The development of reliable, affordable and efficient sensors is a key step in providing tools for efficient monitoring of critical environmental parameters. This review focuses on the use of tapered optical fibres as an environmental sensing platform. Tapered fibres allow access to the evanescent wave of the propagating mode, which can be exploited to facilitate chemical sensing by spectroscopic evaluation of the medium surrounding the optical fibre, by measurement of the refractive index of the medium, or by coupling to other waveguides formed of chemically sensitive materials. In addition, the reduced diameter of the tapered section of the optical fibre can offer benefits when measuring physical parameters such as strain and temperature. A review of the basic sensing platforms implemented using tapered optical fibres and their application for development of fibre-optic physical, chemical and bio-sensors is presented.

Tapered Optical Fibre Sensors: Current Trends and Future Perspectives

A highly sensitive temperature sensor based on an all-fiber Sagnac loop interferometer combined with metal-filled side-hole photonic crystal fiber (PCF) is proposed and demonstrated. PCFs containing two side holes filled with metal offer a structure that can be modified to create a change in the birefringence of the fiber by the expansion of the filler metal. Bismuth and indium were used to examine the effect of filler metal on the temperature sensitivity of the fiber-optic temperature sensor. It was found from measurements that a very high temperature sensitivity of −9.0 nm/°C could be achieved with the indium-filled side-hole PCF. The experimental results are compared to numerical simulations with good agreement. It is shown that the high temperature sensitivity of the sensor is attributed to the fiber microstructure, which has a significant influence on the modulation of the birefringence caused by the expansion of the metal-filled holes.

Highly sensitive temperature sensor using a Sagnac loop interferometer based on a side-hole photonic crystal fiber filled with metal.

Simultaneous measurement of temperature and strain or temperature and curvature based on an optical fiber Mach–Zehnder interferometer

Ammonia is an important indicator among environmental monitoring parameters. In this work, thin-core fiber Mach-Zehnder interferometer deposited with poly (acrylic acid) (PAA), poly (allyamine hydrochloride) (PAH) and single-walled carbon nanotubes (SWCNTs-COOH) sensing film for the detection of ammonia gas has been presented. The thin-core fiber modal interferometer was made by fusion splicing a small section of thin-core fiber (TCF) between two standard single mode fibers (SMF). A beam propagation method (BPM) is employed for the design of proposed interferometer and numerical simulation. Based on the simulation results, interferometer with a length of 2 cm of thin-core fiber is fabricated and experimentally studied. (PAH/PAA)2 + [PAH/(PAA + SWCNTs-COOH)]8 film is deposited on the outer surface of thin-core fiber via layer-by-layer (LbL) self-assembly technique. The gas sensor coated with (PAH/PAA)2 + [PAH/(PAA + SWCNTs-COOH)]8 film towards NH3 gas exposure at concentrations range from 1 to 960 ppm are analyzed and the sensing capability is demonstrated by optical spectrum analyzer (OSA). Experimental results show that the characteristic wavelength shift has an approximately linear relationship in the range 1–20 ppm, which is in accordance with the numerical simulation. Thus, this paper reveals the potential application of this sensor in monitoring low concentration NH3 gas.

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An in-line Mach-Zehnder Interferometer Using Thin-core Fiber for Ammonia Gas Sensing With High Sensitivity.

A compact high temperature sensor utilizing a multipath Michelson interferometer (MI) structure based on weak coupling multicore fiber (MCF) is proposed and experimentally demonstrated. The device is fabricated by program-controlled tapering the spliced region between single mode fiber (SMF) and a segment of MCF. After that, a spherical reflective structure is formed by arc-fusion splicing the end face of MCF. Theoretical analysis has been implemented for this specific multipath MI structure; beam propagation method based simulation and corresponding experiments were performed to investigate the effect of taper and spherical end face on system's performance. Benefiting from the multipath interferences and heterogeneous structure between the center core and surrounding cores of the all-solid MCF, an enhanced temperature sensitivity of 165 pm/°C up to 900°C and a high-quality interference spectrum with 25 dB fringe visibility were achieved.

