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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 10, OCTOBER 2004
Autocorrelation Demodulation Technique for Fiber Bragg Grating Sensor Christophe Caucheteur, Student Member, IEEE, Karima Chah, Member, IEEE, Frédéric Lhommé, Member, IEEE, Michel Blondel, and Patrice Mégret, Member, IEEE
Abstract—In this letter, we describe a very accurate and simple demodulation technique for fiber Bragg grating sensors. The technique is suitable for both single and twin Bragg gratings. A twin grating is composed of two identical gratings located at different positions in the same single-mode fiber. Our demodulation technique evaluates the wavelength position of the reflection spectrum with respect to the spectrum of an undisturbed sensor. To calculate the spectrum shift, it computes the autocorrelation product between the two reflection spectra. The demodulation method, which is very fast, has been tested experimentally with temperature sensors. It gives absolute measurements and provides high accuracy compared to a conventional temperature probe. Index Terms—Fiber Bragg gratings, temperature sensors, twin Bragg gratings.
I. INTRODUCTION
F
IBER BRAGG gratings have been rapidly considered as excellent sensor elements, able to measure temperature, strain, and pressure [1]. Their most significant advantage is that the measurand information is wavelength-encoded, making the sensor independent of fluctuating light levels and immune to source power and connector losses that affect many other types of optical sensors [2]. A big issue is the precise determination of the often small measurand-induced wavelength shift from the reflection spectrum. A lot of techniques have been proposed [2]–[4]. An ideal interrogation method must provide a high resolution within a large wavelength range. It should also be cost-effective compared to conventional optical or electrical sensors. We present here a very accurate demodulation technique which is easy to implement and allows real-time measurements. It evaluates the position of the reflection spectrum with respect to an undisturbed spectrum. The wavelength shift is calculated by autocorrelation between the two spectra. The demodulation method has been tested experimentally with single and twin Bragg gratings for temperature sensing. We focus more on twin Bragg gratings because they provide a greater intrinsic precision than the single one. They are formed by two identical Bragg gratings written at different locations in the core of a photosensitive single-mode fiber. They act as a Fabry–Pérot interferometer. Their reflection spectrum results of the interference between the Manuscript received April 8, 2004; revised June 2, 2004. The work of C. Caucheteur was supported by the Fonds Spécial pour la Recherche (FSR) of the Faculté Polytechnique de Mons (FPMs, Belgium). The work of K. Chah and F. Lhommé was supported by the Région Wallonne through a research grant. The work of P. Mégret was supported by the Belgian Science Policy. The authors are with the Faculté Polytechnique de Mons, Service d’Electromagnétisme et de Télécommunications, B-7000 Mons, Belgium (e-mail:
[email protected];
[email protected]; lhomme@ telecom.fpms.ac.be;
[email protected];
[email protected]). Digital Object Identifier 10.1109/LPT.2004.833106
Fig. 1. Reflection spectrum of a twin Bragg grating. at 25 C = 1543:562 nm, Grating lengths = 0:6 mm, length of fiber between the gratings = 7 mm.
reflections of the two gratings. The lobes of the reflection spectrum, and in particular the main lobe, are narrow, allowing a very accurate determination of the wavelength shift. They have very good sensing properties even if the gratings are weakly reflective [5]. They are, furthermore, important components for temperature-insensitive strain measurements or strain-insensitive temperature measurements [6]. They also constitute a particular case of frequency multiplexing [7]. II. DEMODULATION TECHNIQUE The basic principle of operation used in a fiber Bragg grating sensor is the monitoring of the wavelength shift of the reflection spectrum in response to a change in the measurand (e.g., strain, temperature). The reflection spectrum is easily obtained by launching a broad-band light into the grating through a coupler and by measuring the reflected light within a predefined wavelength range. In the case of twin Bragg gratings, the whole reflection spectrum results of the interference between the reflection spectra of the two single gratings. It is constituted of a cosinusoidal modulation inside an envelope function which is the reflection spectrum of a single Bragg grating. When the length of fiber between the gratings increases, the number of interference fringes also increases and they become narrower. This behavior enables a high intrinsic precision of the sensor [5]. Fig. 1 shows an experimental reflection spectrum of a twin Bragg grating obtained with short gratings (0.6 mm each) separated by about 7 mm of fiber. Each single grating has a period of about 525 nm and an index modulation of nearly . These values have been obtained owing to the numerical synthesis of the grating from its reflection spectrum [5].
