We report a reconfigurable four-channel optical add-drop multiplexer for use in access networks. The optical add-drop multiplexer (OADM) is based on vertically coupled thermally tunable Si/sub 3/N/sub 4/--SiO/sub 2/ microring resonators (MRs) and has been realized on a footprint of 0.25 mm/sup 2/. Individual MRs in the OADM can be tuned across the full free-spectral range of 4.18 nm and have a 3-dB bandwidth of 50 GHz

Baker, Nigel

Driessen, Alfred

Geuzebroek, Douwe H.

Kelderman, Henry

Klein, Edwin J.

Sengo, Gabriel

English

A reconfigurable four channel OADM based on vertically coupled thermally tunable $Si_3N_4/SiO_2$ microring resonators has a footprint of 0.25 mm^2. Each MR can be tuned over 4.3 nm and has a bandwidth >I0 GBit

Klein, E.J.

Geuzebroek, D.H.

Kelderman, H.

Sengo, G.

Driessen, A.

NARCIS 

Reconfigurable optical add-drop multiplexer using microring resonators

We report a reconfigurable four-channel optical add–drop multiplexer for use in access networks. The optical add–drop multiplexer (OADM) is based on vertically coupled thermally tunable $Si_3N_4$ – $SiO_2$ microring resonators (MRs) and has been realized on a footprint of 0.25 mm2. Individual MRs in the OADM can be tuned across the full free-spectral range of 4.18 nm and have a 3-dB bandwidth of 50 GHz

Reconfigurable Optical Add-Drop Multiplexer Using Microring Resonators

A reconfigurable four channel OADM based on vertically coupled thermally tunable $Si_3N_4/SiO_2$ microring resonators has a footprint of 0.25 mm^2. Each MR can be tuned over 4.3 nm and has a bandwidth &gt;I0 GBit

