We report on a refractive index sensor using a photonic crystal fiber (PCF) interferometer which was realized by fusion splicing a short section of PCF (Blaze Photonics, LMA-10) between two standard single mode fibers. The fully collapsed air holes of the PCF at the spice regions allow the coupling of PCF core and cladding modes that makes a Mach-Zehnder interferometer. The transmission spectrum exhibits sinusoidal interference pattern which shifts differently when the cladding/core surface of the PCF is immersed with different RI of the surrounding medium. Experimental results using wavelength-shift interrogation for sensing different concentrations of sucrose solution show that a resolution of 1.62 × 10−4–8.88 × 10−4 RIU or 1.02 × 10−4–9.04 × 10−4 RIU (sensing length for 3.50 or 5.00 cm, respectively) was achieved for refractive indices in the range of 1.333 to 1.422, suggesting that the PCF interferometer are attractive for chemical, biological, biochemical sensing with aqueous solutions, as well as for civil engineering and environmental monitoring applications.
A simple and low-cost long-period fiber grating (LPG) sensor suited for chloride-ion concentration measurement is presented. The LPG sensor is found to be sensitive to the refractive index of the medium around the cladding surface of the sensing grating, thus offering the prospect of development of practical sensors such as an ambient index sensor or a chemical concentration indicator with high stability and reliability. We measured chloride ions in a typical concrete sample immersed in salt water solutions with different weight concentrations ranging from 0% to 25%. Results show that the LPG sensor exhibited a linear decrease in the transmission loss and resonance wavelength shift when the concentration increased. The measurement accuracy for the concentration of salt in water solution is estimated to be 0.6% and the limit of detection for chloride ions is about 0.04%. To further enhance its sensitivity for chloride concentrations, we coated a monolayer of colloidal gold nanoparticles as the active material on the grating surface of the LPG sensor. The operating principle of sensing is based on the sensitivity of localized surface plasmon resonance of self-assembled gold colloids on the grating section of the LPG. With this method, a factor of two increase in the sensitivity of detecting chemical solution concentrations was obtained. The advantages of this type of fiber-optic sensor are that it is compact, relatively simple to construct and easy to use. Moreover, the sensor has the potential capability for on-site, in vivo and remote sensing, and it has potential use as a disposable sensor.
This paper presents the feasibility of utilizing fiber Bragg grating (FBG) and long-period fiber grating (LPFG) sensors for nondestructive evaluation (NDE) of infrastructures using Portland cement concretes and asphalt mixtures for temperature, strain, and liquid-level monitoring. The use of hybrid FBG and LPFG sensors is aimed at utilizing the advantages of two kinds of fiber grating to implement NDE for monitoring strains or displacements, temperatures, and water-levels of infrastructures such as bridges, pavements, or reservoirs for under different environmental conditions. Temperature fluctuation and stability tests were examined using FBG and LPFG sensors bonded on the surface of asphalt and concrete specimens. Random walk coefficient (RWC) and bias stability (BS) were used for the first time to indicate the stability performance of fiber grating sensors. The random walk coefficients of temperature variations between FBG (or LPFG) sensor and a thermocouple were found in the range of −0.7499 °C/ h to −1.3548 °C/ h. In addition, the bias stability for temperature variations, during the fluctuation and stability tests with FBG (or LPFG) sensors were within the range of 0.01 °C/h with a 15–18 h time cluster to 0.09 °C/h with a 3–4 h time cluster. This shows that the performance of FBG or LPFG sensors is comparable with that of conventional high-resolution thermocouple sensors under rugged conditions. The strain measurement for infrastructure materials was conducted using a packaged FBG sensor bonded on the surface of an asphalt specimen under indirect tensile loading conditions. A finite element modeling (FEM) was applied to compare experimental results of indirect tensile FBG strain measurements. For a comparative analysis between experiment and simulation, the FEM numerical results agreed with those from FBG strain measurements. The results of the liquid-level sensing tests show the LPFG-based sensor could discriminate five stationary liquid-levels and exhibits at least 1,050-mm liquid-level measurement capacity. Thus, the hybrid FBG and LPFG sensors reported here could benefit the NDE development and applications for infrastructure health monitoring such as strain, temperature and liquid-level measurements.
