Bending Vector Sensor Research Articles

Two-dimensional vector bending sensor based on single excessively tilted fiber grating

Two-dimensional vector bending sensor based on single excessively tilted fiber grating

Research on vector bending sensors based on taper-drawn seven-core fiber Bragg grating

We present a novel vector bending sensor that enables simultaneous measurement of the bending response in six outer cores of a seven-core fiber (SCF) without the need for fan-in/fan-out configurations. Utilizing femtosecond laser technology, we inscribe seven Fiber Bragg Gratings (FBGs) with distinct wavelengths across the SCF cores. The sensor’s tapering process, tailored for the bending sensing scenario, effectively combines the reflection peaks of the FBGs into one detection channel, thereby substantially improving the measurement efficiency. Our experimental results demonstrate a strong directional dependence of the bending response, with a maximum sensitivity of 127 pm/m−1. The bending angle and curvature magnitude are reconstructed by analyzing the wavelength shifts of any two non-diagonal outer cores, offering a versatile solution for real-time monitoring applications. This sensor design, by eliminating the need for additional fan-in/fan-out devices, simplifies the system architecture and reduces both measurement time and sensor size, making it highly suitable for applications in precision manufacturing, environmental monitoring, and robotics.

Temperature-insensitive vector curvature sensor based on four-core fiber offset structure

Vector curvature measurement is very important in industrial fields with the unavoidable problem of temperature crosstalk. To improve the detection performance, a novel temperature-insensitive vector bending sensor have been proposed in the paper. The sensor is fabricated by core-offset splicing a segment of four-core fiber (FCF) between two single-mode fibers (SMFs). The Mach-Zehnder interference and the recognition ability of bending directions are enhanced by the core-offset structure. The experimental results demonstrate the curvature sensitivities of our sensor reach 38.16 nm/m−1, 37.46 nm/m−1, 4.99 nm/m−1 and −7.12 nm/m−1 in different bending directions within the range of 0.346–0.49 m−1. The average temperature sensitivity is only −6.67 pm/℃ in the range of 30–90 °C. The all-fiber, low cost, high sensitivity, and excellent vectoriality make our sensor more potential in harsh, complex industrial environment.

Study of Vector Bending Sensor in Various Configurations

A triple-core photonic crystal fiber (TCPCF) vector bending sensor has been introduced to study geometric parameter changes. The TCPCF shows a linear relationship between effective refractive index and input wavelength near 1.55µm. An asymmetrical geometry has been proposed for sensing both the direction and amplitude of the bend. The geometry of fiber creates an asymmetrical refractive index profile that leads to shifts in light speed in different fiber sections during the bend. The finite element method is used to study all sensor structures. The variation of the distance between cores, air hole diameter, and the angle between the side cores are studied. This sensor's need for precise and complex fabrication techniques is one of their challenges. Another challenge the design of these types of sensors faces is calibration for different environments and fabrication methods after fabrication is finished.

A reflective multimode fiber vector bending sensor based on specklegram

The vector bending sensor that senses bend orientation and curvature is desperately needed in the smart medical field. However, the existing fiber-optic vector bending sensor is challenging to detect the 360° bending orientation, and both sides of the transmission structure need related equipment resulting in limited application. Here, a reflective multimode fiber vector bending (RMVB) sensor based on the specklegrams is proposed to overcome these challenges. Light source coupling and specklegrams capturing at the same side are achieved by the beamsplitter prism at the proximal end and the reflective coating at the distal end. Specklegrams can be used to estimate the bending state since the bending alters the mode interference. The bending state prediction network is trained using a dataset of 360° bending orientations and 0–5.7 m−1 curvatures. The bending orientation and curvature interval are predicted by the classification model with accuracies of 0.99 and 0.97, respectively. With the mean absolute error lower than 4 % of the measurement range, the regression model can accurately predict the bending orientation and curvature. Therefore, the RMVB sensor with a novel structure capable of curvature sensing in 360° bending orientation provides a promising approach for a single-fiber vector bending sensor and is expected for application in space-tight applications such as interventional operation.

