Optical Network Components Based on Microring Resonators

Douwe Geuzebroek

2006

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6 pages

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In the last years much effort has been made to arrive at optical integrated circuits with high complexity and advanced functionality for application in optical networks. For this aim high index contrast structures, like optical microresonators, are employed that allow for a large number of functional elements within a given chip area: VLSI photonics. Experimental results of work performed at MESA + will be reported including a microresonator-based, ultra-compact reconfigurable optical add-drop multiplexer operating at 40 Gbit/s and fabricated in SiON technology. In addition a discussion will be given of new challenges and possible solutions.

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can be found by considering a single resonance peak in the drop port where the amplitude and finesse are determined by the roundtrip losses. By enhancing the losses and consequently reducing the Q-factor, light can effectively be switched between the drop- and through-ports. The MR can carry out a large number of optical functions. It can be used as a compact filter with high resolution. For Wavelength Division Multiplexing (WDM) applications a MR with add and drop port serves as an ultra-compact building block for an optical add drop multiplexer (OADM). Switching or modulation of light can be done by changing the phase in the resonator by thermal, mechanical [5] or electro-optical [6] means. Besides for applications in the field of communication, MRs can be used as ultra-compact optical sensors [7].

Figure 2. MR-based ROADM, (a) schematic view (b) microscope picture of realized device (area of view 1.2.x 1 mm’), with clearly visible the Q-shaped thin film heaters on top of the MRs.

Figure 3. Spectral response of a MR-based ROADM: a) I, response for two configurations, b) Add response for IAdd1 to IAdd4 measured at Tout most efficient geometry for the OADM. Each of the four MRs can be thermally tuned by a heater across the FSR of 4.2 nm. The heater is Q-shaped for high power efficiency. The MRs have a radius of 50 um, a height of 190 nm and a width of 2.5 um, giving an Nog= 1.517 (TE @1550 nm). The MRs are vertically coupled to the port waveguides which are 2 um wide, 140 nm high and have a Nev =1.505 (TE@1550 nm). Both, the MRs and the port waveguides have been designed for TE operation and are realized in Si3;N,4 by standard contact lithography.

Figure 4. Eye pattern at 40 Gbit/s obtained with the ROADM: a) input (upper trace) and drop port (lower trace); b) demonstration of multicasting: drop 1 (upper trace), drop 2 (lower trace), both tuned to the same input wavelength and measured simultaneously. The pigtailed OADM was measured using a broadband source and an optical spectrum analyzer with a resolution of 0.05 nm. Figure 3 shows the normalized responses measured at J,,, when the broadband source was connected to J;, for two distinct configurations which were set by thermally tuning the MRs. In this setup the OADM drops the channels present on J;, to the desired drop ports IDrop1-IDrop4. These dropped channels are visible as dips in the MR through responses in J,,,;. In the "4 single channels" configuration a total heater power of 446 mW was applied 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 ~12 dB below the normalized input power level. Thus ~94 % of the input power is extracted. A fit of the individual MR responses to a theoretical MR model showed amplitude coupling constants k, and k, of 0.56+0.04 and 0.44+0.04 respectively at ring losses of 1.5+0.5 dB/cm. The measured FSR and Finesse were 4.18 nm and 10.3 respectively, giving a FWHM of 0.41 nm (= 51 GHz). The "single combined channel" configuration in Fig. 3a shows how the responses of the individual MRs could also be shifted to overlap each other. This configuration could be set while dissipating only 20 mW due to the fact that the untuned MRs already had nearly overlapping resonance frequencies, showing good fabrication reproducibility. The minimum of the combined through response is 30 dB (>99 % extracted).

Figure 5. Four-channel WDM Router consisting of 5 connected OADMs, a) schematic view, b) mask lay-out consisting of 4 layers: waveguide, ring, heater and electrodes; depicted area 1.5 x1.8 mm

best solution will be to avoid crossings completely by using a three-level approach [11]. If resonators and eas west as well as north-south waveguides are all fabricated in different layers any undesired interference can | avoided. Reduction of stray light can also be obtained by incorporating absorbing layers (in practice thin met films) at a safe distance (~10 um) from the waveguides. Figure 6. Mask lay-out of |WDM routers (for details see Fig. 5b) and a number of test structures currently being processed by the authors; the area on the wafer is 1 x 1 cm’.