Long-range vibration sensor based on correlation analysis of optical frequency-domain reflectometry signals Zhenyang Ding,1,2 X. Steve Yao,1,2,3 Tiegen Liu,1,2,* Yang Du,1,2 Kun Liu,1,2 Qun Han,1,2 Zhuo Meng,1,2 and Hongxin Chen3 1 2
College of Precision Instrument and Opto-Electronics Engineering, Tianjin University, Tianjin, 300072, China Key Laboratory of Opto-Electronics Information Technical, Tianjin University, Ministry of Education, Tianjin, 300072, China 3 General Photonics Corporation, California 91710, USA *
[email protected]
Abstract: We present a novel method to achieve a space-resolved longrange vibration detection system based on the correlation analysis of the optical frequency-domain reflectometry (OFDR) signals. By performing two separate measurements of the vibrated and non-vibrated states on a test fiber, the vibration frequency and position of a vibration event can be obtained by analyzing the cross-correlation between beat signals of the vibrated and non-vibrated states in a spatial domain, where the beat signals are generated from interferences between local Rayleigh backscattering signals of the test fiber and local light oscillator. Using the proposed technique, we constructed a standard single-mode fiber based vibration sensor that can have a dynamic range of 12 km and a measurable vibration frequency up to 2 kHz with a spatial resolution of 5 m. Moreover, preliminarily investigation results of two vibration events located at different positions along the test fiber are also reported. ©2012 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (060.2430) Fibers, single-mode; (060.2300) Fiber measurements; (290.5870) Scattering, Rayleigh.
References and links 1.
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13. R. G. Duncan, B. J. Soller, D. K. Gifford, S. T. Kreger, R. J. Seeley, A. K. Sang, M. S. Wolfe, and M. E. Froggatt, “OFDR-based distributed sensing and fault detection for single- and multi-mode avionics fiber-optics,” presented at the Joint Conference on Aging Aircraft (2007). 14. http://www.lunatechnologies.com/applications/OFDR-Based-Distributed-Sensing.pdf 15. S. Venkatesh and W. V. Sorin, “Phase noise consideration in coherent optical FMCW reflectometry,” J. Lightwave Technol. 11(10), 1694–1700 (1993). 16. A. Yariv and P. Yeh, Photonics: Optical Electronics in Modern Communications (Oxford Univ. Press, 2007). 17. J. P. Vonder Weid, R. Passy, G. Mussi, and N. Gisin, “On the characterization of optical fiber network components with optical frequency domain reflectometry,” J. Lightwave Technol. 15(7), 1131–1141 (1997). 18. B. J. Soller, D. K. Gifford, M. S. Wolfe, and M. E. Froggatt, “High resolution optical frequency domain reflectometry for characterization of components and assemblies,” Opt. Express 13(2), 666–674 (2005). 19. E. D. Moore and R. R. McLeod, “Correction of sampling errors due to laser tuning rate fluctuations in sweptwavelength interferometry,” Opt. Express 16(17), 13139–13149 (2008).
1. Introduction Fiber optic sensor technology is one of the most promising candidates among the numerous sensing technologies that are suitable for the structural health monitoring (SHM) [1]. This is due to the inherent fiber optic properties such as light weight, small size, non-corrosive, immunity to electromagnetic interference, distributed sensing capability, etc [2]. The vibration is important information for these types of applications because the intrinsic vibration frequencies can be used to evaluate the structural condition and to identify the internal damages at an early stage. Many efforts have been made to fulfill the space-resolved vibration sensing systems [3– 7]. Among them, the widely used approaches are based on either the optical time domain reflectometer (OTDR) [8] or the optical frequency domain reflectometer (OFDR) [9]. For the OTDR methods, a polarization sensitive OTDR method was reported to achieve the vibration measurement to have a response frequency up to 5 kHz with a measurement range of 1 km and a spatial resolution of 10 m [3]. A phase-OTDR method was also described to measure a vibration frequency response up to 1 kHz with a measurement range of 1.2 km and a spatial resolution of 5 m [4]. An improved performance of the phase-OTDR sensor was also realized to measure a vibration frequency response up to 8 kHz to have a spatial resolution as fine as 0.5 m for a measurement range of 1 km by use of the wavelet denoising data processing technique [5] and the polarization-maintaining (PM) configuration [6]. For the OFDR methods, M. Froggatt et al [8] firstly used the OFDR technique for the distributed temperature and strain sensing