Draw Tower Gratings Research Articles

Draw-tower-grating-based Fabry-Perot interferometers for simultaneous measurements of temperature and salinity

In this paper, a fiber optic sensor with draw-tower-grating (DTG)-based Fabry-Perot interferometers (FPIs) are proposed to measure the temperature and salinity of seawater simultaneously. The sensing structure utilizes DTG as the reflector, and two adjacent gratings can be cascaded to compose an FPI. By employing moisture-sensitive materials to the surface of the optical fiber, the variation in salinity can be measured by the axial strain applied by the coating on the sensing optical fiber. The salinity altering causes swelling or shrinking actions of the coating through absorbing or releasing water. Two different moisture-sensitive materials are chosen to eliminate the cross-sensitivity of temperature and salinity. The experimental results show that the sensor exhibits linear response to temperature and salinity changes with good repeatability and stability, and the temperature and salinity sensitivities are 12308.65 rad/°C and 95.02 rad/‰, respectively. The optical path configuration of the DTG sensing array matching compensation interferometer provides the possibility for efficient distributed solutions, which has promising potential for application in marine engineering.

Optimization of surface bonding methods for fiber Bragg grating sensors at cryogenic temperatures

Optical fibre Bragg grating (FBG) sensors have been used for cryogenic temperature and strain measurements in a variety of applications, e.g., hotspot detection in high-temperature superconductor magnets. In such application, the sensitivity of the FBG and its reflection spectrum strongly depend on the methods of application, chiefly the adhesion and the elasticity of the bond, and the birefringence effect. To investigate those features, Draw Tower Gratings (DTG®) with Ormocer coating and FemtoSecond Gratings (FSG) with polyimide coating are mounted in the V-shaped grooves and on the surface of copper substrates using Stycast and Apiezon N. Their performance is investigated between 293 K and 77 K. The FBG sensors mounted in the V-grooves can achieve higher temperature sensitivities than those on the surface of the copper due to enhanced thermal strain transfer. It is found that although there is no obvious difference between the temperature sensitivity of the Ormocer and polyimide coated FBGs at cryogenic temperatures, the polyimide coated FBGs show superior reflection spectra, whose peak intensity remains strong at 77 K. Cryogenic vacuum grease Apiezon N is shown to bond the fibre well to the copper at cryogenic temperatures, which also reduces the birefringence effect induced from the non-uniform strain across the FBG. This paper provides practical knowledge of bonding FBG sensors in cryogenic environments for temperature and/or strain measurements.

Spatial Shape Sensing of a Multisection Continuum Robot With Integrated DTG Sensor for Maxillary Sinus Surgery

Owing to the curved and narrow pathway of the maxillary sinus, reaching blind regions using rigid instruments in the traditional method of treatment is difficult. In this article, a novel multisection continuum robot (CR) integrated with essential instruments and sensors is developed for the maxillary sinus surgery. Moreover, shape sensing is important for the control of the CR, but this is still a challenge due to the confined operating space. To track the spatial shape of the developed CR, a draw tower grating (DTG) sensor is integrated. A shape sensing algorithm is proposed to reconstruct the shape of the CR based on the relation between the DTG sensor and the CR centerline. Simulations and experiments on a maxillary sinus phantom are conducted to test the ability of the CR to reach the blind regions. Different groups of experiments are performed to evaluate the performance of the shape reconstructions, including DTG sensor shape tests and CR shape tests in planar and spatial space. Experiments show a mean planar error of 0.23 mm (0.51%) and a mean spatial error of 0.61 mm (1.36%). The results demonstrate the effectiveness of the multisection CR shape sensing method suggested in this study.

Feasibility of a lung airway navigation system using fiber-Bragg shape sensing and artificial intelligence for early diagnosis of lung cancer.

Currently early diagnosis of malignant lesions at the periphery of lung parenchyma requires guidance of the biopsy needle catheter from the bronchoscope into the smaller peripheral airways via harmful X-ray radiation. Previously, we developed an image-guided system, iMTECH which uses electromagnetic tracking and although it increases the precision of biopsy collection and minimizes the use of harmful X-ray radiation during the interventional procedures, it only traces the tip of the biopsy catheter leaving the remaining catheter untraceable in real time and therefore increasing image registration error. To address this issue, we developed a shape sensing guidance system containing a fiber-Bragg grating (FBG) catheter and an artificial intelligence (AI) software, AIrShape to track and guide the entire biopsy instrument inside the lung airways, without radiation or electromagnetic navigation. We used a FBG fiber with one central and three peripheral cores positioned at 120° from each other, an array of 25 draw tower gratings with 1cm/3nm spacing, 2 mm grating length, Ormocer-T coating, and a total outer diameter of 0.2 mm. The FBG fiber was placed in the working channel of a custom made three-lumen catheter with a tip bending mechanism (FBG catheter). The AIrShape software determines the position of the FBG catheter by superimposing its position to the lung airway center lines using an AI algorithm. The feasibility of the FBG system was tested in an anatomically accurate lung airway model and validated visually and with the iMTECH platform. The results prove a viable shape-sensing hardware and software navigation solution for flexible medical instruments to reach the peripheral airways. During future studies, the feasibility of FBG catheter will be tested in pre-clinical animal models.

