A thermally driven wavelength-selective switch, based on integrated-optic micro-ring resonators is described. This configuration allows high ON/OFF ratios combined with small dimensions. Measurements of the thermal behaviour of a single resonator confirm the switching capability

Driessen, A.

Geuzebroek, D.H.

Kelderman, H.

Klein, E.J.

Klunder, D.J.W.

Tan, F.S.

English

University of Twente Research Information

Planar Lightwave Circuits 11 4.2.5Thermal Wavelength-selective Switch Based on Micro-ring ResonatorsD.H. Geuzebroek, E.J. Klein, H. Kelderman, F.S. Tan, D.J.W. Klunder, A. DressenLightwave Devices Group, MESA + Research Institute, Faculty of Applied Physics, University of TwenteP.O. Box 217, 7500 AE, The Netherlands; e-mail: D.H.Geuzebroek@el.utwente.nlAbstract A thernally driven wavelength-selective switch, based on integrated-optic micro-ring resonators isdescribed. This configuration allows high ON/OFF ratios combined with small dimensions. Measurements of thethermal behaviour ofa single resonator confirm the switching capability.IntroductionCurrently a shift in the deployment of opticaltechniques towards the access network can beobserved. FTTH (Fibre to the Home) is alreadydeployed in some commercial field trials. One of themain issues is the cost of the optical components inthe FTTH system. Since the cost cannot be shared bymany users, the components have a largeeconomical constraint. Typically in a PON (PassiveOptical Network) only several tens of users share thesame equipment. The users home-equipment willeven not be shared. Integrated optics will be able tosatisfy the low-cost challenge, since mass productionis feasible. In this paper a switch based on micro-ringresonators is descrbed. Based on these buildingblocks a compact single chip integrated-optictransceiver for the WDM access network is proposed.( 1 /~ *Figure 1 Schematic cross section and top-view ofa micro resonatorMUicro-ring ResonatorsMicro-ring resonators (MR) can be deployed as WDMfilters, switches, and modulators [1,2]. Their smalldimensions are advantageous for large-scaleintegration -VLSI photonics - and cost limitedapplications. The functional behaviour of a single MRis explained in [3]. The highly selective properties ofthe Lorentzian filter shape of the MR make it a goodcandidate for WDM filter applications. The propertiesof the MR can be exploited even more when multipleresonators are combined to perform specificfunctions. By cascading MRs, for example a moredesirable box-shaped filter response can be created(4]. A schematic picture of a single MR is shown inFig. 1. The MRs described in this work are fabricatedof Si3N4 waveguide channels embedded in SiO2,vertically coupled to a Si3N4 nng with radius R. Withthis technology [5] high Finesse MRs have beendemonstrated [3,6]. The present devices have an aircladding. For thermal tuneability and switching acladding layer as depicted in Fig. 1 can be favourablesince the cladding can be made of a material that hasa high temperature dependent refractive index, e.g. apolymer. Furthermore this configuration allows theplacement of heater structures.Drop ~~~~~Dro-pInput Inpul ~a l - WFigure 2. Single and second order wavelengthselective switch, MRI: passive MR;MR2: tunable MRWavelength Selective SwitchA schematic of the proposed two-stage wavelengthselective switch is shown in Fig. 2. The first stageselects the desired wavelength band. The secondstage, which is thermally tuneable, can switch theselected wavelength band to the drop port. Theswitch can be made up of higher order filter stages toincrease the ON/OFF ratio. Fig. 3 shows thesimulated response of a switch with second orderstages based on MR with a radius of 25 gim, a freespectral range of 10 nm and finesse 100 andpropagation loss 2 dB/cm. In this simulation the MRhas a polymer cladding with WUn/T of -3.3104 K'.When increasing the temperature of the set-up shownin Fig. t by 50 degrees, a difference in effectiverefractive index (Nef) of the ring of 4 10,4 can beobtained, which is sufficiently large to switch betweenthe ON and OFF state.The solid line in Fig. 3 represents the Lorentzianresponse of a single MR. The dotted line shows theresponse of a single stage of a second order switch.The dashed line shows the drop port of the completeswitch in the ON state. The dash-dotted linerepresents the OFF state. As can been seen in thefigure the difference