100 ZR

In the optical access networks, the 400ZR pluggables that have become mainstream in datacom applications are too expensive and power-hungry. Therefore, operators are strongly interested in 100G pluggables that can house coherent optics in compact form factors, just like 400ZR pluggables do. The industry is labeling these pluggables as 100ZR.

A recently released Heavy Reading survey revealed that over 75% of operators surveyed believe that 100G coherent pluggable optics will be used extensively in their edge and access evolution strategy. However, this interest had yet to materialize into a 100ZR market because no affordable or power-efficient products were available. The most the industry could offer was 400ZR pluggables that were “powered-down” for 100G capacity.

By embracing smaller and more customizable light sources, new optimized DSP designs, and high-volume manufacturing capabilities, we can develop native 100ZR solutions with lower costs that better fit edge and access networks.

Making Tunable Lasers Even Smaller?

Since the telecom and datacom industries want to pack more and more transceivers on a single router faceplate, integrable tunable laser assemblies (ITLAs) must maintain performance while moving to smaller footprints and lower power consumption and cost.

Fortunately, such ambitious specifications became possible thanks to improved photonic integration technology. The original 2011 ITLA standard from the Optical Internetworking Forum (OIF) was 74mm long by 30.5mm wide. By 2015, most tunable lasers shipped in a micro-ITLA form factor that cut the original ITLA footprint in half. In 2021, the nano-ITLA form factor designed for QSFP-DD and OSFP modules had once again cut the micro-ITLA footprint almost in half.

There are still plenty of discussions over the future of ITLA packaging to fit the QSFP28 form factors of these new 100ZR transceivers. For example, every tunable laser needs a wavelength locker component that stabilizes the laser’s output regardless of environmental conditions such as temperature. Integrating that wavelength locker component with the laser chip would help reduce the laser package’s footprint.

Another potential future to reduce the size of tunable laser packages is related to the control electronics. The current ITLA standards include the complete control electronics on the laser package, including power conversion and temperature control. However, if the transceiver’s main board handles some of these electronic functions instead of the laser package, the size of the laser package can be reduced.

This approach means that the reduced laser package would only have full functionality if connected to the main transceiver board. However, some transceiver developers will appreciate the laser package reduction and the extra freedom to provide their own laser control electronics.

Co-designing DSPs for Energy Efficiency

The 5 Watt-power requirement of 100ZR in a QSFP28 form factor is significantly reduced compared to the 15-Watt specification of 400ZR transceivers in a QSFP-DD form factor. Achieving this reduction requires a digital signal processor (DSP) specifically optimized for the 100G transceiver.

Current DSPs are designed to be agnostic to the material platform of the photonic integrated circuit (PIC) they are connected to, which can be Indium Phosphide (InP) or Silicon. Thus, they do not exploit the intrinsic advantages of these material platforms. Co-designing the DSP chip alongside the PIC can lead to a much better fit between these components.

To illustrate the impact of co-designing PIC and DSP, let’s look at an example. A PIC and a standard platform-agnostic DSP typically operate with signals of differing intensities, so they need some RF analog electronic components to “talk” to each other. This signal power conversion overhead constitutes roughly 2-3 Watts or about 10-15% of transceiver power consumption.

However, the modulator of an InP PIC can run at a lower voltage than a silicon modulator. If this InP PIC and the DSP are designed and optimized together instead of using a standard DSP, the PIC could be designed to run at a voltage compatible with the DSP’s signal output. This way, the optimized DSP could drive the PIC directly without needing the RF analog driver, doing away with most of the power conversion overhead we discussed previously.

Optical Subassemblies That Leverage Electronic Ecosystems

To become more accessible and affordable, the photonics manufacturing chain can learn from electronics packaging, assembly, and testing methods that are already well-known and standardized. After all, building a special production line is much more expensive than modifying an existing production flow.

There are several ways in which photonics packaging, assembly, and testing can be made more affordable and accessible: passive alignments of the optical fiber, BGA-style packaging, and flip-chip bonding. Making these techniques more widespread will make a massive difference in photonics’ ability to scale up and become as available as electronics. To read more about them, please read our previous article.

Takeaways

The interest in novel 100ZR coherent pluggable optics for edge and access applications is strong, but the market has struggled to provide “native” and specific 100ZR solutions to address this interest. Transceiver developers need to embrace several new technological approaches to develop these solutions. They will need smaller tunable laser packages that can fit the QSFP28 form factors of 100ZR solutions, optimized and co-designed DSPs that meet the reduced power consumption goals, and sub-assemblies that leverage electronic ecosystems for increased scale and reduced cost.