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A Revolutionary 5x200Gbps Microring Modulator Silicon Chip

Introduction

As we enter the era of generative artificial intelligence (AI), extended reality (XR), and full self-driving (FSD) technologies, the demand for faster and more efficient interconnects has skyrocketed. Traditional electrical interconnects are struggling to meet the escalating bandwidth, density, and power requirements imposed by these emerging applications. Optical interconnects, leveraging their high bandwidth, low-loss signal transmission, and wavelength multiplexing capabilities, have emerged as a compelling solution to address these surging data transmission needs.

Among various photonic integration platforms, silicon photonics (SiPh) stands out due to its complementary metal-oxide-semiconductor (CMOS) compatibility and high-volume manufacturing potential. Furthermore, the co-packaged optics (CPO) approach enables seamless integration of electronics and photonics on a single substrate, dramatically shortening link lengths, increasing data density, and reducing energy consumption.

At the heart of SiPh interconnects lies the microring resonator modulator (MRM), a compact and scalable modulation solution that harnesses the wavelength selectivity of microring resonators. By manipulating the free-carrier dispersion effect, MRMs can modulate light via electrical control of carrier concentration. However, traditional MRMs have been hindered by the inherent trade-off between modulation efficiency and bandwidth, limiting their performance.

This tutorial article presents a groundbreaking breakthrough in SiPh technology – a 5x200Gbps microring modulator silicon chip empowered by two-segment Z-shape junctions. This state-of-the-art demonstration shows that all-silicon modulators can practically enable future 200Gb/s/lane optical interconnects, paving the way for the next generation of high-speed data transmission [1].

Device Design

The key innovation in this silicon chip lies in the two-segment Z-shape junction design of the MRMs. Each MRM incorporates two individual modulation segments, the most significant bit (MSB) and the least significant bit (LSB), with a length ratio of approximately 2:1. This two-segment structure is tailored for pulse amplitude modulation with four levels (PAM4), simplifying the complex PAM4 driving signals into two non-return-to-zero (NRZ) signals.

The cross-section of the Z-shape p-n junctions, as shown in Figure 1b, is based on a standard 500nm-wide, 220nm-thick Si waveguide with a 90nm-thick slab. A tailored Z-shape doping profile is devised, incorporating four different implantations: n-type doping with a concentration of 3x10^18 cm^-3, p-type doping of 3x10^18 cm^-3, n^+-type doping of 6x10^18 cm^-3, and p^+-type doping of 6x10^18 cm^-3. This unique doping profile optimizes the overlap between the waveguide mode and the depletion region, enhancing modulation efficiency while reducing free carrier absorption loss and series resistance.

The coupling region of the MRM is designed based on loss simulations, resulting in a 4μm-long coupling region with a gap of 180nm, as depicted in Figure 1e.

Device design of the Si two-segment Z-shape microring modulators (MRMs). a Schematic diagram of the 5-channel Si two-segment MRMs. b Cross-sectional diagram of the Z-shape junction with its corresponding waveguide transverse electric (TE) mode (c) and electric field at -3 V (d). e finite-difference time-domain (FDTD) simulated field distribution of the coupling region.
Fig. 1 | Device design of the Si two-segment Z-shape microring modulators (MRMs). a Schematic diagram of the 5-channel Si two-segment MRMs. b Cross-sectional diagram of the Z-shape junction with its corresponding waveguide transverse electric (TE) mode (c) and electric field at -3 V (d). e finite-difference time-domain (FDTD) simulated field distribution of the coupling region.
Single Microring Modulator Characterization

To evaluate the performance of this innovative design, extensive measurements were conducted on a single-channel MRM. The measured transmission spectrum, shown in Figure 2a, reveals an average off-resonance insertion loss of ~0.9dB, a free spectral range (FSR) of ~5.7nm, a direct current (DC) extinction ratio (ER) of ~16dB, a full width at half-maximum (FWHM) of ~0.35nm, and a quality factor (Q) of ~3700.

The modulation efficiency of the Z-shape junction is characterized by measuring the resonance wavelength shift as a function of the junction bias voltage. As shown in Figures 2g and 2h, the resonance shift is ~11.0pm/V for the LSB segment and ~16.3pm/V for the MSB segment, corresponding to a remarkable modulation efficiency V_π⋅L of ~0.53V⋅cm for the LSB and ~0.69V⋅cm for the MSB. This represents a ~67% improvement in modulation efficiency compared to previous state-of-the-art lateral junction two-segment MRMs.

