Bienstman, Peter

Werquin, Sam

English

Ghent University Academic Bibliography

Interferometric approach tosolve microring resonance splittingin biosensor applicationsS. Werquin1 and P. Bienstman11 Photonics Research Group, Dept. of Information Technology, Gent University,Sint-Pietersnieuwstraat 41, 9000-B Gent BelgiumSilicon-on-insulator microring resonators have proven to be an excellent platform forlabel-free nanophotonic biosensors. The high index contrast of the silicon-on-insulatorplatform allows for fabrication of micrometer size sensors. However, it also limits thequality of the resonances by introducing an intrinsic mode-splitting. Small deviationsfrom perfect symmetry lift the degeneracy of the normal resonator modes. This severelydeteriorates the quality of the output signal. The quality of the resonances is of utmostimportance to determine the performance of the microrings as a biosensor. We suggestan integrated interferometric approach to give access to the unsplit, high-quality normalmodes of the microring resonator.IntroductionSilicon-on-insulator (SOI) microring resonators have proven to be an excellent platformfor label-free nanophotonic biosensors. The high index contrast of the SOI-platform al-lows for fabrication of micrometer size sensors. The small sensor size makes microringresonators excelent candidates for highly multiplexed assays [1]. It also allows for inte-gration of a sensor on an optical fiber tip, opening the door to in-vivo applications [2].By combining microrings in advanced sensing configurations, it is also shown that theirsensitivity can be improved significantly [3]. The high index contrast of the SOI-platformcauses high confinement of the optical fields in the waveguides, which makes the micror-ings very sensitive to changes on the waveguide surface. This explains their very highsensitivity, but at the same time, waveguide roughness causes scattering of the guidedlight. This degrades the quality factor of the resonances and can ultimately lead to split-ting of a resonance [4]. Because the detection limit of a microring biosensor is directlyrelated to the quality of the ring resonance, a high Q-factor is of primordial importance insensing applications [5]. In this paper, we present an integrated interferometric approachto resolve the resonance splitting of a microring resonantor on a single chip. Resulting ina significant improvement of the resonance quality.Origin of resonance splittingA perfectly symmetric microring resonator mode is twofold degenerate. Both clockwise(CW) and counterclockwise (CCW) propagation are possible in the microring and bothmodes are uncoupled. This degeneracy is lifted when the CW-mode and CCW-modebecome coupled. Surface roughness on the waveguide edges and the proximity of buswaveguides for microring interrogation form deviations from circular symmetry. Thesedeviations cause forward propagating light to scatter back into the opposite direction,exciting a CCW-mode from a CW-mode and vice versa. Standing-wave modes as a sym-metric and antisymmetric superposition of the traveling waves can be considered as thenew eigenmodes of the system. They will however, no longer be degenerate as a conse-quence of the symmetry breaking coupling [6]. If the linewidth of the resonance is smallenough to distinguish both modes, the resonance splitting will be visible in the outputsignal. This occurs for high quality resonances. Evidence of this effect is provided in fig-ure 1, which shows both the pass-port and add-port spectrum of a microring in add-dropconfiguration. Only the input port is excited, so ideally, no power is present in the CCW-mode and the add-port remains dark. The measurement shows that backscattering in themicroring waveguide cannot be neglected, resulting in significant power in the add-portand resonance-splitting in the pass-signal.(a) (b)Figure 1: (a) Microring resonator in add-drop configuration (b) Measured spectra showingresonance splitting and backscattered powerIntegrated interferometric circuitAs demonstrated in [7], an interferometric approach can be used to access the normalmodes of the microring resonator in an output signal. We have implemented this in anintegrated circuit on a single SOI-chip. A layout of the circuit is provided in figure 2.Vertical grating couplers are used to couple light from a tunable laser lightsource intoFigure 2: layout of the integrated interferometric circuitthe circuit and collect the power at the output. The input light excites a CW-mode in themicroring resonator, a CCW-mode is excited as a consequence of mode-coupling. Thenormal modes of the microring are a symmetric and antisymmetric superposition of theCW and CCW-mode, so we can express them as follows:a± = 1√2(aCW ±aCCW ) (1)If the coupling per unit time between the bus waveguide and the microring is representedby κ, the fields transmitted and reflected by the resonator aret =1√2ain+κaCW and r = κaCCW (2)Both fields are combined in a multi mode interferometer (MMI), this results in the fol-lowing signals at the different