This paper presents the development and assessment of a liquid level sensor using long-period fiber grating (LPFG)
technology and Shewhart control charts. The 22-mm LPFGs were fabricated with the point-by-point CO2 laser
engraving method. This sensor was designed in such a way that it could be moved up and down with a position
controller. The experimental section covered LPFG position sensing test, liquid level detection capacity and reliability
measurements, and sensing resolution evaluation. LPFG position sensing test was studied and confirmed by the
resonance wavelength shifts which were significantly generated when 75% of the LPFG was immersed in water. There
were ten groups of different liquid level capacity testing and each group underwent ten repeated measurements. Based
on Shewhart control charts including an X-bar chart, s chart, and R chart, the results showed all measurands within the
upper and lower control limits. This sensor was reliable and the liquid level could be measured at least 1000 mm. The
transmission loss versus the percent of immersion of the LPFG sensor for water and green tea was used to study the
sensing resolution. The findings show the LPFG-based liquid sensor had at least 1000-mm level measurement capacity
and about 2-mm resolution.
In this paper, we describe the development of a viscosity sensing system using a simple and low-cost long-period fiber
grating (LPFG) sensor. The LPFG sensor was extremely sensitive to the refractive index of the medium surrounding
the cladding surface of the sensing grating, thus allowing it to be used as an ambient index sensor or chemical
concentration indicator. Viscosity can be simply defined as resistance to flow of a liquid. We have measured asphalt
binder, 100-190000 centistokes, in comparison with optical sensing results. The system sensing asphalt binders
exhibited increase trend in the resonance wavelength shift when the refractive index of the medium changed. The
prototype sensor consisted of a LPFG sensing component and a cone-shaped reservoir where gravitational force can
cause asphalt binders flow through the capillary. Thus the measured time for a constant volume of asphalt binders can
be converted into either absolute or kinematic viscosity. In addition, a rotational viscometer and a dynamic shear
rheometer were also used to evaluate the viscosity of this liquid, the ratio between the applied shear stress and rate of
shear, as well as the viscoelastic property including complex shear modulus and phase angle. The measured time could
be converted into viscosity of asphalt binder based on calculation. This simple LPFG viscosity sensing system is
hopefully expected to benefit the viscosity measurement for the field of civil, mechanical and aerospace engineering.
In this paper, we describe the development and realization of a sensing system using a high-resolution temperature and
strain sensor with fiber Bragg grating (FBG) technology and a simple and low-cost long-period grating (LPG) sensor for
the water level measurement in pavement structures. The FBG sensor consists of a reference fiber grating and a grating
pair scheme that could offer the potential of simultaneous measurement of strain and temperature for monitoring
pavement structures. For FBG sensor, experimental results have shown that measurement errors of 6 micro strains and
0.13 Celsius for strain and temperature could be achieved, respectively. The LPG sensor was extremely sensitive to the
refractive index of the medium surrounding the cladding surface of the sensing grating, thus allowing it to be used as an
ambient index sensor. A LPG-type water level sensor with a resolution was of ~5 mm was demonstrated to distinguish
between in the air and under water. This integrated FBG and LPG sensing system is expected to benefit the health
monitoring of multi-layer pavement structures especially for the evaluation and application of new materials, mix design
procedures or construction technology.
We present a simple, low-cost, temperature- and strain-insensitive long-period gratings (LPGs) written in photonic
crystal fibers (PCFs) that can be used as sensitive chemical solution sensors or bend sensors for a variety of industrial
applications, including civil engineering, aircraft, chemistry, food industry, and biosensing. Three different
configurations of PCFs have been used for this study, including a polarization maintaining PCF, a large mode area PCF
and an endlessly single mode PCF. These LPGs have been characterized for their sensitivity to temperature, strain,
bending, and surrounding refractive index. Transmission spectra of the LPGs were found to exhibit negligible
temperature and strain sensitivities, whereas possessing usable sensitivity to refractive index and bending. This type of
PCF sensor could in principle be designed for optimum sensitivity to desired measurand(s), while minimizing or
removing undesirable cross-sensitivities. The unique sensing features of PCFs are particularly suited for a wide variety
of applications in smart structures, embedded materials, telecommunications and sensor systems.
We propose a simple method to improve the spectral sensitivity and detection limit of long period grating sensor for chemical sensing, in which the grating surface is modified by colloidal gold nanoparticles. The transmission spectra and optical properties of gold nanospheres change with the different refractive index of the environment near the surface of gold nanospheres. The sensor response of gold colloids increases linearly with solvents of increasing refractive index. The results for the measurement of sucrose solutions showed that the slope of wavelength shifts increased from -18 (nm/RIU) to -24 (nm/RIU); while for changes of peak depth, its slope increased from 37 (dB/RIU) to 60 (dB/RIU). The accuracy of concentration measurement for salt water solution was increased from 0.6% to 0.2%, and limit of detection can be improved from 0.04% to 0.02%. When the colloidal gold surface was modified with a dimitrophenyl compound (DNP), results showed that the signal increase linearly with increasing concentration of the analyte, and the detection limit of the sensor for anti-DNP is 9.5×10<sup>-10</sup> M.
In this paper, we describe the development and realization of a newly high-resolution temperature and strain sensor with fiber Bragg grating (FBG) technology. The FBG sensor consists of a reference fiber grating and a grating pair scheme that could offer the potential of simultaneous measurement of strain and temperature for monitoring pavement structures. Experimental results showed that measurement errors of 6 με and 0.13<sup>o</sup>C for strain and temperature could be achieved, respectively. The reliability and long-term stability for temperature measurement with this type of sensor were examined by mounting sensors on the surface of asphalt and concrete specimens. Small root mean square temperature variations (better than 1<sup>o</sup>C) and excellent long-term stability (within 2%) were obtained. The maximum variations in temperature for 48 hours were only 1.94% and 2.32% for asphalt and concrete specimens, respectively. The feasibility of strain measurement for pavement structures was conducted by mounting the packaged sensor on the surface of an asphalt specimen under the indirect tensile loading condition. The measured strains from the packaged FBG sensor agreed linearly with applied loads. A finite-element model (FEM) was conducted to verify the strains obtained from the sensors. In comparison with experimental data and numerical results, the numerical values were all located within FBG measurement error ranges. The strain differences between measurements from the FBG sensor and FEM predictions were between 5% and 7%. This type of simple and low-cost FBG sensor is expected to benefit the developments and applications of pavement structures or transportation infrastructure.
We report the development and demonstration of a simple and low-cost long-period grating (LPG) sensor for chloride ion concentration measurement in concrete structures. The LPG sensor is extremely sensitive to the refractive index of the medium surrounding the cladding surface of the sensing grating, thus allowing it to be used as an ambient index sensor or chemical concentration indicator with high stability and reliability. We have measured chloride ion levels in a concrete sample immersed in salt water solution with different weight concentration ranging from 0 % to 20 %, and results showed that the LPG sensor exhibited a linear decrease in the transmission loss and resonance wavelength shift when the concentration increased. The measurement accuracy for concentration of salt in water solution is estimated to be 0.6 % and the limit of detection for chloride ion is about 0.04 %. To further enhance its sensitivity for chloride concentrations, we have coated gold nanoparticles on the grating surface of the LPG sensor. The sensing mechanism is based on the sensitivity of localized surface plasmon resonance of self-assembled Au colloids on the grating portion of the LPG. With this method, a factor of two increases in sensitivity of detecting chemical solution concentrations was obtained. The advantage of this type of the sensor is relatively simple of construction and ease of use. Moreover, the sensor has the potential capability for on-site, in vivo, and remote sensing, and has the potential use for disposable sensors.