Bending Vector Sensor Research Articles

Highly sensitive vector bending sensor based on an embedded multimode D-shaped LPFG.

In this letter, a highly sensitive bending sensor based on an embedded multimode D-shaped long period fiber grating (EMD-LPFG) is proposed. The novel sensor is applied to carry out vector bending measurement. The proposed LPFG is fabricated by polishing on the prepared structure which is formed by periodically splicing between single mode fiber (SMF) and multimode fiber (MMF). Since the cross section of the embedded MMF is D-shaped, we named it EMD-LPFG. Due to the asymmetric modulation of the refractive index on the fiber by the CO2 laser, the sensor has the ability to distinguish the bending directions, and the MMFs provide higher bending response. The experimental transmission spectrum can match the simulation results well. The experimental results show that the average bending sensitivities in three orthogonal directions are 70.21 nm/m-1 (0°), 9.75 nm/m-1 (90°), -12.04 nm/m-1 (180°) and 9.98 nm/m-1 (270°), respectively. Meanwhile, the temperature sensitivity is 30 pm/°C in the range of 25 °C to 75 °C. According to the ultra-compact structure with the total length of 2.5 mm, high bending sensitivity and ability to distinguish the bending direction, the novel sensor has potential in bending measurement.

High-sensitivity bending vector sensor based on γ-shaped long-period fiber grating

We investigate and prepare a novel long-period fiber grating (LPFG) based on γ-shaped grid for high sensitivity bending measurement. The γ-shaped LPFG is composed of a string of γ-shaped grids, which can provide stronger refractive index modulation than normal planar grids and bring a higher sensitivity. The experimental results show the bending sensitivity of the γ-shaped LPFG reaches −21.66 nm/m−1 within the bending range of 0–2.2 m−1, and the goodness of fit value is over 0.99. Compared with the reported structure, the γ-shaped LPFG not only has high bending sensitivity and good linear fit value, but also makes the structure of LPFG more plentiful and diverse by virtue of its ultracompact structure and adjustable parameters of asymmetric grid. In addition, it is expected that more structural forms of sensor devices can be made based on LPFG to enrich the field of sensing applications and enhance the sensitivity.

Compact Robust Vector Bending Sensor Based on Single Stress-Applying Fiber

We propose a compact assemble-easy cost-effective vector bending sensor consisting of a short section of single stress-applying fiber (SSAF) spliced between two standard single-mode fibers (SMFs). The SSAF is specially designed by introducing a stress-applying part (SAP) into the cladding to achieve the asymmetrical geometry. Because of the mode field mismatch between the SMF and the SSAF, the cladding mode is effectively excited, which is sensitive to the bending and has orientation-dependence. Due to the large effective refractive index difference between the core and cladding mode, a compact dual-mode interferometer can be achieved with a thousands-micron-long structure. When the fiber is bent, the mode fields and the effective refractive indices will be modified, and accordingly, the direction-dependent interference spectral shift is observed. Owing to the well-designed SSAF structure, the cost-effective vector bending sensor can be easily realized by a simple sandwich structure without extra complex alignment procedures or fiber grating inscription. We experimentally verify the vector bending property. The maximum sensitivity of 2.04 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> is obtained, and the experimental results match well with the simulations. Our proposed device is compact, easy to assemble, and cost-effective, which is promising in the field of vector bending measurement.

Compact omnidirectional multicore fiber-based vector bending sensor

We propose and demonstrate a compact and simple vector bending sensor capable of distinguishing any direction and amplitude with high accuracy. The sensor consists of a short segment of asymmetric multicore fiber (MCF) fusion spliced to a standard single mode fiber. The reflection spectrum of such a structure shifts and shrinks in specific manners depending on the direction in which the MCF is bent. By monitoring simultaneously wavelength shift and light power variations, the amplitude and bend direction of the MCF can be unmistakably measured in any orientation, from 0° to 360°. The bending sensor proposed here is highly sensitive even for small bending angles (below 1°).

Temperature-insensitive vector bending sensor based on parallel-integrated fiber Bragg gratings

Parallel-integrated fiber Bragg grating (PI-FBG) arrays use a femtosecond laser line-by-line method. Each FBG in the fiber grating array is fabricated at the same position along the axial direction of a multimode fiber core but at different radial directions. The PI-FBG array can be used to realize a temperature-insensitive vector bending sensor. Such a sensor can not only measure bending curvature but also determine any bending direction. By introducing the demodulation method of equivalent fiber grating, that is, the difference operation of the FBG reflection wavelength changes, the cross-sensitivity problem between the bending and the temperature is overcome, and the bending sensitivity of the sensor is almost doubled; thus, the resolution of the bending sensor is remarkably improved. Therefore, the vector bending sensor can achieve the real-time monitoring of the bending curvature and bending direction of an intelligent engineering structure.

