Fiber Torsion Sensor Research Articles

Using machine learning to enlarge the measurement range and promote the compactness of the optical fiber torsion sensor based on the Sagnac interferometer

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.

Artificial-neural-network-assisted distributed directional optical fiber torsion sensor with the SSAF-based Sagnac interferometer

Artificial-neural-network-assisted distributed directional optical fiber torsion sensor with the SSAF-based Sagnac interferometer

Highly sensitive torsion sensor based on helical eccentric dual-core fiber Michelson interferometer

An optical fiber torsion sensor was academically put forward and experimentally demonstrated by applying a Michelson interferometer (MI) based on a helical eccentric dual core fiber (HEDCF). The sensor consists of an eccentric dual core fiber (EDCF), a taper, and a helical structure based on the EDCF. Since there is a large interval between a centric core and an eccentric core of the EDCF, the helical structure can greatly improve the torsion sensitivity of the sensor. When the twisting direction is consistent with the pre-twisting direction, the optical path difference (OPD) of MI negative increases during the twisting process, and the spectral resonance wavelength shifts blue. Experimental results show that the greater the degree of helical addition, the higher the torsional sensitivity. The torsion sensitivity can reach as high as −7.01 nm/(rad/m) in clockwise (CW) direction in the range of 0–9.595 rad/m. Moreover, the satisfactory sensitivity, stable structure and small temperature crosstalk, let it a valuable select in torsion sensing domain, and is promising to be useful for broad applications across engineering fields.

A highly sensitive torsion sensor based on symmetrically polished SMS fiber structure

We propose and experimentally demonstrate an optical fiber torsion sensor based on symmetrically polished single-multi-single mode (SP-SMS) fiber structure. The polished structure enhances the torsion response effectively, which is fabricated by a high-frequency CO2 laser. Experimental results show that the high sensitivity of the proposed torsion sensor reaches 1.032 nm/(rad m−1) in the range from − 10.261 rad/m to 10.261 rad/m. Meanwhile, the temperature response of the sensor is also investigated, which is 21 pm/°C in the range from 30 °C to 150 °C. Moreover, the structure has the features of low cost, simple fabricating method and small temperature crosstalk.

Seven-core fiber torsion sensor with microcavity structure based on intensity measurement

A seven-core fiber torsion sensor with microcavity structure based on intensity measurement is proposed. It is composed of a seven-core fiber (SCF) and two multi-mode fibers (MMF). The first end of the SCF has a microcavity structure. Because the mode fields between the MMF and SCF do not match at the splicing point, the phenomenon of mode interference will be formed between cladding modes and core modes. Experimental results show that in a clockwise situation, the intensity of transmission spectrum increases in the torsion range of 0∼70°, the torsion sensitivity is 0.098 dB/∘. And in a counterclockwise situation, the intensity of transmission spectrum decreases, the torsion sensitivity is 0.103 dB/∘. The sensor has a simple structure and easy to fabricate, so it has wide application prospects in the field of torsion sensing.

High sensitivity directional torsion sensor based on double-helix long-period grating in tapered polarization-maintaining fiber

We propose a fiber torsion sensor based on double-helix long-period grating (DH-LPG) 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.

Twist-Direction-Discriminable Torsion Sensor Using Long-Period Fiber Grating Inscribed on Polarization-Maintaining Photonic Crystal Fiber

