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Although microwave photonic approaches have been used for fiber sensing applications before, most contributions in the past dealt with evaluating the sensor signal’s amplitude. Carrying this topic on, the authors previously presented a scheme for the interrogation of fiber sensors that was based on a fiber Bragg grating’s phase response for the electrical signal. However, neither has the measurement setup been analyzed nor have the amplitude and phase-based approaches been compared in detail before. Hence, this paper picks up the previously proposed setup, which relies on an amplitude modulation of the optical signal and investigates for sources of signal degradation, an aspect that has not been considered before. Following the incorporation of the microwave signal, the setup is suitable not only for an amplitude-based evaluation of fiber Bragg gratings but also for a phase-based evaluation. In this context, the signal-to-noise ratios are studied for the conventional amplitude-based evaluation approach and for the recently developed phase-based approach. The findings indicate a strong advantage for the signal-to-noise ratio of the phase response evaluation; an 11 dB improvement at the least has been found for the examined setup. Further studies may investigate the consequences and additional benefits of this approach for radio-over-fiber sensing systems or general performance aspects such as achievable sensitivity and sampling rates.

A compact fiber cascaded structure for simultaneous measurement of curvature and temperature based on Fabry-Perot (FP) interference and anti-resonant (AR) mechanism is designed and experimentally demonstrated. The proposed structure is manufactured by sandwiching a segment of hollow core fiber (HCF) with large inner diameter and suitable length between two segments of single mode fibers (SMFs). Three sample sensors were fabricated with different HCF lengths. With theoretical analysis and experimental verifications, an HCF length of 1197 μm was selected for curvature and temperature experiments. The reflection fringes based on FP interference and transmission envelopes based on AR mechanism can be monitored to measure curvature and temperature variations at the same time. From experimental results, the sensitivities of curvature based on AR mechanism and FP interference are 0.2345 nm/m−1 and -0.1021 nm/m−1, and those of sensitivities of temperature are 0.0172 nm/°C and 0.0017 nm/°C, respectively. Therefore, the different responses in curvature and temperature based on two mechanisms enable the proposed structure to simultaneously measure curvature and temperature.

Herein a gold nanosphere (AuNS)-coated wavelength-mode localized surface plasmon resonance (LSPR) fiber sensor was fabricated by a simple and time-saving electrostatic self-assembly method using poly(allylamine hydrochloride). Based on the localized enhanced coupling effect between AuNSs, the LSPR spectrums of the AuNS monolayer with good dispersity and high density exhibited a favourable capability for refractive index (RI) measurement. Based on the results obtained from the optimization for AuNS distribution, sensing length, and RI range, the best RI sensitivity of the fiber modified by 100 nm AuNS reached up to about 2975 nm/RIU, with the surrounding RI range from 1.3322 to 1.3664. Using an 80 nm AuNS-modified fiber sensor, the RI sensitivity of 3953 nm/RIU was achieved, with the RI range increased from 1.3744 to 1.3911. The effect of sensing length to RI sensitivity was proven to be negligible. Furthermore, the linear relationship between the RI sensitivity and plasma resonance frequency of the bulk metal, which was dependent on the interparticle plasmon coupling effect, was quantified. Additionally, the resonance peak was tuned from 539.18 nm to 820.48 nm by different sizes of AuNSs-coated fiber sensors at a RI of 1.3322, which means the spectrum was extended from VIS to NIR. It has enormous potential in hypersensitive biochemistry detection at VIS and NIR ranges.

Nitrogen-vacancy (NV) centers in diamonds play a large role in advanced quantum sensing with solid-state spins for potential miniaturized and portable application scenarios. With the temperature sensitivity of NV centers, the temperature fluctuations caused by the unknown environment and the system itself will mix with the magnetic field measurement. In this research, the temperature-sensitive characteristics of different diamonds, alongside the temperature noise generated by a measurement system, were tested and analyzed with a homemade NV magnetometer in a fiber-optic scheme. In this work, a multi-frequency synchronous manipulation method for resonating with the NV centers in all axial directions was proposed to compensate for the temperature fluctuations in a fibered NV magnetic field sensing scheme. The symmetrical features of the resonance lines of the NV centers, the common-mode fluctuations including temperature fluctuations, underwent effective compensation and elimination. The fluorescence change was reduced to 1.0% by multi-frequency synchronous manipulation from 5.5% of the single-frequency manipulation within a ±2 °C temperature range. Additionally, the multi-frequency synchronous manipulation improved the fluorescence contrast and the magnetic field measurement SNR through an omnidirectional manipulation scheme. It was very important to compensate for the temperature fluctuations, caused by both internal and external factors, to make use of the NV magnetometer in fiber-optic schemes’ practicality. This work will promote the rapid development and widespread applications of quantum sensing based on various systems and principles.

