An intrinsic graded-index multimode optical fibre strain-gauge

Sensors and Actuators A 111 (2004) 210–215

An intrinsic graded-index multimode optical fibre strain-gauge R.M. Ribeiro a,b,∗ , M.M. Werneck b a

Laboratório de Instrumentação e Fotˆonica/COPPE, Universidade Federal do Rio de Janeiro, 21.945-970 Rio de Janeiro RJ, Brazil b Departamento de Engenharia Eletrˆ onica, Telecomunicações of Universidade do Estado do Rio de Janeiro (UERJ) and Universidade Católica de Petrópolis (UCP), Brazil Received 21 November 2002; received in revised form 1 July 2003; accepted 9 November 2003

Abstract We report on a novel strain-gauge based on a graded-index (GI) multimode (MM) optical fibre fed through a fusion spliced singlemode (SM) fibre. The lowest order spatial mode is launched and propagates twice through the MM sensing fibre. Mismatch between the sinusoidal trajectories of the propagating modes relative to GI MM fibre pitch occurs due to the strain and is on-line converted to intensity modulation in the MM/SM interface. Results have shown linear response up to 4250 micro-strains (␮e) range and resolution of 9 ␮e. © 2003 Published by Elsevier B.V. Keywords: Strain-gauge; Optical fibre sensor; Graded-index multimode fibre

1. Introduction An increasing interest has been paid to civil-engineering structures with self-testing capabilities, smart network [1], composite monitoring [2] and real time strain/stress monitoring of vehicles [3]. For those systems, one may observe that multipoint optical strain-gauges play a crucial role. A number of physical mechanisms for optical fibre strain-gauges have been reported in the literature, such as microbend [4], buckling [5], Bragg gratings [6], interferometric [7,8], and many others. All of them feature a number of known advantages relative to the conventional devices [2]. Intensity-modulated optical fibre sensors (as strain-gauges) are the simplest, although featuring some limitations as smaller sensitivity, dependence of light source spectra/ amplitude and losses through the fibre link. A number of techniques has been proposed and demonstrated to overcome such limitations. Interferometric and wavelength encoded modulation have been largely employed for high-sensitivity sensing allowing a number of multiplexing schemes [1,6]. Multimode optical fibres have been widely used in local area networks (LANs) and have two different designs: stepindex (SI) and graded-index (GI) refractive profile. SI fibres were the first used on Telecommunications. Transmission of an optical signal in SI fibres is provided by total internal

reflection of the light rays (modes) from the core-cladding interface. In this case, the total internal reflection produces many optical modes propagating along the fibre core. These modes have different lengths and thus different times of light propagation that distorts (widening) the front of the optical signal that therefore causes modal dispersion that limits the data rate and distance of transmission. As is well known, because of this limitation the GI optical fibres, particularly with parabolic profile, were developed in the past [9]. The core of modal dispersion compensating GI MM fibres has a refraction index gradually declining with parabolic dependence from the core centre to the core-cladding interface. Because the index of refraction decreases away from the centre, the speed of light increases as the cladding is approached, and this tends to compensate for the different paths taken by different modes. Because of refraction instead of total internal reflection, GI MM fibres guide light following sinusoidal trajectories with a definite pitch (spatial period). In this paper, we describe a novel intrinsic optical fibre strain-gauge based on the mismatch between the sinusoidal trajectories of the light propagating modes and the GI MM fibre pitch that can be caused by applied axial strain.

2. Principle ∗

Corresponding author. Tel.: +55-21-2562-8201; fax: +55-21-2562-8200. E-mail address: [email protected] (R.M. Ribeiro). 0924-4247/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/j.sna.2003.11.009

Fig. 1 illustrates the sensing principle here proposed. At first, the probe light is launched and propagates through a SM

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Fig. 1. Illustration of the sensing principle of the sensor operating in the reflective mode.

fibre. The latter is fusion spliced to a GI MM fibre. The core diameter of the GI MM fibre is typically 5–20 times greater than the SM fibre one. After light coupling from SM to the GI MM fibre the fundamental mode propagates following sinusoidal trajectories with a pitch ranging typically from >0.2 mm [10] to around 1 mm [11]. The change in refractive index causes refraction, bending light rays back toward the fibre axis as they pass through layers with lower refractive indexes. The refractive index gradient cannot confine all light entering the fibre, but only rays that fall within a limited confinement angle, as in SI fibre. The refractive-index gradient of the GI MM fibre determines that angle. This causes the GI MM fibre to work as a sequence of lenses that sequentially focuses all modes in spots, or alternatively focusing and defocusing like a graded-index (GRIN) lens effect [12]. The distance (or pitch) between focusing spots in these GI MM fibres also depends on the gradient of the refraction index. The idea and design of these fibres are similar to gradient lenses widely used in optics, but are much smaller. In the reflective mode, the focusing/defocusing rays from the SM/MM splice traverses twice the MM fibre and is re-launched into the SM fibre. Because of some length variation |L0 | of the MM fibre due to an applied strain, there will appear a mismatch between the sinusoidal trajectories of the light propagating modes and the GI MM fibre pitch. Therefore, a longitudinal spatial-encoded modulation of propagating modes arises. The |L0 | dependent spatial modulation of the sinusoidal trajectories is thus on-line converted to intensity modulation after the re-coupling into the SM fibre. The splice works simultaneously as a launcher for transmission and as a spatial filter for receiving that stops a fraction

