WN1 (Invited) 10:30 am -11:00 am Micro-Ring Resonator Filters Vien Van, Brent E Little, Sai T Chu, John V Hrniewicz Little Optics Inc., 9020 Junction Dr., Annapolis Junction, M 20701, USA Introduction Micro-ring resonators are compact wavelength-selective elements that fnd usage in many applications including add-drop filters, demultiplexers, dispersion compensators, modulators [1,2,3]. Their small size and fnctionality make them a key building block in densely-integrated planar lightwave circuits (PLCs). One particularly usefl property of micro-rings is the ease of constructing multi-cavity coupled resonators, allowing high-order flters to be realized that can meet the tight box-like spectal requirement of WDM systems. In addition these micro-rings can be tuned, e.g. by mechanisms such as the thermo optic effect, so that wide-band tunability can be achieved if a verier architecture is employed to overcome the FSR limitation. In this paper we present the latest developments in high-order micro-ring resonators and their application in commercial filters tunable over the C-band. High-order micro-ring resonator flters Micro-ring resonators provide a natural way for synthesizing high-order filters by coupling many cavities together in a serial fashion, as illustrated in Fig. l(a). Standard techniques of filter synthesis developed for microwave flter networks can be straightforwardly applied to micro-ring resonators to synthesize amplitude and phase flters of almost any desired spectal responses. Applications in WDM systems usually call for a Butterworth type of flters with maximally-fat amplitude response and nearly linear phase characteristic within the passband. For an N-th order flter of this type it is found that the bus-to- ring power coupling efciency, K� , and the coupling effciency K� between rings k and k + 1 have to be chosen according to the relationship, K� 1 sin2 (r/2N) K 6 ="' cos(r/N)-cos(2rk/N)' k = 1, 2, . .. N - 1. This condition along with the bandwidth requirement can be used to determine the required coupling parameters of the micro-rings. We have fabricated filters of orders up to 11 using the HydexTM PLC platform with index contrast of 17% [4]. An optical micrograph of a 5t-order flter is shown in Fig. l(b). Fig. 2 shows the theoretical and measured responses of micro-ring filters of orders N = 1, 3, 5 and 11. Sharp filter skirt roll-offs of roughly 6N dB/octave characteristic of Buterworth filters can be observed. Tunable flter based on vernier architecture The use of a high-index contrast material platform such as HydexTM allows micro-rings of very small radii to be fabricated, but the FSRs of these resonators are still far short of the range needed to cover a fll commercial C-band. Here the use of a verier architecture can greatly expand the tunable range of the flter without over-taxing the theral handling capability of the device. Fig. 3 shows an implementation of a 2-stage verier micro-ring flter with polarization diversity [5]. Each stage comprises of a 3fd-order filter with FSR=575GHz for stage 1 and FSR=650GHz for stage 2. The entire circuit including the polarization beam splitter (PBS) and combiner (PBC) was realized on the HydexTM PLC platform. Fig. 4 shows the composite spectra of 20 channels at 50GHz spacing. Average channel insertion loss of 4.1d, PDL of OAdB, and differential group delay better than 5ps were measured. ITU grid alignment better than 0.5GHz was also achieved. Conclusions We presented high-order flter synthesis using micro-ring resonators and experimentally demonstrated flters up to the 11 t order. Using a verier architecture with polarization diversity, a widely tunable WDM flter based on 3fd-order micro-ring resonators was also demonstrated. 0-7803-8557-8/04/$20.002004 IEEE 571