A fiber-optic refractometer for various liquids with refractive indices in the range from 1.33 to 1.43 has been manufactured and tested. The sensor is based on a thin silicon oxynitride (Si3N4-xOx) film coated thinned optic fiber section (taper) obtained in a multimode all-silica optical fiber by chemical etching of the reflective cladding. The film was deposited on the cylindrical surface of the thinned fiber by the surface plasma chemical vapor deposition method (SPCVD). Lossy mode resonance (LMR) was observed in the transmission spectrum of the coated taper at a wavelength dependent on the refractive index of the liquid in which the taper was immersed. We tested the obtained sensors in distilled water, isopropyl alcohol, dimethylformamide, and their aqueous solutions. It was found that with the help of the SPCVD, one can obtain a set of sensors in a single deposition run with the dispersion of sensitivity and spectral position of LMR no more than 5%. Maximum sensitivity of the manufactured sensors to surrounding media refractive index (SMRI) variation exceeds 1090 nm/RIU, which is the highest value recorded to date for a sensor with a non-oxide coating.

This paper presents a novel mismatching fused Mach-Zehnder interferometer (MFMZI) for noncontacted steel corrosion monitoring in real time. Without sputtering or contacting any steel film on the surface of the MFMZI, the corrosion rate of the steel bars can be evaluated with ultrasensitivity even when the concentration of the NaCl solution is only 0.05 wt%. Moreover, simulations have been carried out with variations in the water-based solution to understand the sensitivity response as a function of the effective refractive index (RI). The simulation and experimental results reveal that the sensitivity is up to 10372 nm/RIU, and the speeds of the wavelength shift in 0.05 wt%, 0.1 wt% and 0.2 wt% NaCl solutions are 0.08, 0.12 and 0.17 nm/min under the dynamic corrosion process, respectively.

A robust fiber sensor for salinity measurement based on encapsulated long-period grating in microfiber is proposed. The long-period grating is fabricated in microfiber by inducing periodical deformation with CO2 laser, which is then encapsulated in a holey capillary tube. The encapsulation tube is designed to effectively protect the microfiber from external interference, but does not change the optical properties of the fiber and the response speed of the sensor, which makes the sensor more robust for real applications. Experimental results show that the sensor can achieve a sensitivity of 2.16 nm/% with a good linearity for concentration from 0% to 20%. It is theoretically proved that the sensitivity can be further improved by optimizing the diameter parameters. Such structure may be used as low loss evanescent-wave-coupled optical absorption, fluorescent and gain cells, photoacoustic cells, and etc.

International audience; Despite having vast structural applications, Composites are not exempt from limitations and are susceptible to deforming during operation. Therefore, it is essential to develop in-situ monitoring systems to avoid their catastrophic failure or high repairing cost. So, the objective of this study was to monitor the deformation behavior of composites subjected to cyclic flexural deformation in real-time using a Nylon/Ag fiber sensor. Nylon/Ag fiber sensor was integrated at different direction i.e. 0°, +45°, 90°, -45° gradually between each ply of the composite specimens which were then machined in star shape where each leg signified the direction of the sensor. These specimens were then tested under cyclic flexural deflection at the strain rate of 2mm/min for 10 cycles. Mechanical results of composite specimens and electrical response of each Nylon/Ag sensor fiber showed excellent repeatability however, each Nylon/Ag fiber sensor showed a specific resistance behavior because of their respective position. The increase or decrease in the resistance of the fiber sensor signified the presence of tensile or compressive strain respectively and the intensity of the signal quantified the amount of deformation. The results confirmed that the fiber sensor showed good potential as flexible sensor reinforcement in composites for in-situ monitoring, identification and quantification of the deformation.

In the chain and rope hoist systems of large cranes, instant and monitoring of the strain caused by different loads is of great importance. Strain is a size that cannot be measured directly and whose effect can be measured. Strain can be measured point by point with the help of electronic sensors. However, increasing the number of measuring points causes complexity in electronic systems. Thanks to distributed fiber sensors, this problem has been overcome and standard SMF has been used as tens of thousands of measuring points. In this study, a model of the pulley system used in large cranes is created and the strain occurring at a certain point of a SMF was experimentally measured using the BOTDA technique. SNR was improved by using the wavelet noise reduction technique in the processing of the received signals. In order to monitor the measured strain, a user-friendly interface was designed.

This paper proposed a fiber sensor based on a Silicon core fiber incorporated with a fused silica grating design. The proposed structure is a nice sensitive in refractive index and temperature variation through theoretical calculation and finite element method simulation. The sensitivities of the refractive index in different polarization directions are 1838.7 and 1949.8 nm/RIU, respectively, within the index range from 1.28 to 1.38 RIU. The temperature range within 15 to 40 oC has the sensitivity up to 1.6 nm/oC by using ethanol analyte. The proposed fiber sensor can provide a method or a solution for intelligent sensing systems.

