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High sensitivity vector torsion sensor based on a single Stress-Applying fiber Sagnac interferometer
... The inference spectrum is acquired by an optical spectrum analyzer (OSA, model MS9740A, Anritsu). The output transmission spectrum is expressed by Eq. (1) [28], which responds to the change of the torsion angle of the panda fiber: ...
The support vector regression (SVR) algorithm is presented to demodulate the torsion angle of an optical fiber torsion sensor based on the Sagnac interferometer with the panda fiber. Experimental results demonstrate that with the aid of SVR algorithm, the information in the transmission spectrum of the sensor can be used fully to realize the regression prediction of the directional torsion angle. The full torsion angle ranges from −360° to 360° can be predicted with a mean absolute error (MAE) of 2.24° and determination coefficient (R²) of 0.9996. The impact of the angle sampling interval and wavelength resolution of the spectrometer on the prediction accuracy of the directional torsion angle and the suitability of the SVR algorithm for compact optical fiber sensor and other optical fiber torsion sensors based on the Sagnac interferometer are discussed. Moreover, the multi-objective SVR algorithm is used to eliminate the interference of strain during torsion angle measurement. The SVR algorithm can efficiently enlarge the measurement range of the torsion angle and break through the challenge of demodulating sensing signal for compact fiber torsion sensor. Compared to the prediction accuracy of common machine learning algorithms of artificial neural network (ANN) algorithm, random forest (RF) algorithm, and K-nearest neighbor (KNN) algorithm, the SVR algorithm has the advantages of higher measurement accuracy and shorter testing time.
A broadband edge filter with linear dynamic ranges of ${{40}}\;{\rm{nm}}$ 40 n m in wavelength and ${{93}}\%$ 93 % in transmission loss has been demonstrated both theoretically and experimentally, which is achieved by using a helical long-period fiber grating (HLPG) operating near the dispersion turning point (DTP) of the ${{\rm{LP}}_{1,10}}$ L P 1 , 10 mode. To take full advantage of the wide dynamic ranges, the fabricated edge filter has been successfully used as a power-interrogated torsion sensor, and the torsion responsivity presented is about 0.501 nm/(rad/m), which is higher than most conventional HLPG-based torsion sensors. The results indicate that the proposed edge filter may find applications in any other kinds of power-interrogated sensors as well.
A novel intensity-modulated directional torsion sensor based on a helical taper is proposed and experimentally demonstrated. The tapers are fabricated in standard single-mode fiber by electric-arc discharge, and by rotating one side fiber simultaneously. Experimental results show that the intensity of transmission peak changes inversely when the helical taper is twisted in clockwise and counterclockwise, respectively. The maximum torsion sensitivity can reach -0.484 dB/(rad/m) in the twist rate ranges from -10.67 rad/m to 0 rad/m. Additionally, this torsion sensor is almost insensitive to temperature, which solves the problem of torsion-temperature cross sensitivity greatly. The novel torsion sensor provides a promising candidate for the applications that require accurate rotation, such as civil engineering, the automotive industry, and security monitoring of buildings.
Fiber Optic Shape Sensing is an innovative Optical Fiber Sensing Technology that uses a fiber optic cable to continuously track the 3D shape and position of a dynamic object (with unknown motion) in real-time without visual contact. This technology offers a valid alternative to existing shape sensing methods, thanks to a combination of advantages (including ease of installation, intrinsic safety, compactness, flexibility, electrically passive operation, resistance to harsh environments and corrosion, no need for proximity or computational or numerical models to reconstruct shape) that can bring remarkable improvements to the fields of Civil, Mechanical and Aerospace Engineering, Biomedicine and Medicine. A considerable research effort has been dedicated to this subject in the last twenty years.
This paper presents an ambitious review of the current state of the art of Fiber Optic Shape Sensors (FOSS) based on Optical Multicore Fibers (MCF) or multiple optical single-core fibers with embedded strain sensors and provides a comprehensive analysis of a wide range of aspects, comprising: (1) existing alternative technologies; (2) an overview of optical fiber sensors (3) characteristics and advantages of fiber optic shape sensors; (4) historical achievements; (5) applications; (6) performance and error analysis; and (7) present and future perspectives.
