Pilot Photonics makes a one terabit coherent comb source

Pilot Photonics has produced a four-wavelength laser chip for one-terabit coherent transmissions. 

It is one of several applications the Irish start-up is pursuing using its optical comb source that produces multiple tunable outputs, the equivalent of a laser array.   

The company is using its laser technology and photonic integration expertise to address Next Generation Passive Optical Network 2 (NG-PON2), coherent long-haul transmission, and non-telecom applications such as Light Detection and Ranging (LiDAR) and sensing.

Frank Smyth (right)

“We have a number of chips reaching maturity and we are transitioning from an R&D-focussed company to early commercial activity,” says Frank Smyth, CEO of Pilot Photonics. 

Start-up

Pilot Photonics was founded in 2011 and developed a lab instrumentation product. But its limited market resulted in the company changing tack, adding photonic integration expertise to its optical comb source intellectual property. 

The company secured two grants that furthered its photonic integration know-how. One - Big Pipes - was a European Commission Seventh Framework Programme (FP7) project addressing optical transport and data centre applications using combs. The second, an Irish government grant, helped the start-up to commercialise its comb technology.

But this was also a challenging period for the company which could only employ two full-time staff. “I wasn't even full time for a few years,” says Smyth, who worked evenings and weekends. “We went into a lean period out of necessity.”  

But building a photonic integration capability gained the company a market presence and led to it raising nearly €1million in funding. 

Pilot Photonics now has 11 staff and two products being evaluated by customers. One is a directly-modulated laser for NG-PON2 while the second is a fibre-sensing product. The coherent four-channel source chip will soon be its third evaluation product.

The company is also working on a further funding round of several million Euro that it hopes to close by the year-end.   

Optical comb source

There are several ways to implement an optical comb source. These include solid-state and fibre-based comb sources commonly used for scientific instrumentation but they are unsuited for high-volume applications, says Smyth.

Pilot Photonics’ approach, dubbed gain switching, is suited to high-volume applications and involves the direct modulation of a laser chip. “A close competitor of our technology is mode-locked laser diodes,” he says. This is the technology used by Ranovus for its module designs. 

The start-up claims its technology has distinct advantages. “Our approach gives you better optical properties such as a narrow line-width," he says. The source also offers tunable wavelength spacing, in contrast to most optical combs that use a fixed-cavity design. Pilot Photonics says it can tune the spacing of the sources with sub-kilohertz precision.

The advantage of the comb source for coherent transmission is that a single chip can replace four or eight distinct lasers, saving packaging, size and cost 

Pilot Photonics’ comb sources exploit injection locking between two lasers. Injection locking refers to an effect when two closely matched oscillating systems - in this case, lasers - interact to become synchronised. 

The start-up’s comb source comprises a short-cavity ‘slave’ laser and a long-cavity ‘master’ one. The slave laser is modulated with a sine wave, turning the laser on briefly each cycle, to create a train of optical light pulses. 

Linking the two lasers, injection locking occurs which increases the coherence between the output pulses. As Smyth explains, this reduces the jitter of the slave laser’s output in that the laser is turned on and off at the same exact points each cycle. This turns the slave’s output, when viewed on a spectrum analyser, into equally-spaced narrow line-width light sources. 

The dimensions of the master laser’s cavity set the sources’ line widths while their spacing is dictated by the modulating sine wave. The master laser also determines the central wavelength of the comb sources while the sine wave’s frequency sets the spacings either side. “The master laser gives you a locked centre point and then the tones emanating from the centre can be tuned quite precisely,” says Smyth.

Pilot Photonics’ core intellectual property is making the indium-phosphide optical comb source using its patented gain-switching approach.

Photonic integration

The start-up has built a library of indium-phosphide optical functions in addition to the lasers used for the comb source. The functions include semiconductor optical amplifiers, waveguides, optical couplers, splitters and an active optical filter. 

The splitters are used to place the comb source output on waveguides while an active optical filter on each selects the wanted source. 

“This [active optical filter] is what we use to separate out individual comb lines so we can do fancy things with them,” says Smyth. For example, modulating the source with data, or beating two sources together for frequency multiplication to create sources in the millimetre wave or sub-terahertz ranges. 

Pilot Photonics’ optical circuits are built in an indium-phosphide foundry where the comb source fabrication in done without using regrowth stages. This equates to fewer mask stages to process the indium-phosphide wafer. “There is no regrowth of material back over etched areas,” says Smyth. Fewer steps equates to a less-costly manufacturing process and improved yields.   

The start-up sees NG-PON2, the 10-gigabit four-wavelength PON standard, as the largest and closest market opportunity for the company. Coherent optical transport is another telecom market the company is pursuing.

“The next closest opportunity is optical fibre sensing,” says Smyth, pointing out that there are several optical fibre sensoring techniques that can be made using their laser as a pulse source.

The company is also developing LiDAR technology and is involved with the European Space Agency to develop a light source for high-frequency metrology applications including atomic clocks and gravity meters. 

“It is a very broad range of applications that we can apply the technology to,” says Smyth.   

NG-PON2

Pilot Photonics is not using its source technology as a comb for an NG-PON2 optical line terminal (OLT) but rather as a directly modulated laser for the customer premises equipment’ optical network unit (ONU). 

“What we have done is develop a wavelength-tunable directly-modulated laser for NG-PON2,” says Smyth. The benefit of its design is that the laser chip meets the stringent specification of the ONU by being tunable, meeting a reach of 40km and enabling sub-$100 designs.

The start-up is engaged with several potential NG-PON2 customers including manufacturers, systems vendors and module makers, and has delivered an evaluation board with the chip to its lead customer.    

Two or three network equipment manufacturers are eager to evaluate the chip

Coherent source

The advantage of the comb source for coherent transmission is that a single chip can replace four or eight distinct lasers, saving packaging, size and cost. 

Smyth estimates that a four-channel comb source is a third of the cost of a design using four single-mode lasers. The power consumption is also less; only one thermo-electric cooler is required instead of four.

Pilot Photonics says that it has demonstrated its four-channel comb-source transmitting over hundreds of kilometres. 

The comb source can be used to send 400-gigabit (100 gigabit/wavelength) and 1-terabit (250 gigabit/wavelength) super-channels. “We’ve done two terabits using 16-QAM on most of the channels and QPSK on the outer ones,” says Smyth.

There are also other system performance benefits using a comb source. There is no need for guard bands to separate between the tones. “You are packing them as tight as can be allowed, the ultimate in spectral efficiency,” he says.     

Smyth also points out that non-linear compensation techniques can be used because the frequency spacings are known precisely. Using non-linear compensation methods benefits reach; the laser source can be launched at higher power and the non-linear effects that result can be compensated for.

Pilot Photonics has shown its sources spaced as close as 6.25GHz to 87.5GHz apart. The start-up also says the tones do not need to be evenly spaced. 

The start-up now has its four-channel comb-source chip on an evaluation board that it is about to deliver to interested systems vendors and large-scale data centre operators. 

“Two or three network equipment manufacturers are eager to evaluate the chip,” says Smyth. “They are less forthcoming as to what they are applying it to.”