A coherent roadmap for co-packaged optics
Is coherent optics how co-packaged will continue to scale? Pilot Photonics certainly thinks so.
Part 1: Co-packaged optics
Frank Smyth, CTO and founder of Pilot Photonics, believes the firm is at an important inflection point.
Known for its comb laser technology, Pilot Photonics has just been awarded a €2.5 million European Innovation Council grant to develop its light-source technology for co-packaged optics.
The Irish start-up is also moving to much larger premises and is on a recruitment drive. “Many of our projects and technologies are maturing,” says Smyth.
Company
Founded in 2011, the start-up spent its early years coupled to Dublin City University. It raised its first notable investment in 2017.
The company began by making lab instrumentation based on its optical comb laser technology which emits multiple sources of light that are frequency- and phased-locked. But a limited market caused the company to pivot, adding photonic integration to its laser know-how.
Now, the start-up has a fast-switching, narrow-linewidth tunable laser, early samples of which are being evaluated by several “tier-one” companies.
Pilot Photonics also has a narrowband indium-phosphide comb laser for optical transport applications. This will be the next product it samples.
More recently, the start-up has been developing a silicon nitride-based comb laser for a European Space Agency project. “The silicon nitride micro-resonator in the comb is a non-linear element that enables a very broad comb for highly parallel communication systems and for scientific applications,” says Smyth. It is this laser type that is earmarked for the data centre and for co-packaged optics applications.
Smyth stresses that while still being a small company, the staff has broad expertise. “We cover the full stack,” he says.
Skills range from epitaxial wafer design, photonic integrated circuit (PIC)s and lasers, radio frequency (RF) and thermal expertise, and digital electronics and control design capabilities.
“We learned early on that it’s all well making a PIC, but if no one can interface to it, you are wasting your time,” says Smyth.
Co-packaged optics
Co-packaged optics refers to adding optics next to an ASIC that has significant input-output (I/O) data requirements. Examples of applications for co-packaged optics include high-capacity Ethernet switch chips and artificial intelligence (AI) accelerators. The goal is to give the chip optical rather than electrical interfaces, providing system-scaling benefits; as electrical signals get faster, their reach shrink.
The industry has been discussing co-packaged optics for over a decade. Switch-chip players and systems vendors have shown prototype designs and even products. And more than half a dozen companies are developing the optical engines that surround, and are packaged with, the chip.
However, the solutions remain proprietary, and while the OIF is working to standardise co-packaged optics, end users have yet to embrace the technology. In part, this is because pluggable optical modules continue to advance in data speeds and power consumption, with developments like linear-drive optics.
The ecosystem supporting co-packaged optics is also developing. Hyperscalers will only deploy co-packaged optics in volume when reliability and a broad manufacturing base are proven.
Yet industry consensus remains that optical I/O is a critical technology and that deployments will ramp up in the next two years. Ethernet switch capacity doubles every two years while AI accelerator chips are progressing rapidly. Moreover, the number of accelerator chips used in AI supercomputers is growing fast, from thousands to tens of thousands.
Pilot Photonics believes its multi-wavelength laser technology, coupled with the intellectual property it is developing, will enable co-packaged optics based on coherent optics to address such scaling issues.
Implementations
Co-packaged optics uses optical chiplets or ‘engines’ that surround the ASIC on a shared substrate. The optical engines typically use an external laser source although certain co-packaged optics solutions such as from Intel and Ranovus can integrate the laser as part of the silicon-photonics based optical engine.
Designers can scale the optical engine’s I/O capacity in several ways. They can increase the number of fibres connected to the optical engine, send more wavelengths down each fibre, and increase the wavelength’s data rate measured in gigabits per second (Gbps).
In co-packaged optics designs, 16 engines typically surround the chip. For a 25.6-terabit Ethernet chip, 16 x 1.6-terabit engines are used, each 1.6-terabit engine sending a 100Gbps DR1 signal per fibre. The total fibres per engine equals 32: 16 for the transmit and 16 for the receive (see table).
| Switch capacity/Tbps | Optical engine/Tbps | Optical engines | Data rate/fibre | No. fibres/ engine* |
| 25.6 | 1.6 | 16 | 100G DR, 500m | 32 |
| 25.6 | 3.2 | 8 | 100G DR, 500m | 64 |
| 51.2 | 6.4 | 8 | 400G FR4, 2km | 32 |
| 102.4 (speculative) | 6.4 | 16 | 400G FR4, 2km | 16 |
| 102.4 (speculation) | 12.8 | 8 | 400G FR4, 2km | 32 |
*Not counting the external laser source fibre.
