ISSN 8756-6990, Optoelectronics, Instrumentation and Data Processing, 2008, Vol. 44, No. 2, pp. 178–182. © Allerton Press Inc., 2008. Original Russian Text © Yu.N. Kulchin, O.B. Vitrik, A.V. Dyshlyuk, A.M. Shalagin, S.A. Babin, A.A. Vlasov, 2008, published in Avtometriya, 2008, Vol. 44, No. 2, pp. 113–118.
OPTICAL INFORMATION TECHNOLOGIES
An Interrogation Technique for Fiber Bragg Grating Sensors Based on Optical Time-Domain Reflectometry Yu. N. Kulchin a , O. B. Vitrik a , A. V. Dyshlyuk a , A. M. Shalagin b , S. A. Babin b , and A. A. Vlasov b a
Institute of Automation and Control Processes, Far-Eastern Branch, Russian Academy of Sciences, Vladivostok, Russia, E-mail:
[email protected] b Institute of Automation and Electrometry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia Received July 4, 2007
Abstract—A method for fiber Bragg grating (FBG) interrogation is proposed. The method rests on measuring the light intensity reflected from FBG sensors by means of a standard optical time-domain reflectometer. Multiple FBGs along the fiber optic line are interrogated through time division multiplexing. Specifications of FBGs enabling linear response of the sensors to strain and temperature are determined. The threshold sensitivity to strain is 0.8 × 10 -4 ; the threshold sensitivity to temperature is 5 °C. DOI: 10.3103/S8756699008020131
INTRODUCTION Optical fiber sensors have recently demonstrated considerable potentialities in physical measurements [1–3], namely, immunity to electromagnetic interferences, sensitivity to many physical quantities, chemical stability, long life, simple connection to high-speed and noise-proof optical fiber communication lines, and also the possibility of multiplexing and integrating multiple sensors in distributed information-measurement systems [1]. At present, the leading optical fiber sensors are fiber Bragg grating (FBG) measurement sensors that are commonly applied in monitoring various man-caused constructions such as bridges, tunnels, buildings, towers, dams, marine oil-producing platforms, etc. [1–4]. However, the complexity and, consequently, the high cost of spectral systems intended for detecting the FBG resonance wavelength shift in detecting the external action make application of the FBG sensors problematic [1, 2]. At the same time, most available spectral systems ensure in many cases an accuracy of sensing a FBG temperature and strain, which is excessive for monitoring man-caused constructions. Hence, it seems reasonable to simplify the sensor signal decoding scheme at the cost of decreasing measurement accuracy. This can be achieved by applying the principle of amplitude detecting optical signals reflected from the FBGs. However, earlier investigation of the methods for amplitude demodulation of FBG signals were oriented toward applying continuous radiation sources, which limited the possibilities of sensor multiplexing [1, 3]. Concerning the pulse radiation source, it opens up an opportunity for using well-developed optical time-domain reflectometry (OTDR) for detection and division of FBG signals. Hence, the aim of our paper is to create a reflectometry technique for detecting and multiplexing signals of sensors based on fiber Bragg gratings.
