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A Highly Sensitive Torsion Sensor With a New Fabrication Method
Abstract
A novel long period fiber grating (LPFG) made by using CO
2
laser is proposed as torsion sensor. The fiber sheets fabricated clockwise/anticlockwise in the optical fiber causes the dextrorotation/levorotation of the polarization state of the light during propagation in the sensor. The experimental results indicate that the sensors, with the fiber sheets in different rotation, will appear opposite torsion sensitivities. While twisting the proposed sensor, the resonant wavelength shows good linear relationship with the torsion rate. From -360° to 360°, the LPFG with the clockwise revolving fiber sheets (dextrorotation grating) has a torsion sensitivity of 0.175 nm/(rad/m), whereas the LPFG with anticlockwise revolving fiber sheets (levorotation grating) has a torsion sensitivity of -0.153 nm/(rad/m). In addition, the temperature sensitivities of the two LPFGs are approximately the same, 73 pm/°C and 67 pm/°C, respectively. In conclusion, it is strongly believed that this structure can be used to measure the torsion rate and distinguish the torsion direction, and it may have excellent significance for the development of optical fiber torsion sensors in the future.
... Optical fibers are immune to electromagnetic interference, and their small size and lightweight offer the possibility to embed them in a variety of structures. A number of optical fiber based twist sensors have been demonstrated including the use of fiber Bragg gratings (FBGs) [3,4], long period gratings (LPGs) [5,6] and interferometers [7,8]. Generally, an optical fiber-based twist sensor suffers from cross-sensitivities of the axial strain and temperature and lacks the capability to distinguish the direction of the twist which is not ideal for practical usage. ...
... Generally, an optical fiber-based twist sensor suffers from cross-sensitivities of the axial strain and temperature and lacks the capability to distinguish the direction of the twist which is not ideal for practical usage. Several schemes have been proposed to overcome these drawbacks [5,9]. For example, the sensor based on a spirally etched LPG can eliminate the temperature perturbation and distinguish the direction of the applied torsion, but the fabrication process is overly complex. ...
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.
... To date, various sorts of fiber-optic torsion sensors have been implemented and reported, including Mach-Zehnder interferometers [3], bi-core fibers [4], hollow-core fibers [5], twin-core fibers [6], fiber Bragg gratings (FBGs) [7], and long-period fiber gratings (LPFGs) [8]. Wherein, the LPFG can achieve a high sensitivity by measuring the change in the rotatory vector caused by the twist-induced birefringence [9]. There are a lot of methods for fabricating LPFG, such as ultraviolet laser radiation [10], electric arc [11], and mechanical deformation [12]. ...
... VOLUME XX, 2017 1 (9) here, θ denotes the rotatory angle of the holder and L refers to the distance between the two holders. In the experiment, L is 20 cm. ...
Torsion sensing is a promising technique in a variety of different scientific and industrial applications. In this paper, we present a novel torsion sensor based on the cascaded long-period fiber grating inscribed in a four-mode fiber (CLPFG-FMF). The change in rotatory vector makes resonant wavelength shift, for the twisted CLPFG-FMF. The key factor, which severely impacts on the properties and performance of torsion sensor, is also investigated and discussed. Experimental results show that the resonant wavelength linearly shifts with the torsion rate and the cascaded angle plays an important role in enhancing torsion sensitivity. The proposed torsion sensor achieves optimal sensitivities of -0.519 nm/(rad/m) in counterclockwise and 0.501 nm/(rad/m) in clockwise. The repeatability and stability of the CLPFG-FMF with cascaded angle of 00 are studied as well. The refractive index sensitivity of the CLPFG-FMF with cascaded angle of 00 is -5.20 nm/RIU from 1.3431 to 1.4374 refractive index ranges and the temperature sensitivity is -0.0173 nm/°C from 25 °C to 90 °C. The high sensitivity of torsion sensing is expected to be useful for broad applications across engineering fields.
