Outdoor Experimental Verification of Multicore Fiber Based On Multiparameter Measurement

Cai Sijia

摘要：提出了一種基于多芯光纤的新型结构健康监测传感系统。通过利用多芯光纤，光纤布拉格光栅（FBG）传感器，拉曼光时域反射仪（ROTDR）和基于偏振检测的振动传感器（PVS）的丰富的空间划分通道，成功实现了同时应变，温度和强度的集成。振动感应。为了检验所提出的传感技术在实际应用中的可行性，已将具有不同载荷水平的静态载荷和具有不同振动频率的动态载荷应用于具有表面粘结7芯光纤并受噪声和环境影响的简单支撑钢工字梁外部压力。测量结果表明，基于FBG和PVS的振动传感可同时覆盖低频和高频监测，而FBG和ROTDR组合传感可精确测量应变和温度。提议的传感技术在户外实验中的性能证明了其在结构健康的多参数监测中的潜在应用。

关键词：多芯光纤;振动传感;高频监测

Abstract： A novel structural health monitoring sensing system based on multicore fiber has been proposed in this paper. By utilizing the abundant space-division channels of the multicore fiber， fiber Bragg grating （FBG） sensor， the Raman optical time domain reflectometry （ROTDR） and polarization detection-based vibration sensor （PVS） are successfully integrated to realize simultaneous strain， temperature and vibration sensing. To examine the feasibility of the proposed sensing technology in practical application， static loads of different loading levels and dynamic loads of different vibration frequencies have been applied to a simple supported steel I-beam with surface bonded 7-core fiber with influence of noise and ambient external stress. The measurement results show that the FBG and PVS based vibration sensing covers the both low frequency as well as high frequency monitoring and the FBG and ROTDR combined sensing can precisely measure strain and temperature. The performances of proposed sensing technology in outdoor experiment demonstrate a potential application in multi-parameter monitoring of structural health.

Key words： Disabled;Expression of intrests; Mechanism; Path

CLC number：TP212          Document identification code： A

1 Introduction

Structural Health Monitoring （SHM） can be considered as an integration of sensing and characterization strategies to continuously detect the structure condition and identify structural damage. Optical fiber sensors and sensor networks are well suited for the application as non-destructive sensing paradigms in SHM field due to the particular characteristics of the optical fiber， such as flexibility， durability， immunity to electrical and magnetic interference， and the measuring distance can be very long， as compared with traditional sensors [1-3]. In recent years， optical fiber sensors have been successfully applied to buildings [4，5]， piles [6]， bridges [7，8]， pipelines [9，10]， tunnels [11] and hydroelectric dams [12] to measure strain and temperature， as well as to monitor displacements， cracks， and weight in motion. However， several problems limit the further commercial promotion.

The most prominent problem is that cross-sensitivity of strain and temperature exists in most optical fiber sensors， such as the most widely used Fiber Bragg Grating （FBG） sensors in civil engineering. The strain on the surface of a structure can be inferred by measuring the Bragg resonance wavelength of surface-bonded FBG sensors. Meanwhile， ambient temperature variance also results in the shift in the Bragg resonance wavelength. Thus， single measurement cannot discriminate the temperature and strain effect. An alternative to overcome the cross-sensitivity is to use at least two FBG sensors. A tensioned FBG sensor is bonded tightly along the structure to detect the strain， while another loosely bonded FBG sensor is used to measure only the temperature and compensate for the temperature effect in the first sensor.

A more attractive way to improve the multiparameter discriminative capability is using multicore-fiber based spatial-division-multiplexing （SDM） sensing technologies. The concept of SDM technologies allowing autonomous data streams to be transmitted in parallel spatial channels is not so new in the telecommunications industry [13]. Such spatial channels have recently been used in optical sensing applications where the returned echo is analyzed for the collection of essential environmental information [14]. As one kind of SDM implementation， a multicore-fiber simultaneously has parallel lightwave paths that permit the presence of multiple sensors in the same fiber cross-section. Therefore， the multicore fiber based SDM sensing systems exhibit great potential for simultaneous measurement of temperature and strain by utilizing one single-strand of fiber with many independent cores. Moreover， multicore-fiber based SDM sensing technologies are capable of vibration sensing， which is also a key parameter for SHM.

