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A novel demodulation scheme for high precision quasi-distributed sensing system based on chaotic fiber laser
Accepted Manuscript Title: A novel demodulation scheme for high precision quasi-distributed sensing system based on chaotic fiber laser Author: Jun Zhang Lingzhen Yang Huanhuan Yang Li Zhang Juanfen Wang Zhaoxia Zhang PII: DOI: Reference:
S0924-4247(15)30087-X http://dx.doi.org/doi:10.1016/j.sna.2015.07.033 SNA 9265
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
20-4-2015 27-7-2015 28-7-2015
Please cite this article as: Jun Zhang, Lingzhen Yang, Huanhuan Yang, Li Zhang, Juanfen Wang, Zhaoxia Zhang, A novel demodulation scheme for high precision quasidistributed sensing system based on chaotic fiber laser, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2015.07.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A novel demodulation scheme for high precision quasi-distributed sensing system based on chaotic fiber laser
Jun Zhang a, Lingzhen Yang a, b,*, Huanhuan Yang a, Li Zhang a, Juanfen Wang a, Zhaoxia Zhang a a
College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan, Shanxi030024, China b
Lab of Advanced Transducers and Intelligent Control System, Ministry of
Education, Taiyuan University of Technology, Taiyuan, Shanxi030024, China
*
Corresponding author: Lingzhen Yang
Affiliation: Physics and Optoelectronic Engineering College, Taiyuan University of Technology Detailed permanent address: No.79, Yingzexi Avenue, Wanbailin District, Taiyuan, Shanxi Province, China Email address: [email protected] Telephone number: +86 13754833238
Highlights
A novel demodulation scheme for strain measurement based on chaotic fiber laser is proposed for a high precision quasi-distributed sensing system. Using a cross-correlation algorithm, strain sensing and precise locating can be simultaneously achievedbased on the amplitude variation and time delay of a cross-correlation peak. 1
The relationship between the correlation peaks and strain induced wavelength shifts of weak FBGs is theoretically and experimentally analyzed.
The proposed sensing system demonstrated the strain deviation of 14 με, and high spatial resolution of 7 mm.
2
Abstract A novel demodulation scheme for strain measurement is proposed and demonstrated for a high precision quasi-distributed sensing system based on chaotic fiber laser. Using a cross-correlation algorithm, strain sensing and precise locating can be simultaneously achieved based on the amplitude variation and time delay of a cross-correlation peak. The relationship between the correlation peaks and wavelength shift of weak FBGs induced by strain is theoretically and experimentally analyzed. Experimental results show the strain deviation of 14 με, and the spatial resolution is up to 7 mm. Multi-point synchronous measurements has also been implemented. Key words: Fiber sensor; Chaotic fiber laser; Weak fiber Bragg gratings; Strain;
1. Introduction The FBG as sensing elements has been paid extensive pursue for its intrinsic properties, inherent wavelength-encoded operation and flexible multiplexing capability. Over the past decade, great strides have been made in the application of the FBG
sensors
in
structural
health
monitoring[1],
safety
detection[2]
and
astronautics[3]. For FBG sensing network, the multiplexing capability is one of the most important performances which determines the test range and ability [4, 5]. With the development of FBG sensing technology, a rich variety of the multiplexing schemes receive much focus on the wavelength-division multiplexing (WDM) [6], spatial-division multiplexing (SDM), identical weak fiber Bragg gratings multiplexing [7], time-division multiplexing (TDM) [8] and their combinations [9, 10]. The most commonly used multiplexing technique is WDM technique, in which different FBGs are concatenated while the number of FBGs is directly limited by the spectral ranges 3
of the source. TDM technique can greatly increase the number of FBG sensors in the time domain while it suffers from transmission loss and is limited by the intensity of light source. Recently, Yunmiao Wang et al. proposed a serial TDM sensor network based on ultraweak FBGs [11], but multi-point synchronous measurement has not been implemented. In addition to, the multiplexing capability is dependent on the demodulation techniques of the FBG sensing network. Generally, demodulation techniques can be accomplished mainly by the interference techniques and filter techniques. The interrogation scheme of typical interference techniques is based on an unbalanced Mach–Zehnder interferometer[12] and an unbalanced Michelson interferometer[13]. The typical filter techniques include a matched grating filter[14], a tunable Fabry– Perot filter [15]. Other techniques include cavity ring-down (CRD) spectroscopic technique[16] and a cross-correlation algorithm technique [17]. The cross-correlation algorithm is allowed for the development and demonstration of new technique with a low-cost and high precision for strain measurement system, however, multiplexing capability is still under restrictions. Recently, Yiyang Luo et al. proposed a quasidistributed sensing system based on optical spectrum-limited chaos and CFBG [18, 19]. Compared with CFBG, the identical weak FBGs can be used to significantly improve the multiplexing capacity of sensing system. Meanwhile, the tunable optical narrow-spectrum chaos can increase the measuring range and accuracy of sensing system. In this paper, a quasi-distributed sensing system for a concatenation of identical weak FBGs based on chaotic fiber laser is carried out to implement multi-point synchronous measurement. The strain induced wavelength shifts of weak FBGs can be interrogated by the variation of the cross-correlation peak utilizing chaotic cross4