High-performance vector bending and orientation distinguishing curvature sensor based on asymmetric coupled multi-core fibre

Fibre optic technology is rapidly evolving, driven mainly by telecommunication and sensing applications. Excellent reliability of the manufacturing processes and low cost have drawn ever increasing attention to fibre-based sensors, e.g. for studying mechanical response/limitations of aerospace composite structures. Here, a vector bending and orientation distinguishing curvature sensor, based on asymmetric coupled multi-core fibre, is proposed and experimentally demonstrated. By optimising the mode coupling effect of a seven core multi-core fibre, we have achieved a sensitivity of − 1.4 nm/° as a vector bending sensor and − 17.5 nm/m−1 as a curvature sensor. These are the highest sensitivities reported so far, to the best of our knowledge. In addition, our sensor offers several advantages such as repeatability of fabrication, wide operating range and small size and weight which benefit its sensing applications.

All-Fiber Vector Bending Sensor Based on a Multicore Fiber With Asymmetric Air-Hole Structure

In this article we present an all-fiber vector bend sensor by means of a self-fabricated micro-structured multicore optical fiber. The reported solution is based on differential intensity variations of the light transmitted along the cores whose changes are influenced by the bending angle and orientation. The unique asymmetric structure of the air-holes in the optical fiber provides each core with different confinement losses of the fundamental mode depending on the bending radius and orientation, making each of the cores bend-sensitive in a range of at least 80°. It has been experimentally demonstrated that the reported sensor enables the bending angle and orientation to be detected in a full range of 360° without any dead-zones, and the possibility of end point detection with millimeter precision. Additionally, a reconstruction of the bending vector has been carried out theoretically, and a good match can be observed between the experimental and theoretical data.

Low-temperature crosstalk and surrounding refractive index insensitive vector bending sensor based on hole-assistant dual-core fiber.

A low-temperature crosstalk and surrounding refractive index insensitive vector bending sensor based on resonance coupling is proposed and experimentally demonstrated. The sensor is constructed by directly sandwich-splicing asegment of single eccentric hole-assisted dual-core fiber between two single-mode fibers. The transmission characteristicsof the sensor are theoretically analyzed by combining coupled-mode theory and full-vector finite-element method. The bending responses of the sensor are experimentally measured in the curvature range of0-3.6651 m-1. The bending sensitivities are -15.95 nm/m-1 and 14.87 nm/m-1 at 0° and 180° bending orientation, respectively, which are higher than that of most long-period fiber-grating-based and interferometer-based vector bending sensors. Moreover, the temperature and surrounding refractive index responses of the sensor are investigated. The measured results show that the proposed sensor has a low temperature sensitivity of 21.7pm/°C in the range of 15°C-55°C and is insensitive to the surrounding refractive index in the range of 1.335-1.425, which reduces the cross-sensitivity from temperature and surrounding refractive index.

Two-dimensional microbend sensor based on the 2-core fiber with hump-shaped taper fiber structure

Abstract A novel two-dimensional bending vector sensor formed by cascading two hump-shaped tapers (HSTs) in a segment of 2-core fiber (2CF) is proposed and fabricated, in which the HSTs serve as the mode splitting and coupling. Several modes are excited at the first HSTs, where a part of the light energy is propagated within the center core of the 2CF, and the other part is coupled to the external core. Due to the effective refractive index difference between these core modes, they are recoupled into the lead-out single-mode fiber (SMF) at the second HSTs and interfere with each other. The bulge of the HSTs and two cores of the 2CF are distributed in a triangle. Owing to the asymmetric structure, the sensor can distinguish multiple bending directions. In addition, the linear spectral responses for bending and temperature are observed experimentally. The proposed sensor has many advantages, including compact, inexpensive, and high directional resolution ratio. Experiment results show that the sensor can distinguish bending directions and experience a maximum sensitivity of −6.182 nm/m−1 with a bending range of 0–0.98 m−1 without dead zone.

