DSP- and DAC-Free Optical Transmitter Using Triple-Parallel MZMs

Introduction

Recent data centers and high-performance computers require high-speed and low-power optical interconnects to support ever-increasing data rates. PAM4 signaling has been adopted for data rates beyond 200G Ethernet to increase bandwidth efficiency. However, conventional PAM4 transmitters rely on power-hungry components like digital signal processors (DSPs), digital-to-analog converters (DACs), and linear drivers, each consuming hundreds of milliwatts.

This tutorial discusses a flexible optical transmitter architecture based on triple-parallel silicon Mach-Zehnder modulators (MZMs) that can generate PAM4 signals, perform pre-distortion, and apply pre-emphasis without requiring DSPs or DACs. This innovative approach shifts the signal processing to the optical domain, dramatically reducing power consumption.

Transmitter Architecture

The proposed transmitter architecture, shown in Fig. 1, comprises triple-parallel MZMs driven by thermometer-coded signals.

Fig. 1: Architecture of the proposed flexible transmitter using triple-parallel MZMs and thermometer code.

Triple-Parallel MZMs

The schematic of the triple-parallel MZMs is illustrated in Fig. 2(a). The input light is divided into three arms using a variable coupler, whose ratio can be adjusted via thermal-optic phase shifters (φ1 to φ4). The phase differences between the three arms are controlled by θ1 and θ2 to ensure in-phase combination of the split light.

Fig. 2. Triple-parallel MZMs. (a) Schematic. (b) Thermometer code.

The MZMs in arms i, j, and k are driven by thermometer-coded signals, as shown in Fig. 2(b). In the thermometer code, the eye levels 3, 2, and 1 correspond one-to-one with the bits of i, j, and k, respectively. This independent control over each PAM4 eye height generates equally spaced PAM4 eyes without additional optical loss.

Pre-distortion and Pre-emphasis

The triple-parallel MZMs can generate pre-distorted signals by adjusting the eye height to be wider or narrower, relaxing receiver linearity requirements. Additionally, pre-emphasized NRZ signals can be generated by concentrating the input light into the k arm and driving only MZM k, reducing driver power consumption by one-third compared to PAM4 generation.

All drive signals share the same timing, simplifying control with integrated retiming circuits. Symbol delays, acting as equalizers, can generate pre-emphasized NRZ signals by adjusting the variable couplers' ratios, eliminating the need for power-hungry digital multipliers.

Simulation Results

The proposed transmitter was designed using silicon rib-waveguide MZMs and evaluated through circuit simulations based on an accurate Verilog-A model. The MZMs were driven by push-pull signals with a voltage of 2V.

Fig. 3(a) shows the simulated equally spaced 32-Gbaud PAM4 signal generated by fine-tuning the coupler's ratio. The large eye-opening is sufficient compared to the receiver's noise without incurring additional optical losses.

Fig. 3. Simulated eye diagrams. (a) 32-Gbaud equally spaced PAM4. (b) 32-Gbaud pre-distorted PAM4. (c) 64-Gbaud NRZ without pre-emphasis. (d) 64- Gbaud NRZ with pre-emphasis.

Figs. 3(c) and 3(d) show simulated 64-Gbaud NRZ modulated signals without and with pre-emphasis, respectively, demonstrating the transmitter's versatility in generating various signal formats without DSP or DAC involvement.

Power Consumption Comparison

Table 1 compares the estimated power consumption of the proposed and conventional optical transmitters. The proposed transmitter eliminates the need for DSP and DAC, significantly reducing power consumption. Although three drivers are required instead of one, the use of highly efficient push-pull drivers offsets the increase. The additional power consumption for heater control of the variable couplers is negligible.

Table 1: Estimated power consumption comparison.

Items	Power Consumption [mW]
Conventional	Proposed
DSP	200	0
Electrical DAC	500	0
Driver	500x1	125x3
Optical Control	0	50
Total	1200	425

In total, the proposed transmitter is estimated to reduce power consumption to approximately one-third compared to conventional transmitters, contributing to lower power consumption in data centers and high-performance computing systems.

Conclusion

The proposed DSP- and DAC-free optical transmitter based on triple-parallel silicon MZMs offers a flexible and power-efficient solution for generating PAM4 signals, pre-distortion, and pre-emphasis in the optical domain. By leveraging thermometer coding and variable couplers, this architecture eliminates the need for power-hungry DSPs, DACs, and linear drivers, reducing power consumption to one-third of conventional transmitters. This innovative approach paves the way for more energy-efficient optical interconnects in data centers and high-performance computing systems.

Reference

[1] K. Kawahara and T. Baba, "DAC- and DSP-Free Optical Transmitter Based on Triple-Parallel Si Mach-Zehnder Modulators," Department of Electrical and Computer Engineering, Yokohama National University, Yokohama, Japan, 2024.