We report a highly sensitive twist sensor based on a Sagnac interferometer constructed with a new type of optical fiber which contains an elliptical core and two large semicircular-holes, where the slow axis of the core orthogonal to the air-holes has a large sensitivity towards twist-induced birefringent changes. The novel fiber structure results in a highest twist sensitivity of 5.01 nm/° at a chosen dip over the range from 370°-400°. The resonance dips in the interference pattern respond with different rates in the wavelength shifts in the presence of physical parameters permitting to experimentally distinguish directional torsion, axial strain and temperature.
A highly sensitive optical fiber torsion sensor with femtosecond laser-induced low birefringence SMF-based Sagnac interferometer (SI) is proposed and experimentally demonstrated in this paper. A straight-line waveguide positioned horizontally with respect to the fiber core is inscribed by the femtosecond laser in the cladding of the SMF, which leads to the asymmetry stress distribution in the SMF, and then gives rise to the low birefringence in the SMF. Compared with most of the previous reported SI based torsion sensors, there is no splicing joint in the femtosecond laser-induced low birefringence SMF-based SI, which lowers the transmission loss and makes the SI based torsion sensor more robust simultaneously. The experiment result shows that the proposed torsion sensor exhibits a torsion sensitivity of up to 3.2562 nm/degree, with the high torsion resolution of 0.003 degree. In contrast, the temperature cross-sensitivity and strain cross-sensitivity of the proposed torsion sensor are low, to −0.000055 degree/°C and 0.000013 degree/με, respectively, thus overcoming the cross-sensitivity problem resulting from temperature and strain. Moreover, theoretical analysis are carried out to compare with the experimental results to demonstrate the feasibility and good consistency.
Cascaded coaxial cable Fabry-Perot interferometers (FPI) are studied and demonstrated for distributed torsion measurement. Multiple weak reflectors are implemented on a coaxial cable so that any two consecutive reflectors can form a Fabry-Perot cavity. By fixing the cable sensor in a helical form on a shaft, the distributed torsion of the shaft can be measured by the cascaded Fabry-Perot cavities. A test on a single section shows that the sensor has a linear response with a sensitivity of 1.834 MHz (rad/m)-1 in the range of twisted rate from 0 to 8.726 rad m-1. The distributed torsion sensing capability is useful in drilling process monitoring, structure health monitoring and machine failure detection.
A highly sensitive optical fiber twist sensor has been proposed by employing a Sagnac interferometer based on polarization-maintaining elliptical core fibers (PM-ECFs). The twist effects have been theoretically analyzed and experimentally demonstrated. Based on the photoelastic effect, the resonance wavelength linearly shifts with the increment of twist and the wavelength shift is also dependent on the torsion direction. The maximum torsion sensitivities reach 18.60nm/(rad/m) for clockwise (CW) torsion direction and 15.83nm/(rad/m) for anticlockwise (ACW) torsion direction, respectively. To eliminate the temperature cross-sensitivity effect, a sensor matrix for simultaneous measurement of twist and temperature has also been obtained. Moreover, theoretical and experimental investigations indicate that by optimizing the refractive index difference between the core and cladding, core ellipticity and cladding diameter, the twist sensitivity could be further improved.
An optical fiber twist sensor is proposed by using solidcore lowbirefringence photoniccrystalfiber(LB-PCF)-based Sagnac interferometer. The twist effects on the fiber are theoret- ically analyzed. The results show that the dip wavelength of the transmission spectrum shifts with the twist angle with a high sen- sitivity and resolution of 1.00 nm and 0.01 , respectively. The sensor is also insensitive to environmental temperature change with an ultralow thermal dependent coeffiecient of 0.5 pm C. Index Terms—Birefringence, low-birefringence photonic crystal fiber, optical fiber twist sensor, Sagnac interferometer.
A theoretical and experimental analysis of the polarization properties of twisted single-mode fibers is presented. It is shown that whereas a conventionally twisted fiber possesses considerable optical rotation, a fiber which has a permanent twist imparted by spinning the preform during fiber drawing exhibits almost no polarization anisotropy. It is thus possible to virtually eliminate the commonly observed fiber linear birefringence. As a consequence, fibers made in this way are ideally suited for use in the Faraday-effect current transducer. It is further shown that a permanent twist of a few turns/meter effectively eliminates polarization mode-dispersion. The technique therefore appears attractive for enhancing the bandwidth of very long unrepeatered telecommunication links.
