PAM4 DSPs Battle LPO for OFC Mindshare

Last year, module vendors demonstrated the first 1.6T optical modules, and this year DSP vendors looked ahead to second-generation 1.6T module designs. Whereas the first 1.6T modules connect a 16x100G host interface to 8x200G optics (16:8), next-generation designs will work with forthcoming 200G/lane switch ASICs, as shown in the top row of the figure. Broadcom disclosed its Sian2 1.6T 8:8 DSP at a March investor event, and Marvell followed by announcing its similar Nova 2 at OFC. Not wanting to be left out of the 1.6T landscape, MaxLinear pre-announced Rushmore, which similarly targets 8:8 designs. Although the company withheld product details, it disclosed Samsung Foundry as its manufacturing partner for Rushmore, setting it apart from competitors using TSMC.

Source: Broadcom

Progress on linear pluggable optics (LPO) and other less-than-full-DSP variants was evident at 100G/lane, but vendors also set the stage for 200G/lane. Last November, Credo Semiconductor was first to announce a transmit-only 800G PAM4 DSP for half-retimed modules, which are now known as linear-receive optics (LRO). At OFC, Marvell joined the LRO bandwagon with Spica Gen2-T, a transmit-only version of its 5nm 800G DSP. The company claims the new chip enables sub-8W 800G modules, representing about a 40% power reduction from full-DSP modules. Although Macom demonstrated the components required for 200G/lane LPO, there was a growing consensus that LRO may be required at 200G/lane. In the meantime, the Tx-only 100G/lane DSPs can serve as LRO proof-of-concept vehicles.

The high cost and power of DSPs, combined with customer interest in LPO, has created an opening for startups offering alternative approaches. TeraSignal emerged from stealth at OFC with a unique IC that it calls an intelligent re-driver. Aimed at 400G and 800G LPO modules, the chip is a 100G/lane linear re-driver built in a CMOS process. That process enables added intelligence, such as a digital eye monitor that enables link tuning and monitoring. Another startup, NewPhotonics, demonstrated 200G/lane equalization in the optical domain. In addition to a transmit equalizer, the company's PIC integrates modulators and lasers.

Overall, OFC 2024 proved there's no one-size-fits-all design for PAM4 optics. The massive scale of AI back-end networks has raised the importance of power reduction, particularly for short-reach optics. With the DSP-versus-LPO debate quickly shifting to 200G/lane designs, we can expect new innovations to appear in 2025.

LightCounting published a post-OFC update to its PAM4 and Coherent DSPs report. Subscribers can login to access it: www.lightcounting.com/login


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