Baets, Roel

Taillaert, Dirk

Van Paepegem, Wim

VLEKKEN, J

English

Ghent University Academic Bibliography

A thin foil optical strain gage based onsilicon-on-insulator microresonatorsD. Taillaert*a, W. Van Paepegem ,b  J. Vlekkenc, R. BaetsaaPhotonics research group, Ghent University - INTEC, St-Pietersnieuwstraat 41, 9000 Gent, Belgium bGhent University, Dept. of mechanical construction, St-Pietersnieuwstraat 41, 9000 Gent, BelgiumcFOS&S, Cipalstraat 14, 2440 Geel , BelgiumABSTRACTWe present a novel type of optical strain gage. The strain gage consists of a thin polyimide foil with an integrated opticalcircuit. The strain sensing elements are optical microresonators. The optical response to strain of these microresonators is a wavelength shift of the resonance wavelength. The optical circuit includes several of these resonators to measure strain in different directions. The strain sensor is read-out using a single-mode optical fiber. Because the differentmicroresonators in the optical circuit have different resonance wavelengths, they can be read out using the same fiber.Our strain sensor is some kind of a cross between electrical resistance foil gages and fiber Bragg grating (FBG) sensors.It is a thin foil device, with a thickness of a few tens of micrometers,  but it is an optical device and can be read out in asimilar way as  FBG sensors. We present the working principle, fabrication and first experimental results.Keywords: Strain sensor, silicon, integrated optics1. INTRODUCTIONOptical fiber sensors based on FBGs are well known because of their immunity to electromagnetic interference, opticalmultiplexing and relatively high sensitivity. We report here on a new type of strain sensor that tries to implement theseassets in a small, thin foil, strain gage. This strain sensor uses an optical circuit with several microresonators. In figure 1,a microresonator is shown with a typical transmission spectrum. In this example, the width of the resonance peak is 0.12nm. Because of the nature of the resonator, there is not a single resonance peak, but the spectrum is periodic and the period is called the free spectral range (FSR). In [1], a polymer microresonator was demonstrated that can be used as astrain sensor, but this polymer structure had a very limited FSR. In high refractive index contrast silicon-on-insulatorwaveguides, microresonators with a much smaller diameter and hence larger free spectral range (several tens ofnanometers) can be made [2]. In the silicon-on-insulator (SOI) material, the waveguide core is a crystalline silicon layerwith n=3.45, and it is surrounded by SiO2 (n=1.45) cladding layers. The shift of the resonance wavelength of a resonatoris given byeffeffnnLL '' 'OO where L is the length of the resonator and neff, the effective index of the guided mode.in droppass-35-30-25-20-15-10-50normalized transmission(dB)wavelength (nm)1524 1524.5 1525 1525.5 1526 1526.5passdropFig. 1. Schematic drawing of microring resonator and experimental transmission characteristic of a SOI microresonator.*dirk.taillaert@intec.ugent.be; phone +32 92643445; fax +32 9264 3593; photonics.intec.ugent.beThird European Workshop on Optical Fibre Sensors, Antonello Cutolo, Brian Culshaw, José Miguel López-Higuera, Eds.,Proceedings of SPIE Vol. 6619, 661914, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.738412Proc. of SPIE Vol. 6619  661914-1A circular microresonator (ring resonator) is sensitive to strain in any in-plane direction. By adding two straight sections,a so-called racetrack resonator is created and a different sensitivity to strain in two orthogonal directions can beachieved. In table 1, the theoretical wavelength shift as a function of strain is shown for several SOI racetrack resonators.The geometrical parameters are chosen in such a way that the circumference of the resonator (and hence the FSR) isapproximately the same. The wavelength shift 'O is expressed in pm and the strain H in PH.Table 1. Calculated sensitivity of a SOI mircoresonator for different geometrical parameters.The sensitivity is expressed in pm/PH.'O=0.34Hyy'O=1.30HxxLbend | 12µmLstraight = 24µmR=2µmL=12µm'O=0.45Hyy'O=1.18HxxLbend | 18µmLstraight = 18µmR=3µmL=9µm'O=0.63Hyy'O=0.99HxxLbend | 24µmLstraight = 12µmR=4µmL=6µmFSR | 16 nmLRxyHxx HyyExperimental results on the microresonators with 4 Pm radius, will be presented in this paper. It should be noted that thecross-sensitivity to strain in the transverse direction is still rather high for this structure. This can be improved by usinglonger straight sections and smaller bend radii. There is however a lower limit to the radius of the bend, becauseradiation losses will increase in the bend when the radius becomes to small. We expect that the smallest bend radius withacceptable losses will be around 2 to 2.5Pm in practice. 2. FABRICATION OF THE SENSORFirst the optical circuit is made on a silicon-on-insulator (SOI) wafer. The fabrication of the optical circuit consists ofseveral standard processing steps (lithography, reactive ion etching). This processing is performed in a fabrication line that is also used for the fabrication of CMOS electronics [3]. However the number of processing steps required for theoptical circuit is much lower than for electronics circuits.  A close up of the resonator structures is shown in figure 2.After the fabrication of the SOI circuit is completed, it is not yet suitable as strain sensor because it is made on a thick(approximately 700µm) silicon substrate.  This silicon substrate can be thinned down by grinding to less than 50µm, buteven then it is not suited as strain gage, because the silicon is brittle and has a high elastic modulus. Therefore we havetransfered the SOI optical circuit to a flexible foil and completely removed the original silicon substrate.SiSiO2polyimideSiO215µm3µm0.2µmFig. 