output ports. φ denotes the phase difference between thecombining waves.out1 = 12(1√2ain+κaCW + eiφκaCCW )out2 = 12(1√2ain+κaCW − eiφκaCCW )out3 = 1√2t(3)If we design the circuit such that the phase difference between the reflected and transmit-ted wave equals a multiple of 0 or pi, we see that the signals in output one and two areproportional to the normal modes of the resonator. At the same time, output three is pro-portional to the pass-signal of the microring resonator in the all-pass configuration. Thismeans we have access to the unsplit, high-Q normal modes of the cavity. If the detectionlimit of a biosensor is limited by the quality-factor of the resonance, this provides a toolto improve the detection limit significantly. Higher resonator Q-factors give rise to lowerdetection limits.Experimental resultsThe circuit from figure 2 is designed and processed in a CMOS pilot line at imec. Usingthe vertical in- and outputcouplers on the waveguides, the chip can easily be measured in afiber-to-fiber configuration. For the junction regions where waveguides are combined andsplitted, MMI 2x1 and MMI 2x2 couplers are used. The coupling from the microring tothe waveguide is ensured by weak evanescent coupling to a neighbouring bus waveguide.To obtain the measured spectra, a SANTEC TSL-510 Tunable laser source is used to gen-erate the input signals. Output intensities are measured by a HP-8153 optical powermeter.The laser wavelength is swept in 10pm steps during recording of the power.Figure 3 shows the recorded specta at the three outputs of the circuit. Output one and twoare proportional to the normal modes of the resonator, output three returns the all-passspectrum. We clearly see the all-pass spectrum shows severe splitting of 60pm in theresonance. This values is comparable to 3dB-bandwidth of the normal mode in outputone. The Q-factors of the recorded resonances are listed in table 1. This result showsTable 1: Q-factors of resonancesresonance Q-valueoutput 1 2.20 ·104output 2 1.05 ·104output 3 1.28 ·104an almost twofold improvement of the resonance quality from output three to output one.Notice the distinct asymmetrical shape of the resonance in output two which makes un-ambiguous definition of a Q-value difficult. Additionally, the extracted normal modes donot correspond exactly with the normal modes we can distinguish in the all-pass signal.This non-ideal behaviour is a consequence of fabrication variations. The 2x2 MMI isFigure 3: Measured output spectra of the circuit. Output one clearly shows the improve-ment of the resonance shape compared to the all-pass signal in output three.especially sensitive to this and is not exactly balanced as a result. Also the phase relationbetween the reflected and the transmitted wave differs slightly from the required value ofk ·pi. This can be solved by careful tuning of the waveguide structures.ConclusionWe have succesfully integrated an interferometric setup to resolve the resonance splittingof microrings on the SOI-platform. The access to the unsplit normal modes of the res-onator allows the improvement of the detection limit of microring resonator biosensors.References[1] K. De Vos, J. Girones, T. Claes, Y. De Koninck, S. Popelka, E. Schacht, R. Baets, and P. Bienstman,“Multiplexed Antibody Detection With an Array of Silicon-on-Insulator Microring Resonators,” IEEEPhotonics Journal, vol. 1, no. 4, pp. 225–235, Oct. 2009.[2] C. L. Arce, K. D. Vos, T. Claes, K. Komorowska, D. V. Thourhout, and P. Bienstman, “Silicon-on-insulator microring resonator sensor integrated on an optical fiber facet,” IEEE Photonics TechnologyLetters, vol. 23, no. 13, pp. 890–892, 2011.[3] T. Claes, W. Bogaerts, and P. Bienstman, “Experimental characterization of a silicon photonic biosen-sor consisting of two cascaded ring resonators based on the Vernier-effect and introduction of a curvefitting method for an improved detection limit,” Optics Express, vol. 18, no. 22, pp. 22 747–22 761,2010.[4] W. Bogaerts, P. D. Heyn, T. V. Vaerenbergh, K. D. Vos, S. Kumar, T. Claes, P. Dumon, P. Bienstman,D. V. Thourhout, and R. Baets, “Silicon Microring Resonators,” Laser & Photonics Reviews, vol. 6,no. 1, pp. 47–73, 2011.[5] T. Claes, J. Molera, K. De Vos, E. Schacht, R. Baets, and P. Bienstman, “Label-Free Biosensing With aSlot-Waveguide-Based Ring Resonator in Silicon on Insulator,” IEEE Photonics Journal, vol. 1, no. 3,pp. 197–204, Sep. 2009.[6] T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Modal coupling in traveling-wave resonators.”Optics letters, vol. 27, no. 19, pp. 1669–71, Oct. 2002.[7] J. Knittel, T. G. McRae, K. H. Lee, and W. P. Bowen, “Interferometric detection of mode splitting forwhispering gallery mode biosensors,” Applied Physics Letters, vol. 97, no. 12, p. 123704, 2010.

Interferometric approach ot solve microring resonance splitting in biosensor applications

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Interferometric approach ot solve microring resonance splitting in biosensor applications

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