Although microring resonators can serve as optical filters in integrated photonic circuits, single microring resonator filters do not have a narrow-enough passband to be used in DWDM. One solution is to couple microring resonators in series, analogous to stacking thin-film interference filters to reduce passband width. Researchers at the University of Maryland (College Park, MD) have demonstrated experimentally that this approach works.
For comparison, the researchers made filters consisting of one, two, and three rings (see figure). The devices were fabricated in aluminum gallium arsenide/gallium arsenide (AlGaAs/GaAs) on GaAs substrates and contained a 0.5-µm-thick GaAs core. Using electron-beam lithography followed by etching, narrow single-mode waveguides of 0.42-0.62-µm width were made with a 4.5-µm bend radius and linear loss of less than 10 dB/cm. The resonators were laterally coupled to each other and to the input and output waveguides across an air gap. Light from a tunable external-cavity laser diode was used to characterize the filter transmission spectra.
The researchers compared the filter responses after normalizing the frequency scale to the 3-dB bandwidths of the filters. While the single resonator showed the expected Lorentzian spectral shape, the double-resonator filter exhibited much-steeper rolloff and out-of-band rejection. Power loss was 6 dB for the single resonator, 9.8 dB for the double resonator, and 12.8 dB for the triple resonator. Improvements in design and fabrication will reduce these losses, say the researchers. Varying the interresonator coupling strength made possible the construction of a triple-resonator filter having a passband with a 0.5-nm-wide flat top. For more information, contact Ping-Tong Ho at firstname.lastname@example.org.