Broadcom’s co-packaged optical approach uses eight optical engines around its 25.6-terabit Tomahawk 4 switch chip, each with 3.2Tbps capacity. For the Tomahawk 5, 51.2-terabit Bailly co-packaged optics design, Broadcom uses eight, 6.4Tbps optical engines, sending 400-gigabit FR4, or 4-wavelength coarse WDM wavelengths, across each fibre. Using FR4 instead of DR1 halves the number of optical engines while doubling overall capacity.
The co-packaging solutions used in the next-generation 102.4-terabit switch chip are still to be determined. Capacity could be doubled using twice as many fibres, or by using 200-gigabit optical wavelengths based on 112G PAM-4 electrical inputs, twice the speed currently used.
But scaling routes for the generation after that – 204.8-terabit switch chips and beyond – and the co-packaged optics design become unclear due to issues of dispersion and power constraints, says Smyth.
Scaling challenges
Assuming eight engines were used alongside the 200-terabit ASIC , each would need to be 25.6Tbps. The fibre count per engine could be doubled again or more wavelengths per fibre would be needed. One player, Nubis Communications, scales its engines and fibres in a 2D array over the top of the package, an approach suited to fibre-count growth.
Doubling the wavelength count is another option but adopting an 8-wavelength CWDM design with 20nm spacing means the wavelengths would cover 160nm of spectrum. Over a 2km reach, this is challenging due to problems with dispersion. Narrower channel spacings such as those used in the CW-WDM MSA (multi-source agreement) require temperature control to ensure the wavelengths stay put.
Keeping the symbol rate fixed but doubling the data rate is another option. But adopting the more complex PAM-8 modulation brings its own link challenges.
Another key issue is power. Current 51.2-terabit switches require 400mW of laser launch power (4 x 100mW lasers) per fibre and there are 128 transmit fibers per switch.
“Assuming a wall plug efficiency of 20 per cent, that is around 250W of power dissipation just for the lasers,” says Smyth. “Getting to 4Tbps per fibre appears possible using 16 wavelengths, but the total fiber launch power is 10 times higher, requiring 2.5kW of electrical power per switch just for the lasers.”
In contrast, single-polarisation coherent detection of 16-QAM signals through a typical path loss of 24dB could match that 4Tbps capacity with the original 250W of laser electrical power, he says.
The optimised total laser power improvement for coherent detection versus direct detection as a function of the additional losses in the signal path (the losses not also experienced by the local oscillator). Source: Pilot Photonics
Coherent detection is associated with a high-power digital signal processor (DSP). Are such chips feasible for such a power-sensitive application as co-packaged optics?
Coherent detection adds some DSP complexity, says Smyth, but it has been shown that for pluggable-based intra data centre links using 5nm CMOS silicon, 400-gigabit coherent and direct-detection are comparable in terms of ASIC power but coherent requires less laser power.
“Over time, a similar battle will play out for co-packaged optics. Laser power will become a bigger issue than DSP power,” he says.
The additional signal margin could be used for 10km links, with tens of terabits per fibre and even 80km links at similar per-fibre rates to current direct detection.
“We believe coherent detection in the data centre is inevitable,” says Smyth. “It’s just a question of when.”
Comb-based coherent co-packaged optics
Coherent co-packaged optics brings its own challenges. Coherent detection requires alignment between the signal wavelength and the local oscillator laser in the receiver. Manufacturing tolerances and the effects of ageing in simple laser arrays make this challenging to achieve.
“The wavelengths of a comb laser are precisely spaced, which greatly simplifies the problem,” says Smyth. “And combs bring other benefits related to carrier recovery and lack of inter-channel interference too”.