... The chiral grids fabricated by CO 2 laser provide LPFG different response for bidirectional torsion [22][23]. In 2017, Liu et al. [24] used CO 2 laser to etch four fiber sheets on single-mode fiber (SMF) to build a novel LPFG for torsion sensing, which could distinguish different twisting directions, but the sensitivity was still less than 0.2 nm/(rad·m −1 ) due to the limited RIM of fiber sheet grid. Curved grid shows RIM higher than conventional planar grid in LPFG [25], and its mirror symmetry can be removed by controlling the scanning track, which can be used to design novel chiral fiber device with high sensitivity. ...
... Though the LPFG based on few-mode fiber shows a higher sensitivity, its insert loss is higher than TA-LPFG on SMF, and it requires a section of FMF as long as 5 m, which increases the cost and size of sensor. Tilted LPFG Two-mode fiber 0.37 [23] LPFG and HF-LPFG SMF 0.331 [20] Segmented LPFGs SMF 0.30 [21] Helical LPFG Multi-core fiber 0.198 [11] Fiber sheet LPFG SMF 0.175 [24] Screw-type LPFG SMF 0.162 [22] Twisted LPFG SMF 0.117 [26] Helical LPFGs pair SMF 0.115 [17] Rotary LPFGs pair SMF 0.084 [18] Helix LPFG SMF 0.057 [13] Tilted LPFG SMF 0.049 [27] To evaluate the property of sensor, loop hysteresis cycle and stability are two important indices. In the cycle testing, the No. 3 TA-LPFG is gradually twisted until a torsion rate of 23.20 rad·m −1 , and then relaxed until no torsion. ...
Fiber torsion sensor has been researched for many years due to its various structure and sensitive response. In order to distinguish the torsion direction, fiber sensor still faces some difficult problems, including complex fabricating condition, special fiber structure and limited sensitivity. In this paper, a novel long-period fiber grating (LPFG) formed by tilted-arc grids is designed and fabricated in the normal simple-mode fiber, showing small size and high sensitivity. The asymmetrical tilted-arc grid structure can induce considerable chirality into the tilted-arc LPFG to enable it to distinguish torsion direction, which doesn’t need any equipment to rotate or twist the fiber in the fabrication process. Theoretical analysis indicates that the structure can respond opposite wavelength shifting to the opposite torsion directions, and the torsion sensitivity is related to both the radius and tilted angle of grid. A series of tilted-arc LPFGs are fabricated with CO2-laser scanning and tested in torsion experiment, all of whom can distinguish bidirectional torsion. The maximum sensitivity value can reach 0.514 nm/(rad·m⁻¹), which is higher than many normal tilted LPFGs and twisted fiber structures. The novel LPFG has the potential to be applied in directional torsion field due to its direction-distinguishing ability, high sensitivity and simple fabrication.
... For example, Liu et al. proposed a torsion sensor based on LPG by fabricated clockwise/anticlockwise fiber sheet structures to determine torsion direction. Consequently, the torsion sensor featuring clockwise-revolving fiber sheets (dextrorotation grating) exhibits a remarkable torsion sensitivity of 0.175 nm/rad·m −1 , whereas the LPG with counterclockwise-revolving fiber sheets (levorotation grating) demonstrates a notable 2 of 13 torsion sensitivity of −0.153 nm/rad·m −1 [19]. He et al. proposed a bidirectional torsion sensor based on micro tapered long-period fiber grating with arc discharge technology, and the torsion sensitivity of the sensor is −0.287 nm/rad·m −1 [20]. ...
A torsion sensor with directional discrimination based on a single-mode–twisted multimode–single-mode (STMS) fiber structure is demonstrated. The STMS fiber structure is fabricated by fusion-splicing a segment of multimode fiber (MMF) between two single-mode fibers (SMFs), where the MMF is simultaneously heated and twisted to form paired helical-type structures. The experimental results indicate that the resonance wavelength shifts towards shorter wavelengths as the twist rate increases in the clockwise direction, while an opposite shift occurs in the counterclockwise direction. Within a twist rate range of −5.2 rad/m~5.2 rad/m, the wavelength and transmission sensitivities are determined to be −1.38 nm/rad·m⁻¹ and 3.12 dB/rad·m⁻¹, respectively. Furthermore, the proposed twist sensor exhibits minimal temperature crosstalk of 0.0072 (rad/m)/°C, making it highly desirable for mitigating temperature-induced cross-sensitivity in torsion measurement applications.
