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Variations of resonant wavelengths with the level of water solution with different sugar concentration (% by weight). (a) Cladding mode LP06,out. (b) Cladding mode LP07,out.
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In this study, a long-period fiber grating (LPFG)-based optical fiber sensor device is proposed for simultaneous detection of liquid level and refractive index (RI). When part of the grating was submerged in an unknown liquid, the resonant wavelength of each cladding mode of the LPFG sensor varied linearly with the submerged length and nonlinearly...
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Citations
... Meanwhile, they can be applied in harsh environments. Currently, the most widely used fiber-optic sensors for liquid-level sensing are mainly focused on long-period grating (LPG) [5], [6], [7], [8], Fabry-Perot interferometer (FPI) [9], [10], [11], Mach-Zehnder interferometer (MZI) [15], [16], [17], fiber Bragg grating (FBG) [12], [13], [14], multimode interference (MMI) [18], [19]. In 2007, a fiber-optic etched FBG sensor for liquid-level monitoring was proposed. ...
High-resolution monitoring of liquid level over a wide range has always been a great challenge. In this paper, a fiber ring laser (FRL) sensor system with Fabry-Perot interferometer (FPI) is proposed for the high-resolution measurement of liquid level. The FPI fabricated with a polyimide film is embedded in a FRL for intracavity sensing. It is used as a reflective sensing head, which generates different reflection losses at different liquid levels to modulate the output power of the FRL. With the intracavity sensing, the resolution of the liquid level sensing system has been significantly improved from 37.5 mm to 1.5 mm. The measurement range of the liquid level sensing system is obtained as 0.6 m. The output stability and measurement error of the sensing system have been discussed. The sensor has a compact structure and a broad application prospect in the field of wide-range and high-resolution liquid level monitoring, such as in chemical and marine applications.
... Finally, Huang [60] and Huang et al. [61] used optical fiber to measure the temperature. Despite being a method with a high cost, this technique is usually applied to temperature when the optical fiber is also used to measure other parameters. ...
... Huang et al. [61] 3.1.3. Turbidity ...
Monitoring water quality is an essential tool for the control of pollutants and pathogens that can cause damage to the environment and human health. However, water quality analysis is usually performed in laboratory environments, often with the use of high-cost equipment and qualified professionals. With the progress of nanotechnology and the advance in engineering materials, several studies have shown, in recent years, the development of technologies aimed at monitoring water quality, with the ability to reduce the costs of analysis and accelerate the achievement of results for management and decision-making. In this work, a review was carried out on several low-cost developed technologies and applied in situ for water quality monitoring. Thus, new alternative technologies for the main physical (color, temperature, and turbidity), chemical (chlorine, fluorine, phosphorus, metals, nitrogen, dissolved oxygen, pH, and oxidation–reduction potential), and biological (total coliforms, Escherichia coli, algae, and cyanobacteria) water quality parameters were described. It was observed that there has been an increase in the number of publications related to the topic in recent years, mainly since 2012, with 641 studies being published in 2021. The main new technologies developed are based on optical or electrochemical sensors, however, due to the recent development of these technologies, more robust analyses and evaluations in real conditions are essential to guarantee the precision and repeatability of the methods, especially when it is desirable to compare the values with government regulatory standards.
... Beyond the ease of manufacturing, the transmission spectrum of LPGs is more complex, thus incorporating more information; for this reason, a single LPG can be used in multi-parameter or multi-dimensional sensing [24][25][26][27], whereas similar FBG-based approaches generally require a higher transducer count [28][29][30]. ...
In this work, we present a low-cost tactile sensor based on arc-induced long–period fiber grating (LPG) embedded in polylactic acid (PLA) by fused deposition modeling (FDM). The 3d printing process used to embed the LPG, offers easy customization, flexibility, fast prototyping, and low-cost fabrication. At the same time, optical fiber sensors such as fiber Bragg gratings (FBGs) and LPGs are compact, resistant, and immune to electromagnetic interference. Therefore, the easy fabrication and low cost of arc-induced LPGs coupled to the 3d printing advantages offer low-cost custom force sensing elements with all the advantages of optical fiber sensors. This way, the proposed tactile sensor is a lightweight, small-size, and cost-effective tactile sensing scheme that offers easy implementation and tailoring to specific applications. In this study, we fabricated, embedded, calibrated, and evaluated the proposed LPG sensor tactile sensor. We showed this approach provided high force sensitivity (11.723 nm.N⁻¹) with high linearity (99.28 %), sub-decimal measurement error, and no drift.
... In environments that require ultra-high liquid level sensitivity like medical testing, the common capacitance, resistance, magnetostriction, and ultrasonic sensors have various defects and cannot meet the requirements of liquid level measurement [13] . In recent years, various optical fiber liquid level sensors were proposed, such as fiber Bragg gratings (FBGs) [14,15] , long period fiber gratings (LPFGs) [16,17] , Michelson interferometers (MIs) [18] , and Mach-Zehnder interferometers (MZIs) [19,20] . Especially, the sensor based on a multimode fiber (MMF) structure has attracted more and more attention because of its advantages such as simple structure, low cost, and convenient manufacture. ...
