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MICRO COMB

Micro-combs - optical frequency combs generated by integrated micro-cavity resonators – offer the full potential of their bulk counterparts, but in an integrated footprint. They have enabled breakthroughs in many fields including spectroscopy, microwave photonics, frequency synthesis, optical ranging, quantum sources, metrology and ultrahigh capacity data transmission. Here, by using a powerful class of micro-comb called soliton crystals, we achieve ultra-high data transmission over 75 km of standard optical fibre using a single integrated chip source. We demonstrate a line rate of 44.2 Terabits s−1 using the telecommunications C-band at 1550 nm with a spectral efficiency of 10.4 bits s−1 Hz−1. Soliton crystals exhibit robust and stable generation and operation as well as a high intrinsic efficiency that, together with an extremely low soliton micro-comb spacing of 48.9 GHz enable the use of a very high coherent data modulation format (64 QAM - quadrature amplitude modulated). This work demonstrates the capability of optical micro-combs to perform in demanding and practical optical communications networks.

Download PDF Introduction The global optical fibre network currently carries hundreds of terabits per second every instant, with capacity growing at ~25% annually1. To dramatically increase bandwidth capacity, ultrahigh capacity transmission links employ massively parallel wavelength division multiplexing (WDM) with coherent modulation formats2,3, and in recent lab-based research, by using spatial division multiplexing (SDM) over multicore or multi-mode fibre4. At the same time, there is a strong trend towards a greater number of shorter high-capacity links. Whereas core long-haul (spanning 1000’s km) communications dominated global networks 10 years ago, now the emphasis has squarely shifted to metro-area networks (linking across 10’s–100’s km) and even data centres (< 10 km). All of this is driving the need for increasingly compact, low-cost and energy-efficient solutions, with photonic integrated circuits emerging as the most viable approach. The optical source is central to every link, and as such, perhaps has the greatest need for integration. The ability to supply all wavelengths with a single, compact integrated chip, replacing many parallel lasers, will offer the greatest benefits5,6.

Micro-combs, optical frequency combs based on micro-cavity resonators, have shown significant promise in fulfilling this role7,8,9,10. They offer the full potential of their bulk counterparts11,12, but in an integrated footprint. The discovery of temporal soliton states (DKS—dissipative Kerr solitons)10,13,14,15,16,17 as a means of mode-locking micro-combs has enabled breakthroughs in many fields including spectroscopy18,19, microwave photonics20, frequency synthesis21, optical ranging22,23, quantum sources24,25, metrology26,27 and more. One of their most-promising applications has been optical fibre communications, where they have enabled massively parallel ultrahigh capacity multiplexed data transmission28,29,30.

The success of micro-combs has been enabled by the ability to phase-lock, or mode-lock, their comb lines. This, in turn, has resulted from exploring novel oscillation states such as temporal soliton states, including feedback-stabilised Kerr combs29, dark solitons30 and DKS28. DKS states, in particular, have enabled transmission rates of 30 Tb/s for a single device and 55 Tb/s by combining two devices, using the full C and L telecommunication bands28. In particular, for practical systems, achieving a high spectral efficiency is critically important—it is a key parameter as it determines the fundamental limit of data-carrying capacity for a given optical communications bandwidth2,3.

Recently17,11, a powerful class of micro-comb termed soliton crystals was reported, and devices realised in a CMOS (complementary metal-oxide semiconductor) compatible platform2,3,8,9,31 have proven highly successful at forming the basis for microwave and RF photonic devices32,33. Soliton crystals were so-named because of their crystal-like profile in the angular domain of tightly packed self-localised pulses within micro-ring resonators (MRRs)17. They are naturally formed in micro-cavities with appropriate mode-crossings without the need for complex dynamic pumping and stabilisation schemes that are required to generate self-localised DKS waves (described by the Lugiato-Lefever equation34). The key to their stability lies in their intracavity power that is very close to that of spatiotemporal chaotic states17,35. Hence, when emerging from chaotic states there is very little change in intracavity power and thus no thermal detuning or instability, resulting from the ‘soliton step’ that makes resonant pumping more challenging36. It is this combination of intrinsic stability (without the need for external aid), ease of generation and overall efficiency that makes them highly suited for demanding applications such as ultrahigh-capacity transmission beyond a terabit/s.

Here, we report ultrahigh bandwidth optical data transmission across standard fibre with a single integrated chip source. We employ soliton crystals realised in a CMOS-compatible platform31,32,33 to achieve a data line-rate of 44.2 Tb/s from a single source, along with a high spectral efficiency of 10.4 bits/s/Hz. We accomplish these results through the use of a high modulation format of 64 QAM (quadrature amplitude modulation), a low comb-free spectral range (FSR) spacing of 48.9 GHz, and by using only the telecommunications C-band. We demonstrate transmission over 75 km of fibre in the laboratory as well as in a field trial over an installed network in the greater metropolitan area of Melbourne, Australia. Our results stem from the soliton crystal’s extremely robust and stable operation/generation as well as its much higher intrinsic efficiency, all of which are enabled by an integrated CMOS-compatible platform.

Results Experiment A schematic illustrating the soliton crystal optical structure is shown in Fig. 1a, with the physical chip shown in Fig. 1b and the experimental setup for ultrahigh bandwidth optical transmission in Fig. 1c (also see Methods and Supplementary Note 1). The micro-resonator had an FSR spacing of 48.9 GHz and generated a soliton crystal with this spacing (~0.4 nm) over a bandwidth of >80 nm when pumped with 1.8 W of continuous-wave (CW) power (in-fibre, incident) at 1550 nm. The soliton crystal micro-comb was generated by automatically tuning the pump laser to a pre-set value. The primary comb and generated soliton crystal states are shown in Figs. 2a and b. Figure 2c demonstrates the stability of the soliton crystal comb generation by showing a variation in individual tone powers of < ± 0.9 dB over 10 different generation instances through wavelength sweeping (from 1550.300 to 1550.527 nm). This demonstrates the repeatability of turn-key micro-comb generation from this integrated CMOS-compatible device.

Fig. 1: Conceptual diagram of a soliton crystal micro-comb communications experiment. figure1 a. Illustration of the soliton crystal state used in this paper. We infer from the generated spectrum that the state was a single temporal defect crystal across the ring. The state had a characteristic ‘scalloped’ micro-comb spectrum, corresponding to the single temporal defect crystal state. b Photograph of the fibre-optic packaged micro-ring resonator chip used for soliton crystal generation. The full chip is 5 mm × 9 mm, of which we use devices and access waveguides on ~  ¼ of the area. The AUD $2 coin (20.5 mm diameter) shown for scale is similar in size to a USD nickel or a 10 Euro cent coin. Inset is a microscope image of the ring resonator element, with a scale bar. Visible distortions are due to an overlayer of glue from the fibre array. c Experimental setup. A CW laser, amplified to 1.8 W, pumped a 48.9 GHz FSR micro-ring resonator, producing a micro-comb from a soliton crystal oscillation state. The comb was flattened and optically demultiplexed to allow for modulation, and the resulting data optically multiplexed before the subsequent transmission through fibres with EDFA amplification. At the receiver, each channel was optically demultiplexed before reception. ECL edge-coupled laser, WSS wavelength-selective switch, Rx receiver.