... The CO 2 laser can make the laser energy density focused on the fiber to realize the refractive index changes in the fiber [19]. The process of removing the fiber cladding by computer programming is called polishing [20]. Finally, the LPFG is finished. ...
In this paper, a sensor with high strain sensitivity and low temperature crosstalk is proposed. The proposed sensor is configured by re-modulating the D-shaped long-period fiber grating (D-LPFG). D-LPFG is fabricated by CO2 laser polishing method. Re-modulation refers to etching on the D-LPFG with CO2 laser. A new resonant dip appears after re-modulation. The re-modulation based on CO2 laser etching method changes the distribution of refractive index, which leads to the variation of coupling coefficients. Resonant dip A and dip B with wavelengths at 1301.5 nm and 1565.7 nm are selected to measure strain and temperature. Experimental results show that the strain and temperature sensitivities of the resonant dip A are 9.37 pm/με and 57.00 pm/°C, respectively. The strain and temperature sensitivities of the resonant dip B are -1.38 pm/με and 57.14 pm/°C, respectively. Through calculation, the strain sensitivity of the proposed LPFG reaches 10.75 pm/με with only 0.14 pm/°C temperature sensitivity. The temperature crosstalk is 0.013 με/°C. Therefore, the proposed LPFG can be used as a sensor for strain measurement with extremely low temperature crosstalk.
A cascaded eccentric hole fiber (EHF) sensor with a dumbbell-shaped structure for large-range torsion measurement is proposed and prepared. It is consisting of an EHF and two sections of single-mode fiber (SMF) which are made into spheres by electric discharge. The main sensing structure is the SMF sphere–EHF–SMF sphere. In the first sphere structure and EHF fusion point position, the appearance of mode-field mismatch and refractive index perturbation, the noncircular symmetric (NS) higher order mode (HM) is created, the effective refractive index (ERI) difference between the HM and the fundamental mode (FM) is modulated by the torsion effect during the transmission process, and then, the HM interferes with the FM at the second sphere structure. Experimental results show a maximum sensitivity of 0.759 nm/(rad/m), where the sensor undergoes a torsional change from 0° to ±720°. The blue shift occurs in the torsional range of
to
and
to
rad/m, and a red shift in the range of
to
and
to
rad/m. The periodic shift phenomenon enables the sensor torsion range to be monitored. In addition, the axial strain has a very small impact on the results.
We demonstrated the fabrication of orbital angular momentum (OAM) mode convertors (MCs) operating at 1.3–2.1-μm waveband based on the helical long-period gratings (HLPGs) inscribed in the few-mode fiber (FMF) using a CO
2
laser. The ±1st, ±2nd, and ±3rd OAM mode can be directly generated by the HLPGs with efficiencies of more than 90%. The torsion sensitivities of the first-, second-, and third-order OAM modes in 2-μm waveband are 0.52, 0.55, and 0.36 nm/(rad/m), which has increased by 5.8 times, 2.8 times, and 1.6 times compared to that in 1.3-μm waveband, respectively. The strain sensitivities of the first-, second-, and third- order OAM modes in 2-μm waveband are −4.2, −4.0, and −4.9 pm/
, which has increased by 2.3 times, 2 times, and 2.2 times compared to that in 1.3-μm waveband, respectively. The proposed OAM mode MCs based on the HLPGs can function as not only promotion of optical communication system but also high-torsion sensitivity sensors in the sensing field.