Liquid level measurement in microfluidics is challenging, where a sensor with ultra-high sensitivity but miniature in nature is demanded. In this paper, we propose for the first time a microsized fiber sensor structure in both diameter and length for microfluidics applications, which is capable of sub-micrometer scale liquid level measurement. The sensor is simply fabricated by fusion splicing a short section of a hollow core fiber (HCF) between two singlemode fibers (SMFs). HCFs with different air core diameters (10 µm, 20 µm, 30 µm) were investigated and it is found that for a given length of HCF stronger resonant dips were excited in transmission for the HCF with a smaller air core diameter. Thus the HCF structure with an air core diameter of 10 µm (HCF-10) was used for demonstration of high sensitivity liquid level measurement in microfluidics. Simultaneous excitation of both Anti-Resonant Reflecting Optical Waveguide (ARROW) guiding mechanism and Mach-Zehnder interferometer (MZI) in transmission is demonstrated in such an HCF-10 structure when HCF-10 is longer than the critical length. A maximum sensitivity of 0.042 dB/µm (corresponding to a calculated liquid level resolution of ~0.23 µm) was experimentally achieved with an HCF-10 length of ~867 µm, which is three times higher than that of the previous reported to date of the most sensitive fiber optic liquid level sensors based on intensity modulation. In addition, the proposed sensor shows good repeatability of measurement and a very low cross sensitivity to changes in the surrounding refractive index.

A graphene oxide-coated in-fiber Mach-Zehnder interferometer (MZI) formed with a multimode fiber-thin core fiber-multimode fiber (MMF-TCF-MMF) is proposed and experimentally demonstrated for ammonia gas (NH3) sensing. The MZI structure is composed of two segments of MMF of length 2 mm, with a flame-tapered TCF between them as the sensing arm. The MMFs act as mode couplers to split and recombine light owing to the core diameter mismatch with the other fibers. A tapered TCF is formed by the flame melting taper method, resulting in evanescent wave leakage. A layer of graphene oxide (GO) is applied to the tapered region of the TCF to achieve gas adsorption. The sensor operates on the principle of changing the effective refractive index of the cladding mode of a fiber through changing the conductivity of the GO coating by adsorbed NH3 molecules, which gives rise to a phase shift and shows as the resonant dip shifts in the transmission spectrum. So the concentration of the ammonia gas can be obtained by measuring the dip shift. A wavelength-shift sensitivity of 4.97 pm/ppm with a linear fit coefficient of 98.9% is achieved for ammonia gas concentrations in the range of 0 to 151 ppm. In addition, we performed a repetitive dynamic response test on the sensor by charging/releasing NH3 at concentration of 200 ppm and a relative humidity test in a relative humidity range of 35% to 70%, which demonstrates the reusability and stability of the sensor.

Based on the large evanescent field of microfibers, a novel fiber sensor composed of a microfiber knot resonator and a fiber Bragg grating is proposed for the simultaneous measurement of temperature and refractive index of solutions. Due to the different responses of the microfiber knot resonator and fiber Bragg grating to temperature and refractive index variations, the fiber sensor can measure temperature and refractive index simultaneously by utilizing the sensitivity matrix. The experimental measurement results show that the sensitivities of the microfiber knot resonator and the fiber Bragg grating to temperature are 12.1 pm/°C and 11.6 pm/°C, respectively. The sensitivity of the microfiber knot resonator to the refractive index of solutions is 315 nm/RIU. It is indicated that microfiber knot resonators have a high sensitivity to the refractive index variation of solutions. Considering its small size, low cost, and high stability, the sensor proposed has a prospect in the fields of chemistry and biology.

A cascaded fiber sensor for measuring different curvature ranges based on the cladding mode resonance in double-cladding fiber is proposed. It is formed by double-cladding fiber and single-mode fiber. Based on the evanescent wave coupling principle, the cladding mode resonance phenomenon in double-cladding fiber is illustrated. The transmission spectrum with different length are also analyzed. The experimental results show that the resonance wavelength changes with the length of double-cladding fiber. When the fiber length is 22 mm, the transmission spectrum is blue-shifted as the curvature increases in minor range of 0–0.204 m−1, the corresponding curvature sensitivity can be up to −19376.48 pm/m−1. When the fiber length is 20 mm, the corresponding curvature sensitivity can be up to −4161.73 pm/m−1 as the curvature increases in large range of 0–1.06 m−1. The proposed curvature sensor has the characteristics of simple structure and high sensitivity, so it has a good application prospects in large bridges, industrial buildings and other curvature measurement situations.

