Differential interrogation of FBG sensors using conventional optical time domain reflectometry Yuri N. Kulchin, Anatoly M. Shalagin, Oleg B. Vitrik, Sergey A. Babin, Anton V. Dyshlyuk, Alexander A. Vlasov Institute for Automation and Control Processes (IACP) Far-Eastern Branch of Russian Academy of Sciences (FEB RAS) 5, Radio str., Vladivostok, 69, Russia Institute of Automation and Electrometry (IAE) Siberian Branch of Russian Academy of Sciences (SB RAS), Academician Koptyug Avenue, Novosibirsk, 69, Russia Tel.: + 7 [] -6 Fax: +7 [] -5 E-mail: anton_dys@iacp.dvo.ru Abstract A reflectometric approach is proposed for interrogation of multiple fiber Bragg grating (FBG) sensors recorded in a single fiber optic line, based on the differential registration FBGs response to a short probing laser pulse. A special optical layout has been developed based on a -pass optical fiber circulator and two reference fiber Bragg gratings spaced spectrally by ~ nm and spatially by ~ m, with the initial resonance wavelength of the interrogated FBG being right in between that of the reference gratings. Such a scheme allows transformation of the laser pulse reflected from the interrogated FBG into two time-divided optical signals, one of them being directly proportional to the measured physical value (e.g. FBG temperature or strain), the other being inversely proportional to that. The difference in signals intensity is registered in logarithmic units by a conventional optical time-domain reflectometer. Implementing differential measurement principle allowed us to increase the measurement range as compared to the previously proposed OTDR-based FBG interrogation approaches as well as to eliminate the susceptibility of the system to light power fluctuations induced, for instance, by bending loss in the fiber. Due to its simplicity, efficiency and usage of conventional OTDR equipment the proposed reflectometric technique of FBG interrogation can have a wide range of applications, in particular in the field of structural health monitoring. Keywords: Fiber Bragg grating interrogation, optical time domain reflectometry, OTDR. Introduction Nowadays fiber-optic sensors (FOSs) represent one of the topical trends in the methodology [-5]. This is due to several advantages of FOSs in comparison with conventional measurement devices: total immunity to electromagnetic interference, sensitivity to a variety of physical quantities, chemical stability, long lifetime, natural capability of coupling with high-speed and noiseimmune fiber-optic communication channels, possibility of multiplexing and integration of a large number of sensors in distributed informationmeasurement networks [, ]. As is known the measurement transducers based on fiber Bragg gratings (FBGs) are the leaders among FOSs [, -5]. Such sensors are most widely used for the measurement of temperature and mechanical deformations in the field of structural health monitoring of engineering objects (bridges, tunnels, buildings, towers, dams, oil platforms, ships, airplanes, space vehicles, etc. []). However, a major problem in the practical application of FBG sensors is the complexity and, hence, expensiveness of the spectral systems used to detect the shift of the FBG resonance wavelength upon the measurements of external physical effects on the Bragg grating [, ]. Note that the majority of the existing spectral systems often provide an excessive accuracy for the measurement of FBG -9
temperature and mechanical strain in structural health monitoring applications. Therefore, it is expedient to develop intensity-based approaches to the detection of FBG signals, which will substantially simplify the measurement system used to detect the signals of Bragg gratings at the cost of a possible decrease in the measurement accuracy. Recently, we have proposed reflectometric methods for FBG interrogation based on time and combined time-wavelength separation of the signals [6]. However the susceptibility of the measuring system to the light power fluctuations resulted from pulsed laser source instability or bend loss in the fiber proved to be the major drawback of the methods proposed. The present work is aimed at the development of a reflectometric method of FBG interrogation based on the differential registration of FBG signals which fully eliminates the shortcoming mentioned above and enhances the measurement range of the interrogation system.. Method description A schematic illustrating the method proposed is presented in Fig.. d a d T C, ΔL c P R d l d b P R l Fig.. Differential registration of FBG signals based on optical time-domain reflectometer (OTDR): OTDR, fiber optic circulator, interrogated FBGs, tunable reference FBGs. In the insets: а schematic representation of interrogation pulse spectrum; b reflection of the interrogation pulse from the FBG being interrogated: interrogation pulse spectrum, FBG reflection spectrum; с formation of a differential optical signal:, reference FBGs reflection spectra, interrogation pulse spectrum after reflection from the FBG being interrogated; d schematic representation of OTDR traces obtained upon tuning the wavelengths of reference FBGs so as to interrogate the group of FBGs with resonance wavelength (upper graph) and (bottom graph). Probe pulses generated by a conventional OTDR (Fig., a) pass through fiber optic circulator into the fiber line with the interrogated FBGs recorded with spatial intervals of about m so as to make up groups with the same resonance wavelength within a group. When a probe pulse reaches the first of the FBGs being interrogated, an optical signal is reflected in the spectral band corresponding to the FBG reflection spectrum, which then passes trough circulator onto the reference Bragg gratings. The reference FBGs are previously tuned so that the initial resonance wavelength of the first group of the interrogated FBGs is right in between those of the reference gratings ( and ): - = - (Fig., c). The spectral gap between the reference FBGs is chosen to correspond to the range of possible variations of the interrogated FBGs resonance wavelength associated with strain and/or temperature measurement. The reference FBGs are recorded with a spatial interval of about m so they produce two time-divided optical pulses with the power defined by the -9