Fiber twist sensors based on reflective microfiber couplers (RMCs) with two different fiber types are proposed and investigated. The single-mode fiber RMC (S-RMC) is fabricated by fusing and tapering two single-mode fibers and then connecting two fibers to form a Sagnac loop. The polarization-maintaining fiber RMC (P-RMC) is fabricated by the same method but consist of a segment of polarization-maintaining fiber. The S-RMC and the P-RMC achieve twist sensitivity of 2621.08 (a.u.)/(rad/m) and 2355.59 (a.u.)/(rad/m), respectively. For S-RMC, the wavelength and intensity fluctuations are <0.1 nm (0.05 nm/°C) and 0.2 dB (0.01 dB/°C) at temperatures ranging from 25 °C to 45 °C, respectively. The S-RMC has the characteristics of high twist sensitivity and linearity, thermal stability, low loss as well as easy fabrication.
We propose a fiber torsion sensor based on double-helix long-period grating (DH-LPGs) written in a tapered polarization-maintaining fiber (PMF). A series of DH-LPGs were fabricated in the tapered PMFs with waist diameters of 85 to 67 μm by a CO2-laser heating and twisting system. The torsion, temperature and surrounding refractive index sensing characteristics were investigated experimentally. Thanks to the special structures combined DH-LPGs with tapered PMFs, the proposed sensor can realize directional torsion measurement. The maximum torsion sensitivity of the proposed grating could be up to -2.28 nm/(rad/m) when the tapered waist diameter was 67 μm. The DH-LPGs have potential application in field of all-fiber high sensitivity directional torsion sensing.
We demonstrate the fabrication of a broadband mode converter (MC) operating at 2 m waveband based on long-period fiber gratings (LPFGs) written in the two-mode fiber (TMF) by CO2 laser. The proposed MC offers a 10-dB bandwidth of 58 nm, making it suitable for high-speed broad-band optical communication system at 2 m waveband. The maximum conversion efficiency reaches 98.89% at wavelength of 2 m. The torsion, strain, bending, and polarization characteristics of the broadband MCs were measured experimentally. The torsion sensitivity of the LPFG was measured to be 1.32 nm/(rad/m) at 2 m waveband, which was almost three times higher than that of the grating with the same order mode coupling at 1.55 m waveband. The proposed broadband MC can function as not only a promotion in fiber communication system but also a high sensitivity torsion sensor in fiber sensing system.
In this paper, we propose a sensor with high torsion sensitivity. This sensor has the ability to distinguish the torsion direction. A long-period fiber grating (LPFG) with a unique geometric structure is used to configure this sensor. The proposed LPFG is prepared by CO
2
laser polishing method and etching method. First, we periodically polish the two opposite planes which are parallel to the axis direction of the fiber to get sheets structure. Then, the V-shaped grooves are etched on the surface orthogonal to the sheets to fabricate the proposed LPFG. Because the V-shaped grooves are arranged along the axis direction of the optical fiber, we name it by V-LPFG. This unique geometric structure results in the sensor with high torsion sensitivity. Experimental results prove that the torsion sensitivity of the proposed sensor reaches 0.399 nm/(rad/m) in the range of-10.5 rad/m to 10.5 rad/m. More importantly, the torsion sensitivity is significantly increased from 0.041 nm/(rad/m) to 0.399 nm/(rad/m) after the V-shaped grooves are etched. The temperature sensitivity of the proposed sensor is 50 pm/°C at the temperature range of 30-150 °C. Due to the high torsion sensitivity, relatively easy fabrication process and high price to quality ratio, the proposed sensor has the potential of carrying out torsion measurement with high sensitivity.
In this paper, a novel strain sensor based on Depth-Modulated Long Period Fiber Grating (DM-LPFG) is presented and demonstrated. One side polished deformations with different milling depth are fabricated by a high frequency CO2 laser, so the DM-LPFG possesses a staircase-shaped structure. This kind of structure provides the high strain sensitivity of DM-LPFG. Experimental results clearly demonstrate that the structure has excellent mechanical capacity and repeatability. The sensor has a considerable strain sensitivity, which can reach up to 27.4 pm/με with the range of 0-450 με, it is an order of magnitude higher than conventional CO2 laser induced LPFG. The temperature characteristic is 49 pm/℃ with the range of 30-170 ℃. Furthermore, the strain sensor possesses a compact structure, with a total length of 9 mm.