7-Core Fiber-Based High-Resolution Omnidirectional Vector Bending Sensor Enabled by Microwave Photonics Filter

We proposed and experimentally demonstrated a high-resolution omnidirectional vector bending sensor based on 7-core fiber Mach-Zehnder interferometers (MZIs), which is interrogated by the microwave photonics technique. The sensor is composed of three MZIs in the 7-core fiber, and each MZI consists of two symmetric outer cores. Instead of using the traditional wavelength demodulation method, which has the drawback of very limited resolution due to the used optical spectrum analysis devices, e.g., optical spectrum analyzer (OSA), the proposed sensor is interrogated by high-resolution microwave photonics filter (MPF) system, which is capable of offering up to KHz level measurement resolution. As a result, curvature measurement with resolution of 0.0213 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> is demonstrated when the used resolution of vector network analyzer (VNA) is 500 kHz. Thanks to the ultra-high measurement resolution of VNA (∼KHz), the curvature resolution can achieve 4.27 х 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> in theory, which is 2 orders of magnitude higher than the traditional wavelength demodulation-based fiber bending sensors. The measured curvature sensitivity of the MZI exhibits highly bending direction dependence, attributed to different strain variation under distinct bending angle. In addition, owing to the different angular positions of cores of the MZI in the 7-core fiber, the MZIs have differential response to fiber bending. This feature has been used to develop vector bending sensing, which is manage to determine the bending direction by using any two MZIs, and the obtained average relative errors for the retrieved curvature and bending direction are 2.57% and 0.86%, respectively. The proposed sensor possesses the unique advantages of high sensitivity and high resolution over the state-of-the-art technology, which is very promising for omnidirectional vector bending sensing in the future.

Vector Bending Sensor Based on Hole-Assisted Three-Core Fiber Mach-Zehnder Interferometer

A vector bending sensor based on hole-assisted three-core fiber (HATCF) Mach-Zehnder interferometer (MZI) is demonstrated. The sensor is composed of multi-mode fiber (MMF) - HATCF - MMF structure embedded between two single-mode fibers (SMFs). The bending curvature and direction can be identified by interrogating the wavelength of the interference peak, and the maximum bending sensitivity of -28.726 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> is obtained at the 220° direction. In addition, the sensor has a temperature sensitivity of 44 pm/°C and is insensitive to the external refractive index (RI).

Femtosecond Laser Line-by-Line Inscribed Seven Core Fiber Cascaded Fabry–Perot Cavity and Its Vectorial Bending Sensing Application

Multi-core fibers have been widely used for vector-bending sensing due to their off-axis distributed cores. In contrast to vector-bending sensors based on Bragg gratings, fiber Fabry–Perot (F–P) interferometers are more advantageous due to their ease of fabrication and potential for introducing the Vernier effect to further improve sensitivity. We propose and experimentally demonstrate a cascaded Fabry–Perot (F–P) cavity vector bending sensor. From the experimental results, the sensor has a strong bending dependence with a maximum sensitivity of 123.12 pm/m−1, and the curvature magnitude and direction can be reconstructed from the tilted wavelength shift of the asymmetric fiber-core F–P cavities.

In-Fiber Thermally Diffused Coupler and Fiber Bragg Grating Inscribed in Twin-Core Fiber for Sensitivity-Enhanced Vector Bending Sensing

A vector bending fiber sensor based on core-by-core inscribed fiber Bragg gratings in a twin-core fiber has been proposed and experimentally demonstrated. An in-fiber integrated vector bending sensor is realized by using the thermal diffusion technique to fabricate the coupler. The characteristics of the coupler fabricated by thermal diffusion are simulated and experimented. By inscribing fiber Bragg gratings with different reflection wavelengths in the two cores of a symmetrical twin-core fiber, the curvature sensitivity can be enhanced by tracking the wavelength difference between the fiber Bragg gratings of the two cores. The measured bending sensitivity of the fiber Bragg grating ranges from −161.6 pm/m−1 to +165.5 pm/m−1. The differential sensitivity of the two cores is twice that of a conventional single grating, and the temperature-induced crosstalk is also reduced. The bending sensor proposed in this paper has the advantages of high integration, enhancing the sensitivity and two-dimensional orientation recognizability, and reducing temperature crosstalk, which can be a promising candidate for structural health monitoring or wearable artificial electronics applications.

Orthogonal single-mode helical Bragg gratings created in fiber cladding for vector bending measurement.