correlation algorithm. The relationship between the correlation peaks and wavelength is obtained by curve fitting technology for the demodulation of strain sensing and precise locating which are demonstrated experimentally and numerically. Simulation and experimental results on the capability of strain measurement are presented and discussed. 2. Experimental setup The schematic diagram of quasi-distributed sensing system based on chaotic fiber laser is shown in Fig. 1(a). The presented sensor includes the chaotic fiber laser and quasi-distributed sensing system. The chaotic fiber laser acts as a tunable chaotic light source. Five identical weak FBGs are utilized as the sensing elements. Chaos from the fiber laser is injected into the quasi-distributed sensing system through the optical coupler (OC) with the ratio of 90: 10 and 10% light from OC as reference light is converted into electrical signal by photoelectric detector (PD-1). 90% light from OC is injected into the FBG array and reflected light from FBG array by circulator is converted into electrical signal at the photoelectric detector (PD-2) . The bandwidth of PD is 12 GHz, and the reference signal and reflected signal are collected by the realtime oscilloscope (OSC). Then accorrding to delta function features of chaotic correlaton, the correlation peaks of the cross-correlation between reference light and reflected light can locate the spatical positions to every FBGs. Fig. 1(b) shows a chaotic fiber laser with ring cavity structure. Semiconductor laser of 980 nm is used to pump 8 m erbium-doped fiber (EDF) with the help of wavelength division multiplexer (WDM). Tunable fiber grating (TFBG) as a narrowband filter element has the tuning range from 1540.26 nm to 1571.86 nm with the spectral width of 0.086 nm, which is used to generate chaotic light with different wavelengths. The light goes through the optical coupler (OC) with the ratio of 90: 10, 5
which means 90% light circulates in the ring, and 10% light is used as the detected light injected into the quasi-distributed sensing system. The optical isolator (ISO) ensures the light transports in one direction. The polarization controller (PC) is used to change the polarization states of light. In the experiment, the chaotic signal can be exhibited by adjusting the PC and the pump current. The output characteristics in chaotic fiber laser are shown in Fig. 2 including the time series (a); the radio frequency spectrum (b); the auto-correlation curve(c) and the optical spectrum (d). The chaotic signal is similar to noise in time domain and has a frequency range of 0-12 GHz in the radio frequency spectrum. The autocorrelation curve of chaotic signal is similar to delta function shape. 3. Demodulation principle of sensing system Fig. 3 indicates the chaotic spectrum P ( ) from fiber laser and the reflection spectrum R ( ) of FBG employed to interrogate FBG, can be expressed as
P( ) P0 exp[1 ( 0 )2 ] ,
1 4 ln 2 a 2
(1)
R ( ) RB exp[ 2 ( B ) 2 ] ,
2 = 4 ln 2 b 2
(2)
where 0 and B correspond to the central wavelengths of chaotic spectra and FBG separately, P0 and RB correspond to the reflectivity of TFBG and FBG at their central wavelengths, a and b are their full width at half maximum (FWHM)[20]. The output voltage V1 of the chaotic fiber laser from PD-1 and the voltage V2 of reflected signal by FBG from PD-2 may be obtained by the following integral:
V1
P ( ) d
(3)
1
V2
P ( ) R ( ) d
(4)
2
6
Where 1 and 2 are constants, which are taken loss, splitting ratio and photoelectric conversion coefficients into account. Eqs. (1)-(2) are substituted into Eqs. (3)-(4), then the integrated results can be given by
V1 1 P0
1
V2 2 P0 RB
(5)
1 2 (0 B ) 2 exp[ ] 1 2 1 2
(6)
B 2neff
(7)
(1 Pe )B
(8)
where n eff is the effective refractive index of the core at the Bragg wavelength, and is the modulation period of fiber Bragg Grating, Pe is the effective photoblastic coefficient( Pe 0.2157 ), is the strain applied on the FBG. When strain is applied on FBG, the strain brings the variation to the Bragg period and the effective refractive index of FBG. It cause the variation of center wavelength of FBG as shown in the Eq (8). The variational wavelength( B ) can change the overlap area between the chaotic spectrum and the reflection spectrum of FBG as shown in Fig. 3, So it results the power variation of the reflected signal( V2 ). The wavelength shift of each FBG is converted to the voltage variation. Based on the correlation properties of chaos[21], the cross-correlation coefficient R (t ) between the reference chaos and reflected chaos can be given by R (t ) x (t ) kx(t ) k ( ) ,
k V2 / (rV1 )
Where represent the convolution operation,
(9) x(t ) and
kx (t ) are the
function of the output voltage V1 of the chaotic fiber laser and the voltage V2 of reflected signal in the time domain, which are obtained by OSC. is the delay time, 7
k is the loss coefficient and r is the ratio of optical coupler (OC). Eqs. (5)-(6) are substituted into Eq. (9), the cross-correlation coefficient R (t ) can be given by
2 RB 12 2 (0 B )2 R (t ) exp[ ] ( ) r 1 1 2 1 2
(10)
We can locate the spatial positions of FBG by calculating c / 2n , where c is the light velocity in a vacuum and n is the refractive index of fiber. Under the condition of invariable in the wavelength ( 0 ) of reference chaos, the wavelength ( B ) shift of FBG induced by strain is converted to the cross-correlation coefficient R (t ) change. The cross-correlation coefficient R (t ) is a Gaussian function of strain. According to the Eq (10), the strain measurement and precise locating of FBG can be achieved simultaneously by amplitude change and time delay of cross-correlation peak. 4. Theoretical analysis In order to verify the feasibility of this approach, the Matlab is adopted to simulate the demodulation system based on the above analysis. The chaotic signal obtained in the laboratory is used as the reference signal, and the emitted wavelength of chaotic light source is set to be 1563.7nm. The central wavelength of FBG started at 1562.2nm. According to the Eq (10), the central wavelength of FBG is increased by 0.1nm every time and other parameters are constant. Fig. 4(a) depicts the curves between the correlation peak values and center wavelength of FBG. Fig. 4(b) shows the Gaussian fitting results. The fitting constant is 1. The center wavelength of fitting curve is 1563.7nm, which is agreement with the initial value. 5. Experimental results The initial parameters in Eq (10) are inaccurate based on industrial deviation of FBG and TFBG in practical applications. To improve the accuracy of strain sensing 8