A new cascaded structure based on two long-period fiber gratings (LPFGs) for simultaneous measurement of strain and temperature is proposed and studied. The proposed structure is cascaded by a LPFG and a S-type long-period fiber grating (S-LPFG). The LPFG is fabricated using the CO
2
laser by etched method. The S-LPFG is fabricated using the CO
2
laser by polished method. The grooves of the S-LPFG are staggered in opposite directions, and this special geometry results in high strain sensitivity. The experimental results show that the strain sensitivities of the two resonant dips of the cascaded structure are 16.31 pm/
and −2.42 pm/
with the range of 0-
, respectively. Strain sensitivity varies greatly between two dips and the resonant wavelength shifts in the opposite direction. The temperature sensitivities of the two resonant dips of the cascaded structure are 59.73 pm/°C and 60.38 pm/°C, respectively. Since the difference between temperature and strain sensitivity, matrix method can be used for simultaneous measurement of two parameters. The feasibility of the cascaded structure has been experimentally demonstrated for the simultaneous measurement. Therefore, the cascaded structure has the potential to measure strain and temperature simultaneously.
We propose a novel method to write a long-period fiber grating in helical polished structure (HPLPFG) as a twist sensor. The helical structure is polished on one side of twisting fiber by CO
2
laser following either right-hand or left-hand helix direction. The sensor can discriminate the twist direction and improve the twist sensitivity, due to its asymmetric and helical refractive index distribution. The twist and temperature characteristics of the sensor have been experimentally investigated. The twist sensitivities of the sensor (right-hand direction helix) are 0.199 nm/(rad/m) and 0.141 nm/(rad/m), and the twist sensitivities of the sensor (left-hand direction helix) are −0.119 nm/(rad/m) and −0.196 nm/(rad/m) in clockwise and anti-clockwise directions, respectively. The temperature sensitivities of the two sensors are 0.060 nm/°C and 0.063 nm/°C, respectively. The novel sensor has practical application potential in twist sensing.
A S-shaped long-period fiber grating (SLPFG) is presented in this paper as a new type of strain sensor with ultra-high strain sensitivity. The sensor is fabricated by using the CO2-laser-polished method. It is illustrated by experiments that the WLFPG has structural stability and repeatability. The strain sensitivity of the proposed sensor reaches 29.3 pm/με in the range of 0–1000 με, and the temperature sensitivity is 76.3 pm/°C in the range of 30–170 °C. Compared to other CO2-laser-induced long-period fiber gratings (CO2-LPFGs), the strain sensitivity of the SLPFG is nearly ten times higher.
A new type of spring-shaped long-period fiber grating (S-S-LPFG) written with CO
2
laser is proposed in this letter. A single-mode fiber is etched clockwise in four directions and any two adjacent etched regions are partially coincident in the vertical direction. The special manufacturing method makes that the S-S-LPFG has satisfactory strain sensitivity. The experimental results prove that the strain sensitivity of the S-S-LPFG is up to 30 pm/με with the range of 0-700 με, which is nearly 18 times higher than that of the normal CO
2
-laser-notched long-period fiber gratings (CO
2
-LPFGs). The temperature sensitivity of the S-S-LPFG is also calculated from the experimental data as 37 pm/°C. It is strongly believed that the S-S-LPFG could be an excellent candidate for the strain sensors.
A novel long-period fiber grating (LPFG), fabricated by periodically cascading a series of screw-type distortions, is proposed and experimentally demonstrated. These screw-type distortions are induced by twisting the fiber during CO2 laser beam exposure. The resulting LPFG will either be left- or right-hand helical, depending on the twist rate and direction used during fabrication, with a certain frozen shear strain. Due to the independence between grating pitch and twist rate, this type of LPFG could be more flexible than the helical- or chiral-fiber gratings reported previously. During LPFG twisting, the device displays good directional dependence and an enhanced torsion sensitivity of 0.1604 nm/(rad/m), which implies the structure could be an excellent candidate for torsion sensors.