2. Left : Top view of the optical integrated circuit consisting of several microresonators. The diameter of the bends in the resonators is 8 µm.  Right : Layer stack of the thin foil optical strain sensor. The optical circuit is etched in thesilicon layer. The oxide layers form the cladding of the optical waveguide.Proc. of SPIE Vol. 6619  661914-2On top of the SOI, a polyimide layer is created by spinning and thermal curing. The thickness of this polyimide layer can be chosen anywhere between 5 and 50 µm. Afterwards the original silicon substrate is completely removed bymechanical and chemical processing steps. The final step in removing the substrate is a wet etch using potassiumhydroxide (KOH). This etchant has a very good selectivity Si:SiO2 and consequently the silicon substrate can becompletely removed while the SiO2 layers remain intact. The resulting layer structure is shown in figure 2. This is a thinflexible foil, that can be handled and bent without damaging the integrated optical circuit.It is also possible to spin a second polyimide layer on top of this structure to create a symmetric structure. Polyimide isused because it has good thermal stability, good resistance to solvents and corrosives, low moisture absorption and arelatively low elastic modulus (8.5 GPa in our case). Because polyimide is also used as backing material in electrical resistance strain gages, the bonding materials and procedures for bonding the gages to a workpiece are already wellestablished.3. EXPERIMENTAL RESULTSTo perform measurements, the strain sensor was bonded on an aluminum beam. A photo of the bonded strainsensor isshown in figure 3. The strain sensor was bonded using M-Bond 200 adhesive [4], a commercially available adhesive forbonding of thin foil strain gages. In our first experiments, we have bent the beam by fixing one end and moving the otherend over a known distance using a micrometer srew. The bending induces tensile strain at the upper surface of the plate,that is measured by the strain sensor. To read out the sensor, light is coupled in and out of the optical circuit using single-mode optical fibers. A broadband light source (SLED) is connected to the input fiber and a spectrum analyzer to theoutput fiber. During our experiment, the fibers were mounted on translation stages to align them with the optical circuit,but in the future the fibers will be glued to to the strain gage.Fig. 3. Thin film optical strain sensor bonded on an aluminum test plate. The sensing circuit itself (microring resonator) is not visible with the naked eye, but some other alignment patterns are. The measured shift of the resonance wavelength of the resonators, is plotted in figure 4, as a function of the deflection ofthe aluminum beam. The 4 curves correspond to 4 resonators with a different shape, as indicated on the figure. A linearrelation between the deflection and the measured wavelength shift is obtained. From the deflection of the plate we canestimate the theoretical strain at the top surface. Qualitatively the experimental results agree well with the theoreticalpredictions.  In the near future, a tensile test will be performed to more accurately characterize the sensitivity and make a quantitative comparison with the theoretical results.The largest wavelength shift obtained was 1.7 nm, corresponding to approximately 1700 PH. This limit is due to ourcurrent measurement setup. In this range, the behaviour of the sensor was linear and reversible.We have demonstrated a proof-of-principle device. It should be noted that the structures used are not yet optimized andthat the sensitivity and cross-sensitivity can be improved. We have also not adressed the issue of temperature effects andcompensation in this paper.Proc. of SPIE Vol. 6619  661914-30.00.20.40.60.81.01.21.41.61.82.00 2 4 6 8 10 12beam deflection (mm)wavelength shift (nm)'O'O'O'OOOstrain directionOOFig. 4. Measured wavelength shift of the resonance wavelength as a function of the beam deflection. The 4 curvescorrespond to the 4 resonators with different shape (see also figure 2).4. CONCLUSIONSWe have presented the principle, fabrication and first experimental validation of a new type type of optical strain sensor.Based on a theoretical calculation, we expect a sensitivity up to 1.3 pm wavelength shift per PH using an optimizeddevice design.ACKNOWLEDGEMENTSD. Taillaert thanks the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen) for a postdoctoral grant. This work was supported in part by the European Union through the IST-ePIXnetproject and by the Belgian IAP PHOTON Network.REFERENCES1. B. Bhola, H. Song, H. Tazawa, W. Steier, “Polymer microresonator strain sensors,” IEEE Photonics Technology Letters, vol. 17(4), p.867-869 (2005)2. W. Bogaerts, P. Dumon, D. Van Thourhout, D. Taillaert, P. Jaenen, J. Wouters, S. Beckx, R. Baets, “CompactWavelength-Selective Functions in Silicon-on-Insulator Photonic Wires,” J. Selected Topics in Quantum Electronics,vol. 12(6), 1394-1401 (2006)3. W. Bogaerts, R. Baets, P. Dumon, V. Wiaux, S. Beckx, D. Taillaert, B. Luyssaert, J. Van Campenhout, P.Bienstman, D. Van Thourhout, “Nanophotonic Waveguides in Silicon-on-Insulator Fabricated with CMOS Technology,”Journal of Lightwave Technology , vol. 23(1), p.401-412 (2005)4. “Strain gage installations with M-Bond 200 Adhesive,” instruction Bulletin B-127 from Vishay Micro-Measurements, http://www.vishay.comProc. of SPIE Vol. 6619  661914-4

A thin foil optical strain gage based on silicon-on-insulator microresonators - art. no. 661914

https://biblio.ugent.be/publication/412290/file/447661

A thin foil optical strain gage based on silicon-on-insulator microresonators - art. no. 661914

Abstract

Similar works

Full text

Available Versions

Ghent University Academic Bibliography