On-line writing identical and weak fiber Bragg grating arrays

A fully polymer-based flexible arc-shaped three-dimensional (3D) fiber Bragg grating (FBG) array bionic tactile sensor is presented, in which both the FBG array and the encapsulation layer are made of polymers, and its application in multidirectional force identification of mechanical finger is verified. This innovative design utilizes the unique flexibility, biocompatibility, and mechanical properties of polymers to achieve excellent integration and performance consistency. The FBG array, with three FBGs inscribed in a single-mode polymer fiber using femtosecond laser micromachining, featuring progressively increasing lengths to compensate for the reflectivity degradation typically observed in conventional FBGs. The 3D arc-shaped arrangement of the FBG array endows the sensor with the ability to achieve 3D force perception, including normal stress, left shear stress, and right shear stress. By accurately measuring the wavelength shifts of the FBGs under different force stimuli, the sensor can accurately distinguish multi-directional forces. Experimental results show that the sensor has high force sensitivities to the left shear force, normal force, and right shear force with sensitivities of 4.973nm/N, 4.363nm/N, and 4.746nm/N, and measurement resolutions of 0.02N, 0.023N, and 0.021N, respectively, within a verified linear working range of 0 N to 0.8 N. Its sensitivity is as high as 17 times that of the traditional quartz optical fiber FBG sensor. our sensor can accurately measure multi-dimensional forces with very good sensitivity and significantly reduce the crosstalk caused by humidity, demonstrating very good performance. Its all-polymer design exhibits high flexibility and environmental robustness—completely eliminating crosstalk caused by humidity changes. It is expected to have potential applications in advanced robotics and intelligent tactile sensing systems.

For the improvement of monitoring accuracy, a vibration monitoring for aircraft wing model using a fiber Bragg grating (FBG) array packaged by vacuum-assisted resin transfer molding (VARTM) is proposed. The working principle of the vibration monitoring using FBG array has been explained, which can theoretically support the idea of this paper. VARTM has been explained in detail, which is suitable for not only the single FBG sensor but also the FBG array within a relatively large area. The calibration experiment has been performed using the FBG sensor packaged by VARTM. The strain sensitivity of the VARTM package is 1.35 pm/μϵ and the linearity is 0.9999. The vibration monitoring experiment has been carried out using FBG array packaged by VARTM. The measured rate of strain changes across the aluminum test board used to simulate the aircraft wing is 0.69 μϵ/mm and the linearity is 0.9931. The damping ratio is 0.16, which could be further used for system performance evaluation. Experimental results demonstrate that the vibration monitoring using FBG sensors packaged by VARTM can be efficiently used for the structural health monitoring. Given the validation and great performance, this method is quite promising for in-flight monitoring and holds great reference value in other similar engineering structures.

Over the past years, Thulium (Tm) -doped fiber lasers in the 2μm region have gained a lot of interest due to many potential applications in materials processing and biophotonics. Based on the broad gain regions spanning from 1800nm to 2100nm, they offer the perfect basis to implement broadly tunable and user-friendly light sources like they are increasingly demanded in spectroscopic applications. Recently, a novel tuning mechanism based on a fiber Bragg grating (FBG) array as versatile spectral filter has been reported. This concept combines unique spectral freedom for customized tuning ranges and ultrabroad bandwidths with a fiber-integrated setup in order to maintain the advantages of the waveguide geometry. In this work, we demonstrate such a dispersion tuned and pulsed fiber laser in the Tm domain around 1950nm using a modulator and a discrete FBG array to control the emission wavelength. In order to comply with the demands of potential applications in biophotonics, for the first time, this tuning concept is realized in a polarization maintaining (PM) configuration ensuring linearly polarized output. While a simple FBG array is employed containing five gratings inscribed in PM fiber, we also outline the prospect to implement FBGs fabricated in a standard single mode fiber. The emission characteristics of the system are investigated showing pulse durations down to 11ns and a good spectral signal contrast. In order to highlight the prospect for tunable high-power operation, we have also implemented an amplification stage scaling the average power to more than 25W.

This paper presents the experimental work on distributed temperature sensing making use of Fiber Bragg grating (FBG) array sensor for possible applications in the monitoring of temperature profile in high temperature boilers. A special sensor has been designed for this purpose which consists of four FBGs (of wavelengths λB1 = 1547.28 nm, λB2 = 1555.72 nm, λB3 = 1550.84 nm, λB4 = 1545.92 nm) written in hydrogen loaded fiber in line with a spacing of 15 cm between them. All the FBGs are encapsulated inside a stainless steel tube for avoiding micro cracks using rigid probe technique. The spatial distribution of temperature profile inside a prototype boiler has been measured experimentally both in horizontal and vertical directions employing the above sensor and the results are presented.