Closed-Loop Trajectory Tracking Control of a Cable-Driven Continuum Robot With Integrated Draw Tower Grating Sensor Feedback

Abstract Due to the inherent deformability of continuum robots (CRs), closed-loop control strategies of CRs come with some challenges, including sensor integration within a limited dimension and uncertainties with the CR system. In this paper, to obtain the spatial shape information and tip position of a cable-driven CR, a draw tower grating (DTG) sensor is integrated. A shape sensing algorithm is proposed to reconstruct the shape of the CR based on the relation between the DTG sensor and the CR centerline. Then, with the feedback information of the DTG sensor, a damped least-squares (DLS)-based closed-loop controller is implemented to improve the trajectory tracking performance of the CR. To verify the effectiveness of the proposed scheme, comparative experiments are performed.

High-Resolution Optical Fiber Temperature Sensor Based on Draw Tower Grating Array.

Ocean temperature monitoring is of great significance to marine fishing, aquaculture, and marine operations. Traditional electric sensors lack the potential to multiplex several sensors, and may suffer from electromagnetic interference. Meanwhile, fiber Bragg grating-based sensors have the advantages of high sensitivity, possibility for large-scale multiplexing, and immunity to electromagnetic interference. In this paper, we propose a Fabry–Pérot (FP) interferometer based on the draw tower grating array and combine it with the phase measurement method for demonstration and testing. In the sensor system, two adjacent fiber Bragg gratings (FBGs) are used as mirrors and an optical fiber connects them, forming a sensor unit. The signal was detected through the compensation of the optical path difference via two-arm path differences in an unbalanced interferometer. The sensor is calibrated in the range of 36.00–36.50 °C, and back to 36.00 °C, in steps of 0.10 °C. A thermocouple (DW1222) is used as a reference. Experimental testing demonstrates that under the thermal loop, the temperature and phase can be approximated as a linear relationship, the Pearson square correlation coefficient is 0.9996, and the temperature sensitivity is −9846 rad/°C. To prove that our experimental device can achieve a higher temperature resolution, we measured the background noise of the system. The experimental results indicate that the order of magnitude of our system temperature resolution can reach 10−5 °C. Thus, we believe that the sensor system is promising for the application of ocean temperature detection, and owing to the ultraweak reflection characteristics of the FBG, this method provides the possibility for large-scale multiplexing of the system.

Importance of Internal Tensile Stress in Forming Low-Loss Fiber Draw-Tower Gratings

Internal tensile stress inside fiber core can impact significantly on formation of Draw-tower gratings (DTGs). We carry out online dynamic preparation experiments of DTG to analyze the effect of internal stress on grating reflectivity, as well as offline multi-pulse accumulation experiments to further investigate the underlying mechanisms for grating formation. The optimum laser energy density used to grating writing and the variation of reflectivity with exposure extent were obtained, which is instrumental for their optimization and rapid prototyping. A stress modulation model was proposed as the dominant mechanism to describe the formation of the grating structure by a single laser pulse in the low-loss fiber, which is of great significance for the research of the grating photosensitivity mechanism.

Distributed fiber optic interferometric geophone system based on draw tower gratings

A distributed fiber optic interferometric geophone array based on draw tower grating (DTG) array is proposed. The DTG geophone array is made by the DTG array fabricated based on a near-contact exposure through a phase mask during the fiber drawing process. A distributed sensing system with 96 identical DTGs in an equal separation of 20 m and an unbalanced Michelson interferometer for vibration measurement has been experimentally validated compared with a moving-coil geophone. The experimental results indicate that the sensing system can linearly demodulate the phase shift. Compared with the moving coil geophone, the fiber optic sensing system based on DTG has higher signal-to-noise ratio at low frequency.

Huge capacity fiber-optic sensing network based on ultra-weak draw tower gratings

This paper reviews the work on huge capacity fiber-optic sensing network based on ultra-weak draw tower gratings developed at the National Engineering Laboratory for Fiber Optic Sensing Technology (NEL-FOST), Wuhan University of Technology, China. A versatile drawing tower grating sensor network based on ultra-weak fiber Bragg gratings (FBGs) is firstly proposed and demonstrated. The sensing network is interrogated with time- and wavelength-division multiplexing method, which is very promising for the large-scale sensing network.