We demonstrate a novel, to the best of our knowledge, two-dimensional vector bending sensor based on orthogonal helical Bragg gratings inscribed in the cladding of a conventional single-mode fiber (SMF). The helical cladding fiber Bragg gratings (HCFBGs) are created by using a femtosecond laser direct writing technology and a quarter-pitch graded index fiber (GIF) is used in front of the HCFBGs to diverge the core mode into fiber cladding. In contrast to the multimode resonance observed in conventional cladding Bragg gratings inscribed by using a femtosecond laser point-by-point (PbP) or line-by-line (LbL) technology, the proposed HCFBGs exhibit stable narrowband single-mode Bragg resonance. An HCFBG with a low peak reflectivity of -50.77 dB and a narrow bandwidth of 0.66 nm was successfully fabricated by using a lateral offset of 45 µm between the HCFBG and the fiber core axis. Moreover, two orthogonal HCFBGs were fabricated in the SMF cladding and used for vector bending sensing. Strong orientation dependence could be seen in omnidirectional bending measurement, exhibiting a maximum bending sensitivity of up to 50.0 pm/m-1, which is comparable to that in a multicore FBG. In addition, both the orientation and amplitude of bending vector could be reconstructed by using the measured Bragg wavelength shifts in two orthogonal HCFBGs. As such, the proposed HCFBGs could be used in many applications, such as structural health monitoring, robotic arms, and medical instruments.

Vector bending sensor based on chirped long-period grating with off-axis micro-helix taper

Vector bending sensor based on chirped long-period grating with off-axis micro-helix taper

Two-dimensional vector bending sensor based on a hole-assisted three-core fiber coupler.

We demonstrate a two-dimensional vector bending sensor based on a hole-assisted three-core fiber (HATCF) coupler. The sensor is built by splicing a section of HATCF between two single-mode fibers (SMFs). The resonance couplings between the center core and the two suspended cores of the HATCF occur at different wavelengths. Two completely discrete resonance dips are observed. The bending response of the proposed sensor is investigated over a 360° range. The bending curvature and direction can be identified by interrogating the wavelengths of the two resonance dips, and a maximum curvature sensitivity of -50.62 nm/m-1 is achieved at 0° direction. Moreover temperature sensitivity of the sensor is less than -34.9 pm/°C.

Vector bending sensor for measuring spine curvature

A three-core photonic crystal fiber (TCPCF) vector bending sensor for measuring spine curvature is proposed. Refractive index variations of the TCPCF due to the wavelength differences in the range of 1.25 to 1.85 μm are studied. We show a linear relation between refractive index and operating wavelength. The wavelength of 1.55 μm is chosen for simulation with high energy modules to compensate for bending loss. An asymmetrical fiber is designed to sense both the direction and magnitude of the curve. The asymmetrical geometry of fiber creates an asymmetrical refractive index profile that leads to shifts in light speed in different fiber sections. This geometry asymmetry, in turn, provides the information needed to calculate both variables. Bending effects on the TCPCF has been numerically studied by the finite element method. Data analysis shows that the proposed sensor is sensitive to bending with a sensitivity of 1.2199 pm.m. It also reveals high directional sensing.

Temperature-Insensitive Two-Dimensional Vector Bending Sensor Based on Fabry-Pérot Interferometer Incorporating a Seven-Core Fiber

We proposed and experimentally demonstrated a temperature-insensitive two-dimensional (2D) vector bending sensor on the basis of Fabry-Pérot Interferometer (FPI). The sensor is fabricated by splicing a section of silica capillary optical fiber (COF) between a trench-assisted seven-core fiber (SCF) and a multimode fiber (MMF). For orientation angle from 0° to 360°, the vector bending responses of six outer-core FPIs were studied at a step of 30°. From experimental results, the proposed sensor exhibits strong bending-direction dependence, and the maximum sensitivity is 200.6 pm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> . The curvature magnitude and orientation angle can be reconstructed through the dip wavelength shifts of any two off-diagonal outer-core FPIs. Hence, there are 12 various combinations from six outer-core FPIs, and the average value can be taken in multiple reconstructions to develop exact 2D vector bending measurement. For curvature magnitude and orientation angle reconstructed, the average relative error is 6.2% and 3.3%, respectively. In addition, experiments verified the proposed sensor is insensitive to ambient temperature changes.

Research on Vector Bending SPR Sensor Based on V-Groove Fiber

Bending curvature measurement and bending direction identification are very important in the field of mechanical engineering. This study aimed to realize the high sensitivity direction recognition and curvature sensing based on SPR mechanism by a V-groove fiber SPR vector bending sensor. By CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser, multiple evenly spaced V-grooves were fabricated on the step multimode fiber. The V-groove fiber coupled the core mode to the cladding mode. By rotatably coating 50nm gold film on the cladding behind the V-groove area, and splicing with a multimode fiber with larger diameter to receive the light with SPR signal, the fiber SPR sensor was realized. When the sensing probe was concave inward, the V-groove was squeezed, the angle became smaller, and the excited cladding mode changed with it, resulting in the increase of SPR incident angle, the resonance valley blue shifted, and the sensitivity of curvature sensing wavelength is −5.98nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> . Similarly, when the sensing probe was convex outward, the angle became larger, resulting in the decrease of SPR incident angle, the resonance valley red shifted, and the sensitivity of curvature sensing wavelength is 1.42nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> . The proposed V-groove fiber SPR sensor realized the bending curvature measurement and bending direction identification. The manufacturing method is simple and the sensitivity is high, which opens up a new way for application of the SPR technology in bending measurement.

