Enabling coherent optics down to 2km short-reach links

Silicon photonics luminaries series

Interview 5: Chris Doerr

Chris Doerr admits he was a relative latecomer to silicon photonics. But after making his first silicon photonics chip, he was hooked. Nearly a decade later and Doerr is associate vice president of integrated photonics at Acacia Communications. The company uses silicon photonics for its long-distance optical coherent transceivers.

Chris Doerr in the lab

Acacia Communications made headlines in May after completing an initial public offering (IPO), raising approximately $105 million for the company. Technology company IPOs have become a rarity and are not always successful. On its first day of trading, Acacia’s shares opened at $29 per share and closed just under $31.

Although investors may not have understood the subtleties of silicon photonics or coherent DSP-ASICs for that matter, they noted that Acacia has been profitable since 2013. But as becomes clear in talking to Doerr, silicon photonics plays an important role in the company’s coherent transceiver design, and its full potential for coherent has still to be realised.

Bell Labs

Doerr was at Bell Labs for 17 years before joining Acacia in 2011. He spent the majority of his time at Bell Labs making first indium phosphide-based optical devices and then also planar lightwave circuits. One of his bosses at Bell Labs was Y.K. Chen. Chen had arranged a silicon photonics foundry run and asked Doerr if he wanted to submit a design.

What hooked Doerr was silicon photonics’ high yields. He could assume every device was good, whereas when making complex indium phosphide designs, he would have to test maybe five or six devices before finding a working one. And because the yields were high, he could focus more on the design aspects. “Then you could start to make very complex designs - devices with many elements - with confidence,” he says.

Another benefit was that the performance of the silicon photonic circuit matched closely its simulation results. “Indium phosphide is so complex,” he says. “You have to worry about the composition effects and the etching is not that precise.” With silicon, in contrast, the dimensions and the refractive index are known with precision. “You can simulate and design very precisely, which made it [the whole process] richer,” says Doerr.

Silicon photonics is a disruptive technology because of its ability to integrate so many things together and still be high yield and get the raw performance 

After that first wafer run, Doerr continued to design both planar lightwave circuits and indium phosphide components at Bell Labs. But soon it was solely silicon photonics ICs.

Doerr views Acacia’s volume production of an integrated coherent transceiver - the transmit and receive optics on the one chip - with a performance that matches discrete optical designs, as one of silicon photonics’ most notable achievements to date.

With a discrete component coherent design, you can use the best of each material, he explains, whereas with an integrated design, compromises are inevitable. “You can’t optimise the layer structure; each component has to share the wafer structure,” he says. Yet with silicon photonics, the design space is so powerful and high-yielding, that these compromises are readily overcome.

Doerr also describes a key moment when he realised the potential of silicon photonics for volume manufacturing.

He was reading an academic paper on grating couplers, a structure used to couple fibres to waveguides. “You can only make that in silicon photonics because you need a high vertical [refractive] index contrast,” he says. Technically, a grating coupler can also be made in indium phosphide but the material has to be cut from under the waveguide; this leaves the waveguide suspended in air.

When he first heard of grating couplers he assumed the coupling efficiency would be of the order of a few percent whereas in practice it is closer to 85 percent. “That is when I realised it is a very powerful concept,” he says.

Integration is key

Doerr pauses before giving measured answers to questions about silicon photonics. Nor does his enthusiasm for silicon photonics blinker him to the challenges it faces. However, his optimism regarding the technology’s future is clear.

“Silicon photonics is a disruptive technology because of its ability to integrate so many things together and still be high yield and get the raw performance,” he says. In the industry, silicon photonics has proven itself for such applications as metro telecommunications but it faces significant competition from established technologies such as indium phosphide.  It will require more channels to be integrated for the full potential of silicon photonics as a disruption technology to emerge, says Doerr.

Silicon photonics also has an advantage on indium phosphide in that it can be integrated with electronic ICs using 2.5D and 3D packaging, saving cost, footprint, and power. “If you are in the same material system then such system-in-package is easier,” he says.  Also, silicon photonic integrated circuits do not require temperature control, unlike indium phosphide modulators, which saves power.

Areas of focus 

One silicon photonics issue is the need for an external laser. For coherent transceivers, it is better to separate the laser from the high-speed optics due to the fact that the coherent DSP-ASIC and the photonic chips are hot and the laser requires temperature control.  

For applications such as very short reach links, silicon photonics needs a laser source and while there are many options to integrate the laser to the chip, a clear winning approach has yet to emerge. “Until a really low cost solution is found, it precludes silicon from competing with really low-cost solutions like VCSELs for very short reach applications,” he says.

Silicon photonic chip volumes are still many orders of magnitude fewer than those of electronic ICs. But Acacia says foundries already have silicon photonics lines running, and as these foundries ramp volumes, costs, production times, and node-sizes will continually improve.

Opportunities   

The adoption of silicon photonics will increase significantly as more and more functions are integrated onto devices. For coherent designs, Doerr can foresee silicon photonics further reducing the size, cost and power consumption, making them competitive with other optical transceiver technologies for distances as short as 2km.

“You can use high-order formats such as 256-QAM and achieve very high spectral efficiency,” says Doerr. Using such a modulation scheme would require fewer overall lasers to achieve significant transport capacities, improving the cost-per-bit performance for applications such as data centre interconnect. “Fibre is expensive so the more you can squeeze down a fibre, the better,” he says.

Doerr also highlights other opportunities for silicon photonics, beyond communications. Medical applications is one such area. He cites a post-deadline paper at OFC 2016 from Acacia on optical coherent tomography which has similarities with the coherent technology used in telecom.

Longer term, he sees silicon photonics enabling optical input/ output (I/O) between chips. As further evolutionary improvements are achieved, he can see lasers being used externally to the chip to power such I/O. “That could become very high volume,” he says.

However, he expects 3D stacking of chips to take hold first. “That is the easier way,” he says.