Mee-Koy Chin

Matrix analysis of 2-D microresonator lattice optical filters

We present a transfer matrix analysis of a two-dimensional (2-D) filter to study its frequency response functions. The (M/spl times/N) array consists of N independent columns of microring resonators side-coupled to two channel bus waveguides, with equal spacing between columns and each column consisting of M coupled resonators. We show that such a general 2-D lattice network of lossless and symmetric resonators can approximate an ideal bandpass filter characterized by a flat-top box-like amplitude response without out-of-band sidelobes, and a linear phase response. The bandwidth is determined by the coupling factor between resonators. The 2-D periodic structure exhibits nonoverlapping photonic bandgaps arising from the complementary transmission properties of the row and column arrays. The row array behaves as a distributed feedback grating giving rise to narrow bandgaps corresponding to the flat reflection passbands of the filter with out-of-band sidelobes. The column array, on the other hand, acts as a high-order coupled-cavities filter with broad bandgaps that overlap with the sidelobe regions, thereby effectively suppressing the sidelobes. The phase response is linear except near the band edges, where enhanced group delay limits the usable bandwidth of the filter to about 80%. The minimum size of the array required is about 3/spl times/10, but is ultimately limited by waveguide loss.

High-index-contrast waveguides and devices

We present a theoretical and experimental study of high-index-contrast waveguides and basic (passive) devices built from them. Several new results are reported, but to be more comprehensive we also review some of our previous results. We focus on a ridge waveguide, whose strong lateral confinement gives it unique properties fundamentally different from the conventional weakly guiding rib waveguides. The ridge waveguides have distinct characteristics in the single-mode and the multimode regimes. The salient features of the single-mode waveguides are their subwavelength width, strong birefringence, relatively high propagation loss, and high sensitivity to wavelength as well as waveguide width, all of which may limit device performance yet provide new opportunities for novel device applications. On the other hand, wider multimode waveguides are low loss and robust. In addition, they have a critical width where the birefringence is minimal or zero, giving rise to the possibility of realizing intrinsically polarization-independent devices. They can be made effectively single mode by employing differential leakage loss (with an appropriate etch depth) or lateral mode filtering (with a taper waveguide). Together these waveguides provide the photonic wire for interconnections and the backbone to build a broad range of compact devices. We discuss basic single-mode devices (based on directional couplers) and multimode devices (multimode interferometers) and indicate their underlying relationship.

Theoretical approach to a polarization-insensitive single-mode microring resonator

This paper addresses the polarization sensitivity issue of micro-ring resonators by proposing a novel design of an MMI-coupled resonator with substantially reduced or zero polarization sensitivity, while maintaining singlemode and low-loss conditions. The design is based on polarization-independent, single-mode waveguide obtained by using a judicious combination of critical ridge width and etch depth. The design is limited to relatively large resonators having small FSR (free spectral range). For the first time, it gives designer a handle on the intrinsic polarization-dependent characteristics of waveguide microresonators.

Nested-Ring Mach–Zehnder Interferometer in Silicon-on-Insulator

For the first time, a nested-ring Mach-Zehnder interferometer (MZI) on silicon-on-insulator is realized using a complementary metal-oxide-semiconductor-based process. In this letter, we verify that the device operates in two modes: the inner-loop resonance dominant mode due to strong build-up inside the inner-ring, and the double-Fano resonances mode due to strong light interaction with the outer loop. The results show that the inner-loop resonance is highly sensitive to the MZI arm imbalance compared to the double-Fano resonance mode. Based on these considerations, we obtain a good fit between theory and experiment.

Quasi-stable soliton transmission in dispersion managed fiber links with lumped amplifiers

We study quasi-stable solitons in a cascaded fiber line with alternating positive and negative dispersions, with and without fiber loss. Although more energy is required for forming the quasi-stable solitons than the regular solitons in a uniform fiber system with the same average dispersion, the required soliton energy decreases as the energy loss in one amplification period increases, eventually approaching the same value as in a uniform fiber system. Thus, the predicted advantage of higher soliton energy in dispersion-managed system is diminished in the presence of loss.

Phase engineering for ring enhanced Mach-Zehnder interferometers.

Ring resonators are waveguide realizations of Fabry-Perot resonators which can be readily integrated in array geometries to implement many useful functions. Its nonlinear phase response can be readily incorporated into a Mach-Zehnder interferometer to produce specific intensity output function. We present two generalized array configurations of ring-coupled MZI and discuss their characteristics in terms of the amplitude and phase response of the ring arrays as well as the transmission output of the MZIs. The two types of array have distinct transfer functions and effective phase shifts, and can be tailored to phase-engineer a wide range of MZI transmission functions.

Polarization dependence in waveguide-coupled micro-resonators.

One dominant issue for micro-resonator filters has been polarization sensitivity due to the form asymmetry in nanophotonic waveguides. Differences in the filters transmission intensity for TE and TM polarizations is attributed to the polarization dependent coupling. Complete power transfer in ultra-small directional couplers is demonstrated in agreement with simulations. Polarization dependence is simulated for waveguides of various widths, showing the presence of a critical width at which the propagation constants are equal for TE and TM modes. A design for polarization-independent and single-mode waveguides is discussed, along with implications for the applications of micro-resonators in general.

Boxlike filter response based on complementary photonic bandgaps in two-dimensional microresonator arrays

We propose that a boxlike filter response can be obtained by utilizing complementary photonic bandgap properties of the column and row configurations in two-dimensional microresonator arrays. The filters are fabricated using deep-UV lithography in silicon-on-insulator technology. The observed sidelobes reduction is approximately 10 dB, and the usable bandwidth can be as high as 500-750 GHz.

Finesse enhancement in silicon-on-insulator two-ring resonator system

We demonstrate experimentally the finesse enhancement in a pair of mutually coupled ring resonators coupled to two buses fabricated in silicon-on-insulator technology, as proposed theoretically in an earlier paper. A finesse close to 100 (or Q=30000) is obtained in a two-ring system, with the outer ring double the size of the inner ring, and an external coupling coefficient of 34%. The maximum finesse enhancement relative to the single-ring structure is 14 times, in good agreement with the theoretical prediction.

Proposal for an Ultranarrow Passband Using Two Coupled Rings

We demonstrate theoretically that a simple two-coupled-ring structure yields very narrow resonances in the transmission spectra, provided the loss is very small. The linewidth is smallest when the path lengths of the two rings have a ratio of two. Relative to the single ring, the linewidth is reduced beyond the size scaling by the intensity buildup factor in the second ring. In the lossless case, the linewidth can be two orders of magnitude narrower. The enhancement can still be seen for experimentally demonstrated silicon-on-insulator microrings.