We present an improved design approach for N×N silica-based multimode interference (MMI) devices. By this approach, we could determine a well-defined range of the length of the multimode section that would produce optimal device performance. The range is linked to the propagation constant spacing of fundamental and higher order modes of the multimode waveguide. Related design principles and issues of silica-based MMI devices will be discussed.
Optical ring-resonators could be used to synthesize filters with low crosstalk and flat passbands. Their application to DWDM interleaving has been proposed and investigated previously. However, a number of important factors related to this topic have not yet been considered and appropriately addressed. In this paper, we propose a novel scheme of a symmetrically parallel-coupled ring resonator array with coupling apodisation. We show that it can be used to construct a wavelength interleaver with remarkably improved performance. Various design factors have been considered. An optimization procedure was developed based on minimizing the channel crosstalk in the through and drop ports simultaneously by adjusting the ring-bus coupling coefficients. We show that apodisation in coupling could suppress channel crosstalk effectively, by choosing the optimal coupling coefficients. We also introduced the equalization of both the input and output coupling coefficients to minimise passband ripple. For a 50 - 100 GHz DWDM applications, four rings is found to be the best choice for array size. A four-ring filter achieves crosstalk -24 dB, insertion loss at resonance <1 dB, and good passband flatness (shape factor >0.6).