Coherent Lite is a lower cost, lower power version of the coherent optics that power long haul and metro networks. Designed to work at shorter distances (<20km), Coherent Lite uses less complex DSPs and optics with the goal of bringing the benefits of coherent optics to campus interconnect and, eventually, inside the datacenter. There is currently only one major, uncommitted, potential customer for Coherent Lite (Amazon), which makes sizing the market difficult. However, most vendors view the current stage of Coherent Lite as a necessary steppingstone towards eventual datacenter coherent optics, perhaps at 3.2T.
This report initiates our coverage of Coherent Lite and includes:
- The definition and history as well as the status of the technology
- The benefits and caveats when compared to IM-DD and traditional coherent optics
- Proposed applications
- Vendor implementations
- Market size forecast
Definition and History
Coherent Lite is defined by the OIF, which originally standardized an 800G version based on 400ZR called 800LR. With the move to 1.6T, the designation has officially changed to 1600 Coherent Lite. Cignal AI considers the LR term more accurate and prefers it but has reluctantly accepted the term Coherent Lite.
Google first presented the concept in 2021 (see IEEE presentation). The presentation reviews the convergence of specifications of direct detect PAM-4 and coherent DSPs, particularly as PAM-4 speed reached 200G/lane. There was concern among some at Google that PAM-4 could not deliver adequate reach at 200G/lane for datacenter applications, and these concerns were voiced at OFC 2022 (see OFC 2022 Show Report). This led to the first proposals of Coherent Lite at the OIF.
When originally proposed at the OIF, Coherent Lite at 800Gbps was deemed necessary because the performance of 200G/lane PAM4 was uncertain, especially beyond a few kilometers. 800LR, as it was called, was defined for 10km reach and, originally, C-band operation. Eventually, PAM4 performance at 200G/lane was shown to work at 10km, which met the requirements for datacenter deployment and minimized any need for a Coherent Lite solution at 800G. A few DSPs were built, but no significant volumes were deployed. However, even with a diminishingly small potential market, vendors viewed their experience at 800LR as an important proof point for Coherent Lite at higher speeds.
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