For Bill Gartner, Cisco’s senior vice president and general manager of optical systems and optics, AI’s rapid rise is not just driving bandwidth demand, it’s forcing a rethink of design, reliability, and component integration.
In conversation, Gartner outlines the logic and direction behind Cisco’s evolving optical strategy.
Bill Gartner’s remit encompasses Cisco’s photonics portfolio: inside the data centre, inter-data-centre transport, and technologies such as coherent modems and chips that Acacia, a Cisco business unit, develops.
“Basically, my job covers anything that is optics or photonics,” says Gartner. “It’s a wide brief that increasingly revolves around a single, urgent theme – AI – that is changing everything.”
One consequence of AI, according to Gartner, is a surge in demand for pluggable coherent optics for data centre interconnect (DCI). “For several hyperscalers, coherent interfaces between data centres – DCI – have had to grow significantly to support expanding AI models across sites,” he says. The requirement is boosting demand for coherent pluggables and, in some cases, optical systems and transponders. For Cisco, this is a boon — especially Acacia, which has become central to the company’s optical strategy.
However, AI is also causing a shift with regard to optical modules used inside the data centre, specifically optics at 400 gigabits and 800 gigabits.
“We have been a relatively small player inside the data centre for optics,” says Gartner. “This is an area where Cisco has great potential to deliver.”
For AI, reliability matters
AI’s hunger for parallelism exposes something that traditional data centre networking could hide. In classical IP networks, the TCP/IP protocol can quietly retransmit lost packets. In a graphics processor unit (GPU) cluster, however, every device must remain in sync. “If one link has a burst error or a link flap,” Gartner explains, “it impacts all of them, and the workload has to stop, requiring a back-up to a checkpoint and a restart.” This networking development means that reliability is a significant issue for AI.
Gartner cites a study by SemiAnalysis suggesting that, in a 100,000-GPU cluster, even when the optics having a five-year mean-time-to-failure, the first failure will occur within 26 minutes.
Meta has published data that an AI cluster can experience a 40 per cent cut in computing performance due to such failures.
This is where Cisco is well placed. Gartner points out that Cisco has platform-breadth with its Silicon One ICs, optics, system platforms, and software. With this, Cisco gains insight into how the various elements interact. “The platform approach gives us insights that others, who play only in one of those silos, lack,” says Gartner.
Cisco also examines the components making up the link and factors in ageing effects to ensure sufficient margin over the product’s lifetime. The company also uses rigorous stress testing to push optics beyond formal compliance. In one internal test, Cisco tested 20 third-party optics; and not one of the modules passed Cisco’s qualification regimen.
The implication is simple, says Gartner:
“If you’re buying generic optics, you’d better be sure they’ve really been through a significant qualification process, because in AI, the cost of failure is enormous.”
Cisco has historically qualified third-party optics. However, that is no longer an economical model given hyperscalers ask: “Well, who are you buying that from? I’ll go to them,” he says. And that has been a big part of Cisco’s business. More recently, Cisco has been developing its own technology, including photonic integrated circuits and digital signal processors (DSPs). “That allows us to compete head-on with the suppliers of those optics,” says Gartner.
Co-packaged optics
Some two years ago, at OFC 2023, Cisco showed a co-packaged optics (CPO) prototype in a 25 terabit-per-second (Tbps) switch, an early sign of intent that met, in Gartner’s words, a range of responses, from a yawn to outright negativity.
The criticism, he says, came from suppliers who saw co-packaged optics as collapsing value between the switch silicon and optics. “We put it on the back burner for a bit,” he admits. “But Nvidia’s [co-packaged optics switch] announcement earlier this year has brought it back to the forefront.”
Co-packaged optics’ original driver was power reduction, but linear-drive pluggable optics (LPO) have emerged as another route to efficiency. Cisco is pursuing both, with co-packaged optics activity underway, but measured.
Gartner sees AI workloads as the natural home for co-packaged optics, once manufacturing ecosystems and standards mature: “Only a few companies have all the pieces — optics, switch silicon, systems, and software — and Cisco is one of them.”
Optical circuit switching
Optical switching has been discussed for decades. “I have two patents on optical cross-connects — both expired — which tells you how long this technology’s been looking for a problem to solve,” quips Gartner. However, AI may finally provide a solution to this problem.
In GPU clusters, workloads can require semi-static optical paths between processors. “You could manage that with Ethernet, or you could manage it optically,” he says. Cisco is in “monitoring mode,” assessing how optical circuit switches might complement electrical switching.
