A FBG sensor system with cascaded LPFGs and Music algorithm for dynamic strain measurement

Sensors and Actuators A 135 (2007) 415–419

A FBG sensor system with cascaded LPFGs and Music algorithm for dynamic strain measurement Zeng-Ling Ran a,b,∗ , Yun-Jiang Rao a,b a

Key Lab of Broadband Optical Fiber Transmission & Communication Networks Technology (Ministry of Education), University of Electronics Science & Technology of China, Chengdu, Sichuan 610054, China b Key Lab of Opto-Electronic Technology & Systems (Ministry of Education), Chongqing University, Chongqing 400044, China Received 26 May 2006; received in revised form 17 August 2006; accepted 21 August 2006 Available online 20 September 2006

Abstract A Mach-Zehnder (M-Z) interferometer formed by two cascaded long-period fiber gratings (LPFGs) serving as ‘many’ multi-channel edge filters is used to detect the wavelength shift, induced by measurand of fiber Bragg grating (FBG) sensors with high optical SNR of >60 dB due to the use of a fiber ring laser, for the first time to our knowledge. Advantages of such a demodulation device are low-cost, compact, easy fabrication and implementation, good immunity of vibration and temperature, etc. In addition, in order to improve the resolution for dynamic strain measurement, a spectral analyzing algorithm called Music modern spectrum estimation is adapted. The experimental results show that a dynamic strain resolution of 0.1 ␮␧/Hz1/2 at 700 Hz is obtained, which is 10 times improvement in strain resolution when compared with conventional FFT method. © 2006 Elsevier B.V. All rights reserved. Keywords: FBG sensors; Strain gauge; LPFGs; Edge filter; Fiber ring laser; Music algorithm

1. Introduction In-fiber Bragg gratings (FBGs) have been widely used to optical fiber communication and sensing. Due to high sensitivity, electro-magnetic immunity, compactness, excellent multiplexing capability and ease of fabrication, etc., the FBG sensor has great potential to be applied to health monitoring of large engineering structures, non-destructive testing of composite materials, smart structures, and measurement of strain, pressure, vibration, temperature, etc. [1–3]. Dynamic strain measurement, such as vibration, is of importance for health monitoring of all engineering structures, such as electricity generators, traffic monitoring of highways and bridges, engines, large machines, etc. Numerous techniques for dynamic strain measurement using the FBG sensor have been demonstrated, and detailed reviews of interrogation methods are provided in Refs. [1–3]. Very high strain resolution of 2.5 × 10−15 /Hz1/2 has been achieved by using the interferometric phase detection method, while interro-

∗

Corresponding author. Tel.: +86 28 83206811; fax: +86 28 83206811. E-mail address: [email protected] (Z.-L. Ran).

0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.08.015

gation instruments based on edge filters [4] and scanning fiber Fabry-Perot filters [5], are commercially available. The typical measurement resolution achieved by these non-interferometric method instruments is about 1 ␮␧, and the speed of operation is up to several kHz. The scanning F-P filter approach is relatively complex and expensive while the edge-filter method is simple and inexpensive but it cannot realize the interrogation of multiple FBGs with different central wavelengths simultaneously. Recently, a new method based on an AWG edge filter [6] has been reported to realize the interrogation of multiple FBGs, but it has the constraints of high cost and relatively low optical SNR. In this paper, a low-cost multi-channel interrogation method is proposed, which is implemented by using a multi-channel edge filter based on a M-Z interferometer constructed by two cascaded UV-writing LPFGs with a pitch of 500 ␮m and a period number of 50 [7], combined with a tunable filter to select different FBGs with different wavelengths. An optical SNR of >60 dB is achieved with a 6 km transmission distance only with a 980 nm pump power of 40 mW, this may be the highest optical SNR achieved for FBG sensing systems, to the best of our knowledge [8–10], as the highest optical SNR reported so far is ∼50 dB [8] which is achieved by using a linear-cavity fiber Raman laser