First, the temperature effect on the output polarization degree of the fiber Lyot depolarizer was analyzed in this paper theoretically, which shows that when the environment temperature changes from −40℃ to 80℃, the degree of the output light polarization (DOP) of the fiber Lyot depolarizer will increase until about 70℃, then DOP begin to decrease. The further numerical simulations indicate that this result was caused together by thermally induced group birefringence and thermal coefficient of expansion of the polarization fiber. Using our made fiber Lyot depolarizer (DOP≤0.3% when the environment temperature is 20℃), DOP of the fiber Lyot depolarizer was studied at the condition of a variable temperature experimentally, the results are in agreement with the related theoretically results basically, at last, the difference between theoretical and experimental results is discussed, the possible methods to reduce the influence of the temperature on the output DOP of fiber Lyot depolarizer is presented in this paper. It is a good guiding significance to improve the depolarization performance of the fiber Lyot depolarizer in the fields of fiber sensor, fiber gyroscope, fiber Raman amplifier and so on as we believe.

There is an optical interference noise in the conventional Raman-based fiber-optics distributed sensing, which results in a poor temperature resolution performance. In addition, the traditional whole-fiber demodulation principle complicates the operation steps of the system. In this paper, a novel dynamic difference attenuation recognition (DDAR) principle is operated in the DDP scheme (dual demodulation principle) and the SDP scheme (self-demodulation principle) respectively. It not only helps to eliminate the optical interference noise, but also omits the whole-fiber calibration process. In this experiment, a temperature resolution of 0.30 &deg, C (17.0 km) is achieved through using the DDP scheme based on the DDAR principle, and the measurement time can be shortened to 1.5 s. Meanwhile, a temperature resolution of 0.18 &deg, C (17.0 km) is obtained for the SDP scheme under the DDAR principle. The SNR of DDP and DSP schemes can be optimized to 12.82 dB and 13.32 dB by the proposed DDAR technology. Furthermore, the temperature resolution performance under a large temperature measurement range (0&ndash, 1000 &deg, C) is theoretically analyzed. The results indicate that the temperature responsivity for DDP and SDP schemes are parabolic and linear type respectively, which causes the temperature resolution of the two schemes to show a different trend with the change of temperature. The proposed DDAR method also can improve the temperature resolution in such a large temperature measurement range.

In this article, the goal is to monitor the deformation and damage behavior of composites in real-time using a Nylon/Ag fiber sensor when subjected to dynamic loading. Composite samples are integrated with Nylon/Ag fiber sensors at distinct locations and directions between the plies. Then, these samples are experimentally impacted with low-velocity impact using the Taylor Cannon Gun apparatus at three different velocities i.e. 2.5 m s−1, 3 m s−1, and 6.5 m s−1, respectively. These three sets of tests are conducted to determine the detection performance of the Nylon/Ag fiber sensor when the composite sample experiences no damage, some microdamage, and overall breakage. Besides, the fiber sensor placed in each position showed distinct electrical behavior in all three tests and detected the deformation, damage initiation, quantification, identification, and damage propagation. The results confirmed the ability of the fiber sensor to monitor and identify the mechanical deformation during dynamic loading and showed that the sensor can be used as a flexible sensor reinforcement in composites for in-situ monitoring as well.

We propose and demonstrate a hybrid fiber-based sensor combining a multimode interference (MMI) structure and a surface plasmon resonance (SPR) structure for simultaneous measurement of temperature and refractive index (RI) of a liquid sample. We configure the MMI structure by connecting a single-mode fiber, a no-core fiber, and a single-mode fiber sequentially. We set up the SPR structure by coating a gold film with a thickness of 50 nm on the surface of the no-core fiber. We measure the sensitivity of RI and the temperature of the MMI and SPR structure, respectively. Then we obtain the coefficient matrix to simultaneously measure the temperature and RI of a liquid sample and obtain the highest RI sensitivity of 2061.6 nm/RIU and temperature sensitivity of 37.9 pm/°C. We verify the feasibility of the sensor in liquid alcohol. The testing results indicate that the proposed sensor and testing method are feasible, accurate, and convenient.

This case study describes a new method using a fiber optic Bragg grating sensor to determine muscle fatigue in a patient wearing a resilient occlusal device for the treatment of bruxism symptoms. Four tasks performed at maximum bite force induced rapid exhaustion for the analysis of fatigue. The results show that it is possible to obtain a better understanding of the muscle fatigue process by using the fiber sensor. Differences between the bite patterns are detected at each phase of the induced fatigue. It is possible to compare the bite peak sensitivity coefficients for the exhaustion phase, 1.37 Ns-1, and for fatigue phase, 2.54 Ns-1. Additionally, the increase in the facial thermal profile is correlated to the different bite force pattern performed.