We demonstrate the fabrication of long-period fiber gratings (LPFGs) written in the two-mode fiber (TMF) by CO2 laser. Both uniform and tilted LPFGs were fabricated to provide the light coupling between LP01 mode and LP11 mode with a coupling efficiency of more than 99%. The writing efficiency and the bandwidth of the LPFG mode converter can be adjusted by changing the tilt angle of the tilted TMF-LPFGs. The torsion sensitivity of conventional and tilted LPFG mode converters were measured to be 0.37 nm/(rad/m) and 0.50 nm/(rad/m), respectively. Two orthogonal vector modes (the HEeven 21and HEodd 21 modes) and corresponding orbital angular momentum state were successfully obtained at the resonance wavelength. The proposed LPFG mode converter could be used as not only a high efficiency wavelength tunable mode converter in the mode division multiplexing system but also a high sensitive torsion sensor in the field of optical sensing.
A novel optical fiber torsion sensor head is proposed. A section of polarization-maintaining fiber (PMF) is spliced between single mode fiber (SMF), and a twist taper is fabricated by a commercial electric-arc fusion splicer in the middle of the PMF. The asymmetric characteristics are obtained by the twist taper so that a fiber torsion sensor with directional discrimination is fabricated. Due to the characteristics of the asymmetric structure, the torsion sensitivity for the twist rate from 0 rad/m to −8 rad/m reaches 2.392 nm/rad·m⁻¹ , and for the twist rate from 0 rad/m to 8 rad/m reaches 1.071 nm/rad·m⁻¹ respectively.
We demonstrate the fabrication of long-period fiber gratings (LPFGs) coated with high index nano-film using the atomic layer deposition (ALD) technology. Higher index sensitivity can be achieved in the transition region of the coated LPFGs. For the LPFG coated by nano-film with a thickness of 100 nm, the high index sensitivity of 3000 nm/RIU and the expanded index sensitive range are obtained. The grating contrast of the over-coupled LPFGs and conventional LPFGs are measured and the over-coupled gratings are found to have a higher contrast in the transition region. The cladding modes transition is observed experimentally with increasing surrounding index using an infrared camera. The theoretical model of the hybrid modes in four-layer cylindrical waveguide is proposed for numerical simulation. The experimental results are well consistent with theoretical analysis.
A long period fiber grating (LPFG) coated with a combination of cobalt chloride and gelatin is proposed and demonstrated for the monitoring of relative humidity (RH) levels. The thin overlay of the combination and its spectral properties with respect to different ambient humidity levels are observed. Field emission scanning electron microscope and atomic force microscope have provided detailed evidence of attachment of the amalgamation on LPFG surface using adapted coating methodology. The results show a significant shift in the resonance wavelength of the coated LPFG when the RH is varied from 35% to 90%, with a sensitivity of 0.18 nm/% RH and accuracy of 98.55%.
In this Letter, we present a fiber loop mirror configuration based on a suspended twin-core fiber for sensing applications. Using the suspended twin-core fiber, the fringe pattern is due to the differential optical patch of the light in the two cores associated with a refractive index difference of ~ 10 − 3 , which indicates an advantage of this approach compared with those based on high-birefringent fibers, namely, the possibility of using a small length of fiber. The sensing configuration was characterized for torsion, temperature, and strain. Using the fast Fourier transform technique, it is possible to obtain measurand-induced amplitude variations of the fringe pattern. The results obtained indicate the viability of a temperature- and strain-independent torsion sensor.
We present a novel class of highly sensitive sensors based on long-period fiber gratings that can be implemented with simple and inexpensive demodulation schemes. Temperature, strain, and refractive-index resolutions of 0.65 °C, 65.75 μ∈, and 7.69 × 10⁻⁵, respectively, are demonstrated for gratings fabricated in standard telecommunication fibers.
