Multi-dimensional data transmission using inverse-designed silicon photonics and microcombs

Abstract

The use of optical interconnects has burgeoned as a promising technology that can address the limits of data transfer for future high-performance silicon chips. Recent pushes to enhance optical communication have focused on developing wavelength-division multiplexing technology, and new dimensions of data transfer will be paramount to fulfill the ever-growing need for speed. Here we demonstrate an integrated multi-dimensional communication scheme that combines wavelength- and mode- multiplexing on a silicon photonic circuit. Using foundry-compatible photonic inverse design and spectrally flattened microcombs, we demonstrate a 1.12-Tb/s natively error-free data transmission throughout a silicon nanophotonic waveguide. Furthermore, we implement inverse-designed surface-normal couplers to enable multimode optical transmission between separate silicon chips throughout a multimode-matched fibre. All the inverse-designed devices comply with the process design rules for standard silicon photonic foundries. Our approach is inherently scalable to a multiplicative enhancement over the state of the art silicon photonic transmitters.

Fig. 1. An optical transmitter using the multi-dimensional multiplexing principle of chip-to-chip WDM-MDM data transmission.

A frequency comb laser is evenly distributed to individual WDM transmitters (e.g. cascaded ring modulators) using two-stage Y-junction splitters. The multi-frequency data from each WDM transmitter are routed into spatial modes of a multimode waveguide using an MDM multiplexer (Inset 1). The optical data can then be transmitted through chip-to-fibre couplers and multimode fibre to the receiver, where the mode and wavelength channels are separated by MDM-WDM demultiplexers and detected using photodiodes. The chip-to-fibre coupler is optimised to emit multimode beams in the surface-normal direction and improve coupling efficiencies with orthogonal spatial modes of multimode fibre (Inset 2).

Fig. 2. Inverse design of silicon multimode photonics.

a–c Broadband, low-crosstalk MDM multiplexers: a SEM images of back-to-back MDM multiplexer devices with simulated mode conversion (right). b Measured transmission of the back-to-back MDM multiplexer structure. This broadband structure is utilized in the WDM-MDM data transmission experiments (see Fig. 3). c Insertion loss and crosstalk histograms of the back-to-back MDM multiplexers fabricated in a semiconductor foundry. d, e Multimode splitter which combines a waveguide bend (for TE₃₀) and waveguide crossing (for TE₀₀, TE₁₀ and TE₂₀): d Top: SEM image of a multimode photonic circuit which consists of MDM multiplexer, multimode splitter, and MDM demultiplexer (from left to right). The multimode splitter (white rectangular box) selectively routes multimode signals to different directions. Bottom: Zoomed-in SEM image and simulated spatial mode routing. e Measured channel transmission for the multiplexer-splitter-demultiplexer structure (wavelength: 1540 nm).

Fig. 3. On-chip interconnect: WDM-MDM data transmission with microcombs.

a Data transmission scheme using inverse-desiged MDM and microcombs. The transmitter optical source is generated by pumping the microresonator (Inset: optical microscope images of Si₃N₄ microresonator and Ta₂O₅ photonic crystal ring resonator^,,) with a CW laser. WDM de-multiplxers (W-DEMUX) separate the comb lines and intensity modulators (EOM) encode independent data (NRZ at a symbol rate of 40 GBd/channel). The WDM data were recombined using WDM multiplexers (W-MUX), and are coupled to on-chip MDM inputs simultaneously using a fibre array. At the receiver, all the channels are separated by mode and wavelength demultiplexers and detected using a photodiode (PD). In our experiments, we independently modulate even and odd WDM channels using two EOMs for emulating WDM transmission (see Method section). b Optical spectra of anomalous- and normal-dispersion Kerr soliton combs with Si₃N₄ and Ta₂O₅ resonators, respectively. For the data transmission measurements, 11 and 7 comb lines at the C band were used. c Measured BERs (10¹² bits compared) of the transmitted data channels. For those with BER <10⁻¹², we counted an error occurrence with up to 10¹³ reference bits. d 40-Gb/s eye diagrams of Ta₂O₅ combs data channels directly detected using PD.

Fig. 4. Chip-to-chip multimode link using inverse-designed couplers.

a Schematic of a multimode link (scale bars: 10 μm). A CW laser is coupled to an input single-mode waveguide of the Tx chip via a lensed fibre. The input signal is routed into a specific spatial mode of a multimode waveguide using an MDM multiplexer (Inset 1). Inverse-designed chip-to-fibre coupler emits the multimode signal in the surface-normal direction (Inset 2: Infrared images of multimode beams at the top surface of the device are overlaid on device SEM images), and the multimode beams can be coupled to spatial modes of a multimode-matched fibre (Inset 3: Microscope image of the fibre cross-section). The MDM signal then is sent to the Rx chip, and routed into a specific single-mode output waveguide of the chip throughout fibre-to-chip coupler (Inset 4: multimode beams on the coupler. Infrared images are overlaid on device SEM images, and the multimode beams are from the Tx chip throughout free-space optic alignment) and MDM demultiplexer. All data channels are directly detected using photodiodes at the receiver side. b Measured coupling loss spectra of the fibre-to-chip coupler for spatial mode channels. For this measurement, transmitted power was measured through MDM multiplexer, chip-to-fibre coupler, rectangular core fibre and photodetector. To estimate coupling losses, the insertion loss of the MDM multiplexer and transmission loss of the fibre were subtracted. c Measured chip-to-chip MDM channel crosstalks at 1540 nm (Left: multimode fibre chip-to-chip link, Right: free-space).