system, the curve fitting technology was used to obtain the exact value of the parameters and confirm the original center wavelength of each FBGs in the experiment. 5.1. Strain measurement for single FBG Single FBG is used in strain measurement experiments. The reflectivity of FBG is 10% and the center wavelength of FBG is 1563.738nm. The spectrum of the sensing FBGs is exhibited in Fig. 5. In the initial state, the center wavelength of chaotic fiber laser is 1562.2nm. Tuning the TFBG changes the output wavelength of chaotic fiber laser with a step of 0.1nm. When chaotic wavelength emitted by chaotic fiber laser is matched within the reflection spectrum of FBG, the cross-correlation peak located at this FBG’s spatial position(4.426m) in the cross-correlation curve as shown in Fig. 6(a). By numerous experimental data and Gaussian fitting technology, we obtain the relationship between the correlation peaks and wavelength of chaotic laser source as shown in Fig. 6(b). We get an R-Square of 0.997. The fitting equation is taken as a reference for the strain demodulation and shown as follows: R (t ) 0.02103 0.7682 exp( 140(0 B ) 2 )
(11)
The strain bring the variation to the Bragg period and the effective refractive index. From the Eq (7), we can conclude that the center wavelength of FBG drift. The cross-correlation peak amplitude of FBG observed as a Gaussian function of strain is shown in Fig. 7(a) with a measurement step of 20 με. Fig. 7(b) depicts the strain sensing character of FBG over a strain ra nge from 40 με to 700 με, which shows a good linearity of 0.998 for measurement has been achieved and the maximum strain fluctuation of FBG is less than 14 με. It implies that the minimum detectable quantity is 14 με guaranteeing the precision of the sensor system. Therefore, the strain resolution of the sensing system is about 14 με. 9
The spatial resolution is determined by the full width at half maximum (FWHM) of correlation curve, can be calculated as follows z c FWHM / 2n
(12)
where c is the velocity of light in a vacuum, and n is the refractive index of fiber. As shown in Fig. 8, the spatial resolution is limited by bandwidth of PD. When a PD with bandwidth of 1 GHz is used, the spatial resolution of 4 cm is obtained. When we use a PD with bandwidth of 12 GHz, the spatial resolution is approximately 7 mm. High spatial resolution has been obtained in the sensing system. 5.2. Strain measurement for FBG array Considering the cost of sensing system, we customized a FBG array used in experiment with the spacing of 10 cm whose reflectivity is 10% and the FBG array was attached on a micro-displacement platform to translate displacement to changed strain of the FBGs. When the strain is exerted to the FBGs, two chaotic signals from PD-1 and PD-2 were gathered and made the cross-correlation operation by the OSC. When the wavelength emitted by chaotic fiber laser is included within the reflection spectrum of the concatenated gratings, spatial position of every FBGs is shown by time delay of the cross-correlation. By the above method, we can obtain the Gaussian fit curve of five FBGs separately in Fig. 9. Table 1 shows the important Gaussian fitting parameters of FBG array. The central wavelength of five FBGs without strain can be observed. When the emitted wavelength of the chaotic laser source is 1553.07nm, strain is applied on FBG array. Fig. 10 depicts the correlation peaks of FBG array over a strain range from 20 με to 200 με. The spatial positions of the five FBGs are 2.711m, 2.818m, 2.922m, 3.033m and 3.142m respectively. The correlation coefficient and fitting equation can well demodulate the multi-point strain signals of the FBG sensors 10
network by the method used previously. The strain responses of five FBGs exhibit good linear, one of which is shown in Fig. 11 with R-square of 0.994. The maximum strain measurement disturbance of FBG array is 12 με. It implies that the strain resolution of the FBG array sensing system is about 12 με. 6. Discussion The fitting constant is 1 of Fig. 4 in the numerical simulation, but fitting constant is 0.997 of Fig. 6 in the experiment. That is because the second order reflection between FBGs influence the fit curve by a fake reflection peaks. These fake reflection peaks, appear at the outside of measurement positions, and do not affect the data processing. The issue is when data are superimposed at the true position of a FBG. If the reflectivity of FBGs is 10%, the second order reflectivity is less than 1‰ magnitude compared to the input power after a simple calculation[7].Therefore it is learned that the second order reflection influence the fitting constant in experiment, but the impact of measurements is small. In addition, from Fig. 4, Fig. 6 and Fig. 9, we can see the curves are symmetrical distribution by the center wavelength 0 of initial FBG, which reflect that one correlation coefficient correspond to two wavelengths with the increase of strain exerting to the FBG, so we cannot confirm the center wavelength of the FBG with strain. To solve this problem, the wavelength of chaotic fiber laser is set to be not greater than the center wavelength of FBG in the initial state. When the strain is exerted to the FBG which lead to the center wavelength shift by and the center wavelength is 0 + . So we can choose the part of the decline in Fig. 4, Fig. 6 and Fig. 9. 7. Conclusions In this paper, we have demonstrated a quasi-distributed sensor system for 11
strain measurement. This proposed method utilizes the cross-correlation peak to demodulate a concatenation of identical weak FBG sensors. Strain sensing and precise locating of each FBG can be achieved. The theoretical analysis verified the feasibility of this approach and experimental results show the strain deviation of 14 με, and the spatial resolution is up to 7mm. The multi-point strain synchronous measurement has been implemented. By comparing measured data with practical data, this system is proved to have a good performance on the measurement of strain. Acknowledgments This work was financially supported by the National Natural Science Foundation of China under Project No. 61107033, the Program for the Top Academic Leaders of Higher Learning Institutions of Shanxi Province under Project No. 2012lfjyt05,
the
Programon Social Development by Department of Science and Technology of Shanxi Province under Project No. 20140313023-3. References [1] M. Majumder, T.K. Gangopadhyay, A.K. Chakraborty, K. Dasgupta, D.K. Bhattacharya, Fibre Bragg gratings in structural health monitoring—Present status and applications, Sensors and Actuators A: Physical, 147(2008) 150-64.
