Twist Sensitivity Research Articles

Temperature and Twist Sensor Based on the Sagnac Interferometer with Long-Period Grating in Polarization-Maintaining Fiber.

We utilized a CO2 laser to carve long-period fiber gratings (LPFGs) on polarization-maintaining fibers (PMFs) along the fast and slow axes. Based on the spectra of LPFGs written along two different directions, we found that when LPFG was written along the fast axis, the spectrum had lower insertion loss and fewer side lobes. We investigated the temperature and twist characteristics of the embedded structure of the LPFG and Sagnac loop and ultimately obtained a temperature sensitivity of -0.295 nm/°C and a twist sensitivity of 0.87 nm/(rad/m) for the LPFG. Compared to the single LPFG, the embedded structure of the LPFG and Sagnac loop demonstrates a significant improvement in temperature and twist sensitivities. Additionally, it also possesses the capability to discern the direction of the twist. The embedded structure displays numerous advantages, including easy fabrication, low cost, good robustness, a wide range, and high sensitivity. These features make it highly suitable for applications in structural health monitoring and other related fields.

Three-core fiber-based temperature-insensitive twist sensor

We have experimentally proposed an all-fiber temperature-insensitive twist sensor, which is fabricated by splicing a three-core fiber (TCF) section and two parts of single-mode fibers (SMFs). Because of the special structure of TCF, the fundamental core mode and cladding mode can interfere with each other. The experimental results show that the maximum twist sensitivity is −0.250 nm/(rad/m) in the range of 0–50 rad/m, and 0.109 nm/(rad/m) in the range of −50–0 rad/m. The sensor has features of all-fiber structure, low-cost, easy-fabricated, and temperature insensitive, which makes it a competitive candidate for harsh environment sensing application.

High sensitivity twist sensor based on suspended core fiber Sagnac interferometer with temperature calibration.

A high sensitivity optical fiber twist sensor based on Suspend Core Fiber Sagnac Interference (SCFSI) is proposed and experimentally demonstrated. By filling the air hole of the Suspend Core Fiber (SCF) with alcohol, the twist sensitivity of the twist sensor is greatly improved to 8.37 nm/°. Moreover, the valid angle measurement range of the sensor can be expanded by utilizing the combination of intensity demodulation and wavelength demodulation. The sensor not only has high twist angle sensitivity but also exhibits a capability of temperature calibration. Since the wavelength shifts of the interference fringes of Mach-Zehnder Interferometer (MZI) formed in the suspend core of SCF appears insensitive to twist angle, the parasitic interference formed by MZI can be used for temperature calibration. Due to compact structure, easy fabrication and low temperature cross sensitivity, the proposed sensor has a great potential for structural health monitoring, such as buildings, towers, bridges, and many other infrastructures.

All-Fiber In-Line Twist Sensor Based on a Capillary Optical Fiber

Twist sensors have emerged as crucial tools in the field of structural health monitoring, playing a significant role in monitoring and ensuring the integrity of critical infrastructure such as dams, tunnels, bridges, pipelines, and buildings. We proposed and demonstrated an all-fiber in-line twist sensor which was based on a capillary fiber spliced between two single-mode fibers with a transverse offset. Through a series of experiments, the sensor’s performance was evaluated and quantified. The results showcased remarkable twist sensitivities in both clockwise and anticlockwise directions. With a transverse offset of 8.0 µm, the sensor exhibited twist sensitivities of −0.077 dB/° and 0.043 dB/° in the clockwise and anticlockwise directions, respectively, in the measured twist range from 0 to 90°. Furthermore, it was also demonstrated that the sensor was temperature insensitive at the chosen wavelength of 1520 nm, which can assist in increasing measurement accuracy. Our sensor’s low cost, simplicity of manufacture, and improved performance will push forward its adoption in future engineering applications such as structural health monitoring in dams, tunnels, and buildings.

An ultrasensitive twist sensor based on parallel dual HMSF-SIs with Vernier effect

Sensitivity has always been an important parameter for evaluating a sensor. In pursuit of high-sensitivity sensing, a highly sensitive helical microstructured fiber (HMSF) directional twist sensor based on parallel dual Sagnac interference (SI) rings with the Vernier effect has been first proposed in this paper. The Vernier effect has been realized by superimposing the transmission spectra of dual HMSF-SIs, which have slightly different free spectral ranges (FSR) between the reference and sensing arm. The FSR difference is realized by selecting different pitches of HMSF. Different pitches produce different birefringence, so the FSRs of the reference and sensing arm have a slight difference. The influences of structural parameters of the HMSF on the sensor’s performances are studied by using the finite element method (FEM). The twist sensitivity of parallel HMSF-SIs is significantly enlarged because of the Vernier effect. According to numerical simulation results, the highest twist sensitivity can be up to 3874.34 pm/(rad/m), which is orders of magnitude higher than previous studies. The proposed twist sensor can accurately identify the torsional direction and has a good linear torsional response. The novel sensor has practical application potential in twist detection.