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scheme. Without lasing, SNR of FBG sensors is limited by the side mode suppression ratio (SMSR), normally <30 dB. Furthermore, such a sensor system can support a large number of sensors due to the use of spatial division multiplexing (SDM). In addition, for the spectral analysis of signals of FBG sensors, conventional Fast Fourier Transform (FFT) is normally used to obtain the dynamic strain responses of FBG sensors, but its spectrum resolution and noise immunity are poor, limiting its measurement sensitivity. A Music algorithm is adapted for improving the strain resolution [11]. 2. Experiment The configuration of the sensing system proposed is shown in Fig. 1. The EDFA is pumped by a 980 nm pump laser, and it is used to generate broadband ASE light to illuminate sensing FBGs and amplify the signals reflected by FBGs. After passing through a tunable fiber F-P filter which is used to select the FBGs with different wavelengths, the ASE light selected by the filter is sent into the fiber link via a 90/10 coupler and a circulator. Sensing FBGs are arranged along the fiber link. The reflective signals from the group of FBGs, 1, 2, 3, . . ., N, are amplified by the EDFA. Lasing is generated due to multiple circulations of the reflective signals from the FBGs in the fiber ring. 10% of the lasing light is used to measure the wavelength shifts of the FBGs. In addition, the multiplexing capacity of such a system can be largely enhanced based on SDM [2] by using a 1 × K coupler as shown in Fig. 1. For demonstration, two FBGs with wavelengths of 1556 and 1544 nm were used in the experiment and arranged at 6 km, the reflectivity of FBGs was ∼90%. A ∼13.5 m long EDF was used in the EDFA with an absorption coefficient of ∼6 dB/m at 1530 nm. The spectrum of the tunable fiber F-P filter (Micron Optics Inc., USA) is shown in Fig. 2, with a tunable range of 1520–1590 nm, and a FWHM of 3 nm. A 980 nm pump with a power of 40 mW was used in the EDFA. A C-band circulator was used in the experiment with an insertion loss of ∼0.5 dB at each port. An optical spectrum analyzer (OSA) was just used for the purpose of performance evaluation of the SNR of the FBG. The wavelength shifts of the FBGs are acquired by measuring the ratio between the optical power passing by the multi-channel edge filter and the optical power of the reference light. Such a multi-channel filter is based on an all-fiber in-line M-Z inter-

Fig. 2. Spectrum of the tunable fiber F-P filter.

ferometer formed by two cascaded identical 3 dB-LPFGs with a 100 mm separation. According to the maximum wavelength shift range of the FBG sensors, λm , induced by the measurand, the relationship between the space of the two LPFGs, L and λm is described by λm ≥ λ = λ2 /mL [12], where λ and m are fringe spacing of LPFGs based M-Z interferometer and the difference of effective group index between core and cladding mode, respectively. It shows that the wider the maximum wavelength shift range of the FBG sensors, the closer the space of the two LPFGs is. For being adapt to different sensing wavelength range, the center wavelength of M-Z interferometer could be tuned by using HF acid to etch the fiber to change the wavelengths of LPFGs pair [13]. In addition, the bandwidth of the M-Z filter can be broadened via fabricating chirped LPFGs. The spectrum of the multi-channel filter is shown in Fig. 3, and it has a low temperature coefficient of <5 pm/K due to its relatively small optical path difference between the core and cladding of the fiber that form the two arms of the interferometer, respectively [8]. The spectrum of the FBG measured by the OSA is shown in Fig. 4. SNRs of >60 dB for the FBGs located at 6 km are obtained. The temperature and strain responses of the FBG with a wavelength of 1544 nm are investigated and shown in Fig. 5. Due to high SNR of the FBG, a good linearity of >0.995 for temperature and strain measurement has been achieved, and the temperature and strain coefficients are ∼11 pm ◦ C−1 and ∼1.13 pm ␮␧−1 , respectively.

Fig. 1. Schematic diagram of the FBG sensing system with LPFGs based Mach-Zehnder interferometer and a tunable fiber ring laser configuration.