[2]M. Hassan, N. Tamchek, A.F. Abas, R. Johar, F.M. Adikan, Dual-phase sensing for early detection of prepreg structural failures via etched cladding Bragg grating, Sensors and Actuators A: Physical, 171(2011) 126-30. [3] C. Wang, S.T. Scherrer, Fiber ringdown pressure sensors, Optics letters, 29(2004) 352-4. [4] Z. Wang, F. Shen, L. Song, X. Wang, A. Wang, Multiplexed fiber Fabry–Perot 12
interferometer sensors based on ultrashort Bragg gratings, Photonics Technology Letters, IEEE, 19(2007) 622-4. [5] Q. Sun, D. Liu, L. Xia, J. Wang, H. Liu, P. Shum, Experimental demonstration of multipoint temperature warning sensor using a multichannel matched fiber Bragg grating, Photonics Technology Letters, IEEE, 20(2008) 933-5. [6] D. Sadot, E. Boimovich, Tunable optical filters for dense WDM networks, Communications Magazine, IEEE, 36(1998) 50-5. [7] M. Zhang, Q. Sun, Z. Wang, X. Li, H. Liu, D. Liu, A large capacity sensing network with identical weak fiber Bragg gratings multiplexing, Opt Commun, 285(2012) 3082-7. [8] L. HU, P. ZHANG, D. HUA, X. GONG, Design of DFB Laser's High Performance Temperature Controller in TDM FBG Sensing Array [J], Chinese Journal of Sensors and Actuators, 7(2012) 012. [9] Y.J. Rao, L. Zhang, I. Bennion, A. Lobo Ribeiro, D. Jackson, Combined spatialand time-division-multiplexing scheme for fiber grating sensors with driftcompensated phase-sensitive detection, Optics letters, 20(1995) 2149-51. [10] Y. Rao, A. Lobo Ribeiro, D. Jackson, L. Zhang, I. Bennion, In-fiber grating sensing network with a combined SDM, TDM, and WDM topology, Lasers and Electro-Optics, 1996 CLEO'96, Summaries of papers presented at the Conference on, IEEE1996, p. 244. [11] W. Yunmiao, G. Jianmin, D.Y. Wang, D. Bo, B. Weihong, W. Anbo, A QuasiDistributed Sensing Network With Time-Division-Multiplexed Fiber Bragg Gratings, Photonics Technology Letters, IEEE, 23(2011) 70-2. [12] Q. Yao, H. Meng, W. Wang, H. Xue, R. Xiong, B. Huang, et al., Simultaneous measurement of refractive index and temperature based on a core-offset Mach– 13
Zehnder interferometer combined with a fiber Bragg grating, Sensors and Actuators A: Physical, 209(2014) 73-7. [13] A. Filho, J. De Sousa, G. Guimarães, H. Rocha, A. Ferreira, F. Lima, et al., Adddrop demultiplexer operating in an optical Michelson interferometer based in fiber Bragg gratings for time division multiple access systems, Fiber and Integrated Optics, 29(2010) 239-53. [14] M. Davis, A. Kersey, Matched-filter interrogation technique for fibre Bragg grating arrays, Electronics letters, 31(1995) 822-3. [15] A.D. Kersey, T. Berkoff, W. Morey, Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry–Perot wavelength filter, Optics letters, 18(1993) 13702. [16] N. Ni, C.C. Chan, W.C. Wong, L.Y. Shao, X.Y. Dong, P. Shum, Cavity ring-down long period grating pressure sensor, Sensors and Actuators A: Physical, 158(2010) 207-11. [17] R. Zorn, Deviation from Gaussian behavior in the self-correlation function of the proton motion in polybutadiene, Physical Review B, 55(1997) 6249. [18] Y. Luo, L. Xia, Z. Xu, C. Yu, Q. Sun, W. Li, et al., Optical chaos and hybrid WDM/TDM based large capacity quasi-distributed sensing network with realtime fiber fault monitoring, Optics express, 23(2015) 2416-23. [19] Y. Luo, L. Xia, D. Huang, Z. Xu, W. Li, Q. Sun, et al., Quasi-Distributed Strain Sensing System Based on Optical Spectrum-Limited Chaos and CFBG Intensity Demodulation, IEEE Photonics Journal, 7(2015) 1-7. [20] H. Zou, D. Liang, J. Zeng, Dynamic strain measurement using two wavelengthmatched fiber Bragg grating sensors interrogated by a cascaded long-period fiber grating, Opt Laser Eng, 50(2012) 199-203. 14
[21] L. Xia, D. Huang, J. Xu, D. Liu, Simultaneous and precise fault locating in WDM-PON by the generation of optical wideband chaos, Optics letters, 38(2013) 3762-4.
