Simultaneous strain and temperature measurement using a compact photonic crystal fiber inter-modal interferometer and a fiber Bragg grating

Strain sensing based on resonant states in 2D dielectric photonic quasicrystals

This paper reports the numerical and experimental study of the strain sensing effect of bidimensional quasiperiodic structures made with dielectric cylinders. Structures of around 100 cylinders arranged following a Penrose quasiperiodic disposition were simulated, built and measured, in different states of deformation. The selected quasiperiodic structure contains a symmetric decagonal ring resonator that shows two states in its photonic band gap. The frequency of these states varies linearly in opposite directions as the structure is axially deformed, becoming an interesting sensing principle that can be exploited to build optical strain gauges. As a proof of concept, centimeter-scale glass cylinder (ε_r=4.5) structures were fabricated and their transmission spectra were measured in the microwave range. The same structures were simulated using finite integration time domain showing a good agreement with the measurements. The sensitivity of the prototype built was 12.4 kHz/µε, very linear in a wide range. Therefore, we conclude that the states in the gap of the resonator rings of 2D quasicrystals can find an interesting application in optical strain gauge construction.

High-resolution, large-dynamic-range multimode interferometer sensor based on a suspended-core microstructured optical fiber

The performance of sensors, including optical fiber sensors, is commonly limited by the tradeoff between a large dynamic range and a high resolution. In this Letter, in order to optimize both, we propose an inline multimode interferometer sensor based on a suspended-core microstructured optical fiber. Due to the existence of multiple pairs of mode interferences, the transmission spectrum of the interferometer consists of dense fringes modulated by a lower envelope. Since these mode interferences take place in the uniform material with the same length, the dense fringes and the lower envelope have an identical sensing response without crosstalk. Hence, the sensor integrates the large dynamic range of the lower envelope and the high resolution of the dense fringes. Strain-sensing performance is investigated to validate the characteristic of the large dynamic range and the high resolution of the proposed sensor. The dynamic range, theoretically 0-9200 µɛ, is 12 times larger than for the dense fringes, and the resolution is 17.5 times higher than for the lower envelope.

Simultaneous Measurement of Curvature, Strain and Temperature Using a Twin-Core Photonic Crystal Fiber-Based Sensor

A novel twin-core photonic crystal fiber-based sensor for simultaneous measurement of curvature, strain and temperature is proposed. The fiber sensor is constructed by splicing the homemade twin-core photonic crystal fiber between two segments of single mode fiber. Affected by the coupling between two cores, the transmission spectrum of the fiber sensor has different wavelength responses to curvature, strain, and temperature. The maximal sensitivities to curvature, strain and temperature are 10.89 nm/m^-1, 1.24 pm/με and 73.9 pm/°C, respectively. Simultaneous measurement of curvature, strain and temperature can be achieved by monitoring the wavelength shifts of selected valleys in the transmission spectrum. Contrast experiment based on traditional twin-core fiber is carried out. Experimental results demonstrate that twin-core photonic crystal fiber-based sensor has higher sensitivity and better linearity than traditional twin-core fiber-based sensor.

Optical Fiber Sensors Based on Fiber Ring Laser Demodulation Technology

A review for optical fiber sensors based on fiber ring laser (FRL) demodulation technology is presented. The review focuses on the principles, main structures, and the sensing performances of different kinds of optical fiber sensors based on FRLs. First of all, the theory background of the sensors has been discussed. Secondly, four different types of sensors are described and compared, which includes Mach-Zehnder interferometer (MZI) typed sensors, Fabry-Perot interferometer (FPI) typed sensors, Sagnac typed sensors, and fiber Bragg grating (FBG) typed sensors. Typical studies and main properties of each type of sensors are presented. Thirdly, a comparison of different types of sensors are made. Finally, the existing problems and future research directions are pointed out and analyzed.

Annealing properties of fiber Bragg grating UV-inscribed in boron-germanium codoped fiber

In this work, we mainly focus on the investigation of the feasibility of production of high-temperature stable fiber Bragg grating (FBG) based on reduplicative alternate annealing and hydrogen loading. The experimental results also can demonstrate the significance of the presence of hydrogen to the thermal regeneration of FBGs. The gratings are characterized and variations are compared after each stage, including UV fabrication, annealing, and reduplicative hydrogen-preloaded annealing. In different stages, the spectral and annealing responses of FBG are, respectively, investigated, as temperature increases, the Bragg wavelength consistently shifts to longer wavelengths; nevertheless, the reflection variations are distinctly discrepant. After reduplicative alternate annealing and hydrogen loading, the thermal stability is tremendously improved, and a reborn, stable grating is formed.

