Coherent Modulation vs. PAM4 in 400G/800G Optical Transmission

In the rapidly advancing field of optical transmission, Coherent Modulation and PAM4 (Pulse Amplitude Modulation 4-level) are pivotal technologies for 800G networks. Coherent Modulation is ideal for high-speed, long-distance communication, while PAM4 excels in cost-effective, short to medium-distance data center connections. This article compares their performance in 400G and 800G applications, examining cost efficiency and power consumption to highlight their roles in the future of high-speed optical communication.

What Is Coherent Modulation & PAM4?

Coherent modulation is an advanced optical data transmission technique that uses digital signal processing (DSP) to modulate the amplitude, phase, and polarization of light. This method supports speeds of 100Gbps and higher, enabling terabit-scale data delivery through a single fiber pair. Ideal for long-haul applications, coherent modulation maintains signal integrity over long distances, outperforming traditional intensity detection. It relies on coherent light with precise frequency and phase control, making it the optimal choice for high-speed, long-distance transmissions.

PAM4 was developed to meet the demand for short-haul links, utilizing four distinct pulse amplitudes to convey information. Each amplitude represents two bits, effectively doubling the data rate and making PAM4 twice as bandwidth-efficient as conventional binary models. This makes PAM4 ideal for high-rate, short to medium-distance transmissions, particularly for internal connections within next-generation data centers.

PAM4 in 400G Optical Transceivers

FS 400G PAM4 Optical Transceiver

• 400G QSFP-DD SR8: Adopts 50Gbps PAM4 modulation with a transmission range of up to 100m, widely used in data center 400G direct connections and interconnections.

• 400G QSFP-DD DR4: Modulated with 100Gbps PAM4, offering a transmission distance of up to 500m, suitable for data center 400G direct connections and interconnections.

• 400G QSFP-DD FR4/LR4: Modulated with 100Gbps PAM4, achieving transmission distances of up to 2km and 10km, respectively.

Comparing PAM4 and Coherent Modulation for Long-Range DCI

In long-range Data Center Interconnect (DCI) scenarios, PAM4 competes with coherent modulation, particularly the 400ZR standard. Coherent modulation utilizes dual-polarized 16QAM (DP-16QAM) at approximately 60Gbaud, achieving single-wavelength data rates of 400Gbps or higher. This method requires ultra-narrow linewidth lasers, I/Q modulators, and coherent receivers, which support longer transmission distances compared to PAM4. Coherent transceivers play a crucial role in these applications, integrating these advanced components to deliver high-performance long-distance transmission. While both techniques operate at similar baud rates, coherent modulation encodes more data per wavelength, offsetting PAM4's reliance on multiple wavelengths and simpler components.

Figure 1 : Coherent Modulation vs. PAM4 in 400G Optical Transmission

PAM4 vs. Coherent in 800G Optical Transmission

As data center rates progress to the 800G era, the distinction between PAM4 and coherent technologies diminishes. The competitiveness of each technology depends on factors such as cost and power consumption.

Cost Efficiency: InP vs. Silicon Photonics

One straightforward method to double the data rate, while maintaining the same baud rate, involves upgrading the hardware. For instance, PAM4 can leverage either 4 or 8 wavelengths of 100G/200G, while coherent modulation can utilize two 400G wavelengths.

Another approach is to increase the baud rate, such as doubling it to approximately 110G baud, thereby achieving an overall rate increase from 400 to 800Gbps. In the context of coherent technology, the choice between InP (indium phosphide) or silicon photonic for the I/Q modulator and receiver becomes pivotal. Silicon photonics, despite being cost-effective, exhibits lower performance. It is notable for its high peak voltages and limited bandwidth. Conversely, InP boasts low peak voltages and superior bandwidth but comes with a higher cost.

For PAM4, an indirectly modulated EML (Electroabsorption Modulated Laser) with a built-in indium phosphide (InP) laser is a viable option. Alternatively, an integrated array featuring silicon photonic modulators and InP laser arrays can be employed. Similar to coherent solutions, the drawbacks of silicon photonics, including high peak voltages and inferior bandwidth, are counterbalanced by its cost advantage.

In both PAM4 and coherent technologies, InP transceivers tend to be more expensive, while silicon photonics presents a more budget-friendly alternative.

Power Consumption: Coherent DSP vs. PAM4 DSP

Regarding power consumption, with the evolution of chip technology from 7nm to 5nm and even 3nm, the enhancement is not limited to an increase in DSP processing rates. It also extends to superior power reduction performance. Illustrated in the graph below, 100G coherent technology demonstrates nearly 10 times better power efficiency than 100G PAM4. However, this disparity diminishes notably in 5nm node-based 800G applications. The graph delineates the power performance of Coherent and PAM4 DSPs across various CMOS nodes.

Figure 2 : Coherent Modulation vs. PAM4 for DSP Power Performance

FS argues that as manufacturing yields improve and costs decrease, the coherent approach—requiring only a single laser, modulator, and receiver—can achieve cost competitiveness similar to PAM4. This is true even as optical devices become more complex. Coherent solutions offer increased flexibility and superior performance, making them a viable option. Conversely, PAM4 uses four relatively simple lasers, modulators, and receivers, which could become more complex at 800G. However, PAM4 is still capable of rapidly reducing costs and maintaining competitiveness compared to coherent solutions.

Summary

In 800G optical transmission, coherent modulation and PAM4 each offer distinct advantages. Coherent modulation excels in long-distance, high-speed data transmission with superior performance and flexibility, becoming more cost-competitive as technology advances. PAM4 is ideal for short to medium distances in data centers, valued for its simplicity and cost-effectiveness. As data rates reach 800G, the distinction between these technologies diminishes, with cost, power consumption, and technological advancements influencing optimal choice. The competition and innovation in both will shape the future of high-speed optical communication.