15
Figure Captions Fig.1 (a) Schematic diagram of quasi-distributed sensing system; (b) Experimental setup of chaotic fiber laser. Fig.2 Chaotic dynamics in chaotic fiber laser (a) Time series; (b) radio frequency spectrum; (c) auto-correlation curve; (d) optical spectrum. Fig.3 Spectrum of chaotic fiber laser and reflected spectrum from FBG employed to interrogate FBG. Fig.4 The simulation results (a) the correlation peaks by changing the center wavelength of FBG; (b) Gaussian fitting results of correlation peaks. Fig. 5 The spectrum of the sensing FBGs Fig.6 (a) The correlation peaks of the FBG; (b) Gaussian fitting results of the FBG. Fig.7 (a) Cross-correlation peak versus to changed strain. (b) Strain response of FBG. Fig.8 Relationship between the spatial resolution and bandwidth of PD. Fig.9 The Gaussian fit curve of five FBGs. Fig.10 The correlation peaks of FBG array with different strain. Fig.11 The strain response of FBG1
16
Fig. 1
Fig. 2
17
Fig. 3
Fig. 4
18
Fig. 5
Fig. 6
Fig. 7
19
Fig. 8
Fig. 9
20
Fig. 10
Fig. 11
21
Table.1 The important Gaussian fitting parameters of FBG array Equation
y=y0 + (A/(w*sqrt(PI/2)))*exp(-2*(( 0 - B )/w)^2) FBG-1
FBG-2
FBG-3
FBG-4
FBG-5
R-Square 0.9769
0.9763
0.96101
0.98484
0.99211
B (nm)
1553.19518 1553.12182 1552.96168 1553.07401 1552.79292
w
0.24005
0.37596
0.21376
1.07025
0.17354
A
0.08414
0.14601
0.0542
1.62336
0.1114
y0
0.00433
-0.08864
0.02811
-1.04453
0.02535
22
S0924-4247(15)30087-X http://dx.doi.org/doi:10.1016/j.sna.2015.07.033 SNA 9265
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
20-4-2015 27-7-2015 28-7-2015
Please cite this article as: Jun Zhang, Lingzhen Yang, Huanhuan Yang, Li Zhang, Juanfen Wang, Zhaoxia Zhang, A novel demodulation scheme for high precision quasidistributed sensing system based on chaotic fiber laser, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2015.07.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A novel demodulation scheme for high precision quasi-distributed sensing system based on chaotic fiber laser
Jun Zhang a, Lingzhen Yang a, b,*, Huanhuan Yang a, Li Zhang a, Juanfen Wang a, Zhaoxia Zhang a a
College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan, Shanxi030024, China b
Lab of Advanced Transducers and Intelligent Control System, Ministry of
Education, Taiyuan University of Technology, Taiyuan, Shanxi030024, China
*
Corresponding author: Lingzhen Yang
Affiliation: Physics and Optoelectronic Engineering College, Taiyuan University of Technology Detailed permanent address: No.79, Yingzexi Avenue, Wanbailin District, Taiyuan, Shanxi Province, China Email address: [email protected] Telephone number: +86 13754833238
Highlights
A novel demodulation scheme for strain measurement based on chaotic fiber laser is proposed for a high precision quasi-distributed sensing system. Using a cross-correlation algorithm, strain sensing and precise locating can be simultaneously achievedbased on the amplitude variation and time delay of a cross-correlation peak. 1
The relationship between the correlation peaks and strain induced wavelength shifts of weak FBGs is theoretically and experimentally analyzed.
The proposed sensing system demonstrated the strain deviation of 14 με, and high spatial resolution of 7 mm.
2
Abstract A novel demodulation scheme for strain measurement is proposed and demonstrated for a high precision quasi-distributed sensing system based on chaotic fiber laser. Using a cross-correlation algorithm, strain sensing and precise locating can be simultaneously achieved based on the amplitude variation and time delay of a cross-correlation peak. The relationship between the correlation peaks and wavelength shift of weak FBGs induced by strain is theoretically and experimentally analyzed. Experimental results show the strain deviation of 14 με, and the spatial resolution is up to 7 mm. Multi-point synchronous measurements has also been implemented. Key words: Fiber sensor; Chaotic fiber laser; Weak fiber Bragg gratings; Strain;
1. Introduction The FBG as sensing elements has been paid extensive pursue for its intrinsic properties, inherent wavelength-encoded operation and flexible multiplexing capability. Over the past decade, great strides have been made in the application of the FBG
sensors
in
structural
health
monitoring[1],
safety
detection[2]
and
astronautics[3]. For FBG sensing network, the multiplexing capability is one of the most important performances which determines the test range and ability [4, 5]. With the development of FBG sensing technology, a rich variety of the multiplexing schemes receive much focus on the wavelength-division multiplexing (WDM) [6], spatial-division multiplexing (SDM), identical weak fiber Bragg gratings multiplexing [7], time-division multiplexing (TDM) [8] and their combinations [9, 10]. The most commonly used multiplexing technique is WDM technique, in which different FBGs are concatenated while the number of FBGs is directly limited by the spectral ranges 3
of the source. TDM technique can greatly increase the number of FBG sensors in the time domain while it suffers from transmission loss and is limited by the intensity of light source. Recently, Yunmiao Wang et al. proposed a serial TDM sensor network based on ultraweak FBGs [11], but multi-point synchronous measurement has not been implemented. In addition to, the multiplexing capability is dependent on the demodulation techniques of the FBG sensing network. Generally, demodulation techniques can be accomplished mainly by the interference techniques and filter techniques. The interrogation scheme of typical interference techniques is based on an unbalanced Mach–Zehnder interferometer[12] and an unbalanced Michelson interferometer[13]. The typical filter techniques include a matched grating filter[14], a tunable Fabry– Perot filter [15]. Other techniques include cavity ring-down (CRD) spectroscopic technique[16] and a cross-correlation algorithm technique [17]. The cross-correlation algorithm is allowed for the development and demonstration of new technique with a low-cost and high precision for strain measurement system, however, multiplexing capability is still under restrictions. Recently, Yiyang Luo et al. proposed a quasidistributed sensing system based on optical spectrum-limited chaos and CFBG [18, 19]. Compared with CFBG, the identical weak FBGs can be used to significantly improve the multiplexing capacity of sensing system. Meanwhile, the tunable optical narrow-spectrum chaos can increase the measuring range and accuracy of sensing system. In this paper, a quasi-distributed sensing system for a concatenation of identical weak FBGs based on chaotic fiber laser is carried out to implement multi-point synchronous measurement. The strain induced wavelength shifts of weak FBGs can be interrogated by the variation of the cross-correlation peak utilizing chaotic cross4