Here we propose and experimentally demonstrate an optical fiber torsion sensor with twist direction discrimination capability by incorporating a long-period fiber grating (LPFG) inscribed on polarization-maintaining photonic crystal fiber (PMPCF) by CO2 laser pulses. An LPFG formed on PMPCF has an input-polarization-dependent resonance dip, and two separated resonance dips, designated as the shorter wavelength-dip (SWD) and longer wavelength-dip (LWD), are obtained with respect to orthogonal input polarization. The torsion response of the sensor head was investigated in a wide twist angle range of −220° to 220° for the two resonance dips. Highly linear torsion responses of the SWD and LWD could be obtained with adjusted ${R}^{{2}}$ values of ~0.9960 and ~0.9967, respectively, in a twist angle range from −160° to 160°, when their transmittance levels measured in a logarithmic scale were converted into a linear scale. The torsion sensitivities of the SWD and LWD were measured as −5.067 and 4.830 %/rad in this twist angle range of high linearity, respectively. In particular, its temperature response was also examined in a temperature range from 35 to 85 °C. With pre-torsion applied to the sensor head at twist angles of 220° and −220°, the temperature sensitivities of the SWD and LWD were measured as ~3.64 and ~9.75 pm/°C in terms of the dip wavelength and $-{2.87}\times {10}^{-{3}}$ and ${5.45}\times {10}^{-{5}}$ dB/°C in terms of the dip transmittance, respectively. The experimental results corroborate that our sensor can perform virtually temperature-insensitive torsion measurement with twist direction discrimination capability and good linearity in a wide measurement range.

The theoretical investigation of the proposed optical fiber torsion sensor based on computer-generated-hologram (CGH)

Optical fiber torsion sensors have significant importance for structural health monitoring. In this paper, an optical fiber torsion sensor based on computer-generated-hologram (CGH) is proposed and investigated theoretically. In the proposed structure a dual mode fiber (DMF) is used as a sensing element. An acoustic source can be employed to induce a fiber grating. The acoustic induced fiber grating (AIFG) causes the fundamental LP01 mode of DMF couples to LP11o and LP11e fiber modes. In the presence of torsion in DMF, the profiles of LP01, LP11o and LP11e modes rotate relative to the absence of fiber torsion. At the end of DMF a suitable CGH can be employed to resolve spatially the contribution of each mode and calculate the fiber twist angle. The proposed optical fiber torsion sensor shows the capability of multi event torsion measurement in the interval [0,π∕2) with the resolution of 0.5 milliradian.

Torsion bidirectional sensor based on tilted-arc long-period fiber grating.

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.

Two-mode fiber based directional torsion sensor with intensity modulation and 0° turning point.

In this paper, we report a novel in-fiber Mach-Zehnder interferometer based directional torsion sensor, in which a section of two-mode fiber is sandwiched between two single mode fibers by over core-offset splicing technique. The variety of fringe visibility demonstrates the strong dependence on offset and fiber length. For the first time the near zero visibility at 0° rotating state is obtained by fine offset-modulation. The experimental results show that, with 0° turning point, the counter-clockwise to the clockwise direction can be recognized by the reversal from peak to dip of fringes. Moreover, a competitive sensitivity of 21.485 dB/(rad/cm) is gained with high linearity and low-temperature crosstalk in the range from -40 rad/m to 40 rad/m. Without any pre-twisting, our fiber torsion sensor is small size, ease of fabrication, cost efficiency and very potential in the applications of industrial and artificial intelligence.

Highly sensitive torsion sensor with femtosecond laser-induced low birefringence single-mode fiber based Sagnac interferometer.

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.

Enhanced selfintegration algorithm for fiber torsion sensor based acoustically-induced fiber grating