We demonstrated a novel intensity-modulated interrogation system for an identical weak fiber Bragg grating (FBG) array based on a resonance frequency mapping technique. To compensate for the signal depletion phenomenon because of the increased reflection number of identical weak FBGs, a gain-flattening technique is employed based on the intensity modulation of the applied driving current in the optical fiber laser sensor cavity. Over 60 identical FBGs can be efficiently interrogated by detecting the resonance frequency of each FBG for a distinguished peak position.

A quasi-distributed directional bending sensor based on fiber Bragg gratings (FBGs) array in triangle-four core fiber (TFCF) is proposed and experimentally demonstrated. The proposed sensor was composed of a single mode fiber (SMF) and a TFCF. A piece of TFCF is heated by a thermal diffusion technique (TDT) to fabricate an integrated coupler, which couples the optical power from central core into the side cores. Considering the size of the Gaussian beam and the effective power distribution on the cross section of excimer laser, we fabricated FBGs samples by a defocusing phase mask method. We fabricated some FBGs array samples in TFCF with peak wavelengths in the range from 1528 nm to 1566 nm with a step of 4 nm. The error and variance of peak wavelengths and reflectivity are also measured. Obviously, the consistency of wavelength and strength are satisfactory. The curvature, temperature, and strain responses of FBG array samples with the central core and side cores are studied. Based on the unique characteristics of the proposed sensor, cross-sensitivity of temperature, and strain can be solved by subtracting the wavelength changes of side cores from that of central core. In addition, by adding temperature compensating sensor, we analyzed the combined standard uncertainty for simultaneous measurement of curvature, temperature, and strain. The proposed sensor have high potential to monitor the deformation of the smart wing on the spot and provide information for the flight control system to evaluate the aerodynamic performance of flight conditions.

Local structural health monitoring system in aircraft based on fiber Bragg grating array

As Operators are increasingly seeing the value of in-well monitoring in optimizing production and reducing interventions, Operators are demanding greater accuracy, reliabilty and timeliness of well information above what is available today. This study explores a new method of measuring temperatures in harsh downhole environments that aims to improve on some of the limitations of conventional distributed temperature sensing (DTS). Instead of using Raman backscatter signals to infer temperature, a calibrated fiber Bragg grating (FBG) array is used to measure the temperature. Bragg gratings have previously been deployed as temperature measuring devices, usually as discrete devices individually spliced in a cable. The cable design presented in this study reduces deployment costs compared to discrete Bragg gratings, and can also scale up to 1000 gratings. Additionally, an interrogation method has been developed for measuring the wavelengths of the FBG array. Traditional swept-wavelength interrogation techniques exist to measure FBG arrays, but they are typically limited to approximately 40 gratings. Like the cable, the interrogation method scales up to 1000 gratings at 1 meter grating spacing. Numerous system performance tests were conducted in a laboratory setting and in a simulated well environment. A prototype cable was deployed in a test well alongside a standard DTS cable. The cables were exposed to temperature cycling by circulating hot and cold fluid. Measurements produced by the two cables were recorded and analyzed. The prototype cable test results demonstrated that the grating cable is capable of higher resolution temperature measurement than conventional DTS. The prototype interrogator will be able to perform the measurement in less than 10 seconds, a fraction of the measurement time required by conventional DTS interrogation. Results from other tests have demonstrated the ability to perform well with long lead-in distances without losing accuracy. With this increased range, the interrogator can be located away from the wellhead and in offshore wells, above water.

Drawing-tower inscription of apodized fiber Bragg grating arrays using a rotated focusing cylindrical lens

Fiber Bragg grating (FBG) array, consisting of a number of sensing units in a single optical fiber, can be practically applied in quasi-distributed sensing networks. Serious signal crosstalk occurring between large-serial of identical FBGs, however, has limited the further increase in the number of sensing units, thus restricting applications only for short-distance sensing networks. To reduce the signal crosstalk, we design two novel types of 10-kilometer-long FBG arrays with 10 000 equally spaced gratings, written on-line using a customized grating inscription system, which is affiliated to a drawing tower. Main factors causing signal crosstalk, such as spectral shadowing and multiple reflections, are firstly investigated in theory. Consistent with the theoretical findings, experimental results are proving that ultra-weak (the reflectivity of ∼−40 dB) and multi-wavelength gratings of a number more than 10 000 can be readily identified, with satisfied low crosstalk. The maximum attenuation of grating signal and minimum signal-to-noise ratio (SNR) in a single-wavelength array are 10.69 dB and 5.62 dB, respectively. As a comparison, by increasing the number of central wavelengths to three, the attenuation can be effectively reduced to 5.54 dB and the minimum SNR has been improved to 8.14 dB. The current study significantly enhances the multiplexing capacity of FBG arrays and demonstrates promising potentials for establishing large-capacity quasi-distributed sensing networks.