of the modal field [13]. The high speed of the light through the fibre (∼200,000 km/s) guarantees that the optical signal will experiment the same strain when in and when out. Such principle resembles, but differ from those early reported in the literature for a SM fibre feeding an off-line GRIN lens [14] and micro-curvature/macro-curvature at SM/MM/SM fibre interface [11,15,16].

3. Experimental Fig. 2 shows the experimental set-up for the strain measurements in the static regime. A commercial SM 1310/1550 nm optical time-domain reflectometer (OTDR) was used as the light source/demodulator. The laser light was launched in 600 m length of a 9/125 ␮m SM fibre (NA = 0.08) that was fusion spliced to a 62.5/125 ␮m GI MM fibre (NA = 0.28) both of Telecom-grade. The MM fibre was slightly strained and was bonded at two points at initial length of L0 = 4.5 cm. Therefore, it could be strained by means of a manual linear micro-positioner. The light propagates through the MM fibre and is reflected from it 0◦ angle cleaved end. The cleavage was made by hand (with an Al2 O3 sheet cleaver) or with a commercial high-precision (<0.5◦ ) cleaver. The zoom in Fig. 2 illustrates the sensing principle again (as in Fig. 1) giving an expanded vision of the SM/MM fibre splice where the coupling/filtering occurs. Therefore, the encoded modulation in the spatial domain is on-line converted to intensity-modulation that can be probed by means of the optical time-domain reflectometry technique [17]. Because the OTDR technique

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Fig. 2. The experimental set-up. The zoom illustrates the sensing principle of the device.

is able to measure the Rayleigh backscattering, it can provide a reference for the present sensor that ultimately is of an intensity-modulated kind. In order to avoid the OTDR receiver saturation, the cleaved end of the GI MM fibre was dipped inside water (n = 1.333) thus attenuating the amplitude of the reflected pulses. Therefore, the probe light traverses twice the GI MM fibre, including the strained piece at |L0 |. From the OTDR waveform, the reflectance height H (in dB) was measured as function of the fibre strain |L0 |/L0 (in ␮e). Almost all measurements were performed at room temperature (25 ◦ C).

4. Results and discussion Fig. 3 shows comparatively two measurement results of the optical response H as a function of the GI MM fibre strain |L0 |/L0 up to 9000 ␮e performed at 1310 and 1550 nm wavelength. Both plots clearly show a cyclic trend. Linear response with positive or negative inclination is also observed around points of maximum derivative. By cleav-

ing again the distal end of the GI MM fibre, one can obtain similar cyclic plots as shown at Fig. 3 but with a shifted spatial-domain phase. It was observed that such phase is sensitive to the fibre cleavage. The measurements were reproducible for increase/decrease the strain. The bottom plot of Fig. 3 suggests periodic “fringes” of 5000 ␮e spacing corresponding to 0.23 mm period of applied stretch for 1310 nm wavelength. Similarly the upper plot of Fig. 3 suggests periodic “fringes” with half-spacing of ∼6500 ␮e that corresponds to 0.59 mm period of applied stretch for 1550 nm wavelength. In both calculation the initial fibre length of L0 = 45 mm was taking into account. Both the stretch periods of 0.23 mm and 0.59 mm scales with the ∼0.2–1 mm typical pitch of a GI MM fibre [10,11]. Variations of the GI MM fibre pitch with wavelength are also expected [10]. One might think if the sensing mechanism could be interferometric as a Fabry–Perot configuration. However, this is not true because the fringe spacing of a Fabry–Perot interferometer scales with the wavelength that in the present instance is over 1 ␮m = 0.001 mm. However, the measured “fringes” spacing in our sensor are at least over 230 times

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Fig. 3. The optical response of the strain-gauge at 1310 and 1550 nm wavelength.