Research on performance of fiber twist sensors based on reflective microfiber couplers

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.

Highly sensitive twist and vibration sensing based on polarization-sensitive mode selective coupler

We propose and demonstrate a highly sensitive optical fiber sensor for vibration and twist based on a polarization-sensitive effect of mode selective coupler (MSC). MSC is prepared by melt coupling. Based on the polarization sensitivity, high sensitivity testing of twist and vibration is realized. The proposed MSC can be used for twist sensing with different demodulation methods at the same time, including intensity of spectra and modes. The twist sensitivity is –10.383 a.u./deg ( ∼ −0.02 dB / deg). For vibration sensing, the MSC achieves ultrabroadband sensing frequency response span from infrasound (Hz) to ultrasound (MHz) with good fidelity and high linearity of ≈ 1. The signal-to-noise ratio of single- and dual-frequency vibration detection is as high as ∼108 dB. In addition, the MSC can realize the detection of damped vibration signals steadily. The twist and vibration sensor based on MSC have the advantages of high sensitivity, wide frequency response range, good linearity, high stability, high fidelity, and compact structure. It has potential application prospects in the fields of oil field leakage detection, coal mine, and other structural safety monitoring.

Fabrication and Characterization of Femtosecond Laser Inscribed Long-Period Fiber Grating in Few-Mode Fiber

We experimentally investigated the fabrication and characterization of long-period fiber grating in few-mode fiber (FMF-LPFGs) by using the femtosecond laser direct writing technique. Thanks to the high flexibility and repeatability of this technique, stable FMF-LPFGs fabrication technology has been developed. In addition, the properties of FMF-LPFGs have also been characterized. The coupling of LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> - LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> - cladding mode LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> were investigated by both the transmission spectrum and the mode patterns. The temperature, strain and twist sensitivities of the resonance dips were calibrated by experiments, and the sensitivity difference between the dips was explained. By using their disparate responses to external temperature and strain, a cross coefficient matrix was established and dual-parameter discrimination determination is realized. This work provides a thoroughly theoretical and experimental investigation on the fabrication and characterization of FMF-LPFGs based on femtosecond laser direct writing technique. It is believed that the flexible and efficient LPFGs fabrication technique will benefit a lot in the future for developing various functional specialty optical fiber devices and sensors.

Silver-Coated 45° Radiated Tilted Fiber Grating Based Interferometer and Its Sensing Applications

We proposed a silver-coated 45° radiated tilted fiber grating (45° RTFG) based interferometer, in which the 45° RTFG is working as a beam splitter and a coupler, and the light transmitting in the fiber core is radiated out and resonant in the cladding radial cavity formed by the fully coated silver film, which forms multiple beam interference. We have theoretically and experimentally investigated the proposed fiber interferometer in terms of polarization-dependent transmission spectra, the free spectral range (FSR), spectral visibility and sensing characteristics. Due to the single polarization radiation feature of 45° RTFG, only S-polarized light could form the multiple beam interference and P-polarized light would pass through the grating with low loss. And the FSR of the proposed fiber interferometer is around 6.8 nm, which is determined by the 125 μm cladding diameter. The theoretical results show that the spectral visibility of interference is proportional to the coupling efficient of 45° RTFG and when the coupling efficient reaches 0.92, the spectral visibility is the maximum. In the experiment, the fiber interferometer formed by the 45° RTFG with 11 dB polarization extinction ratio has the spectral visibility of 26 dB. Moreover, we have also investigated the temperature, strain and twist characteristics of the proposed interferometer. The experimental results show that the transmission peak has a red shift with increasing temperature, with a sensitivity of 10 ± 0.1 pm/°C; The strain causes a blue shift for the transmission peak with −0.42 ± 0.01 pm/μϵ sensitivity, and the twist sensitivity reaches 22.408 ± 0.2 dB/rad. The proposed silver-coated 45° RTFG based interferometer has an in-fiber and compact structure, which can be potentially used for optical fiber sensing.

Strain-, curvature- and twist-independent temperature sensor based on a small air core hollow core fiber structure.