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the FBG signal is shown in Fig. 7 and we can see that a dynamic strain resolution of ∼1 ␮␧/Hz1/2 at 700 Hz is achieved. In order to improve the strain resolution without changing any optical arrangement, a modern spectrum analyzing method called Music algorithm [11] is utilized, which is based on eigenvector method. Here, we assume that J is the total length of the observed data. X is the signal vector and expressed by X = [ x(0)

x(1)

· · · x(J − 1) ]

(1)

In general, a complex harmonic model can be described by x(n) = Fig. 3. Spectrum of the LPFGs based Mach-Zehnder interferometer.

As to the dynamic strain sensing performance of the system, one end of the FBG with a wavelength of 1556 nm was mounted on a PZT which has a piezoelectric constant of 0.27 ␮m/V, while the other end was mounted on a translation-stage apart from the PZT by ∼40 cm. The PZT was driven by a triangle wave with a peak-to-peak voltage of ∼70 V at a frequency of 700 Hz. So a strain of ∼50 ␮␧ was applied to the FBG. The waveforms of the PZT driving signal and the detected FBG signal were measured by a 2.5 GHz oscilloscope and are shown in Fig. 6, and the sampling rate and length of the data are 50 kHz/s and 1004. It was found that two waveforms of the FBG signal and PZT driving signal are in good accordance. The FFT spectrum of

P 

Ai exp[j(2πfi n + φi )] + w(n),

i=1

n = 0, . . . , J − 1 (2)

where P is the total number of signal sources and is assumed to be known here, fi the frequency of interest from the ith source, Φi the phase and is assumed to be uniformly distributed in [0, 2π), Ai the unknown amplitude, and w(n) is the zero mean additive noise which is impulsive in nature. Our aim is to find the P frequencies in (2). The MUSIC spectrum is calculated as: Smusic (f ) = M

1

i=p+1 |S

H (f )v

i|

(3)

where M is the dimension of the correlation matrix, s(f ) = T [1, e−j2πf , . . . , e−j2πf (M−1) ] , and vi is the eigenvector corresponding to the ith eigenvalue of the autocorrelation matrix. Note that eigenvalues are sorted in decreasing order since only

Fig. 4. Spectrum of the FBG signal: (a) FBG at 1544 nm and (b) FBG at 1556 nm.

Fig. 5. Temperature and strain response of FBG at 1544 nm: (a) strain and (b) temperature.

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Fig. 6. Waveforms of the PZT driving signal and FBG signal measured by oscilloscope.

the noise-subspace eigenvectors corresponding to M–P smallest eigenvalues are of interest. Since the vector s(f) is orthogonal to vi , i = P + 1, . . ., M at f = fi , i = 1, . . ., P, the summation in the denominator of (3) is zero. Consequently, the MUSIC method estimates the frequencies by picking the P frequencies where SMUSIC(f) attains peaks. As shown in Fig. 8, a dynamic strain resolution of ∼0.1 ␮␧/Hz1/2 at 700 Hz is achieved. Compared with the FFT method, the strain resolution has been improved by 10 times; this is a significant enhancement in sensing system performance. The double-frequency range (1400 Hz) is the second harmonic of the PZT. 3. Summary Fig. 7. FFT spectrum of the FBG signal with a 700 Hz triangle driving voltage.

Fig. 8. Spectrum of the FBG signal with a 700 Hz triangle driving voltage using Music algorithm.

In this paper, an optical SNR of >60 dB is realized for FBGs using the edge-filtering approach, this is because such a sensing system is based on a fiber ring laser configuration with a tunable filter which can enhance signal level and suppress optical noise significantly. A novel, low-cost demodulation method is proposed here, based on a multi-channel edge filter constructed by two cascaded LPFGs to demodulate the wavelength shifts of FBGs. It has some advantages such as low-cost, compact, easy of realizing multi-sensor sensing, good immunity of vibration and temperature, etc. In addition, a modern spectrum estimation method is also demonstrated to further improve the dynamic strain resolution. The experimental results show that a strain resolution of 0.1 ␮␧/Hz1/2 and a strain resolution improvement of 10 times when compared with conventional FFT are achieved, respectively. It is anticipated that such a FBG sensor system with strong spectrum analysis ability could find important applications in health monitoring of various kinds of structures.