greater than the fringes spacing that would be produced by a Fabry–Perot interferometric sensor. Furthermore, with the exception of a Sagnac interferometer, all other fibre interferometer is thermally unstable, including the Fabry–Perot. However, as it will further explained, the sensor is observed to be stable even when the temperature is made to change 20 ◦ C above the room temperature. For launch from SM/MM splice, the GI MM fibre almost works as a single-mode fibre because most of the optical power remained within a 20–30 ␮m diameter circle around the centre of the fibre [10,18]. The diameter of the axial mode in GI MM fibre is close to the core diameter of a SM fibre. Therefore, the GI MM optical fibre acts as a very long flexible lens where the working distances of such lenses

depend on the exact length of the lens. This explains the cyclic dependence of the signal. Fig. 4 shows a measurement result at 1310 nm wavelength for another fibre cleavage comparing with that obtained for Fig. 3. A linear response is observed at a 4250 ␮e range and a sensitivity of −4.6 × 10−4 dB/␮e was calculated after performing a linear regression. Fig. 5 shows a plot of similar measurement result but for 1550 nm wavelength. A linear response is now observed at a 3100 ␮e range and a sensitivity of −1.1 × 10−3 dB/␮e was now calculated. These results show that the sensor is able to measure strains with over 0.01/0.0011 ∼9 ␮e resolution, where 0.01 dB is the resolution of the employed OTDR for the reflective events. One notices that plots shown on Figs. 4 and 5 do not correspond

Fig. 4. The linear optical response of the strain sensor at 1310 nm wavelength.

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Fig. 5. The linear optical response of the strain sensor at 1550 nm wavelength.

to any region of plots of Fig. 3. The sensor response, i.e. the sensitivity, dynamic range and spatial-domain phase of the sinusoidal light trajectories is sensitive to the exact length of the MM fibre and to the cleavage angle of it distal end. The measurements shown on Figs. 4 and 5 were achieved after a new cleavage from those plotted on Fig. 3. Fine adjustment of the sensor can be achieved by a careful polishing of the GI MM fibre end that should be visually controlled with the help of a microscope. The GI MM fibre length should not be too long (<40 cm) otherwise some slightly instability could arise. Although for fibre length >40 cm the observed instability is not actually quite understood, a possible explanation is briefly discussed. The fundamental spatial mode is slightly sensitive to macrobends with radii greater than a critical value for a given spectral region [11,15,16]. Bending and stretching of GI MM fibre as well as geometrical/material imperfections also change polarisation and cause mode mixing of the light running in the fibre. Therefore, we believe that bending and stretching GI MM fibre length >40 cm can cause polarisation changing and mode mixing [13] that ultimately produce some instability in the sensing signal. Reciprocally such fibre should not be too short due to a practical reason. However, for GI MM fibre length of <40 cm the sensor performance was quite stable, thus we believe it is enough for this first trial or to establish the proof-of-principle. The GI MM fibre strain-gauge should be kept strained whatever the temperature of the device operation. Except for losses, there is no limitation on the SM length allowing remote sensing. One may observe that the modulation mechanism here described does not imply in any optical attenuation. Results employing only SM or GI MM fibre instead of the SM/MM splice launcher/filter led to a null result as expected. Because of the relatively low linear thermal expansion of silica (5.2 × 10−7 1/◦ C), small environmental temperature drift (±10 ◦ C) does not imply in

an appreciable error of the measurements. For larger temperature variations, a negative linear expansion material fibre coating may be provided in order to perform thermal compensation. We may compare the results here presented with those reported in the literature. Bhatia et al. describe a long-period grating based strain-gauge featuring 2100 ␮e of linear range and a maximum error of 58 ␮e [19]. A multiplexed in-fibreBragg-grating/fibre-Fabry–Perot strain-gauge have shown ∼5000 ␮e of linear range and 4.2 ␮e resolution when one takes into account the maximum resolution of 0.05 nm for commercial optical spectrum analyser [20]. Further experimental investigations are under development in order to measure the spectral, thermal and dynamic response of the present device sensor as well as to establish the possibility of distributed sensing.

Acknowledgements We would like to thank Financiadora de Estudos e Projetos (FINEP) for the financial support of this research.

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Biographies Ricardo M. Ribeiro holds a BSc degree in Physics from the Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil in 1985, a MSc and DSc degrees in Physics (Photonics) from the Catholic University of Rio de Janeiro (Brazil) in 1989 and 1995, respectively. In 1998, he joined the Instrumentation and Photonics Laboratory of the Federal University of Rio de Janeiro where he performs research and development works on fibreoptic devices and systems. In 2001, he also joined the Electric and Telecommunication Engineering Department of the Catholic University of Petrópolis, Petrópolis and State University of Rio de Janeiro, Brazil. His research interests include photonic materials, microwave photonics, optoelectronics, all-optical devices and systems for Telecommunications and Sensing. He currently has six filed patents and has authored over fourth scientific international papers. Marcelo M. Werneck holds the BSc degree in Electronic Engineering by the Pontif´ıcia Universidade Católica of Rio de Janeiro (PUC-RJ) in 1975. His MSc degree was obtained from the Biomedical Engineering Program (PEB-COPPE) at the Federal University of Rio de Janeiro (UFRJ) in 1977, where he is a lecturer and researcher since 1978. His PhD title was obtained from the University of Sussex (UK) on Biomedical Engineering in 1985. At the moment he is the co-ordinator of the Photonics Laboratory at the Biomedical Engineering Program (PEB-COPPE) of the Federal University of Rio de Janeiro. He centres his research in fibre optic sensors and biomedical instrumentation.