High-sensitivity two-axis vector bending sensor based on side-grooved long period grating in eccentric core fiber

A novel vector bend sensor based on side-grooved Long period grating (SG-LPG) inscribed on eccentric core fiber (ECF) was proposed and investigated. The core of the ECF is close to the surface of the fiber, which makes it very convenient to fabricate different SG-LPGs in ECF by CO2 laser with little damage. Since the direction of the SG-LPG deviates from the direction of the side core of the ECF, the sensor can identify bending directions on two orthogonal axes. At the same time, the strong refractive index modulation caused by grooves makes SG-LPG very sensitive to bending. In the curvature range of 0–2.52 m−1, the bend sensitivity in four directions reaches 27.23 nm/m−1, −25.76 nm/m−1, 14.74 nm/m−1 and −16.83 nm/m−1, respectively. Due to the simple structure, convenient fabrication, high sensitivity, and multi vectors bending detection characters, the sensor has potential in bending measurement.

Two-dimensional vector bending sensor based on Fabry-Pérot cavities in a multicore fiber.

In this work we demonstrate the fabrication and characterization of a temperature insensitive, two-dimensional curvature sensor using a resin based Fabry-Pérot interferometer, constructed using a multicore fiber (MCF). The fabrication simplicity makes this fiber device very attractive compared to the already reported technologies. Furthermore, the sensitivity reached (>400 pm/m-1), 7 times higher than the one reported for fiber Bragg gratings written on a similar MCF. The reconstruction of the amplitude and curvature has been performed for, showing errors lower than 4%. A numerical study has also been developed, allowing us to understand the sensor response at different fiber sensor geometries.

Femtosecond Laser Refractive Index Modulated Multimode Fiber for High Sensitivity Vector Bending Measurement

A femtosecond (fs) laser fabricated refractive index (RI) modulated multimode fiber (MMF) for high sensitivity vector bending measurement is presented. A fs laser-induced local positive RI modulation in the MMF core results in a non-circular symmetric RI distribution of the MMF. Such a RI-modulated MMF used in single-mode-multimode-single-mode (SMS) fiber structure exhibits a bending-dependent spectral shift response. Due to the large core size of the MMF, the RI modulated area can be relatively far away from the fiber core axis, which in turn results in a greater effective RI change during bending. Experimental results show that the highest bending sensitivity of -45.8 nm&#x002F;m-1 is achieved at 0&#x00B0; orientation with the resonant wavelength of 1554.4 nm. In addition to its high sensitivity and simple structure, this vector bending sensor exhibits ultra-low temperature crosstalk in bending measurements, as low as 0.0002 m-1&#x002F;&#x00B0;C, which makes it an excellent candidate for accurate vector bending monitoring applications.

Vector bending sensor based on an edge-core cladding-type fiber Bragg grating.

A two-dimensional vector bending sensor that is both compact and simple is proposed and demonstrated, based on an edge-core cladding-type fiber Bragg grating (ECLFBG) inscribed in an edge-core. The ECLFBG is written parallel to the edge-core using a femtosecond laser point-by-point technique. The reflection spectrum of this ECLFBG varies significantly depending on the magnitude and direction of the fiber's bend. Combining the trend and sensitivity of the wavelength shift and reflection intensity variations of the ECLFBG, the bending magnitude and direction can be measured simultaneously.

A Vector Bending Sensor Based on a Core-Offset Long Period Fiber Grating Induced by an Arc-Discharge

We proposed and experimentally demonstrated a vector bending sensor based on a core-offset long period fiber grating (CO-LPFG). The CO-LPFG is configured by CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser polishing on three mutually orthogonal planes of a single mode fiber (SMF) to different depths and then heating it by the electrical arc-discharge of a fusion splicer. The asymmetrical cladding structure leads to an uneven density distribution of the fiber in the molten state. Under the effects of releasing the residual stress and fiber surface tension, the fiber core deviates toward the polished deeper directions to form a core-offset structure in two dimensions. The experimental results demonstrate that the bending sensitivities of CO-LPFG reach 37.06 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> −5.85 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> -14.59 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> and −12.45 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> for four bending directions within the range of 1.12-2.10 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> The temperature sensitivity of CO-LPFG is 57.90 pm/°C in the range of 40-160°C.