In this paper, a new optical fiber sensor for relative humidity measurements is presented and characterized. The sensor is based on a SnO2 sputtering deposition on a microstructured optical fiber (MOF) low-finesse Fabry-Pérot (FP) sensing head. The feasibility of the device as a breathing sensor is also experimentally demonstrated. The interrogation of the sensing head is carried out by monitoring the Fast Fourier Transform phase variations of the FP interference frequency. This method substitutes the necessity of tracking the optical spectrum peaks or valleys, which can be a handicap when noise or multiple contributions are present: therefore, it is low-sensitive to noise and to artifacts signal amplitude. The sensor shows a linear behavior in a wide relative humidity range (20%–90% relative humidity) in which the sensitivity is 0.14 rad/%; the maximum observed instability is 0.007 rad, whereas the highest hysteresis is 5% RH. The cross correlation with temperature is also considered and a method to lower its influence is proposed. For human breathing measurement, the registered rising and recovery times are 370 ms and 380 ms respectively. The authors are grateful to A. Ortigosa, D. Erro, Dr. M. Bravo and Dr. R.A. Perez-Herrera. We also thank the Spanish Government projects TEC2013-47264-C2-2-R, TEC 2016-76021-C2-1-R, TEC2016-78047-R, TEC2016-79367-C2-2-R, Innocampus and the Cost Action MP 1401, as well as to the AEI/FEDER Funds.

A transverse load fiber sensor based on Mach-Zehnder interferometer constructed by a Bowknot-type taper between a single mode fiber (SMF) and a polarization maintaining fiber (PMF) was proposed. Due to the polarization maintaining fiber’s birefringence, intensities of the two peaks which are corresponding to the fast and slow axis modes changed with the transverse load applied on the PMF. The experimental results showed that the structure with a 2 cm-long PMF has the sensitivities of 104.52 and −102.94 dB/(N/mm) for the fast and slow axis spectral dip wavelengths of 1485 and 1545 nm in the interference pattern, respectively, which are almost 7 times higher than that of the current similar existing transverse load sensor.

This paper proposes a biosensor based on side-hole fibers (SHF) formed by the structure of single-mode fiber (SMF)-coreless fiber (CLF)- SHF-CLF-SMF for the detection of bovine serum albumin (BSA). Among the fibers, the SHF’s misalignment splicing makes one hole exposed, which is very sensitive to the change of the external environment. Due to the unique structure of the SHF, two Mach-Zehnder interferences (MZI) are generated in this fiber sensor, which further improves the sensitivity of the sensor. Experiments show that the refractive index (RI) sensitivity of this sensor reaches 13421.638 nm/RIU, and for the BSA concentration detection, the sensitivity reaches 2.413 nm/(mg/ml). The proposed biosensor is simple, easy to manufacture, cost-effective, and can be used for many chemical and biological analyses.

A novel refractive index (RI) sensor based on an asymmetrical Mach–Zehnder interferometer (MZI) with two different step-like tapers is proposed. The step-like taper is fabricated by fusion splicing two half tapers with an appropriate offset. By further applying offset and discharging to the last fabricated step-like taper of MZI, influence of taper parameters on interference spectrum is investigated using only one device. This simple technique provides an on-line method to sweep parameters of step-like tapers and speeds up the optimization process of interference spectrum, meanwhile. In RI sensing experiment, the sensor has a high sensitivity of −185.79 nm/RIU (refractive index unit) in the RI range of 1.3333–1.3673.

We demonstrated the refractive index sensing characteristics of a miniature fiber sensor composited by silica-hollow-tube (SHT) and Au-micro-sphere (AmS). The high sensitivity is obtained due to the evanescent field effect existing in the SHT with the inner diameter of ∼2.3 μm and the surface plasmon resonance effect excited on the surface of AmS with the diameter of ∼2 μm. Experimental results indicate that this sensor can continuously measure the glucose concentration in range of 0–60% with a good linearity. The high detection sensitivity up to 8.33 μmol/L (47.33 mW/RIU) enables its ability in determining the glucose concentration in either blood or body fluids. Furthermore, the tiny structure is promise to be integrated into the microchip or other injectable structures, and monitor the glucose concentration in real-time.

We proposed a composite microfiber using silica capillary and Ag microparticles, and demonstrated its optical sensing characteristics by changing the refractive index of surrounding environment using different concentrations of NaCl solution. Either the diameter or doping density of Ag microparticles contributes to the sensitivity change. The experimental results reveal that the diameter uniformity and distribution of Ag microparticles exerted impact on the sensing performance. The highest sensitivity of −7246 dB/RIU has been experimentally demonstrated in this work. The selectivity of this composite microfiber sensor will be explored by decorating its surface with some enzyme or other sensitive materials.

Performance parameter of a Bragg fiber waveguide based resonant sensor in presence of a defect layer in cladding regions is theoretically studied. The Bragg fiber waveguide consists of a liquid-core surrounded by alternate high and low refractive indices materials in cladding regions. Reflectivity of the proposed waveguide based resonant sensor is formulated using transfer matrix method for a non-homogeneous multilayer cylindrical system. The waveguide shows a band gap region with a narrow defect mode in the band gap region under the considered wavelength range. Instead of taking a whole band gap as a sensing signal, here the defect peak is taken as the sensing signal. It is observed that the intensity of defect mode is more sensitive for core refractive index than the intensity of traditional band gap region (lobe). This study shows that the higher sensitivity can be achieved by creating the defect at a position in cladding region where the intensity of transmitted light lies between 40% and 90%. Presence of a defect layer is able to increase the detection accuracy of the sensor and, hence increase the overall performance of this sensor.

