We propose a novel micro-ring resonator which uses quasi-TE polarized light in the bus waveguide to excite the quasi-TM polarized modes in a micro-ring. An all-pass filter is demonstrated on Silicon-on-Insulator

Bogaerts, Wim

De Heyn, Peter

Roelkens, Günther

Van Vaerenbergh, Thomas

Vermeulen, Diedrik

English

Ghent University Academic Bibliography

Silicon-on-Insulator polarization rotatingmicro-ring resonatorD. Vermeulen, P. De Heyn, T. Van Vaerenbergh, W. Bogaerts, G. RoelkensPhotonics Research Group, Department of Information Technology,Ghent University - imec, B-9000 Ghent, BelgiumAbstract: We propose a novel micro-ring resonator which uses quasi-TE polarized light inthe bus waveguide to excite the quasi-TM polarized modes in a micro-ring. An all-pass filter isdemonstrated on Silicon-on-Insulator.1. IntroductionResonant devices are considered to be a key technology in integrated photonic circuits. For high index contrast plat-forms such as Silicon-on-Insulator (SOI) the most investigated class of components are definitely the micro-ringresonators [ref]. They can be used as spectral filters for telecommunication and data communication purposes, aswavelength dependent electro-optic modulators and photodetectors, as transducers for label-free biosensing [1], asoptical delay lines, . . . In general quasi-TE polarized light is used in micro-ring resonators since it has the strongestconfinement which enables light guiding in bends with very small radii with negligible bending losses. However, de-pending on the application and exact platform dimensions, the fundamental quasi-TM polarized mode can be preferredto resonate in the micro-ring. The lower confinement of the quasi-TM mode can be for example beneficial for a-thermaldevices [2], sensors based on a cladding refractive index variation, high-Q rings due to a lower backscattering fromthe vertical sidewalls [3] and for large wavelength range tuning using liquid crystals [4]. Most of these applications,however, exhibit waveguides which are vertically asymmetrical.In [5] we showed how an asymmetrical waveguide is a major problem for a grating coupler enabled platform. Duringthe adiabatic transition from a broad multimode waveguide to a single-mode waveguide, several anti-crossings willbe passed, thereby coupling the fundamental quasi-TM mode to higher order quasi-TE modes which are not guidedwhen tapering down to a single-mode photonic wire waveguide. Circumvention of such an adiabatic taper has beendemonstrated by using focusing techniques such as a focusing grating coupler, but with a lower coupling efficiencythan similar grating couplers for quasi-TE polarized light. Secondly, for routing purposes one prefers the polarizationwhich enables the most compact routing functionality, i.e. the TE polarization.A straightforward solution would be to use a cascade of polarization rotators [6], polarization filters and ring-resonator.As a solution we propose a novel micro-ring resonator which uses quasi-TE polarized light in the bus waveguide toexcite the quasi-TM polarized modes in a micro-ring, thus acting as a polarization rotator, polarization filter and ringresonator at the same time.2. DesignThe coupling section of the ring resonator is an asymmetrical directional coupler which acts as a polarization rotator[7, 8]. For a certain width of the bus waveguide, the quasi-TE mode in the bus waveguide will be phase matched to thequasi-TM mode in the micro-ring. In order to achieve successful coupling between these two quasi-polarized modes,a vertical asymmetry is necessary [5].Our design is based on an SOI platform exhibiting a 220nm thick Si core layer on a 2µm thick buried oxide (BOX)layer. The asymmetry requirement is fulfilled by using an air top cladding. A deep etch of 220nm is performed fordefining the waveguides and ring resonator. A shallow etch of 70nm allows us to use grating couplers for character-ization. We choose a bus waveguide width of 320nm and a micro-ring waveguide width of 600nm. The gap of thecoupling section is 300nm and the