University of Twente Research Information

12th EUROPEAN CONFERENCE O N  INTEGRATED O P T I C S  . -.5.LI..-.. -;--. .7 , ',.C--.-..",*-.: ...- . .-.>. .-- Reconfigurable Optical Add-Drop Multiplexer Using Microring Resonators Edwin ~ l e i n ' ,  Douwe Geuzehroek, Henry Kelderman, Gabriel Sengo, Nigel Bake? and Alfred Driessen 'lntegrated Optical MicroSystems Group, MESA+ Research Institute, University of Twente, P.O.Box 217, 7500 AE Enschede, The Netherlands (email: e.i.klein@utwente.nl) ' ~ d v a n c e d  Technology Networks Group, Nortel Networks, Harlow, Essex CMI 7 9NA, United Kingdom Abstract: A reconfigurable four channel OADM based on vertically coupled thermally tunable SilNliSiOz microring resonators has a footprint of 0.25 mm'. Each MR can be tuned over 4.3 nm and has a bandwidth >I0 GBit. Keywords: optical networks, optical add-drop multiplexer, integrated optics, thermally tunable, microring resonator, SijNd Introduction As computers become more powerful and high-bandwidth applications, for instance for streaming media, become more prevalent, the demand for network bandwidth is growing rapidly. In order to satisfy this demand the deployment of optical networks is moving more and more towards the home-user's premises. Since deployment costs are a ma,jor issue in these access networks, low-cost optical filtering and switching functions need to be devised. Integrated optical Microring Resonators (MRs) are viable candidates for these functions as they combine a small footprint with a highly selective filter function [1.2,3]. Furthermore they can he readily combined into more complex structures such as a wavelength selective switch [4,5]. An important component i n  which the filter function and small size of MRs can be applied effectively is a WDM router. Figure I shows a possible 4-channel imvlementation of such a router 161. This router lin ( T I .  > . .consists of five 4-way Optical Add-Drop , . ...,... multiplexers (OADM). In this router the WDM input ; . I &  i .  / +  signal lin is first separated into individual channels (1, ... L) by an OADM. Each of these channels is then guided into one of four additional OADMs. ~hese-OADMS can then add these channels to one of I011t2 ( ZC? .A) four output waveguides lout,. --c The OADMs can be implemented using AWGs or nuH:a:r lout3 ( ZCJ .A) Mach-Zehnders. OADMs based on AWGs are --+ generally large devices since AWGs can only split + 10% ( 1~4.~.L)  channels, and thus require additional coniponenrs, I I '._I + while MZ based OADMs require multiple MZs for C ~ . I . ~ I  c4.3h3 the selection of a single channel. An OADM based on MRs however offers several advantages over these conventional implementations. The use of Figure I :  Schemtic qfu Cchannel WDMRolrrer MRs, which can have radii as small as 2 pm, allows consisring of 5 connecred OADM's for extremely small OADM implementations. In addition, a minimal component implementation of a 4-channel OADM based on MRs can already be realized with only four MRs, due to their highly selective filter characteristic. The first column of figure I shows such an implementation where each MR drops an incoming channel I, to one of the outputs lout, when its resonance frequency corresponds to that of the incoming channel. In this paper the design, fabrication and measurements of a 4-channel thermally tunable MR based OADM are presented. Thursday 7th April 12th  EUROPEAN CONFERENCE O N  INTEGRATED O P T I C S  ,,.- ~ < . ~  -.-..-. ~.~ --,... ,: : - - 7.~., , - ~ ,.-, ~ ~ .~ -.,. ~ ~ . Design and fabrication The OADM was designed as shown in figure 2a. It consists of a central waveguide (Iinilout) and four AddIDrop waveguides. These waveguides are spaced at 250 pm to allow for a standard fiber-array connection. The sizc of the OADM. 1.25 x 0.2 mm' is mainly determined by this spacing. A single MR is located at each intersection of central- and add-drop waveguides. The cross-grid waveguide approach [6.7], in which the two waveguides that couple to the MR cross each other, leads to some crosstalk but is also the most efficient geometry for the OADM. Each of the four MRs can be thermally tuned by a heater. The heater is omega shaped for high power efliciency. A 3D geometry and cross-section of a MR is shown in figure 2b. The MR has a radius of 50 pm, a height of 190 nm and a width of 2.5 pm, giving an Neff=1.517 (TE ($1550 nm) .The 50 pm radius was chosen because it gives a FSR that is smaller than the thermal tuning range, allowing full FSR tuning. In addition, it allows for a MR that is nearly phase matched to the port waveguides. The MR is vertically coupled to these port waveguides whicli are 2 pm wide, 140 nm high and have a Neff-1.505 (TE@1550 nm). Rotli the MR and the port waveguides are designed for TE operation. ir- Iout Figure 2a: Layout of the &/-drop multiplexer 2h: MR top view and coupling region cros~-secrion The high aspect ratio in both the MR and port waveguides stems from lithographical limitations and the use of stoichiometric Si3N4 (n=1.98 @I5SOnm) which is limited to a maximum thickness of 340 nm to avoid material stress. The OADM was fabricated by depositing 140 nm LPCVD Si3N4 on top of 8 pm thermally grown SiO? [2, 81. The SilNr waveguides were then etched using reactive ion etching (RIE). A 1 pm TEOS separation layer was applied next. The ring resonators were then defined by depositing 190 nm SilN4 and RIE. A 4 pm thick layer of PECVD Si02 was deposited next. The device was then annealed at 1150°C. Subsequently the 200 nm thick Chromium heaters were detined using lift-off. Figure 3 shows a close up of a fabricated OADM. The central and addidrop waveguides are clearly discemable as well as the omega shaped heater elements on top of the four micro-ring resonators. The inset shows a pigtailed and packaged OADM chip. The OADM chip itself measures I0 x 9 mm and contains a total of four OADMs of which only the center OADM is connected. :..+ 1 .  + ..