Ultra-weak Fiber Bragg Gratings Research Articles

Terahertz-range interrogated grating-based two-axis optical fiber inclinometer

We report a two-axis fiber inclinometer fabricated using an ultraweak terahertz-range fiber Bragg grating. Three sensing grating structures were inscribed along a single-mode optical fiber using a femtosecond laser, bound together into a sensing array using thermoformed plastic, and fixed to a two-axis rotational stage. Inclination tests were performed in which the fiber was deflected from 0 to 1.7 deg. These tests were repeated at eight azimuthal angles in increments of 45 deg (from 0 to 315 deg). The standard deviation of the largest incli- nation angle error was 0.048 deg and the stability of the inclination angle was 0.030 deg.©2016 SocietyofPhoto-Optical

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.

Field-programmable gate array-based large-capacity sensing network with 1642 ultra-weak fiber Bragg gratings

A field-programmable gate array (FPGA)-based large-capacity sensing network with ultra-weak fiber Bragg gratings (FBGs) is proposed and experimentally studied. The demodulation system is constructed to interrogate 1642 serial time-division-multiplexing FBGs with a peak reflectivity of about −40 dB and equal separations of 2.5 m. Two semiconductor optical amplifiers and an InGaAs linear sensor array controlled by an FPGA are introduced to the demodulation system to achieve fast, precise, and flexible interrogation. The low crosstalk and spectral distortion are investigated through both theoretical analysis and experiments.

Distributed OTDR-interferometric sensing network with identical ultra-weak fiber Bragg gratings.

We demonstrate a distributed sensing network with 500 identical ultra-weak fiber Bragg gratings (uwFBGs) in an equal separation of 2m using balanced Michelson interferometer of the phase sensitive optical time domain reflectometry (φ-OTDR) for acoustic measurement. Phase, amplitude, frequency response and location information can be directly obtained at the same time by using the passive 3 × 3 coupler demodulation. Lab experiments on detecting sound waves in water tank are carried out. The results show that this system can well demodulate distributed acoustic signal with the pressure detection limit of 0.122Pa and achieve an acoustic phase sensitivity of around -158dB (re rad/μPa) with a relatively flat frequency response between 450Hz to 600Hz.

Improved Φ-OTDR Sensing System for High-Precision Dynamic Strain Measurement Based on Ultra-Weak Fiber Bragg Grating Array

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) has been widely used in various applications for its distributed measurement capability of dynamic disturbance along the entire length of sensing fiber. However, traditional Φ-OTDR cannot make high-precision measurement on the external disturbance-induced strain due to the randomly distributed position and reflectivity of scattering points within the optical fiber. In this paper, ultra-weak fiber Bragg grating (UWFBG) array has been proposed to generate strong and controllable reflections while providing acceptable insertion loss. Active laser frequency sweeping and unwrapping algorithms were included to set up a definite relationship between strain value and variation of interference power between neighboring reflection lights. With the assistant of UWFBG array, the capability of Φ-OTDR has been upgraded to be able to take high-precision measurement on strain. In the experiment, multipoint μϵ level dynamic strain variation has been fully captured at the end of a 5-km long sensing fiber with 2 m spatial resolution. The measured strain value was compared to the calibrated one obtained with Mach–Zehnder Interferometer method. These two values fit with each other quite well with a maximum deviation of 6.2 nϵ, which confirmed the validity and accuracy of the proposed method.

Ultra-weak FBG and its refractive index distribution in the drawing optical fiber.

For the online writing of ultra-weak fiber Bragg gratings (FBGs) in the drawing optical fibers, the effects of the intensity profile, pulse fluctuation and pulse width of the excimer laser, as well as the transverse and longitudinal vibrations of the optical fiber have been investigated. Firstly, using Lorentz-Loren equation, Gladstone-Dale mixing rule and continuity equation, we have derived the refractive index (RI) fluctuation along the optical fiber and the RI distribution in the FBG, they are linear with the gradient of longitudinal vibration velocity. Then, we have prepared huge amounts of ultra-weak FBGs in the non-moving optical fiber and obtained their reflection spectra, the measured reflection spectra shows that the intensity profile and pulse fluctuation of the excimer laser, as well as the transverse vibration of the optical fiber are little responsible for the inconsistency of ultra-weak FBGs. Finally, the effect of the longitudinal vibration of the optical fiber on the inconsistency of ultra-weak FBGs has been discussed, and the vibration equations of the drawing optical fiber are given in the appendix.

