Bending Vector Research Articles

Multipoint Temperature-Independent Vector Bending Sensing With Coupled-Core Fibers

Multipoint Temperature-Independent Vector Bending Sensing With Coupled-Core Fibers

A Method for Improving Performance of Vector Bending Fiber Sensor Based on LSTM

A Method for Improving Performance of Vector Bending Fiber Sensor Based on LSTM

Shape measurement by using multicore optical fiber sensor with asymmetric dual cores

Abstract Shape measurement by using multicore optical fiber sensor has attracted more attention in many fields due to the good consistency of the fiber cores. Three symmetrically arranged cores in multicore fiber are usually used for reconstructing shape by calculating the bending vectors, which will not be achieved when one of the cores is damaged or occupied in actual application. A shape measurement method using multicore optical fiber sensor with asymmetric dual cores is proposed in this paper. Based on analyzing the principle of shape reconstruction and the geometric relationship of asymmetric dual cores in multicore fiber sensor, the bending vector is decomposed. The mathematical expressions for bending curvature and orientation of asymmetric dual cores are derived. The two dimensional (2D) and three dimensional (3D) shape reconstruction results both in finite element modelling simulation and shape measurement experiment based on optical frequency domain reflectometry have shown that the proposed method using multicore fiber sensor with asymmetric dual cores is able to achieve shape measurement, and its performance is comparable and even equivalent to the traditional method by using three symmetrically arranged cores. In the experiment, the maximum relative errors of 2D and 3D reconstructed shapes are 2.653% and 5.139%, respectively. The proposed method, which only needs asymmetric dual cores in the multicore optical fiber sensor for shape reconstruction, will be conducive to solve the limitation of multicore optical fiber sensors in shape measurement applications.

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.

Large Curvature Bending Measurable Fiber-Optic Neurons for Multi-Joint Bending Perception

In recent years, dexterous robotic hands have become an indispensable part of intelligent equipment. The introduction of bionic neurons that provide finger motion perception will improve the accuracy of joint control. Here, series- and parallel-integrated inner cladding fiber Bragg gratings (ICFBG) act as bionic neurons to realize multi-joint bending perception in a robotic finger by measuring the reflective intensity changes of the ICFBGs. The fiber-optic neurons (FON) were fabricated using femtosecond laser point-by-point technology in the inner cladding of bend-insensitive single-mode fiber (BIF). The proposed FONs can measure large curvature bending up to 250 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , with a bending sensitivity of ∼0.07 dB/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and a measuring resolution of 0.4 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Additionally, vector bending sensing can be achieved using the two pairs of ICFBGs that are orthogonally distributed to each other. The FONs are insensitive to temperature and axial strain. Furthermore, the proposed FONs were fixed to the knuckles of a robotic finger for multi-joint bending perception. The results show that FONs can potentially be used in robotic gesture recognition and control systems.

Vector Bending MZI Based on Cascaded Application of Graded-Index Fiber and No-Core Fiber

Vector Bending MZI Based on Cascaded Application of Graded-Index Fiber and No-Core Fiber

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).

Directional Bending Sensor and Multiparameter Sensor Based on D-Core Multimode Fiber

For the sensor with interference between fiber core mode and cladding mode, the directional bending sensor can be realized by processing asymmetric structure on the cladding or using eccentric fiber, but the temperature and strain crosstalk of the sensor with interference between fiber core mode and cladding mode is large. In this article, a kind of fiber intermode interference sensor with asymmetric structure is proposed, which can realize vector bending sensing and reduce temperature and strain crosstalk. The designed D-core multimode fiber has cladding and air on both sides of the fiber core. When 0° and 180° bending are applied to it, the interference spectrum moves in different directions, thus realizing the recognition of bending direction. For curvature detection, in the curvature range of 0–0.62 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{1}}$ </tex-math></inline-formula> , the sensitivity of 0° bending is 11.00 nm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{1}}$ </tex-math></inline-formula> , and that of 180° bending is −11.40 nm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{1}}$ </tex-math></inline-formula> . For liquid level detection, the sensing sensitivity is 234 pm/mm with the range of 0–4 cm, and the linearity is 0.993. In addition, the temperature sensitivity is as low as 10 pm/°C, and the strain sensitivity is as low as −1.28 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> . The proposed sensor realized the directional bending sensing and liquid level sensing of intermodal interference. Its low sensitivity to temperature and strain can be used to avoid the cross-sensitivity problems. In the field of water flow detection, the simultaneous detection of water level and water flow direction can be realized by the proposed sensor.

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.

High-sensitivity vector bend sensor based on a fiber directional coupler inscribed by a femtosecond laser.

In this Letter, we demonstrate a high-sensitivity vector bend sensor based on a fiber directional coupler. The fiber directional coupler is composed of two parallel waveguides inscribed within a no-core fiber (NCF) by a femtosecond laser. Since the two written waveguides have closely matched refractive indices and geometries, the transmission spectrum of the fiber directional coupler possesses periodic resonant dips. Such a fiber directional coupler exhibits a good bending-dependent spectral shift response due to its asymmetric structure. Experimental results show that bending sensitivities of -97.11 nm/m-1 and 58.22 nm/m-1 are achieved for the 0° and 180° orientations in the curvature range of 0-0.62 m-1, respectively. In addition, the proposed fiber directional coupler is shown to be insensitive to external humidity changes, thus improving its suitability in high-accuracy bending measurements.

Exoskeleton type long-period fiber grating for cross-talk-resistance single-parameter measurement.

This paper proposes a new, to the best of our knowledge, design framework of long-period fiber grating (LPFG) sensors resistant to multi-parameter cross talk. A section of hollow quartz capillary (HQC), which acts as an exoskeleton, is periodically merged with a single-mode fiber (SMF) by the arc-discharge method. The mechanical stress in the SMF is released while the thermal stress is enhanced after a high-temperature fusion process. Under the influence of the elastic-optical effect, the refractive index of the core is periodically modulated along the axial direction to form an exoskeleton long-period fiber grating (Es-LPFG). The unique exoskeleton structure not only induces mode coupling but also enables the proposed device to resist cross talk among the strain, ambient refractive index, and vector bending. The temperature is able to be measured independently with a sensitivity of 74 pm/ ∘C. The novel Es-LPFG is promising in single-parameter sensing, mode-locked lasers, and frequency-locked gain flattening.

Fabrication and Sensing Characteristics of Long-Period Fiber Grating in Capillary Fiber

We propose a novel fiber device based on arc-induced long-period fiber gratings (LPFGs) in a hollow core silica capillary. This device is made up of a lead-in single-mode fiber (SMF), LPFGs in capillary (CLPFGs), and a lead-out SMF, with a tapered air cavity (TAC) connecting SMF and capillary. The transmissive characteristics of capillary with and without LPFGs are analyzed theoretically and experimentally. Mode coupling of CLPFGs at longer wavelength could be enhanced, as well as the sensing performance increases times over the capillary without gratings. The refractive index (RI) sensitivity can reach to be 15 713.5 nm/RIU in the RI range from 1.451 to 1.456. Moreover, with the asymmetric index distribution induced in the capillary, the CLPFGs present vector bending properties that the bending sensitivities are 79.4 and −8.8 nm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{1}}$ </tex-math></inline-formula> , for bending directions at 0° and 180°, respectively. Furthermore, the proposed device shows negligible change to the external temperature variation, which could solve the cross-sensitivity problem. With the advantages of compactness, low cost, and ease of fabrication, the proposed device is potential in high-sensitivity RI sensing fields and vector bending measurement.

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