Mechanical parameter monitoring based on optical mode detection benefits from its low cross sensitivity and inexpensive instrument. The key to improving detection accuracy is to generate high-quality detection light and use efficient algorithms. We present a strain-independent torsion sensor based on acoustically-induced fiber grating (AIFG) in the dual-mode fiber (DMF) and use the enhanced self-integration algorithm to improve the sensing accuracy. By tuning the radio frequency of driving signal, the LP11 mode generated by the AIFG can be exploited to measure the dynamic torsion variations. Without the complex device such as fiber interferometers and photonic crystal fibers (PCFs), the simple structure built by mode converter and charge coupled device (CCD) can track the dynamic variations and has less cross sensitivity of strain along the transmission direction. The AIFG driven by a radio frequency as a mode converter at specific wavelength does not participate in sensing but generates the high-purity LP11 mode that accounts for more than 90% of total power. With the twist from the rotator stage, the DMF keeps rotating and CCD records the spatial distribution of mode profiles. The features of optical mode is enhanced based on matrix analysis and then the relationship between twist angle and mode features is obtained. Based on image processing, the dynamic variation of spatial beam detected by CCD can be easily tracked and quantified. In experiment, the rotation angle can be obtained by calculating the feature value of the optical mode. Our image detection algorithm is specially designed for the optical fiber mode. Compared with traditional image recognition based on feature learning, it is simple and fast because it is needless to use image segmentation and stochastic processing. Through a series of experiments on angle rotation and parallel strain, we verify the correctness of the enhanced self-integration model and analyse the computational uncertainties that influence the stability of experiment. In the 0 to 180 measurement range, the maximum range of measurement error is less than 11%. When the axial strain is between 100 and 1500 , the sensor is strain-independent. Thus, it is verified that the torsion sensor based on AIFG has high sensitivity and can overcome the cross sensitivity of strain along a certain direction. The pertinent results have significant guidance in designing the multi-parameter sensor. The optical mode detection, instead of the traditional spectrum measurement, enables the whole structure to have the potential to be rebuilt by inexpensive devices that work in visible wavelengths.

All fiber torsion and displacement sensor based on image detection

We propose an all-fiber-optic sensor for simultaneous rotation and displacement measurement based on elastic-optic effect in a dual mode fiber. LP01 and LP11 modes are both excited in the fiber and are distinguished using wavelength-scanning Spatially and Spectrally resolved imaging technique (S2 Imaging) to accomplish rotation and displacement detection. Mode rotation is characterized by the LP11 mode rotation using elastic-optic theory; a displacement area of 120×120μm is detected and measured by recording the trace of LP01 mode pattern. Balance between the detection range and resolution of the sensor is adjustable by simply changing the fiber length or adjusting the relative position of fiber-end, lens and the CCD camera. Real-time detection at ∼0.5s per measurement speed is achieved, and twist rate and moving velocity can also be measured.

Strain-independent fiber torsion and displacement sensor based on acoustically-induced fiber grating

We present a strain-independent torsion and displacement fiber sensor using acoustically-induced fiber grating (AIFG) in dual-mode fiber (DMF). By tuning the radio frequency of driving signal, LP01 and LP11 modes generated by the AIFG can be exploited to measure the dynamic displacement and torsion variations, respectively. Both the twist angle and the twist direction can be monitored via image detection facility at the end of DMF. Between torsion angles of −80° and 80°, the highest twist sensitivity reaches 15pixel/°. The average displacement sensitivity is 5pixel/µm within the recorded two-dimensional movement of 100um×80um. The stable property of sensor is verified when the strain is varied from 100με to 1500με.

Intensity modulated torsion sensor based on optical fiber reflective Lyot filter.

We proposed and experimentally demonstrated a highly sensitive optical fiber torsion sensor based on a reflective Lyot filter for the first time to our knowledge. The reflective Lyot filter is constructed by inserting a section of polarization-maintaining fiber (PMF) between a fiber linear polarizer and a 3dB coupler based fiber loop reflector. Based on the intensity modulation, the proposed torsion sensor exhibits a high torsion sensitivity of up to 20.336 dB/rad, one order of magnitude higher than the achieved in state-of-the-art. In contrast, the temperature cross-sensitivity and strain cross-sensitivity of the proposed torsion sensor are low to -2.0 × 10-4 rad/°C and -6.39 × 10-6 rad/με, respectively, thus overcoming the cross-sensitivity problem resulting from temperature and strain. Moreover, we perform the theoretical simulation of the proposed torsion sensor, and the simulation result obtained agrees well with the experiment results, vividly confirming the viability of the fiber Lyot filter based torsion sensor. Such fiber Lyot filter may find potential applications of highly sensitive torsion sensors in the field of modern smart structure monitoring.