Heterogeneous all-solid multicore fiber based multipath Michelson interferometer for high temperature sensing

High temperature probe sensor with high sensitivity based on Michelson interferometer

The invention relates to an optical fiber grating vibration sensor comprising double cantilever beams with equal strength, a cantilever beam base is arranged on the left side in a housing, the fixing ends of the upper sheet-like cantilever beam and the lower sheet-like cantilever beam, which are parallel to each other are connected with the cantilever beam base into a whole, the free ends are arranged on the upper side surface and the lower side surface of a mass block, cantilever beam position-limiting columns which are vertical to the surfaces are arranged on the surfaces which are respectively parallel to the plane of the upper cantilever beam and the plane of the lower cantilever beam of the mass block, a sealing cover which is equipped with a sensor joint on the outer side is arranged at the open end of the housing, an optical fiber protective casing penetrating the center of the sealing cover and the center of the sensor joint is arranged on the sealing cover, the ratio of the width of the fixing ends of the upper cantilever beam and the lower cantilever beam to the width of the free ends is 5 at least, an upper sensing optical fiber grating is arranged on the upper surface of the upper cantilever beam, a lower sensing optical fiber grating is arranged on the lower surface of the lower cantilever beam, and the series connection output end of the upper sensing optical fiber grating and the lower sensing optical fiber grating penetrates out of the optical fiber protective casing The optical fiber grating vibration sensor has good sensitivity, reliability and stability

Optical fiber grating vibration sensor comprising double cantilever beams with equal strength

The invention provides an optical fiber grating three-dimensional vector vibration sensor which adopts a hollow cylinder structure. The hollow cylinder comprises a hollow sensitive cylinder, an inertial mass block, a base and a sensitive element optical fiber grating, wherein the inertial mass block is located at the top end of the hollow cylinder and is fixed at the top end of the hollow cylinder or connected with the hollow cylinder into an integral whole, and the hollow sensitive cylinder is fixed on the base or connected with the base into an integral whole. The optical fiber grating three-dimensional vector vibration sensor further comprises that the inertial mass block is provided with an optical fiber outlet hole and a base fixing screw hole, wherein the optical fiber outlet hole is used for leading out three optical fibers and is sealed by a sealing material; and the base fixing screw hole is used for fixing a three-dimensional vector vibration sensor. A uniform sensitive strain area of the hollow sensitive cylinder is packaged with a sensing grating, and the sensing grating is connected with an external demodulation unit through the optical fiber outlet hole.

Optical fiber grating three-dimensional vector vibration sensor

The invention provides a temperature insensitive fiber bragg grating acceleration sensor. The temperature insensitive fiber bragg grating acceleration sensor is in a double-rectangular cantilever beam structure; the double-rectangular cantilever beam structure comprises a first cantilever beam, a second cantilever beam, a third cantilever beam, a fourth cantilever beam, an inertia mass block, a first cantilever beam fixed end and a second cantilever beam fixed end, wherein the inertia mass block is located at the geometric center of the four cantilever beams and is fixedly arranged on the four cantilever beams or is connected with the four cantilever beams to form a whole body; the first cantilever beam fixed end and the second cantilever beam fixed end are fixedly arranged at the two ends of the four cantilever beams; the temperature insensitive fiber bragg grating acceleration sensor further comprises a sensor top cover and a base; a certain distance is formed between the sensor top cover and the inertia mass block and the senor top cover and the inertia mass block are fixedly connected with the upper ends of the first cantilever beam fixed end and the second cantilever beam fixed end; a certain distance is formed between the base and the inertia mass block and the base and the inertia mass block are fixedly connected with the lower ends of the first cantilever beam fixed end and the second cantilever beam fixed end; a sensing optical grating is sealed on a non-linear homogeneous strain region of each cantilever beam and is connected with an external demodulating unit through an optical fiber outlet hole.

Temperature insensitive fiber bragg grating acceleration sensor

The invention provides a fiber grating three-dimensional vibration sensor, which adopts a single-diaphragm sensing structure, and comprises a diaphragm, an inertial mass block and sensing gratings, wherein the diaphragm and the inertial mass block are arranged in a casing, the inertial mass block is positioned in the geometrical center of the diaphragm and is fixedly arranged under the diaphragm, the diaphragm is fixedly arranged on the inner wall of a case, and the three sensing gratings are arranged on the upper surface of the diaphragm, and are connected with an external demodulation unit through an optical fiber outlet hole. The fiber grating three-dimensional vibration sensor has the advantages that the wavelength sensing principle of the fiber gratings is used, tension strain and twisting strain are generated in different positions of the upper surface of the sensor sensing diaphragm under the effect of external vibration signals, when the external vibration signals act on the structure, the vibration of the inertial mass block in the sensor is caused, further, the inertia force and the torque are generated, the gratings encapsulated on the inertial mass block generate wavelength drifting, and the detection of vibration signals in three directions is realized through detecting the change of wavelength of the three gratings.

Fu Haiwei

Papers

High temperature probe sensor with high sensitivity based on Michelson interferometer

Optical fiber grating vibration sensor comprising double cantilever beams with equal strength

Optical fiber grating three-dimensional vector vibration sensor

Temperature insensitive fiber bragg grating acceleration sensor

Fiber grating three-dimensional vibration sensor