Optical Fiber Bend Sensor Research Articles

Optical fiber bending sensor based on speckle pattern imaging

In this paper, we propose a new fiber bending sensor based on speckle pattern imaging. The design and implementation of the sensor are demonstrated by simulated studies. The speckle pattern imaging technique by using a multimode fiber can offer high spatial resolution. In this study, we showed that the bending sensor responds very sensitively by using the correlation of the images. The fiber sensing part consists of a curve in a form similar to the S structure. We reached a sensitivity of 0.0295 μm-1 by bending the fiber only 60°. Sensitivity can be further increased by reducing the bending diameter or creating a full loop.

Bend loss fiber optics design based on sinusoidal and ellipse configurations

Bending losses in optical fibers comprise one of the extrinsic attenuations that contribute to optical loss and they are essential for optical fiber bending sensor applications. This work investigated the optical loss in a standard single-mode step-index fiber optics due to fiber bending at 1550 nm wavelength. Variations in macro-bending loss with curvature radius and turn number have been measured. Curvature radius and turn number are examined for sinusoidal and elliptical shaped bending configurations. It has been found that the loss increases as the bending radius and number of turns increase. The result also showed that elliptical shaped bending configuration produced more loss in contrast to that of sinusoidal shaped at bending angles of 180° and 360°. The study on the macro-bending loss in terms of curvature radius and turn number for both elliptical and sinusoidal shaped bending configurations is beneficial for future fiber optic sensor applications.

Highly sensitive optical fiber bending sensor based on hollow core fiber and single mode fiber

We demonstrate a highly sensitive Fabry–Perot interferometer bending sensor based on vernier effect. The sensor device consists of two cascaded fabry-Perot cavities, one is an air-cavity formed by a section of hollow core fiber spliced with single-mode fiber, and the other is formed by two mirrors written in single-mode fiber by femtosecond laser. By accurately controlling the cavity length of the Fabry–Perot interferometer in single mode fiber, achieved by using femtosecond laser micromachining platform, a large and controllable amplification factor can be obtained. The bending sensitivity of our device is ∼52.877 nm/m−1. Such a device is simple in structure, easy in fabrication and stable in operation.

Intensity-modulated bend sensor by using a twin core fiber: theoretical and experimental studies.

A new optical fiber bend sensor is proposed and demonstrated based on a sandwich structure created by splicing a segment of twin core fiber (TCF) between two segments of single mode fibers (SMFs). One core of the TCF is aligned with the cores of two segments of SMFs. An incident beam is directed into the TCF by the lead-in SMF. Light couples back and forth between two cores. The bend sensing performance of the sensor is investigated by intensity-modulated method. The intensity of the operation wavelength is modulated by the change of refractive index and geometrical deformation in the bent TCF. Experimental results show such a bend sensor achieves sensitivity of +0.671 /m-1 and resolution of 0.003 m-1 in the range of 0 to 1.25 m-1. In addition, the sensitivity and bend measurement range can be flexibly adjusted through selection of the length of TCF and sensing configuration. As such, the proposed sensor can be further developed for large or small bend ranges measurement.

An Analytical Model for Describing the Power Coupling Ratio between Multimode Fibers with Transverse Displacement and Angular Misalignment in an Optical Fiber Bend Sensor.

The power coupling ratio between step-index multimode fibers caused by combined transversal and angular misalignment is calculated. A theoretical description of the coupling efficiency between two optical fibers based on geometrical optics is provided. The theoretical calculations are collaborated by experiments, determining the power coupling ratio between three output fibers with an axial offset and angular misalignment with a single input fiber. The calculation results are in good agreement with experimental results obtained using a previously fabricated optical fiber sensor for monitoring physiological parameters in clinical environments. The theoretical results are particularly beneficial for optimizing the design of optical fiber bending sensors that are based on power coupling loss (intensity) as the measurement interrogation requires either axial displacement, angular misalignment, or both.