A fiber twist angle sensor has been theoretically proposed and experimentally verified by employing a fiber Sagnac loop interferometer based on a self-made eccentric dual-core fiber. The asymmetry of the dual-core fiber structure made the deformation of the fiber more obvious under the action of twist and enabled the designed sensor to identify the direction of twist angle. A clockwise (CW) and counterclockwise (CCW) rotation of a 5-cm-long eccentric dual-core fiber in the Sagnac loop generated a phase difference, which induced a red or blue shift of the interference spectrum, and the purpose of twist angle sensing was achieved by monitoring the spectral shift. The size of the twisted object should be larger than 5 cm, so that the sensor can be placed on the surface of the object to measure the twist deformation. In the measuring range from −90° to 90°, the twist angle-based sensitivity and twist rate-based sensitivity of the designed sensor were, respectively, investigated in CW and CCW directions: the former was 45.2 pm/° in CCW direction (−90° to 0°) and 12 pm/° in CW direction (0°–90°), and the latter was 129.49 pm/(rad/m) in CCW direction (–31.4 to 0 rad/m) and 34.38 pm/(rad/m) in CW direction (0–31.4 rad/m). The satisfactory sensitivity, stability, and repeatability make it a competitive alternative in twist angle sensing region, which can be widely used in engineering, biomedical, and other twist angle sensitive fields.
The fabrication and application for twist sensing of an eccentric three-core fiber (3CF) were demonstrated. The fiber was made by a stack-and-draw technique, in which silica rods and core canes were put in a tube and drawn on a fiber drawing tower. Three cores formed a Mach–Zehnder interferometer (MZI), where the lights transmitted in the three cores interfered with each other, resulting in the formation of envelopes on spectrum. Because two of the cores were off-axis, phase differences among the cores varied with twist due to different stretches on each core, which caused shift of the spectral envelopes of the interference signal. Wide-range twist measurement can be realized with relatively high sensitivity by tracking lower dips of the envelopes. Experimental results revealed that the dips shift quadratically with twist angle, which means that the sensitivity increases with twist. The compensation of temperature influence was also implemented by inscribing a Bragg grating on one of the cores with femtosecond laser. Because the fiber can be mass-produced, it is suitable for twist sensing in practical application for its low cost.
We propose a compact and ultrasensitive all-fiber Mach–Zehnder interferometer sensor based on an arc-shaped misaligned structure that can measure strain and torsion sensing. The structure utilizes a single-mode fiber and was fabricated in an optical fiber fusion splicer through simple discharge and fusion splicing steps. The experimental and theoretical investigations of its response revealed an optimal strain sensitivity of up to ~267.17 pm /με¯. In addition, the maximum torsion sensitivities in the clockwise and counterclockwise rotation directions of the fiber cross section were ~ -1.528 dB/(rad/cm) and ~1.2191 dB/(rad/cm), respectively. Further, the proposed device exhibited low temperature cross sensitivity. Therefore, this single-mode-fiber based core-cladding modal interferometer has great potential in the field of photoelectric sensing due to multiple advantages such as compact size, easy fabrication process, low cost, etc.
The transmission characteristics of Sagnac fiber loop interferometer which consists two sections of polarization-maintaining fiber (PMF) spliced at different offset angles were analyzed. Based on the simulation results, an application for twist measurement where PMFs were spliced at different offset angles was designed. When the lengths of two PMFs are equal, a clear spectrum can be obtained for twist or torsion sensing. A phenomenon that the dips would split or merge with twist angle alteration can be used for sensing. Measurement range and sensitivity can be adjusted by changing the PMFs offset angle. The sensitivity was improved by calculating the distance between two adjacent dips, being 0.73 nm/° or 8.37 nm/(rad/m) when the offset angle was 75°.
Torsion sensor based on dual-wavelength laser is proposed. By analyzing the laser intensity difference of two wavelengths in 1550.95 nm and 1552.53 nm, the sensitivity of 3.53 dB/(rad/m) from -1.99rad/m to 2.65 rad/m is achieved.
We demonstrate the fabrication of Mach-Zehnder interferometer twist sensor based on the cascaded long-period fiber gratings inscribed in double cladding fiber. The torsion sensitivity of wavelength and intensity are −0.391 nm/(rad/m) and −1.355 dB/(rad/m), respectively.