overlap integral between the reference FBGs reflection spectra R (), R () and the spectrum of the probe pulse reflected from the interrogated FBG S(,Δ): R d P R S, () R d P R S, () where Δ=n eff Λ mod (α ε + α ΔТ) - FBG resonance wavelength shift as a function of temperature variation (ΔТ) and strain (ε), α, α coefficients defined by the properties of the optical fiber material, n eff optical fiber effective refractive index, Λ mod refractive index modulation depth in the FBG. The two optical pulses then proceed via circulator onto the OTDR where they are represented on the OTDR trace as two reflection peaks with the amplitude proportional to P R and P R and varying with Δ. If the FBG being interrogated is not subjected to strain or temperature change then Δ= and P R =P R ; if Δ < then P R increases, P R decreases and vice versa (Fig., d). After a short delay defined by the spatial interval between the interrogated FBGs the next pulse passes through the circulator onto the reference FBGs, reflected from the second interrogated grating, and two more reflectance peaks appear on the OTDR trace and so on and so forth. In sum there will be N peaks, where N the number of FBGs in the group. In order to interrogate the next group of FBGs the reference gratings are tuned so that - = and the resultant OTDR trace gives the peaks corresponding to the FBGs of the second group, etc (Fig., d). As it was mentioned in [6, 7] the following requirements must be satisfied in order to realize OTDR-based FBG interrogation schemes: - So as to cancel out oscillations of the registered signals resulted from the multimode spectral structure of the probe pulse the FWHM of FBG reflection spectrum (Λ) should exceed the gap between two adjacent longitudinal modes in the probe pulse spectrum ( ). - So as to avoid saturation of the highly sensitive photoreceiver of the OTDR the reflectance at the resonance wavelength of the FBGs being interrogated should be within -%. Fig.. The results of P R /P R calculation: а reflection spectra of the reference FBGs and the spectrum of the probe pulse reflected from the interrogated FBG; b - P R (Δ)/P R (Δ) and P R (Δ)/P R (Δ) in logarithmic units. -95
. Experiment For the purpose of experimental investigation of the proposed technique we used the following setup (Fig. ). Fig.. Experimental setup: OTDR, fiber optic circulator, interrogated FBGs, reference FGBs. Two FBGs with =55,8 nm, =556,7 nm and % reflectance at the resonance wavelength have been employed as the ones being interrogated. Reference Bragg gratings with % reflectance were tuned to = 55, nm, = 55, nm for interrogating the first FBG and to = 555, nm, = 558, nm for interrogating the second one. In the course of the experiment to interrogated FBGs were subjected to calibrated deformation with, - steps. We measured the difference of the corresponding reflection peaks magnitudes in the OTDR trace obtained be ANDO AQ75 OTDR. The experimental results are presented in Fig.. As seen from the figure the dependences are of linear character which proves the above conclusions. The threshold sensitivity in the strain measurement mode amounted to ~5-6. 6 P R / P R, дб 5 Δl/l, * - - - - - - - -5 P R / P R, дб Δl/l, * - - - - - - - -5 Fig.. The dependence of the registered signal on the relative elongation of а the first interrogated FBG, b the second interrogated FBG. Taking into account the possibility of realizing up to spectral channels nm each (which corresponds to the measurement range of ~ С of temperature and ~ - of strain) within the spectrum of the probe pulse (~5 nm around the center of 55 nm), as well as the very low reflectivity of the interrogated FBGs the maximum number of the Bragg gratings -96
that can be multiplexed by the proposed technique is estimated at several hundreds and more which by far surpasses the requirements of most practical applications.. Conclusion Thus a differential reflectometric method for the interrogation and multiplexing of the FBG-based measuring transducers with combined time wavelength multiplexing of the measurement channels is developed and studied. The threshold sensitivity of the method in the strain measurement mode is.5 -, and the maximum number of interrogated Bragg gratings is estimated at several hundreds. Due to the implementation of differential measurement principle the interrogation systems are totally immune to the light power fluctuations resulted from the pulsed laser source instability, bend loss in the optical fiber, etc. Thanks to its inherent simplicity and utilization of conventional OTDR equipment the technique proposed can be widely used for the monitoring of deformation and thermal processes using FBG-based control and measurement systems. 5. Acknowledgement The work was supported by the grant of the President of Russian Federation (МК-9.8.), Russian Foundation for Basic Research (grants 9--985, 8-- 998, 8--9) as well as by the Far-Eastern and Siberian Branches of Russian Academy of Sciences. References. B. Lee. Review of the present status of optical fiber sensors. Optical Fiber Technology., 9, pp. 57-79.. Y.J. Rao. Recent progress in applications of in-fibre Bragg grating sensors. Optics and Lasers in Engineering. 999, v., pp. 97-.. Alan D. Kersey et al. Fiber Grating Sensors. Journal of Lightwave Technology. 997, vol. 5, No. 8, pp. -6.. Jinping Ou. Some recent advances of intelligent health monitoring systems for civil infrastructures in HIT. Proc. SPIE., vol. 585, p. 7. 5. S.A. Vasiljev, O.I. Medvedkov, I.G. Korolev, A.S. Bozhkov, A.S. Kurkov, E.M. Dianov. Quantum Electronics. 5, 5,, pp. 85-. 6. Yu.N. Kulchin, O.B. Vitrik, A.V. Dyshlyuk, A.M. Shalagin, S.A. Babin, A.A. Vlasov. Laser Physics. 7, vol. 7, No., pp. 5-9. 7. Yu.N. Kulchin, O.B. Vitrik, A.V. Dyshlyuk, A.M. Shalagin, S.A. Babin, I.S. Shelemba, A.A. Vlasov. Laser Physics. 8, vol. 8, No., pp. -. -97