Pilot Photonics’ comb laser delivers 16 or 32 wavelengths per fibre, up to 8x more than existing solutions. Smyth says the company intends to fit its comb laser inside the OIF’s standardised External Laser Source pluggable form-factor,
The start-up is also developing a coherent ring resonator modulator for its design. The ring modulator is tiny compared with Mach-Zehnder interferometer modulators used for coherent optics.
Pilot Photonics is also developing IP for coherent signal processing. Because its comb laser locks the frequency and phase of the wavelengths generated, the overall control and signal processing can be simplified.
While it will offer the comb laser, the start-up does not intend to develop the DSP IC nor make optical engines itself.
“A strategic partnership with a company with its own manufacturing facilities would be the most effective way of getting this technology to market,” says Smyth.
ECOC 2023 industry reflections - Part 3
Gazettabyte is asking industry figures for their thoughts after attending the recent ECOC show in Glasgow. In particular, what developments and trends they noted, what they learned and what, if anything, surprised them. Here are responses from Coherent, Ciena, Marvell, Pilot Photonics, and Broadcom.
Julie Eng, CTO of Coherent
It had been several years since I’d been to ECOC. Because of my background in the industry, with the majority of my career in data communications, I was pleasantly surprised to see that ECOC had transitioned from primarily telecommunications, and largely academic, into more industry participation, a much bigger exhibition, and a focus on datacom and telecom. There were many exciting talks and demos, but I don’t think there were too many surprises.
In datacom, the focus, not surprisingly, was on architectures and implementations to support artificial intelligence (AI). The dramatic growth of AI, the massive computing time, and the network interconnect required to train models are driving innovation in fibre optic transceivers and components.
There was significant discussion about using Ethernet for AI compared to protocols such as InfiniBand and NVLink. For us as a transceiver vendor, the distinction doesn’t have a significant impact as there is little if any, difference in the transceivers we make for Ethernet compared to the transceivers we make for InfiniBand/NVLink. However, the impact on the switch chip market and the broader industry are significant, and it will be interesting to see how this evolves.
Linear pluggable optics (LPO) was a hot topic, as it was at OFC 2023, and multiple companies, including Coherent, demonstrated 100 gigabit-per-lane LPO. The implementation has pros and cons, and we may find ourselves in a split ecosystem, with some customers preferring LPO and others preferring traditional pluggable optics with DSP inside the module. The discussion is now moving to the feasibility of 200 gigabit-per-lane LPO.
Discussion and demonstrations of co-packaged optics also continued, with switch vendors starting to show Ethernet switches with co-packaged optics. Interestingly, the success of LPO may push out the implementation of co-packaged optics, as LPO realizes some of the advantages of co-packaged optics with a much less dramatic architectural change.
One telecom trend was the transition to 800-gigabit digital coherent optical modules, as customers and suppliers plan for and demonstrate the capability to make this next step. There was also significant interest in and discussion about 100G ZR. We demonstrated a new version with 0dBm high optical output power at ECOC 2023 while other companies showed components to support it. This is interesting for cable providers and potentially for data centre interconnect and mobile fronthaul and backhaul.
I was very proud that our 200 gigabit-per-lane InP-based DFB-MZ laser won the 2023 ECOC Exhibition Industry Award for Most Innovative Product in the category of Innovative Photonics Component.
ECOC was a vibrant conference and exhibition, and I was pleased to attend and participate again.
Loudon Blair, senior director, corporate strategy, Ciena
ECOC 2023 in Glasgow gave me an excellent perspective on the future of optical technology. In the exhibition, integrated photonic solutions, high-speed coherent pluggable optical modules, and an array of testing and interoperability solutions were on display.
I was especially impressed by how high-bandwidth optics is being considered beyond traditional networking. Evolving use cases include optical cabling, the radio access network (RAN), broadband access, data centre fabrics, and quantum solutions. The role of optical connectivity is expanding.
In the conference, questions and conversations revolved around how we solve challenges created by the expanding use cases. How do we accommodate continued exponential traffic growth on our fibre infrastructure? Coherent optics supports 1.6Tbps today. How many more generations of coherent can we build before we move on to a different paradigm? How do we maximize density and continue to minimize cost and power? How do we solve the power consumption problem? How do we address the evolving needs of data centre fabrics in support of AI and machine learning? What is the role of optical switching in future architectures? How can we enhance the optical layer to secure our information traversing the network?
