The escalating data center traffic driven by applications like machine learning and large-scale data analytics demands networks with low latency, high bandwidth, and scalability. Current data center networks rely on multi-tier fabrics of electrical switches and optical transceivers, consuming considerable power – a problem exacerbated by the slowing of Moore's Law. Optical circuit switching (OCS) emerges as a promising alternative. By eliminating the need for opto-electronic-opto (O-E-O) conversion in the core network, OCS offers high bandwidth and low latency due to the absence of buffers. A flat data center topology further enhances energy efficiency by reducing the number of electrical switches and transceivers. While MEMS-based OCS systems offer high port counts, their slow switching times (~ms) limit bandwidth utilization. Many emerging workloads utilize small packets; for example, in key-value store applications, over 97% of packets are 576 bytes or less. To achieve reconfiguration overhead below 10% at 50 Gbps speeds, nanosecond switching times are crucial. Various photonic chip-based techniques have been explored for fast optical switching (ns), including SOAs, EAMs, and MZIs. Arrayed waveguide grating routers (AWGRs) combined with tunable lasers (TLs) are particularly promising, as their passive core enhances fault tolerance and scalability. However, current tunable lasers have tuning latencies of 10 ns or more, unsuitable for the required speeds. A disaggregated laser architecture, decoupling wavelength generation from selection, offers a path to sub-nanosecond tuning times. One such implementation uses a discrete bank of lasers and a wavelength selector with a photonic integrated AWG and SOAs. This paper proposes and demonstrates a disaggregated tunable transceiver using photonic chip-based soliton microcombs as a multi-wavelength source, leveraging their broadband bandwidth, precise spacing, low power consumption, and scalability.

The paper reviews existing data center network architectures and their limitations in the face of increasing bandwidth demands. It highlights the drawbacks of electrical switching and the potential benefits of optical circuit switching (OCS). The authors discuss prior work on MEMS-based OCS, noting their slow switching speeds. They also review existing fast optical switching techniques based on SOAs, EAMs, and MZIs. The limitations of existing tunable laser technologies for OCS are discussed, motivating the need for a disaggregated laser architecture. Existing work on soliton microcombs and their applications in various areas (LiDAR, microwave photonics, OCT, and coherent communication) is also referenced, emphasizing their potential for ultrafast wavelength switching in data centers. Finally, the literature review highlights the advantages of the proposed soliton microcomb based architecture compared to banks of discrete tunable lasers, pointing out improvements in power efficiency and scalability.

The study employs a novel OCS architecture incorporating a Si3N4-based soliton microcomb as a multi-wavelength source. The soliton microcomb is generated by pumping a packaged Si3N4 microresonator fabricated using the photonic Damascene reflow process. A multi-soliton is initiated by scanning over resonance (forward tuning), followed by single soliton generation via backward switching. The soliton is amplified using a low-noise EDFA, and the resulting comb is de-multiplexed using an AWG. Individual comb channels are switched using discrete SOAs. The 10-90% rise and fall times of a single microcomb carrier are measured. Sub-nanosecond switching of multiple comb channels is demonstrated. For a more compact system, a photonic integrated circuit (PIC) comprising an InP-based SOA array and an AWG is implemented. The wavelength alignment of the comb channels to the AWG is performed by changing the temperature of the PIC. 25 Gbps NRZ and 50 Gbps PAM-4 burst mode data transmissions are demonstrated with ultrafast switching, using both discrete SOAs and the PIC-based system. The bit error ratio (BER) is characterized by changing the received optical power. Digital signal processing (DSP) is used to obtain the BER. The power consumption of the multi-wavelength source is analyzed, considering potential improvements through reduced splicing loss, on-chip actuators, and power-efficient pump lasers. The architecture's scalability is discussed, considering potential deployment scenarios with multiple racks and servers.

The key findings of the study include the demonstration of sub-nanosecond (<520 ps) optical circuit switching using a Si3N4-based soliton microcomb and discrete SOAs. The system achieved successful 25-Gbps NRZ and 50-Gbps PAM-4 burst mode data transmission during the ultrafast switching. Sub-nanosecond switching (<900 ps) was also achieved using a more compact PIC-based system with an integrated AWG and SOAs, further demonstrating 25-Gbps NRZ burst mode transmission. The study shows that over 20 individual comb channels in the C-band (with power >-20 dBm) can be switched at sub-nanosecond speeds. A bit error rate (BER) below the forward error correction (FEC) threshold was achieved for both NRZ and PAM-4 modulation formats, demonstrating the feasibility of the proposed technology for practical data center applications. The power consumption analysis indicates potential for significant power efficiency improvements, making the soliton microcomb-based approach competitive with other multi-wavelength source solutions. The study also shows the scalability of the proposed architecture to thousands of nodes, leveraging the multi-channel capabilities of modern servers and ToR switches.

The results address the research question of achieving nanosecond optical circuit switching for data centers by demonstrating a novel architecture based on soliton microcombs. The sub-nanosecond switching speeds achieved, along with successful high-speed data transmission, show the significant potential of this technology to meet the stringent latency and bandwidth requirements of future data centers. The demonstrated scalability to a large number of nodes addresses the growing demand for energy-efficient and high-throughput data center networks. The findings are relevant to the field of optical communication and data center networking, offering a promising path towards overcoming the limitations of current electrical switching technologies. The power consumption analysis suggests that further optimization could lead to a highly power-efficient solution, making the proposed architecture competitive with existing approaches.

This work demonstrates the feasibility of ultrafast optical circuit switching using integrated soliton microcombs, achieving sub-nanosecond switching speeds and successful high-speed data transmission. The architecture exhibits scalability to large data center networks, addressing crucial power and latency requirements. Future research could focus on further optimizing power efficiency, exploring different modulation formats, and investigating integration with advanced data center networking protocols.

While the study demonstrates significant progress, limitations include the current coupling efficiency of the Si3N4 chip (15%), leading to lower output power. The overshoot observed in the switching signals of the PIC-based system, due to impedance mismatch, could be mitigated through optimization. Further improvements in the AWG crosstalk and insertion loss of the PIC are also warranted. The experimental demonstration was limited to a specific set of wavelengths and modulation formats, suggesting further investigation with broader coverage is beneficial.