1041-1135/04$20.00 © 2004 IEEE
CAUCHETEUR et al.: AUTOCORRELATION DEMODULATION TECHNIQUE FOR FIBER BRAGG GRATING SENSOR
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To determine the wavelength shift, our demodulation technique evaluates the position of the reflection spectrum at the measurand with respect to the spectrum of an undisturbed sensor. When the measurand affects the grating, its reflection spectrum becomes (1) where is the reflection spectrum under the influence of the measurand and is the reflection spectrum of the undisrepresents the wavelength shift due to turbed sensor. the measurand. Equation (1) assumes that the shape of the reflection spectrum remains constant under the influence of the measurand. This approximation, which is commonly adopted, does not affect at all the results provided by the sensor. During the signal acquisition, the reflection spectrum is recorded as a taken at wavelengths set of digitized samples separated by a wavelength step within the working interval . The total number of samples is then equal to
Fig. 2. Evolution of the maximum amplitude of the autocorrelation product between two reflection spectra of a single grating shifted by about 400 pm. at 25 C = 1548:052 nm. Length of the grating = 3 mm,
(2) The wavelength shift between relation product the following equation:
is calculated from the autocorand . We have implemented
(3) where is an integer number varying from to . In our experiments, is of the order of a few hundreds. is positive when the sensor measures an increase of the temperature. For all the wavelengths within the working interval, we make the and autocorrelation product between the amplitudes of shifted by . For each iteration (i.e., each value of ), the autocorrelation product is a vector composed of samples. We then take only the sample of maximum amplitude. . We plot the evolution of We, thus, get a single value called versus the wavelength shift . Typical results are represented in Fig. 2 for a weakly reflective single Bragg grating and in Fig. 3 for a twin Bragg grating. We get a Gaussian profile for a weakly reflective single Bragg grating. There are, however, several peaks for a twin Bragg grating. This behavior is predictable because of the interference fringes in the reflection spectrum at which is of a twin Bragg grating. The value of maximum corresponds to the wavelength shift . The resolution on the determination of is closely linked to and, thus, to the measurement device. To improve the resolution, we realize a rational extrapolation around the samples of maximum amplitude. We take several points around the maximum of (typically ten at each side) and we make a Gaussian profile pass through these points. Then we calculate the values of for the coordinates . Thus, we divide by two the wavelength sampling allowed by the measurement device. It ensures a very weak extrapolation error and also allows a greater resolution for the determination of the wavelength shift. At the main maximum value of , corresponds the wavelength shift .
Fig. 3. Evolution of the maximum amplitude of the autocorrelation product between two reflection spectra of a double Bragg grating shifted by about 500 pm. Gratings length = 0:6 mm, at 25 C = 1543:484 nm, length of fiber between the gratings = 5 mm.
III. EXPERIMENT AND RESULTS In this section, we present our interrogation setup and the results obtained for absolute temperature sensing. We focus here on the double Bragg gratings. The gratings are inscribed in a nonhydrogenated stripped photosensitive fiber (StockerYale) owing to the transverse holographic technique [8], [9] with an Argon laser followed by a frequency doubler emitting at 244 nm. They have a length of about 2.2 mm each, are distant of about 7.2 mm and combined together, have a maximum reflectivity of nearly 30%. Their central wavelength, i.e., the wavelength at which the amplitude of the whole reflection spectrum is maximum, is equal to 1527.13 nm at 25 C. The calibration procedure realized with a tunable laser source HP 8168A, a wavemeter Burleigh WA-1000 and an optical powermeter ANDO AQ 2140 reveals that the wavelength linearly changes with the temperature at a rate of 10.11 pm C [5]. Our experimental setup is very simple and is designed to work as fast as possible. The light coming from a broad-band laser source emitting from 1510 to 1540 nm is launched into the fiber containing the gratings through a 3-dB coupler (Fig. 4). An optical spectrum analyzer ANDO AQ 6317C measures
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Fig. 4. Experimental setup to test the temperature sensor.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 10, OCTOBER 2004
This value yields to a random error on the determination of the temperature of 0.5 C. The values obtained with our sensor have been compared to those given by the Pt 100 probe. The multimeter to which the probe is linked is also connected to the computer. This last one determines the value of the difference between the two measurements. The results obtained for several temperatures are presented in Fig. 5. We have obtained a mean variation of 0.2 C. This value shows the accuracy of our sensor. We believe that its accuracy can still be improved by choosing a tunable laser source allowing wavelength steps as short as 1 pm and by replacing the optical spectrum analyzer by a photodetector. With the local extrapolation, it should be possible to get a subpicometer wavelength resolution. We have presented here experimental data for absolute temperature sensing with twin Bragg gratings. The same setup can, furthermore, be used with uniform Bragg gratings or for strain measurements. IV. CONCLUSION We have presented a very fast and simple digital demodulation algorithm for fiber Bragg gratings sensors which uses autocorrelation so that it can be easily implemented in embedded systems. The demodulation technique can provide very accurate results. The sensor uses low reflective Bragg gratings and is easy to manufacture. The data presented in this letter are related to temperature sensors. The same demodulation technique can also be used for strain measurements.