一种超弱光纤光栅阵列的定位方法

一种超弱光纤光栅阵列的定位方法

Quasidistributed fiber Bragg grating sensor network based on self-heterodyne detection technique

A serial time-division multiplexing optical fiber sensing network with a large multiplexing capacity, which is based on identical ultraweak fiber Bragg gratings (FBGs) and self-heterodyne detection technique, is proposed. An experimental system, which has 10 identical ultraweak FBGs with the same Bragg wavelength of 1550 nm, reflectivity of − 36 dB , and bandwidth of 0.1 nm, is set up to investigate the performance of the proposed scheme. The spectra of 10 ultraweak FBGs are resolved with a high accuracy, and the wavelength–temperature sensitivity and temperature resolution of the system are 10.5 pm /° C , 0.09°C, respectively. A self-heterodyne detection technique is adopted to increase the sensitivity of the receiver, which makes it possible to multiplex over 1000 FBGs along a single optical fiber. Theoretical analyses demonstrate that this sensing scheme can effectively increase the multiplexing capacity and measurement accuracy.

A Novel Interrogation System for Large Scale Sensing Network With Identical Ultra-Weak Fiber Bragg Gratings

A novel interrogation system for large scale sensing network with identical ultra-weak fiber Bragg gratings (FBG) is proposed, theoretically analyzed and experimentally studied. Based on time-division multiplexing (TDM) technique, two semiconductor optical amplifiers are introduced to the interrogation system to achieve fast and flexible demodulation. A simulation depending on the formula of reflected pulse spectra, derived by analyzing the source of disturbance along the optical path, is processed for evaluating the performance theoretically. Experiment was demonstrated on an 843 serial FBGs sensor network, with a peak reflectivity of -40 to -45 dB and a spatial resolution of 2 m along a single optical fiber by launching a 50 kHz scanning frequency with 20 ns effective pulse width to the system. Due to the low crosstalk, it is possible to integrate several thousands of FBG sensors in a single optical fiber.

超弱反射光栅准分布式光纤传感系统研究

超弱反射光栅准分布式光纤传感系统研究

Interrogation system for a sensor array with 1310-nm band ultra-weak fiber Bragg gratings

An interrogation system for the time-division multiplexed sensor array based on 1310-nm band ultra-weak fiber Bragg gratings (FBGs) was proposed in this study. Power spectral density was introduced to budget the optical power and evaluate the interrogation capability of the proposed system. The proposed system consists of one tunable laser and two semi- conductor optical amplifiers, where the former scans the wavelength, and the latter modulates the light and separates the reflected pulses from dif- ferent FBGs through delayed switching. Up to 1000 serial FBGs, with a peak reflectivity of −40 to −45 dB and a spatial resolution of 5 m along an optical fiber, were interrogated to demonstrate the feasibility of the inter- rogation system. © 2013 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI:

A time- and wavelength-division multiplexing sensor network with ultra-weak fiber Bragg gratings

A time- and wavelength-division multiplexing sensor network based on ultra-weak fiber Bragg gratings (FBGs) was proposed. The low insertion loss and the high multiplexing capability of the proposed sensor network were investigated through both theoretical analysis and experimental study. The demodulation system, which consists of two semiconductor optical amplifiers and one high-speed charge-coupled device module, was constructed to interrogate 2000 serial ultra-weak FBGs with peak reflectivity ranging from -47 dB to -51 dB and a spatial resolution of 2 m along an optical fiber. The distinct advantages of the proposed sensor network make it an excellent candidate for the large-scale sensing network.

Online reflectivity measurement of an ultra-weak fiber Bragg grating array

An online measurement method is introduced to ensure the reflectivity of an arbitrary grating in a large-scale ultra-weak fiber Bragg grating (FBG) array. The measurement errors were investigated by theoretical analysis and experiment. In the proposed method, the reflected spectrum of one single FBG was addressed and obtained from the grating array by time domain interrogation, and the FBG reflectivity was calculated by the reference of the end-face reflection in anhydrous ethanol. A measurement error of less than 0.5 dB and a 20 dB dynamic range were obtained in a larger-scale array of 1000 FBGs with an average peak reflectivity of −43 dB, which demonstrated the feasibility of this online measurement method.

A Large Serial Time-Division Multiplexed Fiber Bragg Grating Sensor Network

We demonstrated a wavelength scanning time division multiplexing of 1000 ultra-weak fiber Bragg gratings (FBG) for distributed temperature sensing. The strong multiplexing capability and low crosstalk of the ultra-weak FBG sensors is investigated through both theoretical analysis and experiment. An automated FBG fabrication system was developed for fast 1000-FBG fabrication. The measurement accuracy of Bragg wavelengths for more than 80% FBGs is less than 20 pm, corresponding to 2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}$</tex> </formula> C in temperature.

A Quasi-Distributed Sensing Network With Time-Division-Multiplexed Fiber Bragg Gratings

We propose a serial time-division-multiplexing sensor network based on ultraweak fiber Bragg gratings (FBGs). Twelve ultraweak FBGs are distinguished with a spatial resolution of about 0.2 m, and their spectra are resolved with an accuracy of about 10 pm. The low crosstalk of the gratings makes it possible to multiplex over 1000 gratings along a single optical fiber.