Highly Compact Vector Bending Sensor With Microfiber-Assisted Mach– Zehnder Interferometer

A low-cost and highly compact fiber-optic component is proposed and experimentally demonstrated for vector bending sensing. A segment of microfiber tapered from standard single-mode fibers (SMFs) is spliced between two SMFs with pre-designed lateral offset to construct a sandwich type Mach–Zehnder interferometer of $243.32~\mu \text{m}$ in length. Sensing performances of the proposed vector bending sensor are theoretically analyzed in detail. As the applied curvature increases from 0.3873 to 3.0 m−1, the transmission spectra of the proposed sensor show distinct linear wavelength shift sensitivities for different directions, the maximum of which is up to 3.419 nm/m−1. Besides, temperature test indicates that the proposed sensor possesses a low-temperature cross sensitivity of 33.71 pm/°C, which ensures its applicability for practical uses in temperature-fluctuated environment. Hence, our proposed vector bending sensor possesses such desirable merits as high sensitivity, compact size, low thermal crosstalk, low cost, and orientation-dependent spectral response.

Highly sensitive and temperature-compensated fiber bending sensing based on directional resonance coupling in photonic crystal fibers

Abstract A highly sensitive bending sensor with temperature-compensation based on magnetic fluids (MF) infiltrated photonic crystal fiber (PCF) is presented. Specific cladding air-holes are filled with MF material to form defective channels and introduce new degrees of freedom for fiber sensing. The structure designed in such way can achieve simultaneous measurement of bend-curvature and bend-direction with temperature compensation. The bending sensitivity can reach to −6.544 nm/m−1 with a high linearity within the range of curvature from −10 m−1 to 10 m−1. The proposed two-dimensional (2D) bending vector sensor cannot only improve the sensing precision and measurement range, but also reduce the metrical difficulty and simplify the process.

V-Shaped Long-Period Fiber Grating High-Sensitive Bending Vector Sensor

A novel V-shaped grating grid with a composite structure comprised of two oppositely tilted tracks is firstly proposed, which can provide both asymmetry and high refractive index modulation (RIM) to support the bending vector sensing. A long-period fiber grating formed by the V-shaped grids is fabricated with a high-frequency-CO 2 laser on a single-mode fiber, and it provides different responses of wavelength shift in two-axis directions' bending. The V-shaped grid model is established to theoretically analyze the change of RIM caused by bending. The formula of bending sensitivity is deduced, and the values of sensitivity in two-axis directions are calculated. The experimental bending sensitivities are 16.315 and -23.085 nm/m -1 in the opposite directions within the plane perpendicular to the laser-irradiated direction, which can coincide with the theoretical expectation. Its temperature recycle characteristic is also provided.

Two-dimensional vector bending sensor based on seven-core fiber Bragg gratings.

We demonstrated a two-dimensional vector-bending sensor by use of fiber Bragg gratings (FBGs) inscribed in a homogeneous seven-core fiber. Seven FBGs were simultaneously inscribed in each of all seven cores using a modified Talbot interferometer and a lens scanning method. The vector bending response of six outer-core FBGs was investigated at all 360° directions with a step size of 15°. The bending sensitivities of the six outer-core FBGs display six perfect '8'-shaped patterns in a polar-coordinate system. That is, they exhibit strong bending-direction dependence with a maximum sensitivity of 59.47 pm/m-1. The orientation and amplitude of the vector bending can be reconstructed using measured Bragg wavelength shifts of any two off-diagonal outer-core FBGs. So, the six outer-core FBGs have 12 combinations for bend reconstruction, which can be averaged across multiple reconstructions to develop an accurate two-dimensional vector bending sensor. The average relative error was lower than 4.5% for reconstructed amplitude and less than 2.8% for reconstructed orientation angle θ. Moreover, the seven-core FBGs offer several advantages such as a compact structure, fabrication flexibility, and the temperature compensating ability of central-core FBG.

Two-axis bending vector sensor based on a long-period fiber grating cascading with a hump-shaped taper

A novel fiber-bending vector sensor based on a Mach–Zehnder interferometer (MZI) is proposed and fabricated. The device consists of a long-period fiber grating (LPFG) written by a high-frequency CO2 laser and a hump-shaped taper (HST) in a segment of single-mode fiber (SMF). The HST is an up-taper with the asymmetry perpendicular to the irradiated direction of the LPFG, which induces asymmetry into the fiber in two-axis directions. In the curvature range of 0.3–1.6 m−1, the device can measure bending curvature as well as distinguish the bending direction with two dips at 1480.45 nm and 1493.20 nm, and the sensitivity can reach −6.592 nm m−1. The response for temperature is also investigated and the temperature sensitivity is about 0.05 nm °C−1 in the range of 25 °C–80 °C. By measuring the separation of the two interference dips, the temperature cross-sensitivity can be almost eliminated.