correlation algorithm. The relationship between the correlation peaks and wavelength is obtained by curve fitting technology for the demodulation of strain sensing and precise locating which are demonstrated experimentally and numerically. Simulation and experimental results on the capability of strain measurement are presented and discussed. 2. Experimental setup The schematic diagram of quasi-distributed sensing system based on chaotic fiber laser is shown in Fig. 1(a). The presented sensor includes the chaotic fiber laser and quasi-distributed sensing system. The chaotic fiber laser acts as a tunable chaotic light source. Five identical weak FBGs are utilized as the sensing elements. Chaos from the fiber laser is injected into the quasi-distributed sensing system through the optical coupler (OC) with the ratio of 90: 10 and 10% light from OC as reference light is converted into electrical signal by photoelectric detector (PD-1). 90% light from OC is injected into the FBG array and reflected light from FBG array by circulator is converted into electrical signal at the photoelectric detector (PD-2) . The bandwidth of PD is 12 GHz, and the reference signal and reflected signal are collected by the realtime oscilloscope (OSC). Then accorrding to delta function features of chaotic correlaton, the correlation peaks of the cross-correlation between reference light and reflected light can locate the spatical positions to every FBGs. Fig. 1(b) shows a chaotic fiber laser with ring cavity structure. Semiconductor laser of 980 nm is used to pump 8 m erbium-doped fiber (EDF) with the help of wavelength division multiplexer (WDM). Tunable fiber grating (TFBG) as a narrowband filter element has the tuning range from 1540.26 nm to 1571.86 nm with the spectral width of 0.086 nm, which is used to generate chaotic light with different wavelengths. The light goes through the optical coupler (OC) with the ratio of 90: 10, 5
which means 90% light circulates in the ring, and 10% light is used as the detected light injected into the quasi-distributed sensing system. The optical isolator (ISO) ensures the light transports in one direction. The polarization controller (PC) is used to change the polarization states of light. In the experiment, the chaotic signal can be exhibited by adjusting the PC and the pump current. The output characteristics in chaotic fiber laser are shown in Fig. 2 including the time series (a); the radio frequency spectrum (b); the auto-correlation curve(c) and the optical spectrum (d). The chaotic signal is similar to noise in time domain and has a frequency range of 0-12 GHz in the radio frequency spectrum. The autocorrelation curve of chaotic signal is similar to delta function shape. 3. Demodulation principle of sensing system Fig. 3 indicates the chaotic spectrum P ( ) from fiber laser and the reflection spectrum R ( ) of FBG employed to interrogate FBG, can be expressed as
P( ) P0 exp[1 ( 0 )2 ] ,
1 4 ln 2 a 2
(1)
R ( ) RB exp[ 2 ( B ) 2 ] ,
2 = 4 ln 2 b 2
(2)
where 0 and B correspond to the central wavelengths of chaotic spectra and FBG separately, P0 and RB correspond to the reflectivity of TFBG and FBG at their central wavelengths, a and b are their full width at half maximum (FWHM)[20]. The output voltage V1 of the chaotic fiber laser from PD-1 and the voltage V2 of reflected signal by FBG from PD-2 may be obtained by the following integral:
V1
P ( ) d
(3)
1
V2
P ( ) R ( ) d
(4)
2
6
Where 1 and 2 are constants, which are taken loss, splitting ratio and photoelectric conversion coefficients into account. Eqs. (1)-(2) are substituted into Eqs. (3)-(4), then the integrated results can be given by
V1 1 P0
1
V2 2 P0 RB
(5)
1 2 (0 B ) 2 exp[ ] 1 2 1 2
(6)
B 2neff
(7)
(1 Pe )B
(8)
where n eff is the effective refractive index of the core at the Bragg wavelength, and is the modulation period of fiber Bragg Grating, Pe is the effective photoblastic coefficient( Pe 0.2157 ), is the strain applied on the FBG. When strain is applied on FBG, the strain brings the variation to the Bragg period and the effective refractive index of FBG. It cause the variation of center wavelength of FBG as shown in the Eq (8). The variational wavelength( B ) can change the overlap area between the chaotic spectrum and the reflection spectrum of FBG as shown in Fig. 3, So it results the power variation of the reflected signal( V2 ). The wavelength shift of each FBG is converted to the voltage variation. Based on the correlation properties of chaos[21], the cross-correlation coefficient R (t ) between the reference chaos and reflected chaos can be given by R (t ) x (t ) kx(t ) k ( ) ,
k V2 / (rV1 )
Where represent the convolution operation,
(9) x(t ) and
kx (t ) are the
function of the output voltage V1 of the chaotic fiber laser and the voltage V2 of reflected signal in the time domain, which are obtained by OSC. is the delay time, 7
k is the loss coefficient and r is the ratio of optical coupler (OC). Eqs. (5)-(6) are substituted into Eq. (9), the cross-correlation coefficient R (t ) can be given by
2 RB 12 2 (0 B )2 R (t ) exp[ ] ( ) r 1 1 2 1 2
(10)
We can locate the spatial positions of FBG by calculating c / 2n , where c is the light velocity in a vacuum and n is the refractive index of fiber. Under the condition of invariable in the wavelength ( 0 ) of reference chaos, the wavelength ( B ) shift of FBG induced by strain is converted to the cross-correlation coefficient R (t ) change. The cross-correlation coefficient R (t ) is a Gaussian function of strain. According to the Eq (10), the strain measurement and precise locating of FBG can be achieved simultaneously by amplitude change and time delay of cross-correlation peak. 4. Theoretical analysis In order to verify the feasibility of this approach, the Matlab is adopted to simulate the demodulation system based on the above analysis. The chaotic signal obtained in the laboratory is used as the reference signal, and the emitted wavelength of chaotic light source is set to be 1563.7nm. The central wavelength of FBG started at 1562.2nm. According to the Eq (10), the central wavelength of FBG is increased by 0.1nm every time and other parameters are constant. Fig. 4(a) depicts the curves between the correlation peak values and center wavelength of FBG. Fig. 4(b) shows the Gaussian fitting results. The fitting constant is 1. The center wavelength of fitting curve is 1563.7nm, which is agreement with the initial value. 5. Experimental results The initial parameters in Eq (10) are inaccurate based on industrial deviation of FBG and TFBG in practical applications. To improve the accuracy of strain sensing 8
