coherent

Digital signal processors (DSPs) are the heart of coherent communication systems. They not only encode/decode data into the three properties of a light signal (amplitude, phase, polarization) but also handle error correction, analog-digital conversation, Ethernet framing, and compensation of dispersion and nonlinear distortion. And with every passing generation, they are assigned more advanced functions such as probabilistic constellation shaping.

There are still many challenges ahead to improve DSPs and make them transmit even more bits in more energy-efficient ways. Now that EFFECT Photonics has incorporated talent and intellectual property from Viasat’s Coherent DSP team, we hope to contribute to this ongoing research and development and make transceivers faster and more sustainable than ever. We ask Russell Fuerst, our Vice-President of Digital Signal Processing, how we can achieve these goals.

What’s the most exciting thing about joining EFFECT Photonics?

Before being acquired by EFFECT Photonics, our DSP design team has been a design-for-hire house. We’ve been doing designs for other companies that have put those designs in their products. By joining EFFECT Photonics, we can now do a design and stamp our brand on it. That’s exciting.

The other exciting thing is to have all the technologies under one roof. Having everything from the DSP to the PIC to the packaging and module-level elements in one company will allow us to make our products that much better.

We also find the company culture to be very relaxed and very collaborative. Even though we’re geographically diverse, it’s been straightforward to understand what other people and groups in the company are doing. It’s easy to talk to others and find out whom you need to talk to. There’s not a whole lot of organizational structure that blocks communication, so it’s been excellent from that perspective.

People at EFFECT Photonics were welcoming from day one, making us that much more excited to join.

What key technology challenges must be solved by DSP designers to thrive in the next 5 to 10 years?

The key is to bring the power down while also increasing the performance.

In the markets where coherent has been the de-facto solution, I think it’s essential to understand how to drive cost and power down either through the DSP design itself or by integrating the DSP with other technologies within the module. That will be where the benefits come from in those markets.

Similarly, there are markets where direct detection is the current technology of choice. We must understand how to insert coherent technology into those markets while meeting the stringent requirements of those important high-volume markets. Again, this progress will be largely tied to performance within the power and cost requirements.

As DSP technology has matured, other aspects outside of performance are becoming key, and understanding how we can work that into our products will be the key to success.

How do you think the DSP can be more tightly integrated with the PIC?

This is an answer that will evolve over time. We will become more closely integrated with the team in Eindhoven and learn some of the nuances of their mature design process. And similarly, they’ll understand the nuances of our design process that have matured over the years. As we understand the PIC technology and our in-house capabilities better, that will bring additional improvements that are currently unknown.

Right now, we are primarily focused on the obvious improvements tied to the fully-integrated platform. For example, the fact that we can have the laser on the PIC because of the active InP material. We want to understand how we co-design aspects of the module and shift the complexity from one design piece or component to another, thanks to being vertically integrated.

Another closely-tied area for improvement is on the modulator side. We think that the substantially lower drive voltages required for the InP modulator give us the possibility to eliminate some components, such as RF drivers. We could potentially drive the modulator directly from that DSP without any intermediary electronics, which would reduce the cost and power consumption. That’s not only tied to the lower drive voltages but also some proprietary signal conditioning we can do to minimize some of the nonlinearities in the modulator and improve the performance.

What are the challenges and opportunities of designing DSPs for Indium phosphide instead of silicon?

So, we already mentioned two opportunities with the laser and the modulator.

I think the InP integration makes the design challenges smaller than those facing DSP design for silicon photonics. The fact is that InP can have more active integrated components and that DSPs are inherently active electronic devices, so getting the active functions tuned and matched over time will be a challenge. It motivates our EFFECT DSP team to quickly integrate with the experienced EFFECT PIC design team to understand the fundamental InP platform a bit better. Once we understand it, the DSP designs will get more manageable with improved performance, especially as we have control over the designs of both DSP and PIC. As we get to the point where co-packaging is realized, there will also be some thermal management issues to consider.

When we started doing coherent DSP designs for optical communication over a decade ago, we pulled many solutions from the RF wireless and satellite communications space into our initial designs. Still, we couldn’t bring all those solutions to the optical markets.

However, when you get more of the InP active components involved, some of those solutions can finally be brought over and utilized. They were not used before in our designs for silicon photonics because silicon is not an active medium and lacked the performance to exploit these advanced techniques.

For example, we have done proprietary waveforms tuned to specific satellite systems in the wireless space. Our DSP team was able to design non-standard constellations and modulation schemes that increased the capacity of the satellite link over the previous generation of satellites. Similarly, we could tune the DSP’s waveform and performance to the inherent advantages of the InP platform to improve cost, performance, bandwidth utilization, and efficiency. That’s something that we’re excited about.

Takeaways

As Russell explained, the big challenge for DSP designers continues to be increasing performance while keeping down the power consumption. Finding ways to integrate the DSP more deeply with the InP platform can overcome this challenge, such as direct control of the laser and modulator from the DSP to novel waveform shaping methods. The presence of active components in the InP platforms also gives DSP designers the opportunity to import more solutions from the RF wireless space.

We look forward to our new DSP team at EFFECT Photonics settling into the company and trying out all these solutions to make DSPs faster and more sustainable!