We present a method of helical long-period fiber grating (H-LPFG) fabrication by use of a CO 2 laser for use as an optical torque sensor. A conventional optical fiber grating has periodic vertical index changes along its fiber axis, but a H-LPFG has a screw-type index modulation. The helical index modulation is obtained with the asymmetric index change caused by a single-side laser beam exposure. The H-LPFG shows peak shifts with codirectional or contradirectional torsion to the helix. Also, the polarization-dependent loss is measured to be relatively small compared with that of a conventional long-period fiber grating.
A long-period fiber-grating sensor with a high strain sensitivity of − 7.6 pm ∕ μ ε and a low temperature sensitivity of 3.91 pm ∕ ° C is fabricated by use of focused CO 2 laser beam to carve periodic grooves on a large- mode-area photonic crystal fiber. Such a strain sensor can effectively reduce the cross-sensitivity between strain and temperature, and the temperature-induced strain error obtained is only 0.5 μ ε ∕ ° C without using temperature compensation.
Long-period gratings (LPG’s) written in commercially available boron-codoped fibers operating at wavelengths of < 1.1 µ m are shown to exhibit high temperature and bending sensitivities. Each resonant attenuation band of such LPG’s was observed to split in two when the LPG’s were bent. The split attenuation bands’ separation increased significantly with increasing bend curvature, and the central wavelengths of the split bands provided a measure of temperature. We exploit this effect to allow for simultaneous measurement of temperature and bending in smart-structure applications. The demonstrated novel sensor system is simple and low cost.
We present a novel corrugated long-period fiber grating whose transmission spectra are highly sensitive to the applied tensile strain, torsion, and bending due to the periodical index modulation created and changed by these mechanic forces. The induced index modulation can also be experimentally characterized by using a built-in fiber Bragg grating (FBG). The long period fiber gratings possess the following unique properties when used as sensors. As a tensile strain sensor, its resonance loss varies but resonance wavelength remains stable. As a torsion sensor, the wavelength varies with the applied twist rate. As a bending sensor, the cladding-mode resonance grows with the bending curvature
A novel fiber sensor which enables to simultaneously measure the torsion, strain and temperature is proposed and experimentally demonstrated, which is based on utilization of two successively-cascaded helical long-period fiber gratings (TSCHLPGs). By using the proposed approach, crosstalk effects among temperature, strain and torsion can totally be discriminated for the first time to the best of our knowledge. Moreover, not only the torsion angle, but also the torsion direction can be determined simultaneously. Accuracies for the resolved temperature, the strain and the torsion angle are estimated to be ~0.8 °C within the range of 20-100 °C, ~225 με within the range of 0-2600 με and ~1.2 degree (corresponding to a torsion responsivity of 3.54 nm/rad/m in our case) within the rotation angle of -360° ~ 360°, respectively.
We demonstrate the fabrication of the helical long-period grating (HLPG) in a two-mode fiber (TMF) with CO2 laser. The torsion characteristics of the helical gratings are experimentally investigated. It was found that the resonance wavelength of the TMF-HLPG linearly shifts with a twist sensitivity of 0.47 nm/(rad/m), which is an order magnitude higher than that of the conventional long-period fiber gratings (LPFGs). The temperature sensitivity of the TMF-HPFG was measured to be 23.9 pm/°C, which is one third of that of the conventional LPFGs. The proposed TMF-HLPG-based twist sensor would have a promising application for the twist monitor.
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 magnetic sensor based on a hybrid long-period fiber grating (HLPFG) and an ester-based Fe3O4 magnetic fluid (MF) has been proposed and experimentally investigated. The Fe3O4 MF and HLPFG are encapsulated in a capillary tube using paraffin. The sensor is sensitive to a vertical magnetic field and environmental temperature, while it is not very sensitive to an axial magnetic field. Due to the hybrid structure of HLPFG, the transmission spectrum of the sensor has two groups of resonance dips that have different sensitivities to vertical magnetic field intensity and temperature. This sensor can be used for the simultaneous measurement of temperature and vertical magnetic field intensity, which is desirable to resolve the temperature cross sensitivity issue. The sensor is also advantageous in its ease of fabrication, compactness, and wide linear measurement range.
