Many companies that prepared for the OFC show in March had their plans thwarted due to the COVID-19 pandemic.
OFC did take place in San Diego despite all the hardships. But company withdrawals meant technology demonstrations were scrapped, press releases went unissued and stories left untold.
Ferris Lipscomb
Intel and Ranovus, for example, planned to fanfare their first co-packaged optics designs at OFC. Demonstrations to interested parties did occur but at their offices instead.
Equally, 800-gigabit coherent technologies from Ciena and Infinera would have been showcased at the show, as would industry organisations' interoperability demonstrations.
NeoPhotonics announced in January that it was sampling 400-gigabit coherent pluggable offerings in the CFP2 and OSFP form factors.
At OFC, NeoPhotonics was to show a QSFP-DD module implementing the 400ZR OIF coherent standard, thereby completing its 400-gigabit coherent portfolio.
“A lot of the planned demos involved interoperation in customer switches with other modules,” says Ferris Lipscomb, vice president of marketing at NeoPhotonics. “Many of these demos are now being done in San Jose [its HQ in California] for customers coming individually.”
Chip-stacking and 400ZR
NeoPhotonics’s 400ZR QSFP-DD module is also now sampling.
Acacia Communications has said how it is using 3D-chip packaging technology for its 400-gigabit coherent module designs. Acacia assembles the module’s components - the coherent digital signal processor (DSP), the silicon-photonics photonic integrated circuit (PIC), and the modulator driver and trans-impedance amplifier (TIA) - into one package using chip-stacking techniques.
Is NeoPhotonics doing something similar with its coherent pluggables?
“That has been the end-game for some time and will provide the ultimate size and performance,” says Lipscomb. “But I don’t think 400ZR is the point where it is needed.” The expected volumes for 400ZR modules, while large, will be in the couple of hundred thousand units.
Instead, the “tried-and-true” technology of the COSA and optics, integrating the TIA and driver, along with a separate DSP, does the job for this generation, he says. A coherent optical sub-assembly (COSA) combines the interdyne coherent receiver and the coherent modulator driver (CDM).
Since the DSP generates the most heat within the module, the separation helps disperse the DSP’s heat from the other components.
“We think that when you start to use coherent inside the data centre, and for shorter reaches and higher speeds, such [3D] packaging makes sense and that is where it will end up,” says Lipscomb.
High-bandwidth coherent driver modulator
NeoPhotonics announced the general availability of its high-bandwidth coherent driver modulator (HB-CDM) to coincide with OFC.
The device, operating at a symbol rate of 64 gigabaud (GBd), comprises a quad-channel driver and an indium-phosphide-based quad modulator.
Used alongside NeoPhotonics’s 64GBd interdyne coherent receiver and its narrow linewidth tunable laser, the HB-CDM supports data rates up to 600 gigabit-per-second (Gbps).
At 600Gbps, 64-ary quadrature amplitude modulation (64-QAM) is used and the reach is 80km, suitable for data centre interconnect. At 400Gbps using 16-QAM, metro distances of between 400km and 600km are possible, while at 200Gbps the reach achieved is over 1,000km.
The company also has a 64GBd C++ HB-CDM variant that supports the ‘super-C band’ that extends transmission capacity by using 50 per cent more fibre spectrum than the standard C-band; 6.4THz in total.
Lipscomb says that the HB-CDM design will also support 800Gbps using a symbol rate of 96GBd. "We have not announced general availability of the 96GBd [HB-CDM] but we do it," says Lipscomb.
Other coherent uses
NeoPhotonics is applying its coherent optics know-how for non-communications applications such as Lidar, 3D-sensing and medical diagnostics.
A Lidar system uses light from a laser to sense its environment and can be used as one of several sensors to enable self-driving vehicles.
Earlier this year, the company announced that it was sampling high-power semiconductor optical amplifiers (SOAs) and narrow linewidth distributed feedback lasers (DFB) for automotive Lidar applications. The devices operate at 1,550nm and enable automotive Lidar systems to scan distances beyond 200 meters.
As has happened in optical communications, Lidar systems are evolving from on-off keying - pulse modulation - to coherent detection, says Lipscomb. This boosts sensitivity as the use of a local oscillator acts as an amplifier.
“Any time you need higher sensitivity in optical measurement, coherent is advantageous,” says Lipscomb. “With coherent detection, you can measure things with much higher accuracy and much lower power because you are looking at the phase rather than just, as with a lot of Lidar systems, counting the photons that bounce back.”
Using a coherent-based Lidar, the range and velocity of objects can be sensed. The transmitted frequency of the laser - represented by the local oscillator - and the reflected signal are mixed coherently. This enables phase and amplitude information to be recovered to determine an object’s position and velocity.
The sensing work is at the development stage rather than running businesses but NeoPhotonics is already working with several systems companies.
800ZR
Given the OIF has just published the Implementation Agreement for the 400ZR coherent optical interface and the first 400ZR module products have been announced, what next for coherent pluggables?
The answer is the same OSFP and QSFP-DD form factors implementing 800-gigabit coherent, says Lipscomb.
Such 800-gigabit modules will require co-packaging of the DSP, the optics and associated circuitry. The DSP will also need to be implemented using a 5nm CMOS process node.
Accordingly, such products are likely to be at least three years out. “There is a lot of discussion about it but it is on everyone’s roadmap,” says Lipscomb.
Another goal is the use of coherent technology within the data centre.
“Right now, for 40-80km and above, coherent is a cost-effective solution but inside the data centre it is all PAM-4,” says Lipscomb. “But as the cost of coherent decreases and speeds get higher, you get more of an advantage, and that distance will get shorter.”
PAM-4 refers to 4-level pulse-amplitude modulation.