... To date, there are many efforts devoted to the development of fiber-optic liquid-level sensors owing to their excellent features, such as high sensitivity, small size, immunity to electromagnetic interference, and resistance to harsh environments [4][5][6]. Recently, a number of fiber grating sensing structures, including fiber Bragg gratings (FBGs) [7,8], long period gratings (LPGs) [9,10], and tilted fiber Bragg gratings (TFBGs) [11,12], have been demonstrated for liquid-level measurement. However, the liquid temperature variation will affect the measurement accuracy. ...
We propose and demonstrate an all-fiber liquid-level sensor using an in-line multimode-single-mode-multimode (MSM) fiber structure. A piece of single-mode fiber (SMF) is spliced to two sections of equivalent multimode fiber (MMF) which are used as both mode splitter and mode coupler. The cladding mode will be excited when the light propagates from MMF to SMF, and then it will be combined with fundamental mode to form a Mach-Zehnder interferometer (MZI) when the light propagates from SMF to the other MMF. The liquid level is detected by the selected resonant dips shift of the transmission spectrum. A sensing sensitivity of 264.6 pm/mm is achieved for the proposed sensor with an SMF length of 26mm. Due to its compact structure, easy fabrication, and high sensitivity, the proposed liquid-level sensor is attractive for practical applications in a variety of fields, such as marine detection and chemical processing.
... Stages I and II are dominated by the effect of Fe-C layer thinning and NaCl solution saturation on the evanescent field in the proximity of the LPFG sensor. The saturation effect likely includes two parts: liquid coverage along the length of the LPFG sensor and change in its refractive index from the surrounding Fe-C layer to the NaCl solution [37]. Stage III represents a near completion of corrosion process in the Fe-C layer and the LPFG sensor becomes fully submerged in the NaCl solution. ...
In this study, graphene/silver nanowire (Gr/AgNW)-based, Fe-C coated long period fiber gratings (LPFG) sensors were tested up to 72 hours in 3.5 w.t% NaCl solution for corrosion-induced mass loss measurement under four strain levels: 0, 500, 1000 and 1500 µε. The crack and interfacial bonding behaviors of laminate Fe-C and Gr/AgNW layer structures were characterized using Scanning Electron Microscopy (SEM) and electrical resistance measurement. Both optical transmission spectra and electrical impedance spectroscopy (EIS) data were simultaneously measured from each sensor. Under increasing strains, transverse cracks appeared first and were followed by longitudinal cracks on the laminate layer structures. The spacing of transverse cracks and the length of longitudinal cracks were determined by the bond strength at the weak Fe-C and Gr/AgNW interface. During corrosion tests, the shift in resonant wavelength of the Fe-C coated LPFG sensors resulted from the effects of the Fe-C layer thinning and the NaCl solution penetration through cracks on the evanescent field surrounding the LPFG sensors. Compared with the zero-strained sensor, the strain-induced cracks on the laminate layer structures initially increased and then decreased the shift in resonant wavelength in two main stages of the Fe-C corrosion process. In each corrosion stage, the Fe-C mass loss was linearly related to the shift in resonant wavelength under zero strain and with the applied strain taken into account in general cases. The general correlation equation was validated at 700 and 1200 µε to a maximum error of 2.5% in comparison with 46.5% from the zero-strain correlation equation.
... Since the different resonant peaks of the FM-LPFG have different responses to the measurand changes, it is easy to distinguish the temperature from the SRI [19]. ...
A novel few-mode long-period fiber grating (FM-LPFG) sensor based on dual-peak resonance near the phase-match turning point (PMTP) for simultaneous detection of surrounding refractive index (SRI) and temperature is proposed. The dual-peak resonance near the PMTP of the FM-LPFG is analyzed based on the coupled-mode theory. The cladding modes of adjacent order exhibit dual-peak and single-peak resonances in the wavelength band 1450–1850 nm, respectively. And the response characteristics of different resonant peaks to temperature and SRI are discussed. Results of numerical simulations demonstrate that the SRI sensitivities corresponding to the distance between the dual peaks and single-peak transmissivity are 1114.28 nm/RIU and - 685.42 dB / RIU , respectively, and the temperature sensitivities corresponding to the distance between the dual peaks and single-peak transmissivity are − 0.27 nm / ° C and 0.072 dB/°C. The different SRI and temperature sensitivities of those peaks help to sense multi-parameters.