Cross-sensitivity (crosstalk) to multiple parameters is a serious but common issue for most sensors and can significantly decrease the usefulness and detection accuracy of sensors. In this work, a high sensitivity temperature sensor based on a small air core (10 µm) hollow core fiber (SACHCF) structure is proposed. Co-excitation of both anti-resonant reflecting optical waveguide (ARROW) and Mach-Zehnder interferometer (MZI) guiding mechanisms in transmission are demonstrated. It is found that the strain sensitivity of the proposed SACHCF structure is decreased over one order of magnitude when a double phase condition (destructive condition of MZI and resonant condition of ARROW) is satisfied. In addition, due to its compact size and a symmetrical configuration, the SACHCF structure shows ultra-low sensitivity to curvature and twist. Experimentally, a high temperature sensitivity of 31.6 pm/°C, an ultra-low strain sensitivity of -0.01pm/µε, a curvature sensitivity of 18.25 pm/m-1, and a twist sensitivity of -22.55 pm/(rad/m) were demonstrated. The corresponding temperature cross sensitivities to strain, curvature and twist are calculated to be -0.00032 °C/µε, 0.58 °C/m-1 and 0.71 °C/(rad/m), respectively. The above cross sensitivities are one to two orders of magnitude lower than that of previously reported optical fiber temperature sensors. The proposed sensor shows a great potential to be used as a temperature sensor in practical applications where influence of multiple environmental parameters cannot be eliminated.

The study of second-order coupling of cladding modes of helical long-period gratings inscribed by commercial welding machine

A presentation based on the helical long-period grating (H-LPFG) affected by the change of temperature and twist has been demonstrated. There are two resonant peaks near 1451 nm (resonant peak 1) and 1519 nm (resonant peak 2) when the pitch length is about 757 μm. The resonant peak 1 is caused by second-order diffraction coupling between the fundamental mode and the LP110 cladding mode, and the resonant peak 2 is caused by first-order diffraction coupling between the fundamental mode and the LP15 cladding mode. Their temperature sensitivities are 72 pm/℃ and 45 pm/℃, respectively. The resonant peak 1 twist sensitivities are − 138 nm·mm/rad with co-direction twist and 133 nm·mm/rad with contrary-direction twist, but the resonant peak 2 is insensitive to twist. This indicates that at the resonant peak 2, the H-LPFG can measure temperature without cross-impact by twist. Therefore, this can be applied to measure temperature and twist at the same time.

Compact vector twist sensor using a small period long period fiber grating inscribed with femtosecond laser

We propose and demonstrate a sensitive vector twist sensor based on a small period long period fiber grating (SP-LPFG) fabricated with a femtosecond (fs) laser. The fabricated SP-LPFG is compact in size (2.8 mm) and shows strong polarization dependent peaks in its transmission spectrum due to the vectorial behavior of high-order cladding modes. Twist sensing is realized by monitoring the polarization dependent peaks, since the polarization of input light changes with fiber twist. The proposed sensor can be interrogated by the peak intensity and wavelength, with high twist sensitivity that reaches 0.257 dB/deg and 0.115 nm/deg, respectively.

Two semicircular-hole fiber in a Sagnac loop for simultaneous discrimination of torsion, strain and temperature.

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.

Mode Splitting in ITO-Nanocoated Tilted Fiber Bragg Gratings for Vector Twist Measurement

We proposed and demonstrated a method to improve the birefringence of tilted fiber Bragg gratings (TFBGs) via high-refractive-index indium tin oxide (ITO) coatings. Firstly, we find the mode splitting phenomenon in an ITO film-coated TFBG. Numerical results show that the ITO coating can modulate the wavelength separations between two adjacent p- and s-polarized resonances. A cyclic dependency between the resonance separation and the thickness of the ITO film has been discovered. With proper film thickness selection, two groups of well-separated orthogonally polarized resonances can be achieved. Then we investigated the twist angle sensing capability of such a sensor with an optimal film thickness of 270 nm. The experimental results show that the intensity of the split resonances present reverse responses to applied torsion, which can be used for vector twist measurement. By monitoring the peak-to-peak differential intensity of these two orthogonally polarized resonances, the proposed sensor shows a twist sensitivity of ~0.27 dB/deg and a linear response of 99.8% for the torsion angle range from -45° to 45°, together with the capability of orientation recognization. We believe that this configuration with improved birefringence opens research directions for vector physical parameter measurements such as vibration, bending, and pressure.

A Novel Twist Sensor Based on Long-Period Fiber Grating Written in Side-Helical Polished Structure

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 CO2 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.

