Dan Sadot on coherent's role in the metro and the data centre

Gazettabyte went to visit Professor Dan Sadot, academic, entrepreneur and founder of chip start-up MultiPhy, to discuss his involvement in start-ups, his research interests and why he believes coherent technology will not only play an important role in the metro but also the data centre.

"Moore's Law is probably the most dangerous enemy of optics"

Professor Dan Sadot 

The Ben-Gurion University campus in Beer-Sheba, Israel, is a mixture of brightly lit, sharp-edged glass-fronted buildings and decades-old Palm trees. 

The first thing you notice on entering Dan Sadot's office is its tidiness; a paperless desk on which sits a MacBook Air. "For reading maybe the iPad could be better but I prefer a single device on which I can do everything," says Sadot, hinting at a need to be organised, unsurprising given his dual role as CTO of MultiPhy and chairman of Ben-Gurion University's Electrical and Computer Engineering Department. 

The department, ranked in the country's top three, is multi-disciplinary. Just within the Electrical and Electronics Department there are eight tracks including signal processing, traditional communications and electro-optics. "That [system-oriented nature] is what gives you a clear advantage compared to experts in just optics," he says.

The same applies to optical companies: there are companies specialising in optics and ASIC companies that are expert in algorithms, but few have both. "Those that do are the giants: [Alcatel-Lucent's] Bell Labs, Nortel, Ciena," says Sadot. "But their business models don't necessarily fit that of start-ups so there is an opportunity here." 

MultiPhy  

MultiPhy is a fabless start-up that specialises in high-speed digital signal processing-based chips for optical transmission. In particular it is developing 100Gbps ICs for direct detection and coherent.

Sadot cites a rule of thumb that he adheres to religiously: "Everything you can do electronically, do not do optically. And vice versa: do optically only the things you can't do electronically." This is because using optics turns out to be more expensive.

And it is this that MultiPhy wants to exploit by being an ASIC-only company with specialist knowledge of the algorithms required for optical transmission.

"Electronics is catching up," says Sadot. "Moore's Law is probably the most dangerous enemy of optics."

Ben-Gurion University Source: Gazettabyte

Direct detection

Not only have developments in electronics made coherent transmission possible but also advances in hardware. For coherent, accurate retrieval of phase information is needed and that was not possible with available hardware until recently. In particular the phase noise of lasers was too high, says Sadot. Now optics is enabling coherent, and the issues that arise with coherent transmission can be solved electronically using DSP.

MultiPhy has entered the market with its MP1100Q chip for 100Gbps direct detection. According to Sadot, 100Gbps is the boundary data rate between direct detection and coherent. Below 100Gbps coherent is not really needed, he says, even though some operators are using the technology for superior long-haul optical transmission performance at 40Gbps.

"Beyond 100 Gig you need the spectral efficiency, you need to do denser [data] constellations so you must have coherent," says Sadot. "You are also much more vulnerable to distortions such as chromatic dispersion and you must have the coherent capability to do that." 

But at 100 Gig the two - coherent and direct detection - will co-exist.

MultiPhy's first device runs the maximum likelihood sequence estimation (MLSE) algorithm that is used to counter fibre transmission distortions. "MLSE offers the best possible theoretical solution on a statistical basis without retrieving the exact phase," says Sadot.  "That is the maximum you can squeeze out of direct detection."  

The MLSE algorithm benefits optical performance by extending the link's reach while allowing lower cost, reduced-bandwidth optical components to be used. MultiPhy claims 4x10Gbps can be used for the transmit and the receive path to implement the 4x28Gbps (100Gbps) design. 

Sadot describes MLSE as a safety net in its ability to handle transmitter and/or receiver imperfections. "We have shown that performance is almost identical with a high quality transmitter and a lower quality transmitter; MLSE is an important addition." he says.

Ben-Gurion University Source: Gazettabyte

Coherent metro

System vendors such as Ciena and Alcatel-Lucent have recently announced their latest generation coherent ASICs designed to deliver long-haul transmission performance. But this, argues Sadot, is overkill for most applications when ultra-long haul is not needed: metro alone accounts for 75% of all the line side ports.

He also says that the power consumption of long-haul solutions is over 3x what is required for metro: 75W versus the CFP pluggable module's 24W. This means the power available solely for the ASIC would be 15W. 

"This is not fine-tuning; you really need to design the [coherent metro ASIC] from scratch," says Sadot. "This is what we are doing."

To achieve this, MultiPhy has developed patents that involve “sub-Nyquist” sampling. This allows the analogue-to-digital converters and the DSP to operate at half the sampling rate, saving power.  To use sub-Nyquist sampling, a low-pass anti-aliasing filter is applied but this harms the received signal. Using the filter, sampling at half the rate can occur and using the MLSE algorithm, the effects of the low-pass filtering can be countered. And because of the low pass filtering, reduced bandwidth opto-electronics can be used which reduces cost.

The result is a low power, cost-conscious design suited for metro networks.

Coherent elsewhere

Next-generation PON is also a likely user of coherent technology for such schemes as ultra-dense WDM-PON.

Sadot believes coherent will also find its way into the data centre. "Again you will have to optimise the technology to fit the environment - you will not find an over-design here," he says. 

Why would coherent, a technology associated with metro and long-haul, be needed in the data centre? 

"Even though there is the 10x10 MSA, eventually you will be limited by spectral efficiency," he says. Although there is a tremendous amount of fibre in the data centre, there will be a need to use this resource to the maximum. "Here it will be all about spectral efficiency, not reach and optical signal-to-noise," says Sadot.

Sadot's start-ups

Sadot had a research posting at the optical communications lab at Stanford University. The inter-disciplinary and systems-oriented nature of the lab was an influence on Sadot when he founded the optical communications lab at Ben-Gurion University around the time of the optical boom. "A pleasant time to come up with ideas," is how he describes that period - 1999-2000.  

The lab's research focus is split between optical and signal processing topics. Work there resulted in two start-ups during the optical bubble which Sadot was involved in: Xlight Photonics and TeraCross.

Xlight focused on ultra-fast lasers as part of a tunable transponder. Xlight eventually merged with another Israeli start-up Civcom, which in turn was acquired by Padtek. 

The second start-up, TeraCross, looked at scheduling issues to improve throughput in Terabit routers. "The start-up led to a reference design that was plugged into routers in Cisco's Labs in Santa Clara [California]," says Sadot. "It was the first time a scheduler showed the capability to support a one Terabit data stream, and route in a sophisticated, global manner."

But with the downturn of the market, the need for terabit routers disappeared and the company folded.

Sadot's third and latest start-up, MultiPhy, also has its origins in Ben-Gurion's optical communications lab's work on enabling system upgrades without adding to system cost. 

MultiPhy started as a PON company looking at how to upgrade GPON and EPON to 10 Gigabit PON without changing the hardware. "The magic was to use previous-generation hardware which introduces distortion as it doesn't really fit this upgrade speed, and then to compensate by signal processing," says Sadot.

After several rounds of venture funding the company shifted its focus from PON, applying the concept to 100 Gigabit optical transmission instead.