Highly Sensitive Two-Dimensional Bending Vector Sensor Using an Elliptic Two-Core PCF

A highly sensitive and simple all-fiber interferometric two-dimensional (2D) bending vector sensor is demonstrated. Fiber interferometer works in reflection mode and formed by manually splicing a small section of elliptic two-core photonic crystal fiber at the end of a single-mode fiber. Due to the birefringence nature of the two cores, the fiber device exhibits orientation-dependent bend sensitivity which is different for different polarization states. A high sensitivity of 6.49 nm/mm is achieved for bending along the x-direction, and the rms deviations for bending in the x- and y-directions are estimated to be 0.015 and 0.098 mm, respectively, by using the matrix method. The device is immune to bend-induced power fluctuation.

Two-Dimensional Bending Vector Sensor Based on the Multimode-3-Core-Multimode Fiber Structure

A two-dimensional bending vector sensor based on the multimode-3-core-multimode fiber structure is proposed. The three cores of the 3-core fiber (3CF) distribute in an isosceles triangle. In order to obtain a higher bending sensitivity, two larger diameter external cores are selected in the design of the 3CF. Owing to the asymmetric structure of the 3CF, this sensor can distinguish multiple bending directions. In addition, the linear spectral response for bending and temperature is observed experimentally. The proposed sensor has many advantages. For example, it is compact, inexpensive, and easy to be fabricated. These characteristics of the sensor make it very attractive for two-dimensional bending sensing application.

Temperature insensitive one-dimensional bending vector sensor based on eccentric-core fiber and air cavity Fabry-Perot interferometer

A temperature insensitive directional bending sensor based on an eccentric-core fiber (ECF) cascaded with an air-cavity Fabry–Perot (F-P) interferometer is presented and demonstrated. The ECF-based air cavity F-P interferometer is fabricated by fuse splicing a piece of hollow-core fiber (HCF) in between an ECF and a multi-mode fiber (MMF). The bending sensitivities of the sensor at the two opposite most sensitive directions are 79.5 pm/m−1 and −81.5 pm/m−1, respectively. The temperature sensitivity of the proposed structure is as low as 1 pm °C−1.

Bending Vector Sensor Based on a Pair of Opposite Tilted Long-Period Fiber Gratings

A novel optical fiber grating vector bending sensor head is demonstrated. The sensor head is composed by two cascaded 6° tilted long period fiber gratings, which are separated by a distance d. The asymmetric characteristics of the proposed sensor originate from two tilted long period gratings with angles placed oppositely and asymmetric exposure of the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser pulses. Two selected dips shift toward each other when the sensor head is bent. The interval between the two dips is used for improving the sensitivity of curvature measurement and further decreasing the temperature crosstalk. For the curvature range from 0.5 to 2 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , the bending sensitivities reach -15.4935 and 15.2852 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for the opposite bending directions. The temperature sensitivity of the sensor is one third of the normal long period fiber grating.

Bending vector sensor based on Mach–Zehnder interferometer using S type fibre taper and lateral-offset

A novel, simple and highly sensitive bend vector sensor is proposed and experimentally demonstrated for directional bending measurement. This sensor consists of a lateral-offset and an S fibre taper processed through special fusion splicing method. The asymmetrical fibre structure of S fibre taper and lateral-offset determined a pair of directions along which the bending response to the transmission spectrum is different. Thus, it can be used for bending vector measurement. For a curvature range from −4 to +4 m−1, the bending sensitivities near 1550 nm reach 0.70807 and 0.99695 nm/m−1, respectively.

Ultrasensitive vector bending sensor based on multicore optical fiber.

In this Letter, we demonstrate a compellingly simple directional bending sensor based on multicore optical fibers (MCF). The device operates in reflection mode and consists of a short segment of a three-core MCF that is fusion spliced at the distal end of a standard single mode optical fiber. The asymmetry of our MCF along with the high sensitivity of the supermodes of the MCF make the small bending on the MCF induce drastic changes in the supermodes, their excitation, and, consequently, on the reflected spectrum. Our MCF bending sensor was found to be highly sensitive (4094 pm/deg) to small bending angles. Moreover, it is capable of distinguishing multiple bending orientations.