An optical engine architecture based on ring resonator modulator is proposed to meet the capacity requirements of co-packaged optical applications. It shows the system performance of the core 100Gb/s transmitter unit and 800 Gb/s transmission using 8 wavelengths on a single fiber. （IPC 2020）

Section 2 Optical Engine Architecture

Figure 1 shows the proposed concept of 3.2Tb/s optical engine based on ring resonator. Ring resonator modulators provide many advantages for high-density applications. First of all, its floor area is very small, less than 0.01 mm ^ 2. Therefore, the junction capacitance is less than 100 fF. The modulator can be driven as a lumped element, thus simplifying the driver architecture and limiting power consumption. Secondly, the ring resonator modulator is wavelength selective, allowing the use of wavelength division multiplexing (WDM) scheme architecture, and each fiber pair has multiple channels (wavelengths). Combined with multi-wavelength lasers (such as quantum dot lasers), the number of fibers and lasers required for terabyte capacity optical engines can be reduced to a practical level.

The manufacturing of optical engine must have high yield to meet the cost target of TB level. This requires 2.5D or 3D integration through copper column interconnection, passive alignment of optical fiber and wafer level laser attachment.

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Figure 1-a) The proposed architecture of the 3.2 Tb/s optical engine for co-packaged applications with 51.2 T switch ASICs, which is composed of a ring-based 100G modulator and receiver unit, and has an edge-coupled fiber array; b) SEM of 100G modulator unit, located at 150 μ M between Cu columns.

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Figure 2-a) Block diagram of 800G transmitter composed of driver IC and SiP chip, as well as comb input spectrum and modulation output spectrum; b) The relationship between the extinction ratio and the insertion loss of the ring resonant modulator highlights the working point when the insertion loss is 6dB; c) The 100 Gb/s PAM-4 eye map at the best working point has been captured and applied with 5 tap FFE at Rx.

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Analysis of hot technologies in data center

Author: Guo Liang, et al

JD.COM

Section III Performance of 800 Gb/s transmitter

In order to demonstrate the performance of the 50 Gbaud transmitter unit, an 800 Gb/s transmitter demonstration was made, as shown in Figure 2. It consists of 8 channels, which are modulated by 100 Gb/s PAM-4 and transmitted to a single fiber in WDM configuration. The modulator consists of two phase-shifting segments with 2:1 ring coverage. The longer segment is encoded with the most significant bit (MSB), and the shorter segment is encoded with the synchronized least significant bit (LSB), so that the combined optical modulation is in the PAM-4 format. Each segment is driven by a 22 nm CMOS 3.2 V swing 50Gb/s NRZ driver IC.

The silicon photonic tube core is formed by the input and output edge couplers on the common edge surface, and connected to the tube core through a single bus waveguide. Eight ring modulators are serially oriented on the bus waveguide, and each modulator is controlled by a thermal tuner. The external quantum dot laser source provides input combs of 8 C-band wavelengths at 100 GHz intervals. As shown in Figure 2a, each ring modulator is thermally tuned to a separate wavelength.

Both silicon photonic tube core and driver IC have 150um spacing copper column array, and flip chip is installed on 335 um thick organic intermediate layer. The MSB and LSB drivers are located directly above the ring modulator and are directly interconnected through the via in the intermediate layer.

The driver IC contains an internal PRBS generator for each channel and requires a 28 GHz external clock input. Therefore, the transmitter does not need RF input waveform, thus forming an independent transmitter component.

The insertion loss of each channel is 6 dB, and the extinction ratio is 5 dB, as shown in Figure 2b. This is the analog operating point, which can provide the maximum optical modulation amplitude (OMA) of - 4.2 dBm (relative to 0 dBm input) and the TDECQ of 0.95 dB. The original eye map of this state is also displayed. Please note that there is no Tx pre-emphasis, level setting or FFE in the driver IC. Adding RxFFE with 5 taps in TDECQ calculation will produce an open and uniform eye map.

Each 100 Gb/s channel consumes 2.25pj/bit of electric energy on average and occupies 0.27 mm ^ 2 of area. The bare area required for edge fiber coupling is not considered, which provides a bandwidth density of 370 Gb/mm ^ 2.

Section IV Conclusion

An optical engine architecture is proposed to solve the challenges of high-density optical interconnection faced by data center connectivity and storage in the next decade. The platform based on ring resonator is demonstrated by using 800 Gb/s PAM-4 transmitter with single fiber output, which operates in the C-band and consumes 2.25 pJ/bit of energy.

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