Soft hand perception based on embedded optical fiber bending sensor

The perception capability of soft humanoid hand is very important to the implementation of friendly interactive and dexterous operation. The flexible optical bending sensor which is easy to be embedded into the soft finger is comprised of light emitting diode (LED), optical fiber and photo-sensitive sensor. The roughened optical fibers can generate light overflow. And the bend degree of soft fingers can be obtained by detecting the loss of light energy. The abilities of continuous movement detection, action gestures and object size recognition are implemented according to the following operations: ① calibrating the initial (unbent) and maximum bending statuses of soft fingers with normalization method; ② segmenting the normalized data by uniform, nonuniform and hysteresis segment method; ③ merging together the information of five fingers. The experiment results show that the proposed optical fiber bending sensor is able to detect the continuous sinusoidal movement status, correctly recognize various gestures and grasping postures and the object size. Combining hysteresis subsection method, the shortcoming of single point subsection oscillating near subsection point is overcome, and the recognition rate is improved. This optical flexible bending sensor has the advantages of electromagnetic immunity and easy to be embedded. It has important application value in soft robotic hand or soft actuator.

Orientation-dependent fiber-optic displacement sensor using a fiber Bragg grating inscribed in a side-hole fiber.

We propose and experimentally demonstrate an orientation-dependent fiber-optic bending sensor. The sensing probe consists of a fiber Bragg grating inscribed in both the fiber core and the surrounding cladding of a section of a side-hole fiber. We utilized a side-illumination technique using a femtosecond laser to achieve the grating structure formation. The transmission intensities of both resonances are highly sensitive bending of the fiber, and the bending response shows orientation dependence. The surrounding temperature fluctuation causes a wavelength shift, but not an intensity variation. Therefore, the proposed sensor can be employed for simultaneous measurement of bending and temperature.

Concave-lens-like long-period fiber grating bidirectional high-sensitivity bending sensor.

A novel bidirectional high-sensitivity fiber-optic bending sensor based on the concave-lens-like long-period fiber grating (CLL-LPFG) is designed and demonstrated. The CLL-LPFG is composed by an array of arc-shaped grating planes, and accordingly, its refractive index modulation serves as a concave lens. As a result, the eigencladding mode of the device gets closer to the device surface than the conventional counterpart. Therefore, the proposed sensor provides a more sensitive result. The experimental results show that the bending sensitivities of the CLL-LPFG reach -32.782 nm/m-1 within the bending range of 0-2.08 m-1, which is about sixfold compared to the reported arts. The sensitivity can be potentially improved by optimizing the grating parameters, and the temperature characteristics of the CLL-LPFG can be used to manipulate the grating spectrum.

An Extrinsic Optical Fiber Bending Sensor: A Theoretical Investigation and Validation

This paper presents the theoretical investigation and experimental validation of an extrinsic optical fiber bending sensor based on intensity modulation with automatic compensation. The proposed optical fiber bending sensor can measure continuous bending movement in flexion, extension, and lateral directions, simultaneously. The theoretical study is significant in choosing the most suitable parameter values for the sensor so that its sensitivity and working range are applicable for spine monitoring application. The fabricated bending sensor also shows a minimum signal fluctuation due to temperature effect.

Highly sensitive bending sensor based on multimode–multimode–coreoffset fiber structure

Abstract In this paper, we present a simple fiber optic bending sensor based on the multimode–multimode structure combining with the core-offset fiber structure. The multimode–multimode structure is composed of no core fiber (NCF) with hundreds of micrometers in length as a micro-lens for mode conversion, and single mode fiber (SMF) which can be seen as a section of special multimode fiber (MMF) when considered the cladding modes. The transmission spectrum in the experiment agrees well with the numerical model. The sensitivity of the structure can be achieved as high as 11.104 nm/m −1 in the measuring range. Meanwhile, the sensitivity of the neighboring resonance wavelength around 1546 nm exhibits approximately the same sensitivity which is 10.579 nm/m −1 . Besides, the strain sensitivity is about −0.927 pm/μe within the measuring strain range.