The current research on multicore fibers sensing is mainly focused on their use as shape sensors [15，16] or high temperature sensors [17，18]. In the few studies of simultaneous multi-parameter sensing based on multicore fiber， Newkirk et al. [19] demonstrated a force and temperature sensor. They looked at the dependence of the strain sensitivity on the cladding diameter of the multicore fiber. To decouple the force and temperature they used two multicore fiber sections with different outer diameters. Silva et al. [20] proposed a suspended multicore fiber sensor for simultaneous measurement of curvature and strain. However， these physical parameters were also not simultaneous measured in separate cores in a multicore-fiber， but were discriminated by using matrix method. Aforementioned multicore-fiber based sensors only employed the multicore fiber as a single channel waveguide. When these sensors are applied to SHM， there may be some limitations， such as the measurement of static strain， temperature and vibration of the structure at the same time.

In this paper， by utilizing inherent spatial-division multiplexing property in multicore fibers， we design and fabricate a multi-parameter sensor based on one single optical fiber. The optical fiber with seven cores arranged in a hexagonal array contains fiber Bragg gratings （FBGs）， a distributed temperature sensor and polarization detection-based vibration sensors （PVS）， which allows simultaneous strain， temperature， as well as vibration sensing. For sensors in SHM systems， the main challenge is to assure that the sensor system itself is stable when deployed in the field. For this purpose， an outdoor experiment has been conducted by bonding the multicore fiber to the surface of a simple supported steel I-beam. Results show that designed multi-parameter sensor is reliable and sensitive to detect early structural malfunction.

2 Principle of multi-parameter measurement

The cross-section of the multicore fiber used is shown in Figure 1. The multiparameter measurement is realized by spatially multiplexing the FBG sensor， ROTDR and PVS via different cores of the multicore fiber. The algorithms for detecting the state of the structure are described below.

An Optical Fiber Grating （FBG） can be understood as an optical fiber with a periodic refractive index perturbation pattern inscribed in the core such that it diffracts the optical signal in the guided mode at specific wavelengths. When light is made to pass through the grating， at a particular wavelength， called the Bragg wavelength， the light reflected by the varying zones of refractive indices will be in phase and amplified. The Bragg wavelength is expressed as

where  is the Bragg wavelength， is the effective refractive index of the FBG and  is the grating period. The shift in the Bragg wavelength due to the changes in the temperature and strain is expressed as

where      is the change in temperature experienced at the FBG location and is the longitudinal strain on the FBG， the     and     are coefficients of wavelength sensitivity to temperature and strain， respectively [21]. Due to the fiber bending sensitivity of FBGs， two symmetrical outer cores FBG are designed to eliminate the shift effect caused by bending.

For pure strain measurements， effects of temperature change on the Bragg wavelength has to be suitably compensated. To measure temperature distribution， we propose to monitor the temperature effects of spontaneous Raman backscattering of distributed temperature sensor called Raman Optical Time Domain Reflectometry （ROTDR）. When light get through the optical fiber， collision between photonic and phonon will lead to Raman scattering. The backscattered Raman photons contains information about the temperature distribution along the optical fiber. The relation can be described by [22]：

where      and      are the Stokes and anti-Stokes wavelengths， h and k are Plancks and Boltzmanns constants， c is the speed of light in vacuum，      is the amount of Migration wave， and T is the temperature in Kelvin. Because of it has no effect with strain， ROTDR has a high degree of accuracy， and is easy to fix up distributed.

On the other， using a single FBG sensor in conjunction with an unbalanced fiber interferometer wavelength discriminator， high-resolution vibrations can be detected. The shift in the Bragg wavelength of the FBG detected in this way results in an interferometric signal output         ， which is given by and  where A is a constant， b is the fringe visibility，  φis the phase difference between the interferometer arms，           is the induced phase shift，  is the refractive index of the interferometer medium and    δL is the optical path-difference between the interferometer arms. If the wavelength-shift of the FBG is  δγB and the fibre strain isδσ ， then δγB =kδσ， where k is a constant depending on the strain-to-wavelength-shift responsivity of the FBG. Thus， the dynamic strain-induced change of the reflected wavelength can be modulated [23].

Taking advantage of abundant space-division channels of the multicore fiber， we propose to make use of analysis of polarization for distributed vibration monitoring. Since the polarization of the transmission light is very sensitive to perturbations， the information of external events can be obtained by measuring the change of the state of polarization （SOP） of the Rayleigh backscattering light along the fiber [24]. Assume that Sin， S（z）， and Sb（z） are the SOP of the incident light， the light at the position z ， and the Rayleigh backscattering light from the position z ， respectively. This gives the equations [25]：

where M is the Mueller matrix expressing the evolution of the SOP of the light from fiber initial end to the position z and R is the Mueller matrix for forward propagation. When the fiber is disturbed at a certain position on the fiber， the polarization is thereby modified and then detected by the analyzer via the coupler. Thus， the SOP change of any point z along the fiber， and therefore the vibration， can be detected. This technology can be used to probe vibration for its advantages such as high sensitivity and short response time， etc.