Direct current (DC) characteristics of a single microring modulator (MRM). a Measured transmission spectrum of a single-channel MRM (blue) and a pair of grating couplers (orange). Measured transmission spectrum versus heater voltage ranging from 0 V to 4 V b and resonance wavelength versus heater power. c, d Calculated overlap ratios between the waveguide transverse electric (TE) mode and depletion regions of different junctions. e, f Measured transmission spectrum versus junction voltage from 0.5 V to -4 V at the least significant bit (LSB) and most significant bit (MSB). g, h Extracted resonance wavelengths versus junction voltage for LSB and MSB.
Fig. 2 | Direct current (DC) characteristics of a single microring modulator (MRM). a Measured transmission spectrum of a single-channel MRM (blue) and a pair of grating couplers (orange). Measured transmission spectrum versus heater voltage ranging from 0 V to 4 V b and resonance wavelength versus heater power. c, d Calculated overlap ratios between the waveguide transverse electric (TE) mode and depletion regions of different junctions. e, f Measured transmission spectrum versus junction voltage from 0.5 V to -4 V at the least significant bit (LSB) and most significant bit (MSB). g, h Extracted resonance wavelengths versus junction voltage for LSB and MSB.

The modulation bandwidth of the MRM is determined by the resistance-capacitance (RC) time constant of the junction, the photon lifetime of the microring, and the wavelength detuning from resonance. By analyzing the measured S-parameters, the equivalent circuits for the LSB and MSB segments were derived, as shown in Figures 3b and 3e. The RC-limited bandwidth of the LSB is ~79.1GHz, while the MSB exhibits a bandwidth of ~64.0GHz.

Furthermore, the measured electro-optic (EO) responses, shown in Figures 3c and 3f, reveal a remarkable 3dB EO bandwidth of ~48.9GHz for the LSB and ~48.3GHz for the MSB at the wavelength with maximum modulation slope.

Frequency responses of a two-segment Si microring modulator (MRM). a, d Measured (red) and fitted (blue) Smith charts of S11 for the least significant bit (LSB) and most significant bit (MSB) at -3 V. b, e, Fitted equivalent circuits and corresponding junction resistance-capacitance (RC) responses for LSB and MSB at -3 V. c, f Measured electro-optic (EO) responses (∣S21∣ 2) for LSB and MSB with detuning of Δλ~0.1nm at −3 V.
Fig. 3 | Frequency responses of a two-segment Si microring modulator (MRM). a, d Measured (red) and fitted (blue) Smith charts of S11 for the least significant bit (LSB) and most significant bit (MSB) at -3 V. b, e, Fitted equivalent circuits and corresponding junction resistance-capacitance (RC) responses for LSB and MSB at -3 V. c, f Measured electro-optic (EO) responses (∣S21∣ 2) for LSB and MSB with detuning of Δλ~0.1nm at −3 V.

Thanks to the high modulation efficiency and bandwidth enabled by the two-segment Z-shape design, this MRM can support 200Gb/s PAM4 modulation with clear eye openings, as shown in Figure 4e. The measured transmitter dispersion eye closure quaternary (TDECQ) is 0.2dB at a soft-decision forward error correction (SD-FEC) threshold of symbol error rate (SER) at 1E-2, and the outer ER is 3.6dB.

Remarkably, this high-performance MRM achieves an energy consumption of only ~6.3fJ/bit for 200Gb/s PAM4 modulation with a peak-to-peak radio frequency (RF) swing voltage of 1.6V.

Eye diagrams of a single microring modulator (MRM). a Micrograph of a two-segment Si MRM. Measured eye diagrams of 100 Gb/s non-return-to-zero (NRZ) of the least significant bit (LSB), most significant bit (MSB), both LSB and MSB (b–d); and 200 Gb/s pulse amplitude modulation with four levels (PAM4) of both LSB and MSB (e).
Fig. 4 | Eye diagrams of a single microring modulator (MRM). a Micrograph of a two-segment Si MRM. Measured eye diagrams of 100 Gb/s non-return-to-zero (NRZ) of the least significant bit (LSB), most significant bit (MSB), both LSB and MSB (b–d); and 200 Gb/s pulse amplitude modulation with four levels (PAM4) of both LSB and MSB (e).
Dense Wavelength Division Multiplexing Microring Modulators

Building upon the exceptional performance of the single-channel MRM, a 5-channel dense wavelength division multiplexing (DWDM) MRM array was realized by slightly modifying the microring circumference according to the desired channel spacing. Figure 5a shows a micrograph of this 5-channel DWDM MRM array, where five microrings are connected in series and share a common bus waveguide.