Fiber-Optic Bend Sensor Based on Double Cladding Fiber

We develop and investigate fiber-optic bend sensor, which is formed by a section of double cladding SM630 fiber between standard SMF-28 fibers. The principle of operation of the sensor is based on coupling of the fiber core and cladding modes at the splices of fibers having different refractive index profiles. We use two sources with wavelengths 1328 and 1545 nm to interrogate the sensor. The dependences of transmission on curvature at these wavelengths are significantly different. We show that the proposed sensor is able to perform measurements of curvature with radii from meters to 26 cm with accuracy of about 3%.

Optical Fibre Bending Sensor With Automatic Intensity Compensation

An extrinsic optical fibre bending sensor based on intensity modulation technique aimed for physiological application is presented. This sensing technique is capable of measuring the bending angle continuously in flexion, extension and lateral movements. The sensor working range is between −22° and 22° with 0.4° accuracy and 0.1° resolution. From the theoretical estimation, the bending sensor automatically compensates for variation in the source intensity and various losses in the input and output fibres. The experimental results obtained in this investigation verified the theoretical finding. Among the advantages of the this optical bending sensor are being noninvasive, suitable for continuous measurement and immune from electromagnetic effects.

Simultaneous Measurement of Bend Direction and Degree of Curvature using Micromachined Grating Fibers

We report the design and fabrication of a novel fiber optic bend sensor capable of measuring the bend direction and degree of curvature simultaneously. The sensor has a serial arrangement of two fiber Bragg gratings (FBGs) with two different reflection center wavelengths and similar asymmetrical cross sections with different etching faces on an independent single-mode fiber. These cross sections were fabricated by anisotropic reactive ion etching (RIE) using CF4 plasma. The reflection center wavelengths of the two FBGs shifted sinusoidally with a phase difference of π/2 as a function of the bend direction under constant bend feeder displacement. Because it was shown that the reflection center wavelength shifted linearly as a function of the degree of curvature of an FBG with an asymmetrical cross section in our previous experiment, this sensor could be used to detect both the bend direction and the curvature from a pair of central reflection wavelength shifts for two FBGs. These experimental results were compared with theoretical values calculated from a strain of the core in an etched optical fiber.

Fiber Optic Bending Sensor for Water Level Monitoring: Development and Field Test: A Review

In this paper, we describe a low cost optical sensor of water level based on fiber bending effect associated to the use of an elastomeric membrane. The sensor proposed has a particular design to be simple, reliable, and low cost. It is suitable to be used in tubes of embankment dams, tanks, and reservoirs. The sensor uses a standard single mode fiber and can measure the water levels up to 10 m or more, choosing the appropriate membrane. This paper describes the development of the sensor, a theoretical modeling, and the results of laboratory and field tests. Seven sensors were installed in an embankment dam where they have been used in a real time monitoring system based in optical time domain reflectometry.

Plastic Optical Fibre Sensor for Spine Bending Monitoring with Power Fluctuation Compensation

This paper presents the implementation of power fluctuation compensation for an intensity-based optical fibre bending sensor aimed at monitoring human spine bending in a clinical environment. To compensate for the light intensity changes from the sensor light source, a reference signal was provided via the light reflection from an aluminum foil surface fixed at a certain distance from the source fibre end tips. From the results, it was found that the investigated sensor compensation technique was capable of achieving a 2° resolution for a bending angle working range between 0° and 20°. The study also suggested that the output voltage ratio has a 0.55% diversion due to input voltage variation between 2.9 V and 3.4 V and a 0.25% output drift for a 2 h measurement. With the achieved sensor properties, human spine monitoring in a clinical environment can potentially be implemented using this approach with power fluctuation compensation.

Fiber-optic bend sensor using LP_21 mode operation

A novel fiber-optic bending sensor based upon the propagation of LP21 mode is demonstrated. The sensor, comprised of an S-bend fiber on an elastic film, measures LP21 mode specklegram rotation, which increments linearly with bending angle by the stress-optic effect. The sensor is capable of experimentally achieving a sensitivity as high as 4.13 rad/m(-1). The theoretical analysis of the sensor, which is a combination of fiber coupled-mode theory and elastic-optic theory, validates the accuracy of the sensor. The sensor is also shown to be temperature-immune, and can detect both bending direction and bending angle with a large dynamic range. Furthermore, the sensor implementation incorporates inexpensive single-mode fiber at 650 nm for few-mode operation, enabling low-loss transmission and compatibility with existing interfaces.