3 Experimental setup and results

3.1 Calibration of FBG and ROTDR

Under the constant environmental temperature， the axial tensile strain of the gauge length fiber is changed to calibrate the strain sensitivity of the FBG. Table 1 presents the calibration data measured by the fiber grating demodulator， where  is strain induced wavelength shift and  is the Bragg wavelengths. For an intuitive perception， figure 2 shows the Bragg wavelength as a function of applied strain， while the line represents the linear fitting results of the original data. The results show a good linear relation of with the applied strain， the degree of fitting for which is 0.9996. On the basis of the calculation result， the wavelength-strain coefficient of the used FBG is found to be 1.2 pm/με.

Based on the Raman temperature measurement equation mentioned above， Raman temperature constant need to be calibrated in advance. The calibration results measured by changing the temperature are shown in table 2. Finally， an average of the Raman temperature constant is calculated to be 0.0019.

3.2 Multi-parameter sensing

In order to verify the efficiency of SHM based on multicore fiber， a series of tests has been carried out outdoors. The experiment has been performed on a steel I-beam. The two ends of the I-beam are simply supported with a span length of 9 m. The sensing fiber is tightly glued to the bottom of the web of the steel beam.

3.2.1 Static load test

The purpose of this experiment is to investigate the performance of FBG sensors and ROTDR in quantifying strain and temperature. In the experiment， the strain is applied stepwise to two third-points of the length of the I-beam using masses with a weight of 113 kg each. That is， the loading values under six load levels are 0， 1.13 KN， 2.26 KN， 3.39 KN， 4.52 KN， 5.65 KN， 6.78 KN， respectively. The grating segments of the FBG are inscribed at the positions that coincides with the center span and two third-points of the I -beam. The strain at these positions under different loads is measured with the inscribed FBG sensor， while the deflection of these positions is measured with an electronic level. The results of strain measurement with FBG are then compared with the calculation results of the finite element method （FEM） by fitting the deflection values as shown in Figure 2. For each load level， the ambient temperature measured with ROTDR is compared with a thermometer. The measured results are exhibited in Figure 3.

It can be seen that the temperature results measured with ROTDR are very close to the measurement results with the thermometer. The experiment has been conducted in the afternoon， and both results can reflect a gradual temperature decreasing at that time. Thus， the measured temperature can be readily used to compensate the temperature sensitive of the FBG sensor. On the other hand， strain measurement using FBG displays a satisfied consistence with the results obtained from finite element method under different ambient temperature， which demonstrates that proposed simultaneous measurement of strain and temperature using spatially multiplexed FGB and ROTDR via multicore fiber is reliable.

3.2.2 Dynamic load test

The aim of this experiment is to study the ability of vibration measurement. Different excitation events are generated by hammering on two third-points， center span and vibration motor with speed of 60 r/min， 80 r/min， 100 r/min， respectively. Here the hammering is an impact for the I-beam， and the vibration motor is the source of continuous vibration. For each dynamic load， the response of the structure is simultaneously measured by FBG sensor， FVS and accelerometer. Their measured time domain vibration signal at different frequency and the demodulated frequency spectral using FFT are shown in Figure 4-7. The measured inherent frequencies of the I-beam are compared as presented in table 4， and the change of inherent frequency can reflect the occurrence of early structural malfunction.

As can be seen， the analyzed inherent frequency of the I-beam with the help of the FBG sensor matches very well with the results of accelerometer under different vibrational excitation. With the help of the FVS， there is no obvious result under the vibration motor with a speed of 60 r/min or lower frequency excitation. But it can measure the inherent frequency of the structure under higher frequency excitation. This phenomenon suggests that FVS is more sensitive to high-frequency vibration， which is suitable for application in seismic monitoring.

4 Concluding remarks

In this study， a multicore fiber based multiparameter smart sensing is proposed. By utilizing the spatial multiplexing of FBG， ROTDR and PVS in a 7-core fiber， this sensing technology enables simultaneous measurement of strain， temperature and vibration. To study the feasibility of the proposed sensing technology， experiment under the influence of noise and external stress has been carried out outdoors. The results of static load test demonstrate that the discriminated strain and temperature sensing combining FBG and ROTDR is validated. And the results of dynamic load test indicate that the proposed sensing design combining FBG and PVS can realize accurate measurement of both low frequency and high frequency vibration under complex outdoor environment. All the experimental results show great potential application of the proposed sensing technology for long-term SHM， condition assessment of structures， vibration and seismic response structures and traffic loading assessment on bridges.

（責任编辑：陈之曦）

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