Demonstration and Non-volatile Trimming of a Highly-Parallel, High-Capacity Silicon Microdisk Transmitter

TL;DR

The work targets the data-movement bottleneck in data centers by leveraging silicon photonics to build a highly parallel, high-capacity transmitter based on 64 compact microdisk modulators. It delivers up to $1.28~\mathrm{Tbit/s}$ capacity with per-channel energy as low as $29~\mathrm{fJ/bit}$ and a per-channel EO bandwidth of $19$–$28~\mathrm{GHz}$, enabled by a vertical p–n junction microdisk design with $V_{\pi}L=3.57~\mathrm{V\cdot mm}$ and a tuning efficiency of $90~\mathrm{pm}/\mathrm{V}$. Crucially, the dataset demonstrates a non-volatile, automated laser-trimming platform that permanently shifts microdisk resonances with picometer precision, enabling a fully passive 5-channel DWDM link with $50~\mathrm{GHz}$ spacing and reducing thermal-energy consumption by about $33\%$ while lowering design redundancy. Together, these advances provide a scalable, energy-efficient path toward ultra-dense silicon-photonic interconnects suitable for AI and future computing ecosystems. $1.28~\mathrm{Tbit/s}$ capacity, $50~\mathrm{GHz}$ DWDM, and $33\%$ thermal-energy savings are highlighted, illustrating practical impact for data-center architectures.

Abstract

Optical interconnects are the most promising solution to address the data-movement bottleneck in data centers. Silicon microdisks, benefiting from their compact footprint, low energy consumption, and wavelength division multiplexing (WDM) capability, have emerged as an attractive and scalable platform for optical modulation. However, microdisk resonators inherently exhibit low fabrication error tolerance, limiting their practical deployment. Here, utilizing a CMOS photonics platform, we demonstrate 1.2 Tb/s of off-die bandwidth through a 64 microdisk modulator system. In addition, we develop an automated, close-looped, non-reversible, low-loss, and picometer-precision permanent wavelength tuning technique using laser trimming. The trimming technique reduces 33 % of the energy consumption needed to thermally tune the microdisk resonant wavelength. Using this technique, we achieve a fully passive, 5-channel dense wavelength division multiplexing (DWDM, 50 GHz spacing) transmitter. The integration of the high speed (1.2 Tb/s), low energy consumption (29 fJ/bit) and the permanent wavelength trimming lays a robust foundation for next-generation optical interconnect systems, poised to facilitate scaling of future AI and computing hardware.

Figures (14)

• Figure 1: Artistic vision and microscope image of the high parallelism, high capacity micro-disk transmitter system. (a) Schematic diagram of the 64-channel high parallelism, high capacity microdisk transmitter. The transmitter supports high parallelism SDM and allows for high per-fiber bandwidth WDM transmission. (b) Optical microscope image of the transmitter chip, from input to output, the chip includes the input grating coupler array, microdisk modulators, phase shifters, y-splitter combiner tree, and output edge coupler. (c) High resolution microscope images of the main components of the chip. (d) 2D emission image of the chip shows an 8 by 8 emitter array.
• Figure 2: Direct current characterization of the Microdisk Transmitter chip. (a) Experimental setup of the microdisk transmitter direct current characterization, the TLS and OSA are synchronized to provide high precision characterization, an IR camera is employed to provide parallel characterization. TLS, tunable laser; OSA, optical spectrum analyzer; PC, polarization controller; VGFA, V-groove fiber array; EDFA, Er-doped fiber amplifier. (b) Measured and fitted grating coupler transmission spectrum of the transmitter. (c) Measured broadband transmission spectrum of a single-channel transmitter at 0-V D.C voltage. (d) Measured transmission spectrum versus reversed junction voltage from 0.5 V to -1.5 V, under a 2-V voltage swing at 1536.38 nm, the modulator insertion loss (IL) is 2.6 dB, the extinction ratio (ER) is 9.8 dB, and the optical modulation amplitude (OMA) is -3.2 dB relative to the input optical power, these parameter values are nearly constant among these 64-channel microdisk modulators. (e) Measured transmission spectrum versus heater voltage ranging from 0 V to 5 V, showing a wavelength red shift tunning efficiency of 420 pm/mW. (f) Extracted absorption coefficient and refractive index change of the microdisk transmitter versus junction voltages from 0.5 V to -1.5 V.
• Figure 3: High-speed characterization of the Microdisk Transmitter. (a) Schematic of the high-speed measurement setup. OBPF, optical band-pass filter; PD, photodetector; Oscope, oscilloscope; VGFA, V-groove fiber array; VNA, vector network analyzer; AWG, arbitrary waveform generator. (b) Measured (dash, orange and blue) and calculated (line, green and red) normalized electro-optic $S_{21}$ frequency response. The measured bandwidth of different modulators is between 19 GHz and 28 GHz, which is within the theoretical limitation of the 18 GHz lower band (in resonance wavelength bandwidth) to 42 GHz upper band (RC-only determined bandwidth) bandwidth. Inset: RC circuit model of the modulator that used to fit the measured $S_{11}$ data. (c) Eye diagram of the transmitter at 20 Gbt/s. (d) Time trace of the measured and expected data encoding results among the transmitter at 20 Gbt/s. (e) Experiment-theory difference standard deviation distribution of the transmitter encoding. The measured standard deviation is around 0.007, indicating an over 7 bits bit precision. (f) Distribution of the theory and experimental encoding results, the theory encoding results are designated as $y$ and the experimental encoding results are designated as $y'$.
• Figure 4: Non-volatile trimming of the Microdisk Transmitter. (a) Experimental setup of the non-volatile, close-looped, automatic laser trimming, the 2D grating array was designed to provide in-time monitoring of the emission wavelength. (b) Microdisk resonance wavelength shift versus the trimming time, showing a minimum wavelegnth red shift of 100 pm. (c) Thermal tunning energy saving versus the trimming range. (d) The design redundancy channels needed to achieve the fully passive WDM for pre and post trimmed devices.
• Figure 5: Fully passive DWDM transmission. (a) Wavelength distribution of the foundry fabricated microdisk chip. (b) Selected DWDM wavelength distribution after the trimming, showing uniform wavelength spacing of 25 GHz. (c-h) Schematic and results of the 20 Gbt/s eye-diagram of the trimmed microdisk modulators, which demonstrated a fully passive, dense wavelength division (50 GHz) channel, scalar bar: 40 ps.