system, the curve fitting technology was used to obtain the exact value of the parameters and confirm the original center wavelength of each FBGs in the experiment. 5.1. Strain measurement for single FBG Single FBG is used in strain measurement experiments. The reflectivity of FBG is 10% and the center wavelength of FBG is 1563.738nm. The spectrum of the sensing FBGs is exhibited in Fig. 5. In the initial state, the center wavelength of chaotic fiber laser is 1562.2nm. Tuning the TFBG changes the output wavelength of chaotic fiber laser with a step of 0.1nm. When chaotic wavelength emitted by chaotic fiber laser is matched within the reflection spectrum of FBG, the cross-correlation peak located at this FBG’s spatial position(4.426m) in the cross-correlation curve as shown in Fig. 6(a). By numerous experimental data and Gaussian fitting technology, we obtain the relationship between the correlation peaks and wavelength of chaotic laser source as shown in Fig. 6(b). We get an R-Square of 0.997. The fitting equation is taken as a reference for the strain demodulation and shown as follows: R (t ) 0.02103 0.7682 exp( 140(0 B ) 2 )
(11)
The strain bring the variation to the Bragg period and the effective refractive index. From the Eq (7), we can conclude that the center wavelength of FBG drift. The cross-correlation peak amplitude of FBG observed as a Gaussian function of strain is shown in Fig. 7(a) with a measurement step of 20 με. Fig. 7(b) depicts the strain sensing character of FBG over a strain ra nge from 40 με to 700 με, which shows a good linearity of 0.998 for measurement has been achieved and the maximum strain fluctuation of FBG is less than 14 με. It implies that the minimum detectable quantity is 14 με guaranteeing the precision of the sensor system. Therefore, the strain resolution of the sensing system is about 14 με. 9
The spatial resolution is determined by the full width at half maximum (FWHM) of correlation curve, can be calculated as follows z c FWHM / 2n
(12)
where c is the velocity of light in a vacuum, and n is the refractive index of fiber. As shown in Fig. 8, the spatial resolution is limited by bandwidth of PD. When a PD with bandwidth of 1 GHz is used, the spatial resolution of 4 cm is obtained. When we use a PD with bandwidth of 12 GHz, the spatial resolution is approximately 7 mm. High spatial resolution has been obtained in the sensing system. 5.2. Strain measurement for FBG array Considering the cost of sensing system, we customized a FBG array used in experiment with the spacing of 10 cm whose reflectivity is 10% and the FBG array was attached on a micro-displacement platform to translate displacement to changed strain of the FBGs. When the strain is exerted to the FBGs, two chaotic signals from PD-1 and PD-2 were gathered and made the cross-correlation operation by the OSC. When the wavelength emitted by chaotic fiber laser is included within the reflection spectrum of the concatenated gratings, spatial position of every FBGs is shown by time delay of the cross-correlation. By the above method, we can obtain the Gaussian fit curve of five FBGs separately in Fig. 9. Table 1 shows the important Gaussian fitting parameters of FBG array. The central wavelength of five FBGs without strain can be observed. When the emitted wavelength of the chaotic laser source is 1553.07nm, strain is applied on FBG array. Fig. 10 depicts the correlation peaks of FBG array over a strain range from 20 με to 200 με. The spatial positions of the five FBGs are 2.711m, 2.818m, 2.922m, 3.033m and 3.142m respectively. The correlation coefficient and fitting equation can well demodulate the multi-point strain signals of the FBG sensors 10
network by the method used previously. The strain responses of five FBGs exhibit good linear, one of which is shown in Fig. 11 with R-square of 0.994. The maximum strain measurement disturbance of FBG array is 12 με. It implies that the strain resolution of the FBG array sensing system is about 12 με. 6. Discussion The fitting constant is 1 of Fig. 4 in the numerical simulation, but fitting constant is 0.997 of Fig. 6 in the experiment. That is because the second order reflection between FBGs influence the fit curve by a fake reflection peaks. These fake reflection peaks, appear at the outside of measurement positions, and do not affect the data processing. The issue is when data are superimposed at the true position of a FBG. If the reflectivity of FBGs is 10%, the second order reflectivity is less than 1‰ magnitude compared to the input power after a simple calculation[7].Therefore it is learned that the second order reflection influence the fitting constant in experiment, but the impact of measurements is small. In addition, from Fig. 4, Fig. 6 and Fig. 9, we can see the curves are symmetrical distribution by the center wavelength 0 of initial FBG, which reflect that one correlation coefficient correspond to two wavelengths with the increase of strain exerting to the FBG, so we cannot confirm the center wavelength of the FBG with strain. To solve this problem, the wavelength of chaotic fiber laser is set to be not greater than the center wavelength of FBG in the initial state. When the strain is exerted to the FBG which lead to the center wavelength shift by and the center wavelength is 0 + . So we can choose the part of the decline in Fig. 4, Fig. 6 and Fig. 9. 7. Conclusions In this paper, we have demonstrated a quasi-distributed sensor system for 11
strain measurement. This proposed method utilizes the cross-correlation peak to demodulate a concatenation of identical weak FBG sensors. Strain sensing and precise locating of each FBG can be achieved. The theoretical analysis verified the feasibility of this approach and experimental results show the strain deviation of 14 με, and the spatial resolution is up to 7mm. The multi-point strain synchronous measurement has been implemented. By comparing measured data with practical data, this system is proved to have a good performance on the measurement of strain. Acknowledgments This work was financially supported by the National Natural Science Foundation of China under Project No. 61107033, the Program for the Top Academic Leaders of Higher Learning Institutions of Shanxi Province under Project No. 2012lfjyt05,
the
Programon Social Development by Department of Science and Technology of Shanxi Province under Project No. 20140313023-3. References [1] M. Majumder, T.K. Gangopadhyay, A.K. Chakraborty, K. Dasgupta, D.K. Bhattacharya, Fibre Bragg gratings in structural health monitoring—Present status and applications, Sensors and Actuators A: Physical, 147(2008) 150-64.