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BRAGG SENSOR INTERROGATION BASED ON REFLECTOMETRY The power of radiation reflected from the FBG as a result of sounding by a short laser pulse is determined by the interval of overlapping between the distribution of spectral power density (SPD) of the sounding pulse SLP (l ) and the FBG spectral reflection coefficient r (l, l B ): ¥
Pr (l B ) = ò S LP(l ) r (l, l B ) d l,
(1)
0
where l B is the FBG resonance wavelength. Based on results presented by Medvedkov et al. in [5], we will assume that the dependence of the spectral reflection coefficient for a Bragg diffraction grating with a small depth on the wavelength is described by the Gaussian with the half-width L FBG » l B
æ 0 .4 D nmod çç è neff
2
2
ö æ L mod ö ÷÷ + ç ÷ , ø è L ø
(2)
where L is the length of the Bragg grating, L mod is the modulation period of the refractive index , Dnmod is the modulation depth of the refractive index, and neff is the local effective refractive index [5]. We also assume that the radiation source of the optical fiber time-domain reflectometer is a multimode laser, hence, its power spectrum may be written as SLP (l ) » A(l )F (l ),
(3)
¥ æ æ l - l ö2 ö æ æ l - i l ö2 ö s ÷ ÷ where A(l ) = A0 exp ç- çç F ( l ) exp and = å çç- çç Dl 0 ÷÷ ÷÷ are functions describing the envelope ç è Dl s ÷ø ÷ 0 ø i=0 è ø è è ø and the peaks of SPD distribution of the sounding pulse and characterized by the properties of the active medium and the laser cavity, A0 is the maximum SPD of the sounding pulse, and l s, Dl s, l 0, and Dl 0 are the parameters determined by properties of the laser cavity. Representation (3) is true for a narrow spectral range in which the condition Dl s << l s is fulfilled. It follows from (1) and (3), that due to the discrete behavior of F(l ), the dependence of the reflected light power on the resonance wavelength of the Bragg grating is quasiperiodic, and this can hamper detecting the strain and temperature of FBG by the reflectometry technique. At the same time, the amplitude of oscillations of the reflected signal dPr depends on the relation between L FBG and l 0 , and Fig. 1 shows that when the condition L FBG > l 0 is fulfilled, the contribution to the reflected power from separate spectral components of the sounding pulse is averaged and dPr tends to zero. In this case, expending expression (1) into the Taylor series with an accuracy to the first addend under the condition LFBG << D l s , the change in the reflected light intensity with changing resonance wavelength of the FBG can be calculated by the formula
Fig. 1. The normalized amplitude of signal oscillations versus LFBG l 0. OPTOELECTRONICS, INSTRUMENTATION AND DATA PROCESSING Vol. 44
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(b)
Fig. 2. Experimental system: (a) for investigating the FBG interrogation technique based on reflectometry; (b) for FBG multiplexing (optical fiber reflectometer 1, splitter 2, Bragg gratings fixed on a system that makes it possible to strain them and change the temperature 3, optical spectrum analyzer 4, and broadband optical radiation source 5).
éd ù DP » ê A(l B )ú L FBG D l FBG , ë dl û
(4)
where Dl FBG = 2 neff L mod (a1e + a 2 DT ) is the shift of the resonance wavelength of the Bragg grating, depending on the temperature T and the strain e (a 1 and a 2 are the coefficients determined by the properties of the optical fiber material [6]). Expression (4) takes into account only the main linear terms of the Taylor expansion with respect to Dl FBG . It is seen from (4) that the change in the intensity of the light reflected from the FBG is proportional to temperature and strain of the Bragg grating. In this situation, for achieving the maximum signal amplitude, the resonance wavelength of the Bragg grating should be chosen from the condition of maximum slope dA dl of the envelope of the SPD distribution of the laser. EXPERIMENT According to the described results, for our experiment we fabricated fiber Bragg gratings with the parameters: L FBG » 1 nm and l B » 1530 nm. To prevent saturation of the reflectometer photodetecting device when detecting the FBG signals, we chose a 2% reflection coefficient of the Bragg grating at the resonance wavelength. In this paper, we carried out FBG formation in Lloyd’s interferometer scheme [5, 7]. The UV radiation source was an argon laser with an intracavity doubling of line 488 nm in a BBO crystal with the Gaussian profile of the output beam intensity [8]. We investigated experimentally the FBG signal sensing method based on reflectometry on a system shown schematically in Figs. 2a and 2b. Results of measuring the spectral power density distribution of sounding pulses generated by the reflectometer diode laser by means of the optical spectrum analyzer are shown in Fig. 3. Parameters of the sounding pulse spectrum: l s = 1545 nm, Dl s = 15 nm, l 0 = 0.7 nm, and Dl 0 = 0.15 nm.
Fig. 3. Distribution of spectral power density of sounding pulses generated by the optical fiber time-domain reflectometer. OPTOELECTRONICS, INSTRUMENTATION AND DATA PROCESSING Vol. 44
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(b)
Fig. 4. Intensity of light reflected from FBG versus: (a) relative extension and (b) grating temperature.
Fig. 5. A reflectogram of an optical fiber line with three FBGs.