Loosening Monitoring of Bolted Joints Using Optical Fiber Bending Sensor for Aircraft Lug Assembly

Bolted joints of aircraft lug assembly play an important role in connecting the wing and fuselage of an aircraft structure. Generally, the bolted joints get loosened because they are frequently exposed to the dynamic loads induced when an aircraft is in service. For this reason, it is important to monitor the condition of the bolted joints to avoid any critical defect that will lead to any risk in human life. However, it is difficult to conduct the bolted joints’ loosening inspection by an operator. In past few decades, optical fiber based sensor has been widely used due to its advancement over a conventional piezoelectric (PZT) sensor, especially due to its small size and light weight. With regard to this, a loosening monitoring of bolted joints using optical fiber bending sensor for aircraft lug assembly is proposed in this paper. The lug assembly specimen, which consists of a stainless steel lug, CFRP/Nomex honeycomb sandwich panel, carbon-steel bolt, nut and washer, and the monitoring system, which consists of a single mode fiber with ten optical fiber bending sensor nodes and an Optical Time-domain Reflectometer (OTDR) were used to determine the bolt loosening at every 1 degree interval.

Multifiber optical bend sensor to aid colonoscope navigation

A colonoscopy's near-blind navigation process frequently causes disorientation for the scope operator, leading to harm for the patient. Navigation can be improved if real-time colonoscope shape, location, and orientation information is provided by a shape-tracking aid, such as a fiber optic bend sensor. Fiber optic bend sensors provide advantages over conventional electromechanical shape-trackers, including low cost and ease of integration. However, current fiber optic bend sensors lack either the ability to detect both bending direction and curvature, or the ability to detect multiple localized bends. An inexpensive multifiber bend sensor was developed to aid users in navigation during colonoscopy. The bend sensor employs active-cladding optical fibers modified with fluorescent quantum dots, bandpass filters, and a complementary metal-oxide-semiconductor imager as key components. Results from three-fiber sensors demonstrate the bend sensor's ability to measure curvature (error of 0.01 mm), direction (100% accuracy), and location (predetermined distance) of a bend in the fiber bundle. Comparison with spectroscopy data further confirmed the accuracy of the bending direction measurement for a three-fiber sensor. Future work includes improvements in fiber manufacturing to increase sensor sensitivity and consistency. An expanded 31 fiber bundle would be needed to track the full length of a colonoscope.

Highly sensitive bend sensor based on Mach–Zehnder interferometer using photonic crystal fiber

Optical fiber bend sensor with photonic crystal fiber (PCF) based Mach–Zehnder interferometer (MZI) is demonstrated experimentally. The results show that the PCF-based MZI is sensitive to bending with a sensitivity of 3.046 nm/m −1 and is independent on temperature with a sensitivity of 0.0019 nm/°C, making it the best candidate for temperature insensitive bend sensors. To that end, another kind of bend sensor with higher sensitivity of 5.129 nm/m −1 is proposed, which is constructed by combining an LPFG and an MZI with zero offset at the second splice mentioned above.

基于迈克尔逊干涉的光纤弯曲传感器

A novel high sensitivity fiber optic bending sensor based on Michelson interferometry was developed.The sensing probe utilizes a fiber ribbon affixed onto an elastic structure to generate interference signals as a function of curvature.Further a micro-cavity filled with refractive index matching liquid was added to the optical path length and modulated in audio frequency to modify interference signal,which was demodulated using phase generated signal carrier technology to obtain a highly sensitive detection of curvature change.Experimental measurement of prototype shows that the fiber optic bending sensor can achieve a sensitivity of 43.96 rad/m-1 and a high resolution of 0.004 m-1.