Low-cost temperature- and strain-insensitive twist sensor based on a hybrid fiber grating structure.

We proposed and experimentally demonstrated a temperature- and strain-insensitive twist sensor based on a hybrid fiber grating structure, in which the hybrid grating structure is constructed with a 45°-tilted fiber grating and a chirped fiber Bragg grating UV-inscribed in a single-mode fiber in series. The sensing performance has been evaluated by experimental and numerical analyses, which are in good consistency. The experimental results show that the hybrid-grating-structure-based twist sensor has a maximum twist sensitivity up to 15.037dB/rad. Moreover, due to the invariability of the fiber birefringence and the state of polarization of the input light, such sensor has intrinsically low temperature and strain sensitivities of 7.86×10-3 dB/°C and 6.7×10-5 dB/με, corresponding to the maximum twist measurement error resulting from temperature and strain of 5.2×10-4 rad/°C and 4.5×10-6 rad/με, respectively.

A new ultra long-period fiber grating for measuring torsional characteristics

ABSTRACTIn this paper, we proposed a novel kind of periodic helical ultra-long period fiber grating (U-LPFG) for measuring the torsion rate and distinguishing the torsion direction. The sensor was fabricated by spirally deforming a common fiber by local heating for 4 periods. This method of fabrication was not only easy to implement, but also further increased the sensitivity of the grating to the torsion because of the spiral structure. This paper introduced the torsional characteristics of this sensor in detail and drew some relevant conclusions. Experimental results showed that the twist sensitivity of 0.085 nm/(rad/m). Meanwhile, the temperature sensing characteristic of the proposed U-LPFG was also tested as 68 pm/°C within the range of 30 °C ∼210 °C. Therefore, this proposed sensor could achieve good results in simultaneously measuring the torsion rate and distinguishing the torsion direction, and it may have great potential in some practical applications.

Fiber laser twist sensor with hybrid few-mode tilt Bragg grating and few-mode long period grating

A twist sensor with hybrid few-mode tilted fiber Bragg grating (FM-TFBG) and few-mode long period grating (FM-LPG) in fiber laser cavity is demonstrated. The FM-LPG is utilized to excite LP11 core mode. The FM-TFBG is used for sensing. The transverse modes at 1 553.9 nm and 1 550.5 nm are LP01 and LP21 core modes, respectively, which are coupled from forward-propagating LP11 core mode. These two excitation wavelengths have opposite variation tendencies, which participate in sensing. The twist sensitivity of 0.16 dB/° from -40° to 40° is achieved. The proposed sensor has potentially used for structure monitoring in many areas.

Strain-insensitive twist and temperature sensor based on seven-core fiber

We propose a fiber-optic sensor for simultaneous measurement of twist and temperature using a Mach-Zehnder interferometer by sandwiching a segment of seven-core fiber (SCF) between two sections of very short multimode fibers via fusion splicing technique. The cladding diameter and the pitch between adjacent cores of the SCF are measured to be 150 and 41.5 μm, respectively, larger than those of conventional SCFs, which lead to a higher twist sensitivity. Within twist range from 4.758 to 40.439 rad/m and temperature range from 30 to 100 °C, this sensor shows a twist sensitivity of −400 pm/(rad/m) and a temperature sensitivity of 123 pm/°C, respectively. The influence of strain is negligible since the strain sensitivity is much lower. The sensor configuration possesses other advantages such as ease of fabricate, high contrast, and low-cost, therefore it exhibits practical applications potentials in engineering machinery and building structural health monitoring.

High Sensitive Directional Twist Sensor Based on a Mach–Zehnder Interferometer

A new type of Mach–Zehnder interferometer (MZI) is proposed to measure twist and temperature. The sensor contains a pretwisted taper and a ball. The pretwisted taper is made by pretwisted, and then, thermally tapering along a single-mode fiber, to induce permanent spiral distortion in the fiber. The experimental results indicate that the sensor has higher twist sensitivity. Twisting the sensor for counterclockwise and clockwise direction, the twist sensitivities of Dip 1584 nm are –0.378 and 0.083 nm/(rad/m), respectively, and the twist sensitivities of Dip 1685 nm are –0.117 and 0.089 nm/(rad/m), respectively. Meanwhile, the twist direction can be determined by the value of the twist sensitivities. The temperature sensitivities of Dip 1584 and Dip 1685 nm are 63 pm/°C and 74 pm/°C, respectively. The feasibility of measuring the torsion and temperature characteristics has been experimentally confirmed. Thus, the proposed MZI sensor can be used to measure twist and temperature and discriminate twist direction.