This paper presents the development and assessment of two types of Long Period Fiber Grating (LPFG)-based sensors including a mobile liquid level sensor and a reflective sensor for the measurement of liquid level and fluid-flow velocity. Shewhart control charts were used to assess the liquid level sensing capacity and reliability of the mobile CO2-laser engraved LPFG sensor. There were ten groups of different liquid level experiment and each group underwent ten repeated wavelength shift measurements. The results showed that all measurands were within the control limits; thus, this mobile sensor was reliable and exhibited at least 100-cm liquid level measurement capacity. In addition, a reflective sensor consisting of five LPFGs in series with a reflective end has been developed to evaluate the liquid level and fluid-flow velocity. These five LPFGs were fabricated by the electrical arc discharge method and the reflective end was coated with silver by Tollen's test. After each liquid level experiment was performed five times, the average values of the resonance wavelength shifts for LPFG Nos. 1–5 were in the range of 1.35–9.14 nm. The experimental findings showed that the reflective sensor could be used to automatically monitor five fixed liquid levels. This reflective sensor also exhibited at least 100-cm liquid level measurement capacity. The mechanism of the fluid-flow velocity sensor was based on analyzing the relationship among the optical power, time, and the LPFG's length. There were two types of fluid-flow velocity measurements: inflow and drainage processes. The differences between the LPFG-based fluid-flow velocities and the measured average fluid-flow velocities were found in the range of 8.7–12.6%. For the first time to our knowledge, we have demonstrated the feasibility of liquid level and fluid-flow velocity sensing with a reflective LPFG-based sensor without modifying LPFGs or coating chemical compounds.
Optical fiber sensors based on waveguide technology are promising and attractive in chemical, biotechnological, agronomy, and civil engineering applications. A microfluidic system equipped with a long-period fiber grating (LPFG) capable of measuring chloride ion concentrations of several sample materials is presented. The LPFG-based microfluidic platform was shown to be effective in sensing very small quantities of samples and its transmitted light signal could easily be used as a measurand. The investigated sample materials included reverse osmosis (RO) water, tap water, dilute aqueous sample of sea sand soaked in RO water, aqueous sample of sea sand soaked in RO water, dilute seawater, and seawater. By employing additionally a chloride ion-selective electrode sensor for the calibration of chloride-ion concentration, a useful correlation (R2 = 0.975) was found between the separately-measured chloride concentration and the light intensity transmitted through the LPFG at a wavelength of 1,550 nm. Experimental results show that the sensitivity of the LPFG sensor by light intensity interrogation was determined to be 5.0 × 10−6 mW/mg/L for chloride ion concentrations below 2,400 mg/L. The results obtained from the analysis of data variations in time-series measurements for all sample materials show that standard deviations of output power were relatively small and found in the range of 7.413 × 10−5−2.769 × 10−3 mW. In addition, a fairly small coefficients of variations were also obtained, which were in the range of 0.03%–1.29% and decreased with the decrease of chloride ion concentrations of sample materials. Moreover, the analysis of stability performance of the LPFG sensor indicated that the random walk coefficient decreased with the increase of the chloride ion concentration, illustrating that measurement stability using the microfluidic platform was capable of measuring transmitted optical power with accuracy in the range of −0.8569 mW/ h to −0.5169 mW/ h. Furthermore, the bias stability was determined to be in the range of less than 6.134 × 10−8 mW/h with 600 s time cluster to less than 1.412 × 10−6 mW/h with 600 s time cluster. Thus, the proposed LPFG-based microfluidic platform has the potential for civil, chemical, biological, and biochemical sensing with aqueous solutions. The compact (3.5 × 4.2 cm), low-cost, real-time, small-volume (∼70 μL), low-noise, and high-sensitive chloride ion sensing system reported here could hopefully benefit the development and applications in the field of chemical, biotechnical, soil and geotechnical, and civil engineering.
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