A novel long period fiber grating (LPFG) is inscribed in few mode fibers by CO2-laser-irradiation method. The experiment results and theoretical calculation demonstrate that the LPFGs are originated from the strong coupling between the fundamental core mode LP01 and the high-order core mode LP11. The resonant wavelength is found to shift toward the lower wavelength with temperature or force increasing, which is quite different from LPFGs fabricated in common single mode fibers. The gratings are quite sensitive to strain and slightly insensitive to temperature. Such structures are easy to be fabricate and can be promising candidates as strain sensors.
The rejection band of a long-period fiber grating written in a heavily twisted single-mode fiber by a CO2 laser can split into two, when the twist applied to the fiber is removed after the writing of the grating. We attribute the wavelength-splitting effect to the generation of a rotary frozen-in torsion strain along the fiber in the writing process. The wavelength split increases with the twist rate and the effect is independent of the polarization state of light. We present a simple expression to estimate the wavelength split, which agrees reasonably well with the experimental results. We also measure the temperature and torsion characteristics of the grating. Such a grating could find applications as an optical filter or a temperature-insensitive torsion sensor.
A fiber-optic sensor for torsion measurement, based on a two-linearly polarized (LP)-mode operation in ultrahigh birefringent photonic crystal fiber is described. The structure of the photonic crystal fiber presents two large asymmetric holes adjacent to the core fiber. When linearly polarized light is injected in x - and y -directions, respectively, two separate interferometers can be obtained. In one of these cases, as torsion is applied to the sensing head a beat between the two interferometers is formed due to the simultaneous excitation of the two polarization states. The detection technique to read the torsion sensor is based on the analysis of the fast Fourier transform, which proved to be an effective and simple solution. The sensor exhibited reduced sensitivity to temperature.
We report on enhanced torsion sensitivity by using a highly birefringent photonic crystal fiber (HB-PCF)-based Sagnac interferometer. In order to increase the torsion sensitivity, we introduced an anisotropic microstructure into the cross section of an HB-PCF by enlarging the size of air holes of one row. This can result in a high birefringence of the order of 10-3 and low sensitivities to bending and temperature. The torsion sensitivity was measured to be high with ~0.06 nm/°.
In this paper, we report a novel long-period fiber grating (LPFG) fabricated by using a new writing technique that is mainly based on the thermal shock effect of focused high-frequency CO2 laser pulses at several kilohertz. A number of unique characteristics of such a LPFG, such as bend, torsion, and transverse load, are observed by experiments, for the first time, to our knowledge. Based on these unique features, a novel bend-insensitive LPFG sensor that could solve the problem of cross-sensitivity between bend and other measurands, a novel torsion sensor that can realize absolute measurement of twist rate, and a load sensor that can achieve simultaneous measurement of transverse load and temperature using a single LPFG element are proposed and demonstrated. These unique features of the LPFGs are mainly due to the asymmetrical distribution of the refractive index on the cross section of the LPFG induced by high-frequency CO2 laser pulses.
We present a detailed investigation into the sensitivity of long-period fiber gratings (LPFGs) as a function of temperature, strain, and surrounding refractive index, with particular attention to the higher order cladding modes and the possibilities for ultrasensitive sensors. From a general theoretical analysis, we identify a general sensitivity factor which offers new physical insight into LPFG behavior and represents a useful design aid in conjunction with a set of measurand-specific sensitivity factors. Our analysis reveals the existence of turning points in the mode dispersion characteristics at which ultrasensitive operation may be obtained. In an extensive set of coordinated experiments, we verify the theoretical predictions with close agreement and provide demonstrations of the device behavior close to the turning points. Alternative sensor schemes for temperature, strain, and refractive index based, respectively, on measurement of the dual resonance characteristic of the modes and on the transmission characteristics close to the turning points, utilizing higher order modes of the LPFG, are presented. For two variables at least, we record the highest LPFG sensitivities yet reported