Fig. 5. Comparison between the temperatures measured by the twin grating sensor and given by a Pt 100 probe.
the reflection spectrum within the selected wavelength range 1525–1530 nm with a wavelength sampling of 2 pm. We have, thus, samples. The optical spectrum analyzer is controlled by a computer in which the algorithm described above is implemented. The end of the fiber and the not used port of the coupler are placed in a gel to prevent back reflections. The gratings and a Pt 100 temperature probe are placed in an oven whose temperature is externally controlled so that they are equally affected by the temperature. The temperature probe is connected to a digital multimeter Rhode & Schwartz UDS 5. The precision of the probe is equal to 0.05 C. The reference spectrum (at 25 C) is recorded in the computer. The reflection spectrum at the measurand is stored for every temperature. Then the wavelength shift between the reference and the disturbed spectra is calculated by autocorrelation as described in Section II. The resolution is increased to 1 pm of maximum ampliby extrapolation around the samples tude. This value allows us to detect temperature shifts of about 0.1 C. The data obtained during the calibration procedure are then used to deduce the temperature shift from the wavelength shift. The computation time is only a few seconds for the whole process which confers a real time operation to the sensor. The wavelength reproducibility of the optical spectrum analyzer is guaranteed by the manufacturer to be equal to 5 pm.
ACKNOWLEDGMENT The authors Prof. T. Dutoit.
appreciate
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helpful
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REFERENCES [1] A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, and E. J. Friebele, “Fiber grating sensors,” J. Lightwave Technol., vol. 15, pp. 1442–1463, Aug. 1997. [2] A. Othonos and K. Kalli, Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing, pp. 301–389, 1999. [3] B. Lee and Y. Jeong, “Interrogation techniques for fiber grating sensors and the theory of fiber gratings,” in Fiber Optic Sensors. New York: Marcel Dekker, 2002, pp. 297–381. [4] Y. Zhao and Y. Liao, “Discrimination methods and demodulation techniques for fiber Bragg grating sensors,” Opt. Lasers Eng., vol. 41, pp. 1–18, 2004. [5] C. Caucheteur, K. Chah, F. Lhommé, M. Blondel, and P. Mégret, “Characterization of twin Bragg gratings for sensor application,” in SPIE Proc. Photonics Europe, vol. 5459, 2004, to be published. [6] V. V. Spirin, S. V. Miridonov, M. G. Shlyagin, I. Marquez, and R. M. Lopez, “Differential twin Bragg grating sensor for temperature and strain gradient measurement,” SPIE Proc. Optical Fiber Sensors, vol. 4074, pp. 262–270, 2000. [7] M. G. Shlyagin, S. V. Miridonov, I. M. Borbon, V. V. Spirin, P. L. Swart, and A. A. Chtcherbakov, “Multiplexed twin Bragg grating interferometric sensor,” in 15th OFS 2002, vol. 1, 2002, pp. 191–194. [8] G. Meltz, W. W. Morey, and W. H. Glenn, “Formation of Bragg gratings in optical fibers by a transverse holographic method,” Opt. Lett., vol. 14, pp. 823–825, 1989. [9] P. E. Dyer, R. J. Farley, and R. Giedl, “Analysis and application of a 0/1 order Talbot interferometer for 193 nm laser grating formation,” Opt. Commun., vol. 129, p. 98, 1996.
