100G DWDM Optical Module with PAM4 and Coherence Technology
1207 2021-10-20
By using standard optical modules (such as CFP and QSFP28), 100G transfers within the data center are possible. Although they are well suited for transmitting 100G traffic within racks and data centers, this can become a problem when 100G traffic needs to be transmitted over long distances (for example, long connections between different data centers (for example, over 40km)). This requires a 100G DWDM optical module.

Since DWDM SFP transceivers are widely used for scaling up 10G network capacity, DWDM technology is not new to the industry. In terms of functionality, the 100G DWDM is very similar to its predecessor. However, it is often used for longer distances in a 100 gigabyte network.

PAM4 and Coherence are two industry-leading solutions that offer greater bandwidth and transmission distance. When comparing 100G DWDM PAM4 to a coherent optical module, it depends on what functions the network needs and benefits from. In this article, we will analyze both options to help businesses make informed decisions.

PAM4 optical module
The Pulse Amplitude Modulation (or PAM4 optical device) was created to respond to the need for optical modules for short distance links. It uses four different pulse ranges to convey information. Each amplitude has two digits, which doubles the data rate and makes PAM4 twice as bandwidth efficient as the traditional binary model.

The advantage of PAM4
Prior to PAM4, the binary NRZ modulation format was used for 100G long distance and 40G data transmission networks. PAM4 comes with four different code types for encoding two bits of data, so the bandwidth of the connection can be doubled with this modulation format. Today, single-wavelength PAM4 modulation technology is being used because it is considered to be the most efficient and cost-effective driver of 100 gigabytes of data transmission.
PAM4 optical modules can be directly used in switches in embedded DWDM networks. This is a cost-effective and simple solution for customers willing to build embedded DWDM data networks.

The disadvantage of PAM4
One of its drawbacks is that the PAM4 optical module requires an amplifier to transmit over 5km. In this case, a separate DWDM multiplexer equipped with dispersion compensation and an amplifier is required to connect the data center. In addition, if the PAM4 optical module is used in conjunction with an existing DWDM network, the DCM and amplifier must be prepared first to avoid future problems.

Another drawback is that PAM4 is susceptible to noise. Its extra voltage level requires a reduced level interval, leading to a higher signal-to-noise ratio, which is why PAM4 works best in short-range optical systems.