Comparative study of long period and fiber Bragg gratings under gamma irradiation

We evaluated comparatively the radiation sensitivity of two long period gratings (LPGs) manufactured in a SMF28™ optical fiber through the melting-drawing method based on CO2 laser assisted by a micro-flame, of a commercial fiber Bragg grating (FBG), and of a commercially available Draw Tower Grating (DTG), all exposed to gamma radiation. The tests were run on-line using the optical frequency domain reflectometry (OFDR) technique, in order to provide unequal sensitivity, the gratings being wavelength multiplexed. The dose rate dependence of the LPGs response was evaluated for dose rates of 0.37kGy/h and 0.24kGy/h, up to the total dose of (33.9±0.8)kGy and (21.5±0.5) kGy. The FBG and DTG proved to be radiation hardened having a radiation sensitivity of 12pm/kGy, while the LPGs radiation sensitivity was as high as 1.2nm/kGy under the same irradiation conditions. A recovery of the radiation induced wavelength shift for LPGs was noticed at room temperature, at a rate of 2.2pm/h, over a 211h period. The effect of LPG wavelength change after exposure to gamma-ray can be exploited in the development of radiation dosimeters, up to the dose of 10kGy, when saturation appears. The temperature sensitivity of the LPG2 changed from 48pm/°C before the irradiation, to 50 pm/°C after the irradiation for a total dose of 21kGy.

Signal-to-Noise Ratio Evaluation of Fibre Bragg Gratings for Dynamic Strain Sensing at Elevated Temperatures in a Liquid Metal Environment

Vibration measurements of the fuel assembly of a nuclear reactor are a very useful tool to determine the health and lifetime of the reactor core. The importance of these measurements is exacerbated in the new generation of heavy liquid metal reactors, where the fuel assembly is exposed to a corrosive molten metal coolant at 300 °C and where the space between the individual fuel pins is limited to a few millimeters. In this paper, we consider fibre Bragg gratings as potential candidates for carrying out fuel pin vibration measurements in such an environment. We describe a dedicated method to integrate fibre Bragg gratings in a fuel pin, and we subject this pin to conditions close to those encountered in a real heavy liquid metal reactor. More specifically, we report on the performance of draw tower gratings used as a vibration sensor when the fuel pins are immersed in heavy liquid metal at 300 °C for up to 700 h. The performance evaluation is based on monitoring the signal-to-noise ratio of the grating's spectral response as a function of time. We show that accurate detection of the Bragg peak becomes very challenging after 400 h of exposure. Additionally, we succeed to extend the useful lifetime with a factor of two by using an appropriate integration of the fiber in the fuel pin and by using an alternate peak detection algorithm.

Automated location of fiber Bragg gratings in draw tower grating arrays using a thermal scanning technique

For some critical applications, the location of fiber Bragg gratings (FBGs) in draw tower grating (DTG) arrays needs to be determined to sub-mm accuracy; for example, successful packaging of the FBGs in mm scale packages. The DTG manufacturing process leaves no external visible identification marks on the fiber, hence location needs to be determined prior to packaging. This work presents an automated fiber marking system that can accurately locate the positions of the FBGs within 0.1 mm. The simple, low cost, and automated system avoids manual fiber handling and allows accurate packaging of the FBGs for sensing applications. © 2014 Society of

Post-hydrogen-loaded draw tower fiber Bragg gratings and their thermal regeneration

The idea of Bragg gratings generated during the drawing process of a fiber dates back almost 20 years. The technical improvement of the draw tower grating (DTG) process today results in highly reliable and cost-effective Bragg gratings for versatile application in the optical fiber sensor market. Because of the single-pulse exposure of the fiber, the gratings behave typically like type I gratings with respect to their temperature stability. This means that such gratings only work up to temperatures of about 300 °C. To increase temperature stability, we combined DTG arrays with hydrogen postloading and a thermal regeneration process that enables their use in high-temperature environments. The regenerated draw tower gratings are demonstrated to be suitable for temperatures of more than 800 °C.

Effect of the Fiber Coating on the Radiation Sensitivity of Type I FBGs

We have studied the influence of the fiber coating on the radiation sensitivity of fiber Bragg gratings (FBGs). These FBGs are draw tower gratings written before applying the coating in a fiber with a photosensitive 18-mol.% GeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -doped core. Using a Co <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> source we irradiated polyimide, acrylate, and ormocer coated gratings as well as mechanically stripped ormocer coated FBGs up to a 40-kGy dose. This total dose level corresponds to a near-earth space radiation environment mission. We show that the coating type must be taken into account for correctly interpreting ionizing radiation effects on FBGs.

High-reflectivity draw-tower fiber Bragg gratings—arrays and single gratings of type II

Fiber Bragg gratings (FBG) were manufactured during the fiber drawing process [draw tower grating (DTG)] with excellent reflectivity values. This was done in the region of 1550 nm by single pulses of a 248-nm excimer laser applied during the fiber drawing process of single mode fibers. An improved setup for the writing process and special photosensitive fibers enable the manufacture of type I DTG arrays with a reflectivity of up to 40% and type II DTGs with a reflectivity near 100%. Details of the setup and results of the DTG arrays and DTGs of type II are reported.