Coherent Modulation vs. PAM4 in 800G Optical Transmission

Coherent modulation, employed in coherent optical communication, involves altering the frequency, phase, and amplitude of the optical carrier to transmit signals. Unlike intensity detection, coherent modulation requires coherent light, with defined frequency and phase, and is used primarily for high-speed and long-distance transmissions.

PAM4 finds application in high-rate, short to medium-distance transmissions, making it well-suited for the internal connections of next-generation data centers.

Taking the FS 400G optical transceiver as an example:

• 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.

In long-range Data Center Interconnect (DCI) scenarios, PAM4 faces competition from coherent modulation based on the 400ZR protocol. This coherent modulation operates at baud rates around 60Gbaud with dual-polarized 16QAM (DP-16QAM) modulation, supporting single-wavelength rates of 400Gbps or higher. Coherent modulation demands ultra-narrow linewidth lasers, I/Q modulators, and coherent receivers, enabling longer transmission distances compared to PAM4. Despite similar baud rates, coherent transmission allows single-wavelength encoding of more data, compensating for PAM4's use of multiple wavelengths and simpler lasers.

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.

Choosing Between InP and 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.

Coherent vs. PAM4 in High-Speed Transmission

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.

Multiple companies have experimentally validated these approaches. FS contends that as yields improve and costs decrease, the coherent approach, requiring only one laser, modulator, and receiver, can achieve cost competitiveness comparable to PAM4. This holds true even as optical devices become more intricate. The increased flexibility and performance achievable with coherent solutions can then come to the forefront. While PAM4, with four relatively straightforward lasers, modulators, and receivers, might face increased complexity at 800G, it remains adept at swiftly reducing costs and maintaining competitiveness compared to coherent solutions.

In summary, the competition between coherent and PAM4 transmission technologies is ongoing, with future developments poised to determine the prevailing approach.