> :-- . . I.... . ' l i  Thursday 7th April 12th EUROPEAN CONFERENCE ON INTEGRATED OPTICS Measurement results and discussion The pigtailed OADM was measured using a broadband source and an optical spectrum analyzer with a resolution of 0.05 nm. Figure 4 shows the normalized responses measured at Iout when the broadband source was connected to Iin for two distinct configurations which were set by thermally tuning the MRS. In this setup the OADM now drops the channels present on Iin to the desired drop ports IOropl-ID@. These dropped channels are visible as MR through responses in Iout. In the "4-channel configuration" a total heater power of 446 mW wasapplied to set the MR resonance frequencies on a 100 GHz ITU Grid (spaced at 0.8 nm). The minima of the individual MR through responses are -12dB below the normalized input power level. Thus =94 % of the input power is extracted. A fit of - single Ccmblned channel -35 1 1548 154E 1550 1552 1554 1556 WavelengIh (nm) Figure 4: Iout response for a combined-channel and 4- channel OADM configuration the individual MR responses to a theoretical MR model showed amplitude coupling constants KI and KZ of 0.56 k0.04 and 0.44 a . 0 4  respectively at ring losses of 1 SM.5 dBIcm. The measured FSR and Finesse were 4.18 nm and 10.3 respectively, giving a FWHM of 0.41 nm (=5 1 GHz). The single channel configuration in figure 4 shows how the responses of the o individual MRs could also be shifted to overlap each other. This configuration -5 could be set while dissipating only 20 mW due to the fact that the untuned MRs E -10 '0 already had nearly overlapping resonance - & frequencies, showing good fabrication 2 -1s reproducibility. The minimum of the n. x combined through response is 30 dB (>99 .z -20 % extracted). 2 2 -2s Figure 5 shows the normalized responses measured at Iout when the broadband -30 source is connected to the different add- ports (IAddl-IAdd4). In this configuration the -35 1544 1548 1548 1550 1552 1554 1556 MRs select a channel from these ports and add it to lin. Wavelength (nm) The figure shows that minima of the individual MR drop responses are -17dB Figure 5: Add Responses for IMl-Iw4 measured at lout. below the normalized output power level ( ~ 8 9 %  of input power dropped). The effects of the adjacent MRs on an add channel can also be observed, for instance in the add response of the first resonator. Here the three (indicated by arrows 2,3,4) MRs that follow the first MR drop power from the main channel which shows up as dips in the Iout response. Using the parameters obtained from the fit to the responses in figure 4, the channel crosstalk [9] - - PD,,,(~ = 2, + A ~ I T U )  sin (n.AAITU / FSR) CT = lolo phOp (2 = 2,) sin2(n/2~) of the device can be calculated to be - I  1.7 dB. The OADM could be reconfigured in <1 ms due to the fast thermal tuning response of the MRs [lo]. Figure 6 shows that a single MR has a linear tuning response and has a maximum tuning range (without heater damage) Thursday 7th April 12th EUROPEAN CONFERENCE ON INTEGRATED OPTICS of a . 3  nm at a power dissipation of 380 mW, yielding a thermally induced wavelength shift of I 1 prn/mW of dissipated heater power. The device shows no measurable thermal crosstalk due to the small heater area, wide (=I 50 pn) heater separation and the high thermal conductivity (1 6 1 WImiK) of the silicon substrate. Bandwidth measurements on the OADM have yet to be performed. Measurements carried out at Nortel on single MRs with the same geometry of the MRs used in the OADM have shown however that the bandwidth is >IOGBit/s [5] while theory predicts a bandwidths >20 GBit/s. Conclusions 5.0 - The feasibility of a reconfigurable add-drop multiplexer with thermally tunable Si3N4 MRs has been shown. Out of a large number of - possible configurations a single- and a 4-channel g 3 5 -  - configuration have been measured. A change between the different OADM configurations could be made in <1 ms. Individual rings showed up to 12 dB extinction in the through port and 17 dB in the drop port. The rings have already been tested for bandwidth and showed good performance at 10 GBit/s [5] while even higher values are expected. The relatively low FSR of 4.18 nm of the MRs in the OADM k lk &, & 3k a will pose problems in network applications since Dissipated heater power (mw) multiple channels will be dropped within the 3d telecom window for which this OADM is Figure 6: MR Tuning response intended. It is however possible to solve this issue by reducing the ring radius or utilizing the Vernier effect by combining multiple MRs of different radii [I 11. Using multiple rings will also further reduce the crosstalk which, with -1 1.7 dB is still too high for practical applications. A cascade of two identical rings of the type used in the OADM for instance, will reduce the crosstalk to -23 dB. Acknowledgements The work has been carried out within the EC funded IST-project NAIS; Next Generation Advanced Integrated- Optic Subsystems, IST-2000-280 18. References 1) S.Suzuki et al., 'Integrated -optic ring resonators with two stacked layers of silica waveguide on Si', Photonic Technology Letters., vol. 4(1 I), pp. 1256-1258,1992 2) F.C. Blom et al., 'Experimental study of integrated-optics microcavity resonators: Toward an all-optical switching device', Applied. Physics. Letters, vol. 71, pp. 747-749, 1997 3) B.Little et al., 'Microring resonator channel dropping filters', Journal of Lightwave Technology, vol. 15. pp. 998- 1005,1997 4) E.J.Klein et al., 'Wavelength Selective switch using thermally tunable microring resonators', Proceedings LEOS annual meeting, p. MM 1,2003 5) D.H.Geuzebroek et al., 'Compact Wavelength-Selective Switch for Gigabit Filtering in Access Networks', IEEE Photonic Technology Letters, Accepted for publication in February 2005. 6) B.Little et al., 'Microring resonator arrays for VLSl photonics', IEEE Photonic Technology Letters, vol. 12, pp. 323-325.2000 7) Sai T . C ~ U  et al., 'An Eight Channel Add-Drop Filter Using Vertically Coupled Microring Resonators over a Cross Grid', IEEE Photonic Technology Letters, Vol. 1 1. No. 6, pp. 691-693, June 1999 8) K.Worhoff et al., 'Silicon oxinitride - a versatile material for integrated optics applications', J. Electrochemical Society, vol. 149(8), pp. F85-F91,2002 9) D.Klunder et al., 'Vertically and laterally waveguide-coupled cylindrical microresonators in Si3N4 on Si02 technology', Applied Physics B, vol. 73, pp. 603-708,2001 10) R. Meijerink et al., 'Thermally tunable, wide FSR switch based on micro-ring resonators', Proceedings Symposium IEEElLEOS Benelux, pp. 155-158,2002 1 1) R.Grover et al. 'Parallel-Cascaded Semiconductor Microring Resonators for High-Order and Wide-FSR Filters', Journal of Lightwave Technology, vol. 14, no. 10, pp. 2260-2267, 1996 Thursday 7th April 

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