Figure 4 represents results on measuring the dependence of the optical intensity reflected from the FBG on the relative extension and temperature of the Bragg grating, which were obtained using the optical fiber time-domain reflectometer. For decreasing the level of reflectometer noises, the results were averaged over more than 100 tests. The figure shows that the functions are linear; this is in agreement with the earlier obtained conclusions. The threshold sensitivity of the method to sensing the relative FBG extension was 0.8 × 10 -4 ; the threshold sensitivity to temperature was 5 °C. For investigating the FBG multiplexing method based on reflectometry, three Bragg gratings with the same resonance wavelength were recorded on one fiber line with a 20 m interval. A reflectogram of the line is depicted in Fig. 5. Three of out five reflection peaks correspond to the Brag gratings, whereas the first and last peaks correspond to the connector and to the end of the fiber line, respectively. The value of each peak of reflection from the Bragg gratings is determined by the resonance wavelength of the corresponding FBG, and as any of the gratings extends, the amplitude of the corresponding peak changes, as is shown in the figure. In the course of our experiment, we have found that if more than eight Bragg gratings with the same resonance wavelength are placed successively on the fiber line, the form of the envelope of the sounding pulse spectrum A(l ) is disturbed, hence, the linear behavior of the detected dependence Pr (l B ) is disturbed. Taking into account that the spectrum of the radiation source contains two ranges of 10 nm each, with the maximum envelope slope (1535 ± 5 nm and 1555 ± 5 nm), assuming that the ranges of measured temperatures and strains are about 500 °C and 4 × 10 -3 , respectively, and also taking into account the possibility of the reflectometer to operate at a wavelength of 1.3 mm, the maximum number of multiplexed Bragg gratings, applying the reflectometer used in this work, is estimated as 64. CONCLUSIONS Thus, we have investigated experimentally and theoretically the method for optical fiber grating sensor interrogating and multiplexing based on reflectometry. The threshold sensitivity of the method to relative OPTOELECTRONICS, INSTRUMENTATION AND DATA PROCESSING Vol. 44
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FBG extension was 0.8 × 10 -4 and 5 °Ñ to FBG temperature, respectively. The experimentally obtained characteristics allow us to conclude that since the proposed approach is simple and efficient it can be commonly used for monitoring strain processes in elements of constructions in order to provide their operating safety. ACKNOWLEDGMENTS This research was supported by the Russian Foundation for Basic Research (grants nos. 06-02-96902-ð_ofi, 07-02-91015-AF_a, and 08-02-00064-a), the Far-Eastern Branch of RAS (grants nos. 06-II-UO-02-003, 06-III-V-02-049, 06-02-96002 r_vostok_a, and 07-III-B-02-007), and integrated FEB and SB RAS grant no. 06-II-SÎ-02-005/no.3.8 and INTAS N 04-78-7227. REFERENCES 1. B. Lee, “Review of the Present Status of Optical Fiber Sensors,” Opt. Fiber Technol. 9, 57 (2003). 2. Y. J. Rao, “Recent Progress in Applications of In-Fibre Bragg Grating Sensors,” Opt. and Lasers in Eng. 31, 297 (1999). 3. A. D. Kersey, M. A. Davis, H. J. Patrick, et al., “Fiber Grating Sensors,” Journ. Lightwave Technol. 15 (8), 1442 (1997). 4. J. Ou, “Some Recent Advances of Intelligent Health Monitoring Systems for Civil Infrastructures in HIT,” Proc. SPIE 5851, 147 (2004). 5. O. I. Medvedkov, I. G. Korolev, and S. A. Vasilyev, Recording Fiber Bragg Gratings in a Scheme with Lloyd’s Interferometer, Preprint No. 6, SCFO (Prokhorov Inst. of General Physics, Moscow, 2004), p. 46. 6. S. A. Vasilyev, O. I. Medvedkov, I. G. Korolev, et al., “Refractive Index Fiber Gratings with Applications,” Kvantovaya Elektron. 35 (12), 1085 (2005). 7. G. Meltz, W. W. Morey, and W. H. Glenn, “Formation of Bragg Gratings in Optical Fibers by a Transverse Holographic Method,” Opt. Lett. 14, 823 (1989). 8. S. R. Abdullina, S. A. Babin, A. A. Vlasov, and S. I. Kablukov, “Intracavity Doubling Generation Frequency in a Wide-Aperture Ar Laser,” Kvantovaya Elektron. 35, 857 (2005).
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