Ultra-compact strain- and temperature-insensitive torsion sensor based on a line-by-line inscribed phase-shifted FBG.

A novel temperature- and strain-independent optical fiber torsion sensor based on a phase-shifted fiber Bragg grating (PSFBG) inscribed by the line-by-line (LbL) technique in a standard single-mode fiber with a femtosecond laser has been proposed and experimentally demonstrated. The strong birefringence created by the LbL inscription technique leads to the significant polarization splitting of the transmission peak of the PSFBG. By simply monitoring the variation of the amplitude difference between the two polarization-peaks, the fiber torsion angle and the fiber torsion direction can be simultaneously deduced without temperature and strain confusion. The torsion sensor exhibits a high torsion sensitivity of up to -1032.71 dB/(rad/mm), with the distinct advantages of low manufacture cost, small dimension (just ~1.72mm), and extremely robust and simple structure, which make it very attractive for practical applications. To the best of our knowledge, this is the smallest torsion sensor ever reported.

Fiber torsion sensor based on a twist taper in polarization-maintaining fiber.

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.

Fiber optic stress-independent helical torsion sensor.

Femtosecond laser-fabricated waveguides have been formed into helical paths throughout the cladding of single-mode optical fibers to demonstrate a strain-independent fiber torsion sensor. A comparison between a Bragg grating sensor and a Mach-Zehnder based on helical waveguides (HWs) showed a much weaker twist sensitivity of 1.5 pm/(rad/m) for the grating in contrast with a value of 261 pm/(rad/m) for the interferometer. The HW geometry provided an unambiguous determination of the rotational direction of the twist while facilitating a convenient and efficient means for optical coupling into the single-mode core of the fiber. The flexible three-dimensional writing by the femtosecond laser fabrication method enabled the direct inscription of compact and robust optical cladding devices without the need for combining or splicing multiple-fiber segments.

In-fiber torsion sensor based on dual polarized Mach-Zehnder interference.

This paper presents a novel optical fiber torsion sensor based on dual polarized Mach-Zehnder interference (DPMZI). Unlike the conventional fiber sensor, the proposed sensor is composed of a sensor part and a demodulator. The demodulator is made by a bared single mode fiber (SMF) loop, and the sensor part is a segment of a coated SMF placed before the loop. A mathematical model is proposed based on DPMZI mechanism and from the model when the sensor part is twisted, the E-field rotational angle will bring a quasi-linear impact on the resonance dip wavelength in their matched detecting range. A proof-of-concept experiment was performed to verify the theoretical prediction. From the experimental data, a sensitivity of -0.3703, -1.00962, and -0.59881 nm•m/rad is achieved with the determining range of 12.0936, 7.6959, and 10.4444 rad/m respectively. The sensor which is composed only of the SMF has the advantages of low insertion loss (~-2dB), healthy structure, low manufacture cost, and easy assembly and application.

Optical Fiber Torsion Sensor with Mechanically Induced Long Period Fiber Gratings in Rare-Earth Doped Fibers

In this work wavelength sensitivity in mechanically induced long period fiber gratings (MLPFG) is analyzed. This analysis is first carried out both in standard single-mode fiber SMF-28 and in Er-doped fibers. The mechanical analysis for both types of fibers under different torsion conditions is presented. In order to apply the torsion one of the fiber ends is fixed while torsion is applied on the other end. A MLPFG whose period is 503 μm is used to press the fiber after torsion is applied. This allows for micro curvatures to be formed on the fiber, which in turn generates a periodical index perturbation on it. Here, it was noted that the sensitive wavelength shift of the rejection bands is bigger for Er-doped fibers. For a torsion of 6 turns applied to 10 cm of doped fiber the wavelength peaks can be moved up to 25 nm, which is longer to what was detected on standard fibers. Therefore, by using Er-doped fibers to monitor torsion on structures will give more sensitive and accurate results than using standard fibers. These results can be employed for sensing applications, especially for small to medium size structures, which can be mechanical, civil or aeronautics.