A three element, 15.3 cm, fiber Bragg grating array (FBGA) operating at 1550 nm wavelength is fabricated using a single mode photosensitive fiber. The FBGA is initially simulated using in-house developed software based on the Transfer Matrix Method, then fabricated using a double frequency Argon laser and a phase mask technique, and interrogated using Optical Frequency Domain Reflectometry. A single fiber Bragg grating (FBG) is accurately strain calibrated using a Fabry-Perot interferometer and piezoelectric actuation. The piezoelectric is linearly ramped, and the shifts in the Bragg wavelength along with the fringe count from the Fabry-Perot interferometer are recorded. The fringe count is then used to determine the strain on the FBG and compared to changes in the Bragg wavelength in-order to calculate the strain gage factor. This result is used to calibrate the FBGA for strain measurements. The FBGA is then bonded to a cantilever beam with three electric strain gages attached next to each FBG in the array. The axial strain results obtained from the electric strain gages and FBGA are compared for various displacements of the cantilever beam. The Fabry-Perot interferometer and piezoelectric calibration method is a non-destructive process that eliminates the need to bond the FBG to an external support during the calibration process, and can also be used to calibrate electric strain gages.

On-line fabricated fiber Bragg grating (FBG) array and its sensing potentials have attracted plenty of attention in recent years. In this paper, FBG arrays are written on-line on a two-mode fiber, and this two-mode fiber Bragg grating (TM-FBG) is further experimentally investigated for temperature and curvature sensing. The responses of this sensor were characterized by 11.2 pm/°C and −0.21 dB/m−1 for temperature and curvature, respectively. Based on the measurements, a dual-parameter fiber sensing system was developed, which can realize the quasi-distributed, simultaneous detection of temperature and curvature, making it suitable for structural health monitoring or perimeter security.

We demonstrate a quasi-distributed sensor for cantilever health inspection measurements using a fiber Bragg grating (FBG) array inscribed in a polymer optical fiber. The FBGs were characterized and calibrated for axial strain, temperature, and relative humidity prior to their mounting on a carbon cantilever beam, the tail rotor of a helicopter. By using the zero-crossing demodulation algorithm, we recovered the time-dependent, wavelength response from each Bragg grating sensor and the vibration response of the beam was extracted. We used the response of the beam to study how the addition of masses at different positions on the beam influences the vibrational behavior and mimics the location of “damage” through the time-dependent results. We show that health inspection measurements are feasible with polymer-based fiber Bragg gratings, offering accurate and rapid detection of damage points on a structural beam.

We present a novel method to discretely tune the emission wavelength of pulsed fiber-integrated lasers. As spectral filter, a step-chirped fiber Bragg grating (FBG) array is employed combining a monolithic structure with an unrivaled design freedom enabling large tuning bandwidths as well as tailored spectral characteristics towards fingerprint tuning features. Together with an electrical control mechanism ensuring programmable operation, this tuning method promotes fiber-integrated lasers to access new fields of applications e.g. in biophotonics and distributed sensing. The potential of this tuning concept is investigated based on an Ytterbium-doped fiber laser. The system shows superb emission properties including excellent wavelength stability, high spectral signal contrast (up to 50dB) and narrow linewidth (15GHz) as well as adjustable pulse durations in the nanosecond range with peak powers up to 100W. Additionally, the unique spectral potential of this method is demonstrated by realizing filter designs enabling e.g. a record tuning range of 74nm for fiber-integrated lasers.

We propose a novel method for fabrication of ultra-weak Fiber Bragg Grating (UWFBG) Arrays, which grating array is UV-inscribed with single exposure pulses in the UV-Transparent Coating Optical Fiber (UV-TCF) assisted by a fiber winding system. Compared with the drawing tower grating technique, such system has a more flexible grating control, such as wavelength multiplexing, space distribution.