Device architecture and precision nanofabrication of microring-resonator filter banks for integrated photonic systems

To achieve the maximum benefit of electronic-photonic integrated circuits wavelength-division multiplexing must be used. This requires the design and fabrication of a highly integratable photonic device, capable of performing multiplexing/demultiplexing operations with low loss and minimal crosstalk. A filter bank consisting of high-index-contrast microring-resonator filters, with accurately spaced resonant frequencies can meet these requirements. This paper describes the basic architecture of microring-resonator filter banks, and how to maximize performance while keeping fabrication challenges reasonable. The greatest challenge in fabricating such devices is achieving the dimensional precision, on the scale of tens of picometers, needed to attain accurately spaced resonant frequencies. To do this, a fabrication method based on varying the electron-beam dose during scanning-electron beam lithography is used. This approach is used to create a dual twenty-channel filter bank, comprised of second-order silicon-rich silicon nitride microring resonators. The average resonant frequency spacing is off from the target spacing by only 3 GHz, corresponding to a dimensional precision of 75 pm. This approach is also shown to be compatible with the fabrication process for silicon microring resonators. Furthermore, it is shown that any remaining resonant frequency errors can be corrected with postfabrication thermal tuning. Also, a method of using the contra-propagating mode of a microring-resonator filter is demonstrated, enabling a single filter bank to multiplex/demultiplex two signals at the same time.