An Overview of 4WDM Optical Transceiver

In current optical communication market, optical transceiver products are developed in the trend of high data transmission rate, longer reaches, lower power consumption, etc. To keep pace with the trend better, many new optical module products with high transmission rate and longer reaches emerge, such as the latest one, QSFP28 4WDM(4-wavelength Wavelength Division Multiplexing) optical transceiver. In this article, we will mainly talk about it.

What is 4WDM MSA?

The 4WDM MSA is an industry consortium dedicated to defining optical specifications and promoting adoption of interoperable 100G (4x25G) optical transceivers for 10 km based on the CWDM4 wavelength grid, and for 20 km and 40 km based on the LAN-WDM wavelength grid, over duplex single-mode fiber (SMF). These extended reaches are important for modern datacenter interconnects and mobile backhaul applications. The 4WDM MSA participants are responding to previously unmet industry needs for longer reaches, lower costs, and lower power consumption, as compared to previously available standards, in small form factors.

An Introduction to 4WDM Optical Module

4WDM(4-Wavelength Wavelength Division Multiplexing) optical module is defined by 4WDM MSA, targeted for longer reaches, lower costs, and lower power consumption, smaller form factor(QSFP28 form factor is usually preferred).

4WDM optics have three specifications including 4WDM-10, 4WDM-20, 4WDM-40. 4WDM-10 is one type of 100G (4x25G) optical transceivers for the 10 km based on the CWDM4 wavelength grid; 4WDM-20 and 4WDM-40 are types of 100G (4x25G) optical transceivers respectively for 20kms and 40kms based on the LAN-WDM wavelength grid over duplex single-mode fiber (SMF). With respect to the benefits of 4WDM, its main advantages are lower in cost and power dissipation, and longer in transmission distance.

Specifications of 4WDM Optical Module: 4WDM-10, 4WDM-20, 4WDM-40

The 100G-4WDM-10 is based on the CWDM4 wavelength grid. To some degree, the 100G-4WDM-10 technical specification leverages the success of the CWDM4 2 km specification that has found broad acceptance in its target datacenter market. Like CWDM4, the 100G-4WDM-10 specification employs 4 lanes of 25Gb/s using Coarse Wavelength Division Multiplexing (CWDM) technology to transport 100G optical traffic across duplex single mode fiber (SMF).

Both specifications take advantage of the Forward Error Correction (FEC) on the host port, in accordance with IEEE 802.3bj KR4 RS FEC. One key advantage of CWDM is that the lasers do not need to be cooled or temperature controlled, resulting in lower power consumption and simplicity of manufacturing. 100G 4WDM-10 transceivers share these advantages and furthermore are specified to be fully interoperable with CWDM4 products. The 100G-4WDM-10 specification does not restrict the form-factor although high-density QSFP28 modules are expected to be dominant.

4WDM-20 and 4WDM-40 are added on the basis of the 100G 4WDM-10. They employ LAN-WDM wavelength grid on the basis of IEEE 100GBASE-LR4 and ER4, over duplex single-mode fiber (SMF). Simultaneously, they also utilize the IEEE 802.3 KR4 RS FEC used on the host interface to reduce the cost.

This specification defines 4 x 25Gbps Local Area Network Wavelength Division Multiplex (LAN WDM)optical interfaces for 100Gbps optical transceivers for Ethernet applications including 100GbE. Forward Error Correction (FEC) is a link requirement in order to ensure reliable system operation. Two transceivers communicate over single-mode fibers (SMF) of length from 2 meters to at least 20 or 40 kilometers. The transceiver electrical interface is not specified by this MSA but can have, for example, four lanes in each direction with a nominal signaling rate of 25.78125Gbps per lane.

In addition, the 4WDM-20 specification, which is an extension of the 100G-4WDM-10 10 km specification, enables customers to increase their reach using the same kind of optical components as 100GBASE-LR4 products. Similarly, the 100G-4WDM-40 enables 40km-reach with lower power consumption and in a smaller form factor than existing 100GBASE-ER4 compliant products that utilize a power-hungry SOA (Semiconductor Optical Amplifier).


It is commented by related expert that the new 20kms and 40kms specifications are an important milestone to enable this market. Thus, the prospect of 4WDM optical transceivers is worth expecting. For most optical component suppliers, it means another opportunity.

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