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Acknowledgements This work is supported by the key project of National Natural Science Foundation of China under Grant No. 60537040 and the key project of the Natural Science Foundation of Chongqing. References [1] A.D. Kersey, M.A. Davis, et al., Fiber grating sensors, J. Lightwave Technol. 15 (1997) 1442. [2] Y.J. Rao, In-fibre Bragg grating sensors, Meas. Sci. Technol. 8 (1997) 355. [3] Y.J. Rao, Recent progress in applications of in-fibre Bragg grating sensors, Opt. Laser Eng. 31 (1999) 297. [4] S.M. Melle, K. Liu, R.M. Measures, A passive wavelength demodulation system for guided-wave Bragg grating sensors, IEEE Photon. Technol. Lett. 4 (1992) 516. [5] A.D. Kersey, T.A. Berkoff, W.W. Morey, Multiplexed fibre Bragg grating strain-sensor system with a fibre Fabry-Perot wavelength filter, Opt. Lett. 18 (1993) 1370. [6] P. Niewczas, A.J. Willshire, L. Dziuda, J.R. McDonald, Performance analysis of the fiber Bragg grating interrogation system based on an arrayed waveguide grating, IEEE Trans. Instrum. Meas. 53 (2004) 1192. [7] X.J. Gu, Wavelength-division multiplexing fiber filter and light source using cascaded long-period fiber gratings, Opt. Lett. 23 (1998) 509. [8] P.C. Peng, H.Y. Tseng, S. Chi, Long-distance FBG sensor system using a linear-cavity fiber Raman laser scheme, IEEE Photon. Technol. Lett. 16 (2) (2004) 575–577. [9] J.H. Lee, Y.G. Han, Y.M. Chang, S.B. Lee, Raman amplifier based longdistance, remote FBG strain sensor with EDF broadband source recycling residual Raman pump, Electron. Lett. 40 (18) (2004) 1106–1107.

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[10] Z.L. Ran, Y.J. Rao, Long-distance fiber Bragg grating sensor system based on hybrid Raman/Erbium-doped fiber amplifier, in: Proceedings of the 17th International Conference on Optical Fiber Sensors, Belgium, Proc. SPIE 5855 (2005) 583. [11] S.M. Kay, Modern Spectrum Estimation: Theory and Application, Prentice Hall, Englewood Cliff, NJ, 1988. [12] B.H. Lee, J. Nishii, Dependence of fringe spacing on the grating separation in a long-period fiber grating pair, Appl. Opt. 38 (15) (1999) 3450– 3459. [13] Y.J. Rao, A.Z. Hu, Y.C. Niu, A novel dynamic LPFG gain equalizer written in a bend-insensitive fiber, Opt. Commun. 244 (1–6) (2005) 137– 140.

Biographies Ran Zeng-Ling was born in Chongqing, China, in 1977. He received the MEng degree in Department of Optoelectronic Engineering from Chongqing University, China, in 2003. He is currently pursuing his PhD degree at University of Electronic Science & Technology of China. He is a staff member of the Key Lab of Broadband Optical Fiber Transmission and Communication Networks Technology (Ministry of Education), University of Electronic Science and Technology of China. His research interests are distributed fiber-optic sensing systems and optical fiber transmission technology. Rao Yun-Jing was born in YunNan, China, in 1962. He received his MEng and PhD degree in Department of Optoelectronic Engineering from Chongqing University, China, in 1986 and 1990, respectively. He is the supervisor of the Key Lab of Broadband Optical Fiber Transmission and Communication Networks Technology (Ministry of Education), University of Electronic Science and Technology of China. His currently research interests are optic fiber technology and optoelectronic devices.