As I revisited my home city and stood on the banks of the river Clyde – at a location once the shipbuilding centre of the world – I remembered visiting my grandfather’s workshop where he built ships’ compasses and clocks out of brass.
It struck me how much the area had changed from my childhood and how modern satellite communications had disrupted the nautical instrumentation industry. In the same place where my grandfather serviced ships’ compasses, the optical industry leaders were now gathering to discuss how advances in optical technology will transform how we communicate.
It is a good time to be in the optical business, and based on the pace of progress witnessed at ECOC, I look forward to visiting San Diego next March for OFC 2024.
Dr Loi Nguyen, executive vice president and general manager of the cloud optics business group, Marvell
What was the biggest story at ECOC? That the story never changes! After 40 years, we’re still collectively trying to meet the insatiable demand for bandwidth while minimizing power, space, heat, and cost. The difference is that the stakes get higher each year.
The public debut of 800G ZR/ZR+ pluggable optics and a merchant coherent DSP marked a key milestone at ECOC 2023. For the first time, small-form-factor coherent optics delivers performance at a fraction of the cost, power, and space compared to traditional transponders. Now, cloud and service providers can deploy a single coherent optics in their metro, regional, and backbone networks without needing a separate transport box. 800 ZR/ZR+ can save billions of dollars for large-scale deployment over the programme’s life.
Another big topic at the show was 800G linear drive pluggable optics (LPO). The multi-vendor live demo at the OIF booth highlighted some of the progress being made. Many hurdles, however, remain. Open standards still need to be developed, which may prove difficult due to the challenges of standardizing analogue interfaces among multiple vendors. Many questions remain about whether LPO can be scaled beyond limited vendor selection and bookend use cases.
Frank Smyth, CTO and founder of Pilot Photonics
ECOC 2023’s location in Glasgow brought me back to the place of my first photonics conference, LEOS 2002, which I attended as a postgrad from Dublin City University. It was great to have the show close to home again, and the proximity to Dublin allowed us to bring most of the Pilot team.
Two things caught my eye. One was 100G ZR. We noted several companies working on their 100G ZR implementations beyond Coherent and Adtran (formerly Adva) who announced the product as a joint development over a year ago.
100G ZR has attracted much interest for scaling and aggregation in the edge network. Its 5W power dissipation is disruptive, and we believe it could find use in other network segments, potentially driving significant volume. Our interest in 100G ZR is in supplying the light source, and we had a working demo of our low linewidth tunable laser and mechanical samples of our nano-iTLA at the booth.
Another topic was carrier and spatial division multiplexing. Brian Smith from Lumentum gave a Market Focus talk on carrier and spatial division multiplexing (CSDM), which Lumentum believes will define the sixth generation of optical networking.
Highlighting the approaching technological limitation on baud rate scaling, the ‘carrier’ part of CSDM refers to interfaces built from multiple closely-spaced wavelengths. We know that several system vendors have products with interfaces based on two wavelengths, but it was interesting to see this from a component/ module vendor.
We argue that comb lasers come into their own when you go beyond two to four or eight wavelengths and offer significant benefits over independent lasers. So CSDM aligns well with Pilot’s vision and roadmap, and our integrated comb laser assembly (iCLA) will add value to this sixth-generation optical networking.
Speaking of comb lasers, I attended an enjoyable workshop on comb lasers on the Sunday before the meetings got too hectic. The title was ‘Frequency Combs for Optical Communications – Hype or Hope’. It was a lively session featuring a technology push team and a market pull team presenting views from academia and industry.
Eric Bernier offered an important observation from HiSilicon. He pointed to a technology gap between what the market needs and what most comb lasers provide regarding power per wavelength, number of wavelengths, and data rate per lane. Pilot Photonics agrees and spotted the same gap several years ago. Our iCLA bridges it, providing a straightforward upgrade path to scaling to multi-wavelength transceivers but with the added benefits that comb lasers bring over independent lasers.
The workshop closed with an audience participation survey in which attendees were asked: Will frequency combs play a major role in short-reach communications? And will they play a major role in long-reach communications?