between the ON and OFF statecan reach 30 dB, which looks very promising forapplications in a WDM system.For the higher order filter stages the parallel set-uphas been chosen instead of a serial one forCladdingI Si3N II R2SiPlanar Lightwave Circuits II 4.2.5-3001553.1 1554.1Wavelength (nm)Figure 3 Response of second order switchtwo reasons. The first is that small differences incentre wavelength caused by technologicalimperfections affects the filter shape only slightly [7].Furthermore in the parallel set-up the bandwidth ofthe higher order filter stage can be controlled bychanging the coupling constants between the bussesand the rings [7,8]. A broadening of the drop filtershape decreases the ON/OFF ratio given a certainshift. So by minimizing the bandwidth the highestpossible ON/OFF ratio can be reached for the lowesttemperature changes.00 00oo 00Figure 4 Array of switches. Bold MRs: active, thinMRs: passiveTransceiverThe MR based switch looks promising for applicationsin a transceiver as proposed in [1,9]. This transceiverutilizes the small foot print area and the highselectivity of the MR to create a low cost fullyintegrated optic transceiver for WDM accessnetworks. Fig. 4 shows two of an array of n switches,functioning as a receiving WDM filter. Each switchcan handle one ITU channel. This way the transceiveris able to select the appropriate wavelength channelassigned to the user within the access system. Ascan be seen in Fig. 4 the different switches areconnected through their in- and output busses. Thewaveguides between the two stages of differentswitches are not connected. Otherwise the OFF stateof one switch could overlap the ON stage from theneighbouring switch. Now a larger shift of theresonant peak is allowed, causing a higher ON/OFFratio. The rings can be thermally tuned because inthis application fast switching is not necessary.Thermal measurementsThe response of a single MR with air cladding (seeFig 1.) has been measured as a function oftemperature. Fig. 5 shows the power in the throughport as a function of wavelength at differenttemperatures. Clearly a shift of the spectral responsecan be seen with increasing temperature: 8YJT = 21pm/OC, about twice as much as our computersimulations. The difference can be explained becausein the simulations only the thermal effect on the indexn is taken into account. By heating the device also theeffect of thermal expansion of the complete devicemust be taken into account and consequently also theexpansion of the diameter. Taking into account thethermal expansion of the Si substrate an additionalshift of the centre wavelength of 13 pm/OC can beexpected. Therefore, the total predicted shift is ingood agreement with the measured value.1.00.910Cf 0.8CB 0.7E0Z 0.60.5 4-_1548 1549 1550Wavelength (nm)Figure 5 Thermal influence on troughportresponse of a MRConclusionA thermally tuneable switch is described which giveshigh ON/OFF ratios and has an extreme smallfootprint, circa 0.05 mm2. Experiments show thatthermo-optic switching is feasible. In future work theresonators will be heated locally by the use of heaterson top of the MRs. Furthermore Electro-Opticmodulators based on MRs will be fabricated.Financial support by the EC [9] is acknowledged.References1 Driessen et al, Proc. Symposium IEEE/LEOSBenelux Chapter (2001), 25-28.2 Rabiei et al, Proceedings OFC, (2002), 31-33.3 Klunder et al, Applied Physics B, 73 (2001), 603-608.4 Madsen et al, Optical Filter Design and Analysis,Wiley-lnterscience, New York (1999).5 Ridder, de et al, IEEE JST Quantum Electronics, 46, 1998.6 Tan et al, submitted for IEEE/LEOS workshop onfiber and optical passive components, Glasgow,June (2002).7 Little et al, Optics Letters, 25 5 (2001), 344-346.8 Griffel et al, Photonic Techn. Letters, 12 7 (2000),810-812.9 NAIS; Next-generation Advanced Integrated-OpticSubsystems, project EC-IST-2000-28018.a. I..--" ,@' .~ *.. ,,R '.. I - .I I: .... I'la* .0.4 S1 #IA %

Thermal wavelength-selective switch based on micro-ring resonators

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Advanced Integrated-Optic Subsystems, project EC-IST-2000-28018. a. I ..--&quot; ,@'

Optical Filter Design and Analysis, Wiley-lnterscience,

submitted for IEEE/LEOS workshop on fiber and optical passive components,

Thermal wavelength-selective switch based on micro-ring resonators

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