This article describes experimental tests to determine PM fibers Panda style responses to a thermal source with different initial temperature. The aim of this study was to determine the sensitivity of a polarization maintaining fiber to the radiating heat, and to upgrade the space configuration and time response when using the 635 nm light. The sensitivity of the polarization maintaining fiber during excitation of both polarization modes is the principle of this sensor function. This excitation is caused by temperature change and by absorption of thermal radiation. This mechanism is used as an indicator for detection of temperature field disturbance. This article also provides links to previously published results and compares them to the results in this article.

Having the correct seeding rate for a unit area is vital to crop yields. In order to assess the desirable seeding rate, the number of discharged seeds needs to be monitored in real-time. However, for small seeds, the miscounting of seeds during monitoring happens frequently when using conventional seeding quantity sensors, which have wide light beam intervals. Thus, a seeding quantity sensor, which enables small seeds to pass through the light beam steadily, was developed. Based on the seed-shading time, a seed-counting algorithm was proposed. Moreover, the key structure parameters of the proposed sensor were ascertained using an optimization experiment. Finally, the developed seeding quantity sensor was tested against a photoelectric sensor and a fiber sensor to compare the seed monitoring accuracies. The results show that the average monitoring accuracy of the developed sensor, photoelectric sensor, and fiber sensor were 97.09%, 56.79%, and 91.10%, respectively. Furthermore, the factorial analysis shows that the forward velocity of the experimental apparatus and the rotational speed of the seeding plate did not significantly change the monitoring accuracies obtained by the developed sensor. Therefore, the developed sensor can be applied to monitor the seed quantity for the precision seeding of small seeds accurately and robustly.

The effect of &gamma, ray irradiation on the surface plasmon resonance (SPR) sensing capability of refractive index (n = 1.418&ndash, 1.448) of the silica glass optical fiber comprised of germano-silicate glass cladding embedded with Au nano-particles (NPs) was investigated. As the &gamma, ray irradiation increased from 1 h to 3 h with the dose rate of 1190 Gy/h, the morphology of the Au NPs and the SPR spectrum were found to change. The average diameter of Au NPs increased with the aspect ratio from 1 to 2, and the nano-particles became grown to the clusters. The SPR band wavelength shifted towards a longer wavelength with the increase of total dose of &gamma, ray irradiation regardless of the corresponding refractive indices. The SPR sensitivities (wavelength/refractive index unit, nm/RIU) also increased from 407 nm/RIU to 3553 nm/RIU, 1483 nm/RIU, and 2335 nm/RIU after the &gamma, ray irradiation at a total dose of 1190 Gy, 2380 Gy, and 3570 Gy, respectively.

As a clean and sustainable energy source, hydrogen gas is highly demanded in industrial and commercial fields. Accordingly, the swift and accurate detection of hydrogen gas is becoming increasingly important for the safe production and operation of hydrogen. This review presents recent advancements of optical hydrogen sensors based on metal hydrides, focusing on the sensing material, sensor structure, and sensing principles. Starting with an overview of the significance of hydrogen sensing, this paper lists the demands for ideal hydrogen sensors and addresses the practical advantages of optical hydrogen sensors. Secondly, seven types of optical hydrogen sensors based on the gasochromic effect, micro-mirror, fiber grating, interferometer, evanescent field, surface plasmon resonance and localized surface plasmon resonance are comprehensively discussed. Then the preparation methods, advantages and disadvantages of different metal hydrides for hydrogen sensing are summarized. Lastly, the practical challenges, possible solutions and future prospects are outlined.

A photonic crystal fiber (PCF) with high relative sensitivity was designed and investigated for the detection of chemical analytes in the terahertz (THz) regime. To ease the complexity, an extremely simple cladding employing four struts is adopted, which forms a rectangular shaped core area for filling with analytes. Results of enormous simulations indicate that a minimum 87.8% relative chemical sensitivity with low confinement and effective material absorption losses can be obtained for any kind of analyte, e.g., HCN (1.26), water (1.33), ethanol (1.35), KCN (1.41), or cocaine (1.50), whose refractive index falls in the range of 1.2 to 1.5. Besides, the PCF can also achieve high birefringence (∼0.01), low and flat dispersion, a large effective modal area, and a large numerical aperture within the investigated frequency range from 0.5 to 1.5 THz. We believe that the proposed PCF can be applied to chemical sensing of liquid and THz systems requiring wide-band polarization-maintaining transmission and low attenuation.