Crescent shaped Fabry-Perot fiber cavity for ultra-sensitive strain measurement

Optical Fabry-Perot interferometer sensors based on inner air-cavity is featured with compact size, good robustness and high strain sensitivity, especially when an ultra-thin air-cavity is adopted. The typical shape of Fabry-Perot inner air-cavity with reflection mode of operation is elliptic, with minor axis along with and major axis perpendicular to the fiber length. The first reflection surface is diverging whereas the second one is converging. To increase the visibility of the output interference pattern, the length of major axis should be large for a given cavity length. However, the largest value of the major axis is limited by the optical fiber diameter. If the major axis length reaches the fiber diameter, the robustness of the Fabry-Perot cavity device would be decreased. Here we demonstrate an ultra-thin crescent shaped Fabry-Perot cavity for strain sensing with ultra-high sensitivity and low temperature cross-sensitivity. The crescent-shape cavity consists of two converging reflection surfaces, which provide the advantages of enhanced strain sensitivity when compared with elliptic or D-shaped FP cavity. The device is fabricated by fusion splicing an etched multimode fiber with a single mode fiber, and hence is simple in structure and economic in cost.

Strain and high-temperature discrimination using a Type II fiber Bragg grating and a miniature fiber Fabry-Perot interferometer

A novel method for simultaneous measurement of strain and high temperature using a Type II fiber Bragg grating (FBG) and a miniature fiber Fabry-Perot interferometer (MFFPI) is proposed. The MFFPI is produced by fusion splicing a short section of quartz capillary tube with two single-mode fibers, and then it is exposed by a focused femtosecond laser and a phase mask to inscribe a Type II FBG nearby. The reflection spectrum of this sensor is the superposition of the reflection spectrum of the FBG and the interference fringe of the MFFPI. This sensor shows perfect high-temperature and strain responses. Because of the different responses to the uniform variations of strain and temperature, by measuring the reflection peak of FBG and one of the interference dips of the MFFPI, strain and temperature can be simultaneously determined. The resolutions of this particular sensor in measuring strain and temperature are estimated to be ±8.4  μϵ and ±3.3°C, respectively, in the range from 0 to 1122 μϵ and from 23°C to 600°C.

Overview of Fiber Optic Sensor Technologies for Strain/Temperature Sensing Applications in Composite Materials

This paper provides an overview of the different types of fiber optic sensors (FOS) that can be used with composite materials and also their compatibility with and suitability for embedding inside a composite material. An overview of the different types of FOS used for strain/temperature sensing in composite materials is presented. Recent trends, and future challenges for FOS technology for condition monitoring in smart composite materials are also discussed. This comprehensive review provides essential information for the smart materials industry in selecting of appropriate types of FOS in accordance with end-user requirements.

Simultaneous strain and temperature sensing using a slightly tapered optical fiber with an inner cavity

An ultracompact optical fiber mode interferometer capable of performing simultaneous strain and temperature sensing is demonstrated.

Up-taper-based Mach-Zehnder interferometer for temperature and strain simultaneous measurement

A novel all-fiber sensing configuration for simultaneous measurements of temperature and strain based on the up-taper Mach-Zehnder interferometer (MZI) with an in-line embedded fiber Bragg grating (FBG) is proposed and experimentally demonstrated. This configuration consists of two up-tapers fabricated by an excessive fusion splicing method and a short segment of inscribed FBG. Due to the different responses of the up-taper MZI and the FBG to the uniform variation of temperature and strain, the simultaneous measurement for these two variables could be achieved by real-time monitoring the transmission spectrum. For 0.01 nm wavelength resolution, a resolution of 0.311°C in temperature can be achieved, and the average strain resolution is 10.07 με.

Simultaneous measurement of temperature and force with high sensitivities based on filling different index liquids into photonic crystal fiber

A double-filled photonic crystal fiber (PCF) was fabricated by filling liquids of different indexes into two air holes in the cladding. The core mode coupled to the local cladding modes LP(01) and LP(11) in the 1310 and 1550 nm wavebands, respectively. Due to the unique characteristics of the mode coupling, the resonant peaks in different resonance areas shifted to the opposite directions with the variations of the temperature or the force. The double-filled PCFs achieved in this work showed useful applications in the simultaneous measurement of both the temperature and the force.

