  Nyquist wavelength division (de)multiplexing (N-WDM) is a highly promising
technique for next-generation high-speed elastic networks. In N-WDM, Nyquist
filtering is an essential function that governs the channel spectral
efficiency. However, most Nyquist filter implementations to date require either
expensive, power-hungry digital electronics or complex arrangements of bulky
optical components, hindering their adoption for important functions such as
Nyquist channel shaping and reconfigurable optical add-drop multiplexers
(ROADMs) for Nyquist super-channels. Here, we present a distinctive solution
with low-cost, power-efficient, and simple-device natures, which is an on-chip
optical Nyquist-filtering (de)interleaver featuring sub-GHz resolution and a
near-rectangular passband with 8% transition band. This unprecedented
performance is provided by a simple photonic integrated circuit comprising a
two-ring-resonator-assisted Mach-Zehnder interferometer, which features high
circuit compactness using high-index-contrast Si3N4 waveguide, while showing
sub-picosecond optical delay accuracy enabling full C-band coverage with more
than 160 effective free spectral ranges of 25 GHz across a bandwidth over 4
THz. Such devices show clear potential for chip-scale realization of N-WDM
transceivers and ROADMs.