Unsurprisingly, given an audience interested in comb lasers, the majority’s response to both questions was yes. However, what surprised me was that the short-reach application had a much larger majority on the yes side: 78% to 22%. For long-reach applications the majority was slim: 54% to 46%.
Having looked at this problem for many years, I believe the technology gap mentioned is easier to bridge and delivers greater benefits for long-reach applications than for short-reach, at least in the near term.
Natarajan Ramachandran, director of product marketing, physical layer products division, Broadcom
Retimed pluggables have repeatedly shown resiliency due to their standards-based approach, offering reliable solutions, manufacturing scale, and balancing metrics around latency, cost and power.
At ECOC this year, multiple module vendors demonstrated 800G DR4 and 1.6T DR8 solutions with 200 gigabit-per-lane optics. As the IEEE works towards ratifying the specs around 200 gigabit per lane, one thing was clear at ECOC: the ecosystem – comprising DSP vendors, driver and transimpedence amplifier (TIA) vendors, and VCSEL/EML/silicon photonics vendors – is ready and can deliver.
Several vendors had module demonstrations using 200 gigabit-per-lane DSPs. What also was apparent at ECOC was that the application space and use cases, be it within traditional data centre networks, AI and machine learning clusters and telcom, continue to grow. Multiple technologies will find the space to co-exist.
Pilot Photonics makes a one terabit coherent comb source
Pilot Photonics has produced a four-wavelength laser chip for one-terabit coherent transmissions.
It is one of several applications the Irish start-up is pursuing using its optical comb source that produces multiple tunable outputs, the equivalent of a laser array.
The company is using its laser technology and photonic integration expertise to address Next Generation Passive Optical Network 2 (NG-PON2), coherent long-haul transmission, and non-telecom applications such as Light Detection and Ranging (LiDAR) and sensing.
Frank Smyth (right)
“We have a number of chips reaching maturity and we are transitioning from an R&D-focussed company to early commercial activity,” says Frank Smyth, CEO of Pilot Photonics.
Start-up
Pilot Photonics was founded in 2011 and developed a lab instrumentation product. But its limited market resulted in the company changing tack, adding photonic integration expertise to its optical comb source intellectual property.
The company secured two grants that furthered its photonic integration know-how. One - Big Pipes - was a European Commission Seventh Framework Programme (FP7) project addressing optical transport and data centre applications using combs. The second, an Irish government grant, helped the start-up to commercialise its comb technology.
But this was also a challenging period for the company which could only employ two full-time staff. “I wasn't even full time for a few years,” says Smyth, who worked evenings and weekends. “We went into a lean period out of necessity.”
But building a photonic integration capability gained the company a market presence and led to it raising nearly €1million in funding.
Pilot Photonics now has 11 staff and two products being evaluated by customers. One is a directly-modulated laser for NG-PON2 while the second is a fibre-sensing product. The coherent four-channel source chip will soon be its third evaluation product.
The company is also working on a further funding round of several million Euro that it hopes to close by the year-end.
Optical comb source
There are several ways to implement an optical comb source. These include solid-state and fibre-based comb sources commonly used for scientific instrumentation but they are unsuited for high-volume applications, says Smyth.
Pilot Photonics’ approach, dubbed gain switching, is suited to high-volume applications and involves the direct modulation of a laser chip. “A close competitor of our technology is mode-locked laser diodes,” he says. This is the technology used by Ranovus for its module designs.
The start-up claims its technology has distinct advantages. “Our approach gives you better optical properties such as a narrow line-width," he says. The source also offers tunable wavelength spacing, in contrast to most optical combs that use a fixed-cavity design. Pilot Photonics says it can tune the spacing of the sources with sub-kilohertz precision.
The advantage of the comb source for coherent transmission is that a single chip can replace four or eight distinct lasers, saving packaging, size and cost
Pilot Photonics’ comb sources exploit injection locking between two lasers. Injection locking refers to an effect when two closely matched oscillating systems - in this case, lasers - interact to become synchronised.
The start-up’s comb source comprises a short-cavity ‘slave’ laser and a long-cavity ‘master’ one. The slave laser is modulated with a sine wave, turning the laser on briefly each cycle, to create a train of optical light pulses.