Miniature all-fiber Fabry-Perot sensor for simultaneous measurement of pressure and temperature

This article presents a miniature, high-sensitivity, all-silica Fabry-Perot fiber-optic sensor suitable for simultaneous measurements of pressure and temperature. The proposed sensor diameter does not exceed 125 μm and consists of two low-finesse Fabry-Perot resonators created at the tip of an optical fiber. The first resonator is embodied in the form of a short air cavity positioned at the tip of the fiber. This resonator utilizes a thin silica diaphragm to achieve the sensor's pressure response. The second resonator exploits the refractive index dependence of silica fiber in order to provide the proposed sensor's temperature measurement function. Both resonators have substantially different lengths that permit straightforward spectrally resolved signal processing and unambiguous determination of the applied pressure and temperature.

Simultaneous temperature and force measurement using Fabry-Perot interferometer and bandgap effect of a fluid-filled photonic crystal fiber

A novel fiber sensor capable of simultaneously measuring force and temperature is proposed and investigated. A section of high-index-fluid-filled photonic bandgap fiber (HIFF-PBGF) is inserted in a fiber loop to act as the sensing head. Photonic bandgap effect of the HIFF-PBGF as well as Fabry-Perot interferometer (FPI) introduced by controlling the splicing between the HIFF-PBGF and single mode fiber is used for achieving force and temperature discrimination. Taking advantage of the bandgap being high sensitivity to the temperature, a high temperature sensitivity of more than -1.94 dB/°C is achieved, which is the highest based on the intensity measurement, to our best knowledge. Meanwhile, a force sensitivity of 3.25 nm/N (~3.9 pm/με) is obtained, which could be enhanced by controlling the FPI shape. The device also has the strong points of easy fabrication, compact structure and high interference fringe contrast.

Discrimination between strain and temperature by cascading single-mode thin-core diameter fibers

A simple fiber-optic sensor capable of discrimination between temperature and strain is proposed and experimentally demonstrated. The sensor head is formed by cascading two sections of single-mode thin-core diameter fibers (TCFs) that act as two different inter-modal interferometers (IMIs). Due to the different sensitivity responses of the two IMIs to strain and temperature, it is possible to discriminate temperature and strain by monitoring the resonant wavelength shifts. The experimental results indicate that the measured strain and temperature resolutions are 37.41 με and 0.732 °C within a strain range of 0-1333.3 με and a temperature range from 26.9 °C to 61.7 °C. The sensing sensitivities of strain and temperature are -1.03 pm/με and 30.74 pm/°C, respectively. The proposed sensor features the advantages of easy fabrication, low cost and high sensitivity, and it exhibits great potential in dual-parameter measurement.

Reflective fiber-optic refractometer based on a thin-core fiber tailored Bragg grating reflection

A novel reflective refractometer based on a thin-core fiber (TCF) sandwiched between a leading single-mode fiber (SMF) and a fiber Bragg grating (FBG) imprinted SMF stub was demonstrated. The reflection from the fiber stub occurs in two well-defined wavelength bands, corresponding to the Bragg core mode and cladding modes. The TCF section functions as a tailorable bridge between the FBG core mode reflection and the surrounding refractive index (SRI). Linear response with enhanced sensitivity of 133.26 dB/refractive index unit for temperature-immune SRI measurement within the biologically desirable sensing range of 1.33-1.41 has been achieved via cost-effective power detection.

Simultaneous strain and temperature fiber grating laser sensor based on radio-frequency measurement

We propose and experimentally demonstrate a novel simultaneous strain and temperature fiber optic sensor. The sensing head is formed by two concatenated ultra-short distributed Bragg reflector lasers that operate in single longitude mode with two polarization modes. The total length of the sensing head is only 18 mm. The two lasers generate two polarization mode beat notes in the radio-frequency range which show different frequency response to strain and temperature. Simultaneous strain and temperature measurement can be achieved by radio-frequency measurement. This approach has distinctive advantages of ease of interrogation and avoidance of expensive wavelength detection.

Long period grating assistant photonic crystal fiber modal interferometer

A novel in-fiber modal interferometer based on a long period grating (LPG) inscribed in a two-mode all-solid photonic bandgap fiber (AS-PBGF) is presented. After inserting a small piece of the AS-PBGF into two sections of standard single-mode fiber (SMF) via being spliced slight core offset, LPG is inscribed in the AS-PBGF. The LPG is especially designed to realize the coupling between two core modes of LP01 and LP11 in the AS-PBGF. Two core modes LP01 and LP11 of the AS-PBGF are excited firstly at the input spliced point and actualized energy exchange when they pass through the LPG. Then the two beams will interfere at the output spliced point to form a high-contrast in-fiber modal interferometer. The proposed interferometer has some advantages such as configuration compact, high interference contrast and the wavelength spacing well controlled by changing the position of the LPG without changing the total length of AS-PBGF.