100G single lambda is an optical specification using PAM4 (four-level pulse amplitude modulation) signaling to transmit 100G data stream over one single laser/wavelength. It is first standardized by the 100G Lambda MSA (Multisource Agreement), which is an industry consortium with a common focus to provide a new set of optical interface specifications. These specifications are developed around an optical channel data rate of 100Gb/s, aiming to use in 100G and 400G applications in a cost-effective way.
The appearance of 100G single lambda is not a coincidence. Most optical transceivers like 100GBASE-LR4, 100G-CWDM4, 100G-PSM4 and 100GBASE-SR4 today, operating at 100Gb/s, consist of four sets of transmitters and receivers operating in parallel lanes of 25Gb/s. Those four optical signals are coupled either through parallel fibers or are optically multiplexed to a single fiber for transport, which requires a series of costly optical components and packaging. To reduce the total cost and get higher transmission efficiency, 100G single lambda transceiver specification was proposed. Transceivers with this specification use the 100G PAM4 signaling for 100G per wavelength, which decreases the optical complexity and cost by reducing the number of optical transmitters and receivers from 4 to 1.
Optical components take more than 60% of the whole cost inside a 100GE pluggable optical module. According to the principles of 100G transceiver transmission, the common 100G QSFP28 transceivers like 100GBASE-SR4 use discrete optical components to realize 25Gbps at each lane. Normally, optical components cannot be scaled to the same degree because they usually consist of discrete components. But as the demand for strict module density and size grows, the final transceiver cost will be determined by the number of discrete components. Within the same volume, the fewer the discrete optical components used in the transceiver, the lower the cost.
From the figure above, 100GBASE-LR4 could support 10km links over 4 wavelengths multiplexed together onto a single fiber. Since the wavelengths were spaced closely together with LAN-WDM wavelength spacing, it was not convenient for laser temperature controlling, which needed costly hermetic packaging. Therefore, 100GBASE-CWDM4 was proposed to solve this problem. The 100GBASE-CWDM4 spaced the laser wavelengths far apart, saving much cost. However, it limited the transmission distance to 2 km. To eliminate the complexity of multiple wavelength lasers, 100GBASE-PSM4 uses 4 pairs of fibers and the specification for just one wide laser wavelength. However, it added fiber complexity to the module and to the interconnecting cable infrastructure compared to LR4 or CWDM4. And 100GBASE-PSM4 could only support 500m transmission.
Then here comes the 100G single lambda transceiver to simplify the optical module structure and improve the efficiency. The 100G single lambda transceivers include 100GBASE-DR, 100GBASE-FR (100G-FR) and 100GBASE-LR (100G-LR). Those transceivers adopt the 4x 25G electrical signal from the host and use a DSP to translate the signal to a PAM4 modulation instead of using NRZ signals as is done with LR4, CWDM4 or PSM4. The use of the PAM4 signal on a single wavelength means that the full 100G data stream is transmitted by a single laser, with no WDM or parallel fiber, reducing the number of optical components like transmitters and receivers from 4 to 1. The 100GBASE-DR is specified for 500m links. Later, the 100G Lambda MSA extended the reach to 2 km with 100GBASE-FR, allowing longer links or higher loss environments. Until now, the MSA has extended the reach to 10 km with 100GBASE-LR, addressing the same applications as 100GBASE-LR4.
The new 100G single lambda standard not only reduces the complexity of the optical components inside the modules, but also reduces the cost of 100G links. According to IEEE, the ability to support 100G per lambda (wavelength) could reduce the cost of a 100GE optical signal by at least 40% with a single optical lane. That means, transition from 4 wavelengths/lambda to one wavelength/lambda results in relative cost reduction over 40%.
As traffic continues to grow, the need for simpler, more cost-effective pluggable optical modules will become the key to transceiver market development, especially in high-speed and high-density applications like 200G and 400G. It makes the transition from 100G to 400G applications easier, and reduces the internal complexity of 400G modules, which is more conducive to the development of 400G. At the same time, the fiber count is reduced, which is good for cost-saving.
Since PAM4 modulation has made single-lane 100G possible, upgrading from 100G to 4x 100G becomes a reality. For example, the IEEE leveraged 100GBASE-DR for the 400GBASE-DR4 optical standard. The 400GBASE-DR4 could break out into four parallel 100GBASE-DR modules and provide 400G connectivity over 500m. With 100G single lambda transceivers, 100G breakout connections from a 400G port is easy.
The 100G Lambda MSA group also helps upgrade the existing IEEE 400G standards. In the existing 400G standards, the 400G transceivers rely on 8 lanes of 50G PAM4 optical signals on the LAN-WDM wavelength grid, which have the same issues as 100GBASE-LR4 explained above. The number of wavelength channels required and the complexity of the multiplexer and demultiplexer seem costly. Realizing such shortcoming, the MSA decided to use 100G per wavelength instead of 50G and use four channels on the CWDM wavelength grid with a wider window of tolerance, which provides a lower-cost path for 400G.