Tunable optical-path correlator for distributedstrain or temperature-sensing application

Yonggui Yuan, Bing Wu, Jun Yang, and Libo Yuan*Photonics Research Center, School of Science, Harbin Engineering University, Harbin 150001, China

*Corresponding author: lbyuan@vip.sina.com

Received July 12, 2010; accepted August 12, 2010;posted September 9, 2010 (Doc. ID 131416); published October 8, 2010

Based on a cavity-length tunable fiber-loop resonator, a multibeam optical path difference is generated. It can beused to match and correlate the reflective signals from the partial reflective ends of each sensing fiber gauge. Thecorrelation signals correspond to the sensing gauge lengths, and the shift of the correlation peak is related with thefiber sensing gauge elongation caused by strain or temperature. Therefore, it can be used to measure distributedstrain or deformation for smart structural monitoring. © 2010 Optical Society of AmericaOCIS codes: 060.2370, 060.4230.

Optical sensing systems based on low-coherence inter-ferometry have been intensively investigated in pastyears [1–9]. The advantage of this approach over conven-tional interferometric sensors is the ability to coherentlymultiplex many sensors onto a single optical signalwithout requiring the use of relatively complex time orfrequency multiplexing techniques [10,11]. The critical is-sue for building a low-coherence multiplexing fiber opticsensing system is to design a tunable optical path genera-tor. Based on the Fabry–Perot interferometer, a tunablecavity as an optical-path correlator has been proposed[12]. In the present work, by using a three-port circulator,we developed a tunable optical path correlator and con-figured a low-coherence interferometric multiplexingsensors system that measures the absolute optical pathlength elongations between reflectors of fiber segments.The fiber optic white-light interferometric sensing sys-

tem, which we have based on a cavity length tunable fiberloop resonator, is shown in Fig. 1. It consists of a super-luminescent light-emitting diode light source working atwavelength 1550 nm, two three-port fiber optic circula-tors, a photodiode detector, and a number of fibersegments (sensor gauges) connected in series. To distin-guish each fiber sensor, the gauge length is different ineach. In the sensing system, a cavity length tunable fiberoptic loop resonator has been used to generate an opticalpath difference (OPD) nL0 þ 2X , as shown in Fig. 1(b).The OPD of the cavity length tunable fiber-loop resonatorcan be varied through the use of a scanning mirror. Thescanning mirror is used to adjust the OPD of the tunablefiber-loop resonator to match and trace the change of thefiber length in each sensing segment; see Fig. 1(a). Wemake the OPD of the tunable fiber loop resonator nearlyequal to the fiber sensor gauge length, so the two reflectedlight waves from the two end surfaces of each sensinggauge can be matched with the cavity length of the reso-nator. Thatmeans theoptical path delay of one loop lengthof the resonator can be compensated by the sensing gaugelength, as shown in Fig. 1(b).When theOPDof the tunablefiber loop resonator is equal to the gauge length of a par-ticular sensor, a white-light fringe pattern is produced. Asthe optical path of the fiber sensor is modulated by, forexample, strain or temperature, the perturbation para-meter that is related to the optical path change resultsin the shift in the center peak position of the interference

fringe. Thus, it can be traced and recorded by moving thescanning mirror. Therefore, the strain or temperature canbe measured.

When one sensor, for example, Lj , has been disturbedby ambient stress or temperature, then the deformationof the sensor j can be measured by moving the scanningmirror to tracing the change. Thus the variation can berecorded as

ΔXj ¼ ΔðnljÞ; ð1Þwhere n is the refractive index of the fiber guide mode.Thus, the strain or temperature can be calculated as

εj ¼ΔXj

nequivalentlj; ð2Þ

ðTi − T0iÞ ¼ΔXi

liðT0iÞnðλ; T0iÞ½αf þ CT �; ð3Þ

where nequivalent ¼ nf1 − ð1=2Þn2½ð1 − νÞp12 − νp11� repre-sents the equivalent refractive index of the fiber guide

Fig. 1. (Color online) Working principle of the two-beamoptical-path correlation.