Inter-data centre connectivity
When it comes to inter-data centre connectivity, the bulk of the coherent pluggables shipping are the OIF-defined 400ZR and the 400-gigabit ZR+ versions, which offer greater reach.
Hyperscalers account for the bulk of deployments, linking data centres and scaling AI workloads across geographically distributed sites. However, Cisco claims that it also has 350 service providers deploying the optics, with almost all of them using the 400ZR+ standard.
Meanwhile, deployment of 800ZR and 800ZR+ has begun. Cisco had alpha samples one year ago and now is in full production. “It’s a mixed bag,” says Gartner, “with two hyperscalers transitioning to 800-gigabit coherent pluggables, while other hyperscalers are waiting for the OIF-defined 1600ZR and 1600ZR+ specifications to be completed and will skip 800ZR and 800ZR+ to deploy 400ZR and 400ZR+ in the meantime.”
“The innovation has been beyond belief,” says Gartner. “I worked on the very first dense wavelength-division multiplexing (DWDM) system that was ever deployed, and it had a capacity of 20 gigabit-per-second,” he says. “When I came to Cisco, the total capacity on a system was 400 gigabit, and now we are putting 400 gigabit in a pluggable.”
The OIF is currently working on defining 1600ZR and, for the first time, a specification for enhanced ZR+ (1600ZR+). The industry organisation is also developing a pared-down 1.6-terabit version, known as ‘coherent-lite’, for up to 10km.
The decision for coherent players is how best to address the applications and how to design the coherent DSP chips needed. At the recent ECOC 2025 show, companies discussed various possibilities: developing a single DSP for all three standards; two DSPs, one for the 1600CL and 1600ZR, and one for 1600ZR+; or even a distinct DSP for the 1600CL and one addressing 1600ZR/ZR+. Given that coherent DSPs will be implemented in 3nm or even 2nm CMOS, these are very costly undertakings.
“It’s natural for hyperscalers to ask, if I only need 20 kilometres, can I reduce power or cost?” he says. “Coherent-lite has promise there, but you have to avoid fragmenting the market.”
Cisco is committed to the OIF’s 1600ZR and 1600ZR+ standards.
“I think coherent-lite will have applications as well,” says Gartner. “We have not announced any product, but we are certainly investigating coherent-lite.”
Embedded coherent modems
Gartner maintains that Cisco’s early bet on coherent pluggables is being vindicated. When Cisco acquired Acacia, competitors dismissed pluggables as a niche for 400ZR. “Since then, we’ve seen 400ZR+, 800ZR+, and even ultra-long-haul pluggables,” he notes. “In every dimension — bit rate and reach — the application space has expanded.”
Even so, embedded coherent optics continue to offer spectral-efficiency advantages for long-haul and subsea systems. “We’ve hit Shannon’s limit,” he says. “The future innovation is about reducing cost, power, and size — all of which favour pluggables,” he says, but admits that embedded coherent modem designs will still serve the high-end use cases where fibre is scarce.
Pluggable form factors
Form-factor debates still animate the industry. Cisco championed the QSFP-DD pluggable module form factor for backward compatibility with 100-gigabit optics, even as the high-speed alternative form factor, the OSFP, gained traction at 800 gigabit. And now, work is being undertaken for a new high-density OIF pluggable.
But talk of yet another form factor makes Gartner wary. “Whenever someone proposes a new form factor,” he says, “we have to ask: does it support copper as well as optics? Is it backwards compatible? Are we retiring existing infrastructure prematurely?” The wrong choice, he warns, can strand customers.
Silicon photonics and 400-gigabit lanes
Cisco remains a believer in silicon photonics, but Gartner acknowledges the industry’s search for new modulator technologies as optical lane speeds approach 400 gigabits.
“People never thought we’d get silicon photonics to where it is today,” he says. “We are believers — but we are also exploring other approaches.”
The question is whether Cisco continues to push lane speed or moves to wider, slower buses. “Both options are on the table,” says Gartner.
Beyond optics
On his LinkedIn profile, Gartner lists his role as a kids soccer coach. Does he still coach? “My kids are grown,” he says, with a smile, “so I’ve stopped coaching soccer.” Since the COVID-19 pandemic, however, he has been learning to play the guitar.
Playing guitar requires controlled adjustments, as does Gartner’s role regarding Cisco’s photonics’ strategy. Somehow, though, mastering six strings and frets sound simpler than orchestrating a symphony of photonic technologies and business challenges.