Full-C-band, sub-GHz-resolution Nyquist- filtering (de)interleaver in photonic integrated circuit

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 subpicosecond 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. Introduction The constant growth in demand for high data-rate telecom services, coupled with the need for an increasingly flexible and versatile network, poses an imminent need for a new generation of optical fiber networks. These networks must be not only able to support large capacity transmission, but also allow for flexible bandwidth resource management [1, 2]. In this critical fiber infrastructure, highly spectrally-efficient (de)multiplexing and optical channel control and routing with high frequency granularity are paramount for optimal operation. As such, Nyquist wavelength division multiplexing (N-WDM) is well placed to become a key multiplexing technology, as its well-defined rectangular channel spectrum intrinsically allows for add-drop operation by simple optical filtering [3-11]. A rectangular pass-band, high granularity optical filter is an important enabling technology for spectrally efficient fiber networks. In N-WDM, a sharp ‘Nyquist’ filter with a bandwidth equal to the channel baud rate is required, which is usually implemented by means of digital root-raised cosine (RRC) filters. Although digital implementations of Nyquist filtering are able to provide nearly perfect RRC filter responses with flexible roll-off factors [12], an optical Nyquist filter offers several advantages for practical applications. Optical Nyquist filters can be designed with multiple passbands to allow for multi-channel processing using a single device, promising a dramatically simplified system configuration – particularly when the filter provides wide frequency/wavelength coverage, e.g. over the full C-band as desired in applications. Moreover, optical filtering provides “line-rate” process capability without the need of bit-level access to the signals and avoids the use of high-speed digital electronics for digital signal processing and digital-to-analog conversion, which can considerably lower system latency, with potential savings in power consumption and cost. Perhaps most critically,