... As simultaneous measurement is considered an effective way to solve the cross-sensitivity problem, it is of great importance in fiber grating devices. To address different requirements in various research fields, sensing characteristics have been selected for simultaneous measurement, such as simultaneous measurement of temperature and strain [3]- [5], temperature and refractive index (RI) [6], temperature and torsion [7], temperature and magnetic field [8], liquid level and RI [9], shape and temperature [10], pressure and temperature [11], and others [12]- [14] Simultaneous measurement of strain and temperature is more widely used in some fields than other dual-parameter measurements, including automobiles, spacecraft, nondestructive evaluation of civil infrastructure, and environmental monitoring. Hence, several structures that can realize simultaneous measurement of strain and temperature have been proposed in recent years, including cascade long period fiber grating (LPFG) [15]; cascade fiber Bragg grating [16]; a fiber grating inscribed on a special optical fiber [17]- [18]; an LPFG induced by electric-arc discharge [19]; an LPFG cascading another fiber structure, as combined with a tapered three-core fiber [20]; hybrid LPFG/MEFPI sensor [21]; micro-tapered fiber grating [22]; asymmetrical fiber Mach-Zehnder interferometer [23]; and others [24]- [25]. ...
A novel sensor structure has been proposed and experimentally investigated for simultaneous strain and temperature measurement. The structure is fabricated by weak power modulation of CO2 laser exposure on tapered long period fiber grating (LPFG). Compared with the transmission spectrum of the tapered LPFG, two peaks appear in the transmission spectrum of the novel structure. These resonance peaks exhibit different sensitivity responses; thus, simultaneous measurement of strain and temperature is realized by monitoring the wavelength shift of the two peaks. Experiment results indicate that strain sensitivities of the two peaks are 1.82 pm/μE and 8.17 pm/μE, and temperature sensitivities are 47.9 pm/°C and 65 pm/°C, respectively.
... In Experiment #1 to #3, water (n s = 1.33), decane (n s = 1.41), and propylene glycol (n s = 1.43) was gradually added into the graduated cylinder, respectively as shown in Figure 3a. Figure 4a shows the example spectrum changes of the LPFG sensing unit as the water level increases. In addition to laboratory experiments, numerical simulation was also performed for the cladding mode of LP 07 following the procedures in reference [18]. Figure 4b compares the experimental resonant wavelength changes of the cladding mode of LP 07 to simulations for a water level from 0 to 30 mm (full length of the grating) according to Equation (1). Figure 4b shows that the experimental and simulated results on the resonant wavelength changes agree with each other very well. ...
... In Section 2.1, Equation (2) predicts that a higher cladding mode of LPFG sensor can result in a larger Γ s , leading to a higher sensitivity to measure the surrounding medium changes such as liquid interface or level changes. From previous study [18], although the resonant wavelength changes with liquid nonlinearly, linear approximation can be used to estimate the liquid level changes. More details about the nonlinearity of the liquid level to the resonant wavelength change refer to [18]. ...
... From previous study [18], although the resonant wavelength changes with liquid nonlinearly, linear approximation can be used to estimate the liquid level changes. More details about the nonlinearity of the liquid level to the resonant wavelength change refer to [18]. In addition, if the cladding mode is too high, it may show a turning point in spectrum, which is undesired for this application [28]. ...
Insoluble liquids show layers such as water and oil. The detection of the exact interface locations and the level changes for layered liquids are of paramount importance for chemistry purifications, liquid storage in reservoirs, oil transportation, and chemical engineering. However, accurately measuring liquid layers is challenging. This paper introduces a multi-parameter sensing device based on a long-period fiber grating (LPFG) sensor simultaneously detecting boundary and level changes of layered liquids. Laboratory experiments demonstrated that the sensor device would respond to the liquid interface change as a sharp and sudden resonant wavelength change, while it would show a gradual and steady resonant wavelength change to the level changes of layered liquids. The lab experiments also showed that the sensor device has a higher sensitivity when a higher LPFG cladding mode is used.
... Type II FBGs can work at a temperature up to 1000 °C , but its fabrication requires expensive high power lasers such as femtosecond laser or excimer laser [6][7][8][9]. Comparing with FBGs, LPFGs exhibit much higher temperature sensitivity [10][11][12][13][14], but their cross-sensitivity to external refractive index (RI) can't be overcome [15][16][17][18]. ...
... Comparing with previously reported fiber temperature sensors, the HCF-based interferometer exhibits a high temperature sensitivity that is four times higher than that of the multimode-fiber-based interferometer [19][20][21], and an excellent temperature stability that is much better than that of traditional FBGs [3][4][5]. In addition, this configuration is immune to external RI that is irrealizable for LPFG temperature sensors [15][16][17][18]. ...
We report a new fiber optic sensor for temperature measurement at a temperature range of up to 900°C with excellent stability and repeatability. The sensing head is comprised of a short hollow-core fiber segment spliced between two single-mode fibers using a commercial splicer. Optical power from the lead-in fiber is partly coupled to silica cladding of the hollow-core fiber with the remainder propagating in air core by short distance. The two beams are then re-coupled to the core of the lead-out fiber owing to the offset splicing joint. Due to the effective index difference between the two beams, an interference pattern in the transmission spectrum is obtained. The distinct thermo-optic coefficients between silica and air result in a high temperature sensitivity of 41 pm/°C, and the enclosed structure ensures its immunity to the external refractive index.