Linking the two lasers, injection locking occurs which increases the coherence between the output pulses. As Smyth explains, this reduces the jitter of the slave laser’s output in that the laser is turned on and off at the same exact points each cycle. This turns the slave’s output, when viewed on a spectrum analyser, into equally-spaced narrow line-width light sources.
The dimensions of the master laser’s cavity set the sources’ line widths while their spacing is dictated by the modulating sine wave. The master laser also determines the central wavelength of the comb sources while the sine wave’s frequency sets the spacings either side. “The master laser gives you a locked centre point and then the tones emanating from the centre can be tuned quite precisely,” says Smyth.
Pilot Photonics’ core intellectual property is making the indium-phosphide optical comb source using its patented gain-switching approach.
Photonic integration
The start-up has built a library of indium-phosphide optical functions in addition to the lasers used for the comb source. The functions include semiconductor optical amplifiers, waveguides, optical couplers, splitters and an active optical filter.
The splitters are used to place the comb source output on waveguides while an active optical filter on each selects the wanted source.
“This [active optical filter] is what we use to separate out individual comb lines so we can do fancy things with them,” says Smyth. For example, modulating the source with data, or beating two sources together for frequency multiplication to create sources in the millimetre wave or sub-terahertz ranges.
Pilot Photonics’ optical circuits are built in an indium-phosphide foundry where the comb source fabrication in done without using regrowth stages. This equates to fewer mask stages to process the indium-phosphide wafer. “There is no regrowth of material back over etched areas,” says Smyth. Fewer steps equates to a less-costly manufacturing process and improved yields.
The start-up sees NG-PON2, the 10-gigabit four-wavelength PON standard, as the largest and closest market opportunity for the company. Coherent optical transport is another telecom market the company is pursuing.
“The next closest opportunity is optical fibre sensing,” says Smyth, pointing out that there are several optical fibre sensoring techniques that can be made using their laser as a pulse source.
The company is also developing LiDAR technology and is involved with the European Space Agency to develop a light source for high-frequency metrology applications including atomic clocks and gravity meters.
“It is a very broad range of applications that we can apply the technology to,” says Smyth.
NG-PON2
Pilot Photonics is not using its source technology as a comb for an NG-PON2 optical line terminal (OLT) but rather as a directly modulated laser for the customer premises equipment’ optical network unit (ONU).
“What we have done is develop a wavelength-tunable directly-modulated laser for NG-PON2,” says Smyth. The benefit of its design is that the laser chip meets the stringent specification of the ONU by being tunable, meeting a reach of 40km and enabling sub-$100 designs.
The start-up is engaged with several potential NG-PON2 customers including manufacturers, systems vendors and module makers, and has delivered an evaluation board with the chip to its lead customer.
Two or three network equipment manufacturers are eager to evaluate the chip
Coherent source
The advantage of the comb source for coherent transmission is that a single chip can replace four or eight distinct lasers, saving packaging, size and cost.
Smyth estimates that a four-channel comb source is a third of the cost of a design using four single-mode lasers. The power consumption is also less; only one thermo-electric cooler is required instead of four.
Pilot Photonics says that it has demonstrated its four-channel comb-source transmitting over hundreds of kilometres.
The comb source can be used to send 400-gigabit (100 gigabit/wavelength) and 1-terabit (250 gigabit/wavelength) super-channels. “We’ve done two terabits using 16-QAM on most of the channels and QPSK on the outer ones,” says Smyth.
There are also other system performance benefits using a comb source. There is no need for guard bands to separate between the tones. “You are packing them as tight as can be allowed, the ultimate in spectral efficiency,” he says.
Smyth also points out that non-linear compensation techniques can be used because the frequency spacings are known precisely. Using non-linear compensation methods benefits reach; the laser source can be launched at higher power and the non-linear effects that result can be compensated for.
Pilot Photonics has shown its sources spaced as close as 6.25GHz to 87.5GHz apart. The start-up also says the tones do not need to be evenly spaced.
The start-up now has its four-channel comb-source chip on an evaluation board that it is about to deliver to interested systems vendors and large-scale data centre operators.
“Two or three network equipment manufacturers are eager to evaluate the chip,” says Smyth. “They are less forthcoming as to what they are applying it to.”