Brillouin scattering properties of silica fibers at 850-nm wavelength were experimentally investigated using a Brillouin optical time-domain analysis scheme. In the single-mode 780-HP fiber, the Brillouin frequency shift (BFS) and full width at-half maximum of the Brillouin gain spectrum were found to be ~19795 and 90 MHz, respectively. The estimated peak Brillouin gain is 0.32 (mW) -1 . The temperature and strain dependences of BFS were determined as well. Distributed sensing capability was demonstrated at a minimum spatial resolution of 1.5 m and a fiber length of 100 m. As a proof-of-principle, the temperature and strain dependences of the BFS in SFM-28 fiber have been determined at 850- and 1550-nm wavelengths, suggesting its possible use for temperature/strain discrimination.

We report distributed temperature measurements based on Raman scattering performed during steady state $\gamma $ -ray irradiation at a dose rate of 1 kGy( ${\rm SiO}_{2}$ )/h and up to a total ionizing dose (TID) of $\sim 0.1\ \hbox{MGy}$ . We characterize on-line the evolution of the performances of a single-ended Raman distributed temperature sensor (RDTS) during the $\gamma $ -ray exposure of different classes of commercial multimode fibers (MMFs) acting as the sensing element. RDTS is influenced by the radiation-induced attenuation (RIA) phenomena leading to both large errors in the temperature measurements and a diminution of the useful sensing length. The amplitude of the radiation-induced temperature error strongly depends on the fiber choice and on the irradiation conditions. For the single-ended RDTS operation in the targeted Cigeo application the selection of a radiation tolerant sensing fiber will be mandatory, but not sufficient, to overcome the expected severe ambient conditions around radioactive wastes. For efficient temperature sensing up to an accumulated dose of 0.1 MGy, pre-irradiation of the selected radiation resistant (RR) fibers appears also necessary to improve the sensor performances.

In this paper, a tapered polymer fiber sensor (TPFS) is employed to detect the crack of Concrete Beam (CB). The sensing principle for crack detection is simply described based on V-number theory. The experiments are carried out by cement mixture mixed with high reactive powder to form the CB, in which the TPFSs are embedded and surface glued. Thermocouples and strain gauges are also embedded to calibrate and determine the ambient temperature and applied strain, meanwhile, the Linear Variable Differential Transformer ( LVDT) sensors are used to measure the deflection of the CBs. Four points loading test is applied for several samples to evaluate the sensors’ ability for monitoring the beam deflection and crack. Experimental results also indicate that the TPFSs can be used for post-crack detection.

An in-line fiber sensor for simultaneous measurement of refractive index (RI) and temperature is proposed and demonstrated. The sensor head is composed of an asymmetric Mach–Zehnder interferometer (MZI), which is combining the single mode-multimode-single mode (SMS) fiber structure and a peanut-shape structure. The transmission dips of the sensor will shift as the ambient RI or temperature variation due to the phase changing. The different wavelength transmission dips formed by the cladding modes interfering with the core mode have different sensitivity responses, so the simultaneous measurement of the RI and temperature can be achieved by monitoring the wavelength shifts of the two transmission dips. The experimental results show that the sensing sensitivities of RI and temperature are −86.7434 nm/RIU and 0.0590 nm/°C in the RI range from 1.3105 to 1.3517 and temperature range from 25 °C to 85 °C, respectively. The novel sensor processes easy fabrication, low cost, and high sensitivity, making it offers high potential applications in physical, biological and chemical sensing.

A compact refractive index sensor is proposed and investigated based on multimode fiber-micro tapered single-mode fiber-multimode fiber (MTSM) structure. The micro tapered single-mode fiber excites the high-order cladding mode and induces a compact size of 2.2 mm. The influence of multimode fiber (MMF) for the interference fringe is studied numerically and verified experimentally. The experimental and simulated results show that the appropriate MMF length is conducive to form a clear interference fringe. At the same time, the results of refractive index test show that the maximal sensitivity reaches 1879.87 nm/RIU under external refractive index of 1.437. Due to the advantages of small size and high sensitivity, the novel fiber sensor has the potential in the application of refractive index sensing.

The rapid and accurate detection of cellular hydrogen peroxide (H2O2) is of great significance in cell monitoring and drug screening. We have developed a porous hollow fiber sensor with a cell-in-lumen configuration to detect H2O2 released by cells in real-time. In contrast to the conventional two-dimensional electrode structure, the sensing unit was located at the outer surface of the hollow fiber, while cells grew in the lumen, and the released H2O2 can diffuse through the porous wall of the fiber. The sensing layer wrapped outside the porous hollow fiber can capture the released H2O2 in a flow-through mode, resulting in a high sensitivity to the nanomolar level. In short-term monitoring of cell respiratory burst, the current response of the cell-in-lumen sensing configuration was six times greater than that of a conventional cell-out-electrode configuration. The porous wall also separated the cells and electrodes, minimizing the impact of electrical field on the cells. The high sensitivity and stability of detection ensured the cell-in-lumen configuration could be used to monitor the cellular H2O2 gradual release process induced by anticancer drugs. Such in-situ monitoring of the cellular release of H2O2 provides a new in-vitro drug screening platform for personalized medicine and cancer therapy.