A highly sensitive torsion sensor based on side-hole fiber (SHF) Sagnac interferometer is proposed and experimentally demonstrated in this paper. The theoretical analyses on sensing performances of the proposed interferometric sensor are in good accordance with their experimental measurement counterparts. Due to photo-elastic effect, the birefringence of SHF in the loop would vary with the torsional rate, giving rise to linear wavelength shift and sinusoidal power variation responses to torsion applied on the SHF. Based on wavelength and intensity interrogations of transmission dips, the torsional sensitivities reach up to about 2.2 nm/(rad/m) and ±10 dB/(rad/m), respectively. Opposite wavelength shift responses for clockwise and counterclockwise torsion states equip the proposed sensor with good twisting direction distinguishability. Besides, experimental results indicate that our proposed sensor has a low axial strain cross sensitivity of 4.3 pm/μe. The side-hole-fiber-based interferometric torsion sensor possesses such desirable merits as high sensitivity, low axial strain cross sensitivity, good torsional direction distinguishability, and good mechanical robustness, ensuring its viability for practical use on torsion sensing occasions.
An in-line Mach-Zehnder interferometer based on a dual-tapered photonic crystal fiber (PCF) for bending vector and strain sensing is proposed and experimentally demonstrated. This sensor consists of a 25.7-cm homemade asymmetric twin-core PCF sandwiched between two tapers. Due to the asymmetry of twin cores, two opposite bending directions can be distinguished with the maximum bending sensitivities of 18.29 and -18.13 nm/m
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, respectively. By applying axial strain to the sensor, the strain sensitivity reaches up to -10.65 pm/με. Because of the excellent linear response, the effect of temperature on strain and bending sensing can be eliminated by using a measurement matrix. The advantages of bending direction recognition, high strain and bending sensitivities, and compact structure make this sensor attractive for structural deformation monitoring.
A segmented long period fiber grating (LPFG)-based torsion sensor with both high sensitivity and ability to discriminate torsion direction is proposed and experimentally investigated. The sensor is made up of three spatial orthogonal short LPFGs by using a CO
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laser notched method. Experimental results show that the torsion sensitivities can reach -0.54 and 0.33 dB/(rad/m) in dip depth and 0.30 and -0.14 nm/(rad/m) in resonance wavelength for clockwise and counterclockwise, respectively, in a single polarization state. Torsion rate and torsion direction can be measured simultaneously by the sensor. Moreover, the temperature sensitivity of the sensor has been determined as 0.067 nm/°C. Therefore, the proposed segmented-LPFG would have great potential applications in engineering and fiber sensors.
With the evolution of technology, it is now possible to build more flexible surgical instruments with multiple degrees of freedom (DOFs) that can be deployed in Natural Orifice Translumenal Endoscopic Surgery (NOTES), further reducing tissue damage, blood loss and recovery time. However, the limits of flexible robots are mainly represented by the lack of accurate position feedback during the surgical procedure. The most critical problems of shape sensors are size and quantity limits, as well as sterilizability. This paper presents a novel helical configuration of single optic fiber with multiple fiber Bragg grating (FBG) sensors to measure the shape of flexible robots. Detailed design of helix parameters, simulation and verification experiments were performed. We have shown the feasibility of helical configuration to reduce the effect of temperature, and detect the torsion and bending of the flexible shaft. The experimental results demonstrated the potential to provide a shape detection for flexible robot.
A novel torsion sensor is proposed by cascading two identical helical long-period fiber gratings. On the basis of interference theory, the separation between the dips in the transmission spectrum will be affected by the applied torsion. In addition, a linear response of the split dependent on the torsion rate in different directions can be deduced. Moreover, the ambient temperature has a limited influence on the sensor result. Confirmation experiments were carried out, and good agreement of derivations and experimental results is obtained.
A fiber-optic twist sensor based on a pressure-induced birefringence singlemode fiber loop mirror is proposed. The birefringer SMF is made by applying a transverse force against a short length of singlemode fiber. The sensitivity of the twist angle measurement of 0.19 nm/o is achieved experimentally. The proposed sensor is more convenient and simple than that of standard polarization-maintaining fibers.
Implementation of successful civil structural health monitoring strategies requires selection and placement of sensors suitable for measurement of key parameters that influence the performance and health of the structural system. Optical fiber sensors have been successfully implemented in aeronautics, mechanical systems, and medical applications. Civil structures pose further challenges in monitoring mainly due to their large dimensions, diversity as well as heterogeneity of materials involved, and hostile construction environment. This article provides a summary of basic principles pertaining to practical health monitoring of civil engineering structures with optical fiber sensors. The issues discussed include basic sensor principles, strain transfer mechanism, sensor packaging, sensor placement in construction environment, and reliability and survivability of the sensors.