200G AOC Mainstream Technology Solutions

NADDOD Jason Data Center Architect Jan 12, 2023

Active optical cable (AOC ), because of its low cost, high performance and other characteristics, is widely used in large-scale data center and high performance computing (HPC) market applications, AOC also continue to upgrade to higher speed, 100G, 200G and even 400G AOC are constantly new products launched. The 200G AOC is undoubtedly the most popular product in recent years, so what are the 200G AOC technology solutions?

The 200G AOC on the market uses the same technical solutions, including 200G AOC package mostly QSFP-DD and QSFP56, modulation from NRZ transition to PAM4, and lasers are almost all VCSEL lasers, so as to achieve 200G optical interconnect.

200G AOC Package - QSFP-DD/QSFP56

The 200G AOC mainly adopts the new generation of mainstream packaging mode QSFP56 and QSFP-DD to achieve forward and backward compatibility with QSFP.

200G aoc

The QSFP56 package features four transmit and receive ports, but with transmission rates of up to 56Gbps per channel and modulation of PAM4, a common solution for 200Gb/s.

The QSFP-DD pluggable package supports a minimum of 12 watts of thermal capacity in a typical system design. The higher thermal capacity reduces the module’s requirement for heat dissipation features, thus avoiding unnecessary costs. Its electrical interface uses eight channels at rates up to 25Gb/s (NRZ modulation) or 50Gb/s (PAM4 modulation) per channel, aggregating to provide solutions of up to 200Gb/s or 400Gb/s.

200G AOC Modulation - NRZ/PAM4

The QSFP-DD-based 200G AOCs use the traditional NRZ (Non-Return-to-Zero) modulation, while the QSFP56-based series mostly use PAM4 (Pulse Amplitude ModulaTIon: Pulse Amplitude Modulation) modulation. What is the difference between these two modulation methods?

The most traditional digital signal is NRZ signal, that is, the use of two signal levels to represent the digital logic signal 1, 0 information, each symbol cycle can transmit 1bit of logic information; PAM4 signal can use four different signal levels to signal transmission, each symbol cycle can represent 2bit of logic information (0, 1, 2, 3).

Therefore, to achieve the same signal transmission capability, the symbol rate of PAM4 signal only needs to reach half of NRZ signal, so the transmission channel loss caused by it is greatly reduced.

PAM signal is a popular signal transmission technology for next-generation data centers to do high-speed signal interconnection, and is widely used for electrical or optical signal transmission for 200G/400G interfaces.


200G AOC Lasers-VCSEL Lasers

VCSEL lasers are widely used in optical communication, optical interconnection, optical storage and other fields due to their small size, easy integration, and current coupling efficiency of up to 90% with multimode fibers.

VCSEL laser is a compound semiconductor active device. The VCSEL laser is mainly composed of an optical cavity with multiple quantum wells (MQW) active layers and two sets of high reflectivity distributed feedback Bragg mirrors (DBR), which generate bias currents through the laser driver to produce “0” and “1” pulses. The VCSELs can be used to perform quality checks on the chip in the manufacturing process, which in turn leads to direct troubleshooting and reduced manufacturing costs.

What are the advantages of VCSELs over other light sources?

  • Small size
  • Low price
  • Single longitudinal mode output
  • Low threshold current
  • Round output spot
  • Easy to integrate into large area arrays
  • Current coupling efficiency with multimode fiber exceeds 90%

And NADDOD has been leading the industry in the development of AOC based on VCSEL technology, and formed an automated production platform.

Using the above mainstream technology solutions, NADDOD borrows its own advantages to create a full range of 200G products not only to help customers solve transmission problems, but also to promote the evolution of domestic 200G rate products forward to contribute resources and strength.