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interferometer sensors based on ultrashort Bragg gratings, Photonics Technology Letters, IEEE, 19(2007) 622-4. [5] Q. Sun, D. Liu, L. Xia, J. Wang, H. Liu, P. Shum, Experimental demonstration of multipoint temperature warning sensor using a multichannel matched fiber Bragg grating, Photonics Technology Letters, IEEE, 20(2008) 933-5. [6] D. Sadot, E. Boimovich, Tunable optical filters for dense WDM networks, Communications Magazine, IEEE, 36(1998) 50-5. [7] M. Zhang, Q. Sun, Z. Wang, X. Li, H. Liu, D. Liu, A large capacity sensing network with identical weak fiber Bragg gratings multiplexing, Opt Commun, 285(2012) 3082-7. [8] L. HU, P. ZHANG, D. HUA, X. GONG, Design of DFB Laser's High Performance Temperature Controller in TDM FBG Sensing Array [J], Chinese Journal of Sensors and Actuators, 7(2012) 012. [9] Y.J. Rao, L. Zhang, I. Bennion, A. Lobo Ribeiro, D. Jackson, Combined spatialand time-division-multiplexing scheme for fiber grating sensors with driftcompensated phase-sensitive detection, Optics letters, 20(1995) 2149-51. [10] Y. Rao, A. Lobo Ribeiro, D. Jackson, L. Zhang, I. Bennion, In-fiber grating sensing network with a combined SDM, TDM, and WDM topology, Lasers and Electro-Optics, 1996 CLEO'96, Summaries of papers presented at the Conference on, IEEE1996, p. 244. [11] W. Yunmiao, G. Jianmin, D.Y. Wang, D. Bo, B. Weihong, W. Anbo, A QuasiDistributed Sensing Network With Time-Division-Multiplexed Fiber Bragg Gratings, Photonics Technology Letters, IEEE, 23(2011) 70-2. [12] Q. Yao, H. Meng, W. Wang, H. Xue, R. Xiong, B. Huang, et al., Simultaneous measurement of refractive index and temperature based on a core-offset Mach– 13
Zehnder interferometer combined with a fiber Bragg grating, Sensors and Actuators A: Physical, 209(2014) 73-7. [13] A. Filho, J. De Sousa, G. Guimarães, H. Rocha, A. Ferreira, F. Lima, et al., Adddrop demultiplexer operating in an optical Michelson interferometer based in fiber Bragg gratings for time division multiple access systems, Fiber and Integrated Optics, 29(2010) 239-53. [14] M. Davis, A. Kersey, Matched-filter interrogation technique for fibre Bragg grating arrays, Electronics letters, 31(1995) 822-3. [15] A.D. Kersey, T. Berkoff, W. Morey, Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry–Perot wavelength filter, Optics letters, 18(1993) 13702. [16] N. Ni, C.C. Chan, W.C. Wong, L.Y. Shao, X.Y. Dong, P. Shum, Cavity ring-down long period grating pressure sensor, Sensors and Actuators A: Physical, 158(2010) 207-11. [17] R. Zorn, Deviation from Gaussian behavior in the self-correlation function of the proton motion in polybutadiene, Physical Review B, 55(1997) 6249. [18] Y. Luo, L. Xia, Z. Xu, C. Yu, Q. Sun, W. Li, et al., Optical chaos and hybrid WDM/TDM based large capacity quasi-distributed sensing network with realtime fiber fault monitoring, Optics express, 23(2015) 2416-23. [19] Y. Luo, L. Xia, D. Huang, Z. Xu, W. Li, Q. Sun, et al., Quasi-Distributed Strain Sensing System Based on Optical Spectrum-Limited Chaos and CFBG Intensity Demodulation, IEEE Photonics Journal, 7(2015) 1-7. [20] H. Zou, D. Liang, J. Zeng, Dynamic strain measurement using two wavelengthmatched fiber Bragg grating sensors interrogated by a cascaded long-period fiber grating, Opt Laser Eng, 50(2012) 199-203. 14
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15
Figure Captions Fig.1 (a) Schematic diagram of quasi-distributed sensing system; (b) Experimental setup of chaotic fiber laser. Fig.2 Chaotic dynamics in chaotic fiber laser (a) Time series; (b) radio frequency spectrum; (c) auto-correlation curve; (d) optical spectrum. Fig.3 Spectrum of chaotic fiber laser and reflected spectrum from FBG employed to interrogate FBG. Fig.4 The simulation results (a) the correlation peaks by changing the center wavelength of FBG; (b) Gaussian fitting results of correlation peaks. Fig. 5 The spectrum of the sensing FBGs Fig.6 (a) The correlation peaks of the FBG; (b) Gaussian fitting results of the FBG. Fig.7 (a) Cross-correlation peak versus to changed strain. (b) Strain response of FBG. Fig.8 Relationship between the spatial resolution and bandwidth of PD. Fig.9 The Gaussian fit curve of five FBGs. Fig.10 The correlation peaks of FBG array with different strain. Fig.11 The strain response of FBG1
16
Fig. 1
Fig. 2
17
Fig. 3
Fig. 4
18
Fig. 5
Fig. 6
Fig. 7
19
Fig. 8
Fig. 9
20
Fig. 10
Fig. 11
21
Table.1 The important Gaussian fitting parameters of FBG array Equation
y=y0 + (A/(w*sqrt(PI/2)))*exp(-2*(( 0 - B )/w)^2) FBG-1
FBG-2
FBG-3
FBG-4
FBG-5
R-Square 0.9769
0.9763
0.96101
0.98484
0.99211
B (nm)
1553.19518 1553.12182 1552.96168 1553.07401 1552.79292
w
0.24005
0.37596
0.21376
1.07025
0.17354
A
0.08414
0.14601
0.0542
1.62336
0.1114
y0
0.00433
-0.08864
0.02811
-1.04453
0.02535
22