based on the analysis of the spectral shift of the local Rayleigh backscattering spectra in the wavelength domain. The OFDR for the distributed temperature and strain sensing application has advantages of a high spatial resolution and a simple configuration [10–14]. Based on this technique a dynamic vibration measurement with a high spatial resolution of 10 cm has been demonstrated [7]. As the method requires to measure a spectral shift of the local Rayleigh backscattering spectra continuously, disadvantageously a measurable vibration frequency is limited to 32 Hz. In this method, the measurement range is limited to 17 m due to a short laser coherent length [15]. All the previously reported methods have a significantly limitation for the measurement range, for example, the OTDR vibration sensing can achieve the longest measurement range about 1 km while the OFDR vibration sensor measurement length is only 17 m. Such short measurement range is limited to many practical applications in the field such as for the large structural health monitoring that typically requires a measurement distance of several kilometers to tens kilometers. In this paper, we report a novel method to achieve a space-resolved long-range vibration detection system based on the correlation analysis of OFDR signals by using a standard single-mode optical fiber. By two separate measurements of the vibrated and non-vibrated states for the test fiber, the vibration frequency and position information can be extracted by the cross-correlation analysis of the beat signals between the vibrated state and non-vibrated state of the local Rayleigh backscattering (RB) in the spatial domain. In the previous reported methods [7], the vibration frequency information was obtained by analyzing the Rayleigh backscattering spectra in an optical frequency domain. However, in our method the vibration frequency information are extracted from the RB beat signal analysis in a spatial domain,
#176532 - $15.00 USD
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Received 19 Sep 2012; revised 14 Nov 2012; accepted 14 Nov 2012; published 6 Dec 2012
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thereby advantageously a maximal measurable vibration frequency can be increased up to 2 kHz or even higher with a spatial resolution of 5 m. By using of a highly coherent tunable laser source (TLS), i.e. with a narrow linewidth of ~1 kHz, our OFDR sensor measurable range can be up to 12 km. Moreover, we also studied two vibrations located at different positions of the test fiber and preliminarily experimental results show that it may be possible to further realize a distributed vibration sensor. To author’s best knowledge, this is the longest measurement range and the highest frequency response ever achieved by using the OFDR for a vibration sensor. 2. Operation principle and signal processing 2.1 Operation principle An OFDR interferometer provides a beat signal that is produced by the optical interference between two light signals originating from the same linearly frequency chirped highly coherent light source. One signal E s (t ) is reflection light such as Rayleigh backscattering (RB) light from the fiber under test (FUT) along the test path, while another E r (t ) follows a reference path of the interferometer (see below Fig. 1). For a tunable laser source (TLS) having a linear optical frequency tuning speed γ , the optical field E r (t ) from the reference path can be written as E r (t ) = E 0 exp { j [2π f 0t + πγ t 2 ]} ,
(1)
where f 0 is an initial optical frequency. By assuming that there is a vibration event occurring at a round-trip time delay τ of the FUT, the vibration can cause phase change in E s (t ) along the test fiber path that can be written as δ sin(2π f m t ) , where f m is the vibration frequency and δ is the phase modulation amplitude. Assuming that a reflection reflectivity is r (τ ) at τ and α is the fiber attenuation coefficient, a reflectivity with fiber attenuation can be written as R (τ ) = r (τ ) exp(−ατ c / n ) , where c is the light speed in vacuum and n is the refractive index of fiber. Then E s (t ) of reflection in a vibration state with the reflectivity and the fiber attenuation ( R (τ ) ) can be expressed as E s (t ) = R (τ )E 0 exp { j [2π f 0 (t − τ ) + πγ (t − τ ) 2 − δ sin 2π f m t ]} .
(2)
According to the discussion in [16] exp(− j δ sin 2π f m t ) =
n =+∞
J
n =−∞
n
(δ ) exp(− jn 2π f m t ),
(3)
Equation (3) can also be approximated as exp(− j δ sin 2π f m t ) = J 0 (δ ) + J 1 (δ ) exp(− j 2π f m t ) − J 1 (δ ) exp( j 2π f m t ). (4) Assuming that the phase modulation amplitude δ is small (e.g., δ
