CWDM8

Andrew Rickman, founder and CEO of silicon photonics start-up, Rockley Photonics, discusses the new joint venture with Hengtong Optic-Electric, the benefits of the company’s micron-wide optical waveguides and why the timing is right for silicon photonics. 

Andrew Rickman

The joint venture between Rockley Photonics and Chinese firm Hengtong Optic-Electric is the first announced example of Rockley’s business branching out.

The start-up’s focus has been to apply its silicon photonics know-how to data-centre applications. In particular, Rockley has developed an Opto-ASIC package that combines optical transceiver technology with its own switch chip design. Now it is using the transceiver technology for its joint venture.

“It was logical for us to carve out the pieces generated for the Opto-ASIC and additionally commercialise them in a standard transceiver format,” says Andrew Rickman, Rockley’s CEO. “That is what the joint venture is all about.”

Rockley is not stopping there. Rickman describes the start-up as a platform business, building silicon photonics and electronics chipsets for particular applications including markets other than telecom and datacom. 

 

Joint venture

Hengtong and Rockley have set up the $42 million joint venture to make and sell optical transceivers.

Known for its optical fibre cables, Hengtong is also a maker of optical transceivers and owns 75.1 percent of the new joint venture. Rockley gains the remaining 24.9 percent share in return for giving Hengtong its 100-gigabit QSFP transceiver designs. The joint venture also becomes a customer of Rockley’s, buying its silicon photonics and electronics chips to make the QSFP modules.

“Hengtong is one of the world’s largest optical fibre cable manufacturers, is listed on the Shanghai stock market, and sells extensively in China and elsewhere into the data centre market,” says Rickman. “It is a great conduit, a great sales channel into these customers.”   

The joint venture will make three 100-gigabit QSFP-based products: a PSM4 and a CWDM4 pluggable module and an active optical cable. Rickman expects the joint venture to make other module designs and points out that Rockley participates in the IEEE standards work for 400 gigabits and is one of the co-founders of the 400-gigabit CWDM8 MSA.

Rockley cites several reasons why the deal with Hengtong makes sense. First, a large part of the bill of materials used for active optical cables is the fibre itself, something which the vertically integrated Hengtong can provide.

China also has a ‘Made in China 2025’ initiative that encourages buying home-made optical modules. Teaming with Hengtong means Rockley can sell to the Chinese telecom operators and internet content players.

In addition, Hengtong is already doing substantial business with all of the global data centres as a cable, patch panel and connector supplier, says Rickman:“So it is an immediate sales channel into these companies without having to break into these businesses as a qualified supplier afresh.”

 

A huge amount of learning happened and then what Rockley represented was the opportunity to start all over again with a clean sheet of paper but with all that experience

 

Bigger is Best?

At the recent SPIE Photonics West conference held in San Francisco, Rickman gave a presentation entitled Silicon Photonics: Bigger is Better. His talk outlined the advantages of Rockley’s use of three-micron-wide optical waveguides, bucking the industry trend of using relatively advanced CMOS processes to make silicon photonics components.      

Rickman describes as seductive the idea of using 45nm CMOS for optical waveguides.“These things exist and work but people are thinking of them in the same physics that have driven microelectronics,” he says. Moving to ever-smaller feature sizes may have driven Moore’s Law but using waveguide dimensions that are smaller than the wavelength of light makes things trickier.

To make his point, he plots the effective index of a waveguide against its size in microns. The effective index is a unitless measure - a ratio of a phase delay in a unit length of a waveguide relative to the phase delay in a vacuum. “Once you get below one micron, you get a waveguide that is highly polarisation-dependent and just a small variation in the size of the waveguide has a huge variation in the effective index,” says Rickman.

Such variations translate to inaccuracies in the operating wavelength. This impacts the accuracy of circuits, for example, arrayed-waveguide gratings built using waveguides to multiplex and demultiplex light for wavelength-division multiplexing (WDM).

“Above one micron is where you want to operate, where you can manufacture with a few percent variation in the width and height of a waveguide,” says Rickman.“But the minute you go below one micron, in order to hit the wavelength registration that you need for WDM, you have got to control the [waveguide’s] film thickness and line thickness to fractions of a percent.” A level of accuracy that the semiconductor industry cannot match, he says. 

A 100GHz WDM channel equates to 0.8nm when expressed using a wavelength scale. “In our technology, you can easily get a wavelength registration on a WDM grid of less than 0.1nm,” says Rickman. “Exactly the same manufacturing technology applied to smaller waveguides is 25 times worse - the variation is 2.5nm.” 

Moreover, WDM technology is becoming increasingly important in the data centre. The 100-gigabit PSM4 uses a single wavelength, the CWDM4 uses four, while the newer CWDM8 MSA for 400 gigabit uses eight wavelengths. “In telecom, 90-plus wavelengths can be used; the same thing will come to pass in the years to come in data centre devices,” he says.

Rockley also claims it has a compact modulator that is 50 times smaller than competing modulators despite them being implemented using nanometer feature sizes. 

 

We set out to generate a platform that would be pervasive across communications, new forms of advanced computing, optical signal processing and a whole range of sensor applications

 

Opto-ASIC reference design

Rockley’s first platform technology example is its Opto-ASIC reference design. The design integrates silicon photonics-based transceivers with an in-house 2 billion transistor switch chip all in one package. Rockley demonstrated the technology at OFC 2017.

“If you look around, this is something the industry says is going to happen but there isn't a single practical instantiation of it,” says Rickman who points out that, like the semiconductor industry, very often a reference design needs to be built to demonstrate the technology to customers.“So we built a complete reference design - it is called Topanga - an optical-packaged switch solution,” he says.

Despite developing a terabyte-class packet processor, Rockley does not intend to compete with the established switch-chip players. The investment needed to produce a leading edge device and remain relevant is simply too great, he says.

Rockley has demonstrated its in-package design to relevant companies. “It is going very well but nothing we can say publicly,” says Rickman.  

 

New Markets

Rockley is also pursuing opportunities beyond telecom and datacom.

“We set out to generate a platform that would be pervasive across communications, new forms of advanced computing, optical signal processing and a whole range of sensor applications,” says Rickman.

Using silicon photonics for sensors is generating a lot of interest. “We see these markets starting to emerge and they are larger than the data centre and communications markets,” he says. “A lot of these things are not in the public domain so it is very difficult to report on.”

Moreover, the company’s believes its technology gives it an advantage for such applications. “When we look across the other application areas, we don’t see the small waveguide platforms being able to compete,” says Rickman. Such applications can use relatively high power levels that exceed what the smaller waveguides can handle.

Rockley is sequencing the markets it will address. “We’ve chosen an approach where we have looked at the best match of the platform to the best opportunities and put them in an order that makes sense,” says Rickman.

Rockley Photonics represent Rickman’s third effort to bring silicon photonics to the marketplace.Bookham Technology, the first company he founded, build different prototypes in several different areas but the market wasn't ready. In 2005 he joined start-up Kotura as a board member. “A huge amount of learning happened and then what Rockley represented was the opportunity to start all over again with a clean sheet of paper but with all that experience,” says Rickman.

Back in 2013, Rockley saw certain opportunities for its platform approach and what has happened since is that their maturity and relevance has increased dramatically.

“Like all things it is always down to timing,” says Rickman. “The market is vastly bigger and much more ready than it was in the Bookham days.”