ring radius is 30µm. The bus waveguide has the same curvature as the micro-ring,resulting in a plane symmetric coupling section.3. MeasurementsThe measurement result is shown in Figure 1. Because the grating couplers act as a coupling method for TE-polarizedlight and as polarization filter with an extinction ratio of 50dB, we know that only the quasi-TE mode is excited in thebus waveguide. By calculating the theoretically expected Free Spectral Range (FSR) of the quasi-TM resonant mode1500 1505 1510 1515 1520 1525 1530 1535 1540Wavelength (nm)3530252015105Insertion Loss (dB)Fig. 1: Measurement result of the polarization rotating micro-ring resonator in an all-pass filter configuration.and comparing it with the measured FSR, we can conclude that quasi-TE polarized light from the bus waveguideis coupled to the quasi-TM resonant mode in the ring. The mean extinction ratio (ER) over the 40nm wavelengthrange is −16.2dB, which means that the ring is critically coupled. The mean Q is 32660 and the mean finesse is 88.Furthermore, no peak splitting was detected.4. ConclusionWe proposed a new kind of micro-ring resonator which makes use of a polarization rotating directional couplingsection to excite the quasi-TM resonant mode. A critically coupled all-pass filter showing very good performance wasdemonstrated. This polarization micro-ring resonator is expected to be useful for a whole class of applications whereTM-polarized light is preferred to resonate.5. AcknowledgementsDiedrik Vermeulen thanks the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT)for a grant. Thomas Van Vaerenbergh is supported by the Flemish Research Foundation (FWO-Vlaanderen) for a PhDgrant.References and links1. K. De Vos, I. Bartolozzi, E. Schacht, P. Bienstman, and R. Baets, “Silicon-on-Insulator microring resonator for sensitive and label-freebiosensing.” Optics express, vol. 15, no. 12, pp. 7610–5, Jun. 2007. [Online]. Available: http://www.ncbi.nlm.nih.gov/pubmed/195470872. J. Teng, P. Dumon, W. Bogaerts, H. Zhang, X. Jian, X. Han, M. Zhao, G. Morthier, and R. Baets, “Athermal Silicon-on-insulator ringresonators by overlaying a polymer cladding on narrowed waveguides.” Optics express, vol. 17, no. 17, pp. 14 627–33, Aug. 2009. [Online].Available: http://www.ncbi.nlm.nih.gov/pubmed/196879413. P. D. Heyn, B. Kuyken, D. Vermeulen, W. Bogaerts, and D. V. Thourhout, “High-Performance Low-Loss Silicon-on-Insulator MicroringResonators using TM-polarized Light,” in Optical Fiber Communication Conference. Optical Society of America, 2011.4. W. D. Cort, J. Beeckman, R. James, F. A. Fernandez, R. Baets, and K. Neyts, “Tuning silicon-on-insulator ring resonators with in-planeswitching liquid crystals,” JOSA B, vol. 28, no. 1, pp. 79–85, 2011.5. D. Vermeulen, K. V. Acoleyen, S. Ghosh, S. Selvaraja, W. A. D. D. Cort, N. A. Yebo, E. Hallynck, K. D. Vos, P. P. P. Debackere, P. Du-mon, W. Bogaerts, G. Roelkens, D. V. Thourhout, and R. Baets, “Efficient Tapering to the Fundamental Quasi-TM Mode in AsymmetricalWaveguides,” in European Conference on Integrated Optics, 2010.6. D. Vermeulen, S. Selvaraja, P. Verheyen, W. Bogaerts, D. V. Thourhout, and G. Roelkens, “High Ef-ficiency Broadband Polarization Rotator on,” in Group IV Photonics, 2010, pp. 42–44. [Online]. Available:http://ieeexplore.ieee.org/xpl/freeabs all.jsp?arnumber=5643432&abstractAccess=no&userType=inst7. L. Liu, Y. Ding, K. Yvind, and J. r. M. Hvam, “Silicon-on-insulator polarization splitting and rotating device for polarization diversity circuits,”Optics Express, vol. 19, no. 13, pp. 12 646–12 651, 2011.8. D. Dai and J. E. Bowers, “Novel concept for ultracompact polarization splitter-rotator based on silicon nanowires.” Optics express, vol. 19,no. 11, pp. 10 940–9, May 2011. [Online]. Available: http://www.ncbi.nlm.nih.gov/pubmed/21643354

Silicon-on-insulator polarization rotating micro-ring resonator

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Silicon-on-insulator polarization rotating micro-ring resonator

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