Based on the large evanescent field of microfibers, a novel fiber sensor using a microfiber knot resonator embedded in a Hi-Bi fiber loop mirror is proposed for simultaneous measurement of the temperature and refractive index of solutions. Due to the different responses of the microfiber knot resonator and fiber loop mirror to temperature and refractive index variations, the fiber sensor can measure temperature and refractive index simultaneously by utilizing the sensitivity matrix. The experimental measurements show that the sensitivities of the microfiber knot resonator and the fiber loop mirror to temperature are 5.6 pm/°C and −464.1 pm/°C, respectively. The sensitivity of the microfiber knot resonator to the refractive index of dilute solutions is 116 nm/RIU. The sensor has a higher sensitivity to refractive index variations than LPGs and has potential for use in the field of biochemistry.

This study describes the deployment of machine learning algorithm called generalized linear model (GLM) to improve the temperature prediction performance in Brillouin optical time domain analysis (BOTDA) fiber sensor for distributed temperature sensing application. In GLM, the temperature prediction is made from the Brillouin gain spectrum (BGS) and the link function, without the need to determine the Brillouin frequency shift (BFS). In this proof-of-concept experiment, the performance of GLM was investigated by collecting the BGS and comparing it to the conventional Lorentzian curve fitting (LCF) method. From the experimental results, we have found that the GLM method produced a more consistent temperature prediction than the conventional LCF method. Furthermore, the proposed GLM method could still retain an accurate temperature measurement regardless of low signal-to-noise ratio (SNR) and large frequency scanning step while collecting BGS, which is difficult to be achieved by the conventional LCF method at certain level. In addition to that, the prediction obtained is 655 times faster than the conventional LCF method. The small and negligible deterioration to the temperature resolution confirmed the robustness of GLM in performing fast and accurate temperature measurement for BOTDA.

We observed that the sensitivity of a fiber sensor is changed by introducing fast and slow light. In this fiber sensor configuration, fast and slow light are generated by a double Lorenz fiber Bragg grating (DL-FBG) and the probe pulse through DL-FBG will have different delays by varying the input laser wavelength. The maximum delay is 180 ps (slow light) and the maximum advance is 90 ps (fast light). Experimental result shows that the fiber sensor sensitivity obtains enhancement by introducing slow light, while the sensitivity of fiber sensor decreases when combining with the fast light.

A new type of fiber interferometer based on two short thin-core fiber insertions with a section of standard fiber in between is proposed. The thin-core fiber insertions excite cladding modes and provide energy exchange between different fiber modes. We have simulated and measured transmission through a single thin-core fiber section. The standard fiber section is used to accumulate phase differences between core and cladding modes propagating through the structure. Transmission spectra are measured for various lengths of the interferometer. Separate control of thin-core and standard fiber insertions adds flexibility in obtaining spectra desirable for sensing applications.

Fiber optic sensors for industrial, consumer, medical and military applications have been reported. Their working principle is majorly based on non-linear optics. For the first time to our knowledge, we propose a cost-effective real time monitoring polarization-based digital signal processing (DSP)-assisted sensor that relies on linear optics for application in earth mass movements due to environmental factors. A sensitivity of 0.075 kg−1 is achieved. This is realized on an average SOP speed of 0.83°/s yielding 598° angular drift on its polarization changes. A sensing performance comparison of single mode fiber (SMF) and large effective area fiber (LEAF) fiber is reported.

Soft robotics is an emerging field, since it offers distinct opportunities in areas where conventional rigid robots are not a feasible solution. However, due to the complex motions of soft robots and the stretchable nature of soft building materials, conventional electronic and fiber optic sensors cannot be used in soft robots, thus, hindering the soft robots&rsquo, ability to sense and respond to their surroundings. Fiber Bragg grating (FBG)-based sensors are very popular among various fiber optic sensors, but their stiff nature makes it challenging to be used in soft robotics. In this study, a soft robotic gripper with a sinusoidally embedded stretchable FBG-based fiber optic sensor is demonstrated. Unlike a straight FBG embedding configuration, this unique sinusoidal configuration prevents sensor dislocation, supports stretchability and improves sensitivity by seven times when compared to a straight configuration. Furthermore, the sinusoidally embedded FBG facilitates the detection of various movements and events occurring at the soft robotic gripper, such as (de)actuation, object holding and external perturbation. The combination of a soft robot and stretchable fiber optic sensor is a novel approach to enable a soft robot to sense and response to its surroundings, as well as to provide its operation status to the controller.