The measured transmission spectrum of the 5-channel MRM array, depicted in Figure 5b, exhibits a uniform distribution of resonance wavelengths within one FSR, enabling direct modulation of each channel without thermal tuning.

Leveraging the same experimental setup as the single-channel MRM, open 200Gb/s PAM4 eye diagrams were measured for all five channels, as shown in Figures 5c-5g. The measured TDECQ values (at SER=1E-2) for channels 1 to 5 are ~1.02dB, ~0.83dB, ~0.93dB, ~1.00dB, and ~0.67dB, respectively, with an outer ER of ~3.6dB for all channels.

The channel crosstalk, a crucial figure of merit for DWDM modulators, was quantified by measuring the EO response of an MRM at adjacent and its own channel wavelengths. As illustrated in Figure 5h, the channel 2 MRM exhibits an average crosstalk of ~-33.4dB and ~-57.7dB with respect to channels 1 and 3, respectively, even without thermal tuning. This remarkably low crosstalk can be further improved by precisely tuning the channel spacing.

This groundbreaking 5-channel DWDM MRM array demonstrates an aggregated data rate of 1Tb/s (5x200Gb/s PAM4), paving the way for next-generation high-speed optical interconnects.

Eye diagrams and crosstalk performances of the 5-channel Si microring modulator (MRM) array. a Micrograph of a 5-channel dense wavelength division multiplexing (DWDM) Si MRM array. b Measured transmission spectrum of the 5-channel Si MRM array. c–g Measured eye diagrams of 200 Gb/s pulse amplitude modulation with four levels (PAM4) of channel 1, channel 2, channel 3, channel 4, and channel 5 (C1-C5). h Measured channel crosstalk of the DWDM MRM array.
Fig. 5 | Eye diagrams and crosstalk performances of the 5-channel Si microring modulator (MRM) array. a Micrograph of a 5-channel dense wavelength division multiplexing (DWDM) Si MRM array. b Measured transmission spectrum of the 5-channel Si MRM array. c–g Measured eye diagrams of 200 Gb/s pulse amplitude modulation with four levels (PAM4) of channel 1, channel 2, channel 3, channel 4, and channel 5 (C1-C5). h Measured channel crosstalk of the DWDM MRM array.
Conclusion

This tutorial article has presented a revolutionary 5x200Gbps microring modulator silicon chip empowered by two-segment Z-shape junctions. By combining an optimized Z-shape doping profile and a two-segment architecture, this innovative design mitigates the inherent trade-off between modulation efficiency and bandwidth, enabling high-speed modulation with energy consumption of sub-ten fJ/bit.

Key achievements of this state-of-the-art demonstration include:

  • A high 3dB electro-optic bandwidth of ~48.6GHz

  • A large modulation efficiency with V_π⋅L of ~0.6V⋅cm

  • Open 200Gb/s PAM4 eye diagrams with only 1.6V peak-to-peak voltage and 6.3fJ/bit energy consumption

  • A 5-channel DWDM MRM array supporting an aggregated data rate of 1Tb/s (5x200Gb/s PAM4)

  • Remarkably low channel crosstalk of <-33dB, with potential for further improvement to ~-57dB

This groundbreaking silicon photonics technology demonstrates that all-silicon modulators can practically enable future 200Gb/s/lane optical interconnects, addressing the surging data transmission demands of emerging applications such as generative AI, XR, and FSD. As a scalable and energy-efficient solution, this 5x200Gbps microring modulator silicon chip paves the way for the next generation of high-performance computing systems and data centers.

Reference

[2] Yuan, Y., Peng, Y., Sorin, W.V. et al. A 5 × 200 Gbps microring modulator silicon chip empowered by two-segment Z-shape junctions. Nat Commun 15, 918 (2024). https://doi.org/10.1038/s41467-024-45301-3

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