatability and stability. [ABSTRACT FROM PUBLISHER]

A novel fiber optic hybrid device for simultaneously measuring temperature and relative humidity (RH) is proposed and demonstrated in this paper. The device is composed of a fiber Bragg grating (FBG) and a reflection type photonic crystal fiber interferometer (PCFI) infiltrated with humidity sensitive material Agarose. The Agarose infiltrated PCFI is used to monitor the RH and an in-line FBG is used to monitor the temperature. The temperature is measured by detecting the shift in the Bragg wavelength and the humidity is measured by monitoring the change in the optical power of the reflected spectrum. The sensor shows a temperature sensitivity of 9.8 pm/^\circC and an optical power variation of >7~dB for an RH change of 75%, with less cross sensitivity. [ABSTRACT FROM PUBLISHER]

A compact fiber sensor based on fiber Bragg grating (FBG) embedded in a fiber modal interferometer (MI) for simultaneous measurement of strain and temperature is proposed. Because the total length of the sensor is 12 mm and can be further reduced, the spatial resolution of the sensor is improved. Due to the different responses of the fiber MI and the FBG to strain and temperature, the simultaneous measurement at the same point is achieved. For 0.01-nm wavelength resolution, the resolution of the sensor is 5.96 \mu\varepsilon and 0.31^\circC in strain and temperature, respectively. [ABSTRACT FROM PUBLISHER]

A temperature sensor based on modal interference in a hollow-core photonic bandgap fiber (HC-PBGF) is presented. The modal interferometer is fabricated by splicing the two ends of a 12-mm HC-PBGF to the standard single-mode fibers. The airholes in the core and cladding of the HC-PBGF are completely collapsed during the fusion splicing, and the length of the collapsed area of about 300 μm is observed. The transmission spectra under different temperatures are measured. The sensitivity of 31.8 pm/°C is achieved in the range of 25 °C-60 °C. The influence of the length of HC-PBGF on the sensitivity is also investigated. [ABSTRACT FROM PUBLISHER]

An all-fiber modal interferometric torsion sensor based on simple splicing configuration is presented wherein a photonic crystal fiber is spliced in between multimode fiber and single mode fiber. Appreciable torsion sensitivity of ~79.83 pm/° is obtained in the long dynamic range of 180°. It is also found that there is no effect of strain and temperature on torsion sensitivity in the range of 0-4500 με and 30-200°C, respectively, leading to zero cross-sensitivity of torsion with strain and temperature. [ABSTRACT FROM PUBLISHER]