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mode. For silica materials at wavelength λ ¼ 1550 nm,the parameters are n ¼ 1:46, Poisson ratio ν ¼ 0:17, andphotoelastic constants p11 ¼ 0:12 and p12 ¼ 0:27, takenfrom [13]. nðλ; T0iÞ is the refractive index of the opticalfiber under the condition of wavelength λ and the ith sen-sor ambient temperature T0i. Here, αf and CT are thethermal expansion coefficient and the refractive indextemperature coefficient of the optical fiber, respectively.αT ¼ 5:5 × 10−7=°C, CT ¼ 0:811 × 10−5=°C at wavelengthλ ¼ 1550 nm are taken from [14].In our experiment, convenient fiber patch cords have

been chosen as fiber optic sensors. Each patch cord isapproximately 1 m long, but each is a different lengththan the others. By connecting ten fiber optic patch cordsin a series, a ten-sensor array has been tested in ourlaboratory. The ten-sensor output signals from the corre-lator are plotted in Fig. 2(a), and each signal peak corre-sponds to a particular fiber sensing gauge. To display thecorrelation measurement trace, one of an enlarged peakenvelope curve (fiber sensor 7) is also given in Fig. 2(b),and each sensor’s response performance of strain or tem-perature is similar. This is shown in Figs. 3 and 4, respec-tively. In the system, the scanning mirror was mountedon a high-resolution positioning system with a step reso-lution of 0:1 μm, and the accuracy was limited by the dif-ficulty in identifying the central fringe of the white-lightinterferometric pattern, which is estimated as less than�one fringe, equivalent to half wavelength (0.775 μm–0.8μm) for the 1550 nm light source. The moving mirrorscanning range is 150 mm, and the insertion loss betweenthe collimator and the reflect mirror is changed from 1to 2 dB.

Sensitivity of the system depends on the fiber sensorgauge length, as shown in Eq. (2). For example, if thegauge length of the fiber optic sensor is about 1 m, thenthe strain sensor’s sensitivity is about 0:8 με=m, while, ifthe gauge length is 10 m, then the sensitivity would beimproved to 0:08 με=m.

For evaluation of the total number of fiber optic sen-sors that can be multiplexed in the proposed sensing sys-tem, we assume that the light power launched into thefiber is P0, and the minimum power that can be detectedby the photodiode is Pmin. Then, the maximum number ofthe total multiplexed fiber optic sensors can be estimatedby the condition

PDðjÞ ≥ Pmin; j ¼ 1; 2;…; N: ð4Þ

For arbitrary fiber optic sensor j of the multiplexing ar-ray, the signal intensity amplitude detected by the photo-detector is proportional to the coherent mixing betweenthe reflected signals from the two partial reflectors ofeach sensor. The maximum number of the fiber sensorscan be estimated according to the calculated method gi-ven in [11,12]. For our case, the light source power is1 mW; the maximum number of the fiber sensors canbe calculated as Nmax ¼ 22.

In conclusion, based on the proposed cavity length tun-able fiber-loop resonator, a multiplexed fiber optic strainor temperature sensing system suitable for smart struc-ture applications has been designed and demonstrated.This three-port circulator scheme improves stability bysimplifying the optical arrangements of the tunable

Fig. 2. (Color online) Output signals from the correlator.

Fig. 3. (Color online) Strain-measuring experimental result.

Fig. 4. (Color online) Temperature-measuring experimentalresult.

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optical path correlator. The sensor’s multiplexing capa-city is more then 20. The linear sensor arrays are comple-tely passive, and the length measurements are absolute.Elongations can be obtained for each sensing fiber seg-ment so that it can be used to measure quasi-distributionstrain or temperature for large-scale smart structures.

This work was supported by the key project of theNature Science Foundation of Hilongjiang Province(No. ZD200810), the Key Project Foster Program for Uni-versity and College Science and Technology Innovation(No. 708030), and the National Natural Science Founda-tion of China (NSFC), under grants 60877046, 60707013,and 60807032, to Harbin Engineering University.

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