Silicon photonics economics set to benefit III-V photonics
Silicon photonics promises to deliver cheaper optical components using equipment, processes and fabrication plants paid for by the chip industry. Now, it turns out, traditional optical component players using indium phosphide and gallium arsenide can benefit from similar economies, thanks to the wireless IC chip industry.
Valery TolstikhinSilicon photonics did a good thing; it turned the interest of the photonics industry to the operational ways of silicon
So argues Valery Tolstikhin, head of a design consultancy, Intengent, and former founder and CTO of Canadian start-up OneChip Photonics. The expectations for silicon photonics may yet to be fulfilled, says Tolstikhin, but what the technology has done is spark interest in the economics of component making. And when it comes to chip economics, volumes count.
“For III-V photonics - indium phosphide and related materials - you have all kinds of solutions, designs and processes, but all are boutique,” says Tolstikhin. “They are not commercialised in a proper way and there is no industrial scale.” The reason for this is simple: optical components is a low-volume industry.
This is what Tolstikhin seeks to address by piggybacking on high-volume indium phosphide and gallium arsenide fabrication plants that make monolithic microwave integrated circuits (MMICs) for wireless.
“To take photonics out of boutique fabs, you need to do some standardisation and move to a fabless model, then you can load the fabs day and night with wafers,” says Tolstikhin. “That is the only way to make a process mature, reproducible and reliable.”
Tolstikhin has spent the last decade pursuing this approach. “The idea is to use something available in indium phosphide which is relatively close to a pure-play foundry.” A pure-play foundry is a fab that makes chips but does not design, market or sell them as its own products.
Tolstikhin’s first involvement was at start-up OneChip Photonics which developed an indium-phosphide platform that used a variety of photonic devices to make photonic integrated circuits (PICs), based on a commercial MMIC process.
The issue with III-V integrated photonics is that to implement different functions - a passive waveguide and a laser, for example - different materials are needed. “What makes a low-loss passive waveguide, does not work for the laser,” says Tolstikhin.
To overcome this, the wafer is repeatedly etched in certain areas, to remove unwanted material, and new layers grown instead with the required material, a process known as selective-area etch and regrowth. This is a complicated and relatively low-yield process that is custom to companies and their fabs, he says: “This is how all commercial lasers and PICs are made.”
In contrast, MMICs using indium phosphide do not need regrowth, simplifying the process considerably. To use a MMIC fab for an optical design, however, it must be developed in a way that avoids the need for regrowth stages.
“At OneChip we believe we did the first commercial laser - not just the laser but the PIC with it - regrowth-free,” says Tolstikhin. “It was made in a MMIC fab, that is the key.”
“To take photonics out of boutique fabs, you need to do some standardisation and move to a fabless model, then you can load the fabs day and night with wafers”
Wafer economics
To understand the relative economics, Tolstikhin compares the number of wafers - wafer starts - processed in silicon, indium phosphide and gallium arsenide.
One large TSMC fab has 400,000 12-inch CMOS wafer starts a year whereas globally the figure is equivalent to some 70 million such wafers a year. For MMICs, one fab Tolstikhin works with has 15,000 4-inch indium phosphide wafer starts a year whereas a large optical component company uses just a couple of thousand 3-inch indium phosphide wafers a year.
“In photonics, the [global] volumes – even for components going into the most massive markets like PON and the data centre interconnects – are still very low,” says Tolstikhin.
Gallium arsenide is somewhere in between: Win’s fab in Taiwan, which makes power amplifiers for wireless and other MMICs, has 250,000 6-inch wafers starts a year, while TriQuint’s fab in USA, with similar product line in wireless, totals 150,000 6-inch wafer starts a year.
Such volumes are not negligible and exceed all the needs of photonics, he says, enabling photonics to make claims similar to those trumpeted for silicon photonics: a mature process with a well-established quality system and, with its volumes, delivers better economics.
Moreover, if applications that currently are based on indium phosphide could be transferred to gallium arsenide, that would give an order of magnitude economies of scale, says Tolstikhin: “One example is mid-reach single-mode optical interconnects with an operating wavelength around 1060 nm, with gallium arsenide used for the transmitter, receiver and transceiver PICs”.
And while the scale of III-V semiconductor manufacturing may still be much lower than CMOS, the up-front cost involved in using a III-V fab is also much less.
Using III-V semiconductors for analogue electronics like the laser /modulator drivers or the trans-impedance amplifier also delivers a speed advantage: heterojunction bipolar transistors (HBTs) in indium phosphide have been demonstrated working at up to 400 GHz, and these, being vertical devices, do not have their speed scaled with lithography. In contrast, CMOS analog electronics is much slower and its device speed is scalable with lithography resolution. A 130 nm CMOS process, the starting point for silicon photonics, cannot support optical components with bit rates beyond 10 Gbps.
Design house
Intengent, Tolstikhin’s company, acts as a bridge between OEMs building optical components and sub-systems and the III-V foundries making photonic chips for them.
He compares Intengent to what application-specific IC (ASIC) companies used to do for the electronic chip industry. Intengent works with the OEM to specify and design the photonic chip based on its system application and then works with the fab to develop and turn the chip into a product by meeting its design rules and process capabilities.
“The aim is that you can go and design within existing fabs and processes something that meets the customer’s application and requirements,” he says.
Tolstikhin is also working with ELPHiC, a Canadian start-up that is raising funding to develop single-mode mid-board optics. The indium-phosphide design combines analogue electronic circuitry with the photonics.
“It appears the best way [to do mid-board optics] is based on electronic and photonic integration onto one substrate and indium phosphide is a natural choice for such a substrate,” he says.
Tolstikhin makes clear he is not against silicon photonics. “It did a good thing; it turned the interest of the photonics industry to the operational ways of silicon: standardised processes, pure-play foundries, device designs separate from the semiconductor physics, and circuit designs separate from the wafer processing.”
As a result, something similar is now being pursued in III-V photonics.
MACOM acquires Mindspeed to boost 100 Gig offerings
Ray MoroneyThe Mindspeed acquisition increases the serviceable addressible market for MACOM, both geographical - the company will strengthen its presence in Asia Pacific - and by gaining new equipment vendor accounts. It also broadens MACOM's 100 Gigabit physical device portfolio.
"We are targeting the 100 Gig buildout and the growth coming from that," says Ray Moroney, product line manager, opto-device business unit at MACOM.
Mindpeed also makes a broad portfolio of crosspoint switches used in datacom equipment, and several families of communications processors.
With the acquisition of Mindspeed we have the full electronics bill-of-materials for CFP2 and CFP4 [module] client-side applications
MACOM entered opto-electronics with the acquisition of Optimai in 2011 that had long-haul and client-side modulator drivers and trans-impedance amplifiers (TIAs). Now with Mindspeed's products, MACOM can capture client-side designs with clock data recovery chips and quad-channel TIAs for 100 Gig modules. "With the acquisition of Mindspeed we have the full electronics bill-of-materials for CFP2 and CFP4 [module] client-side applications," says Moroney.
MACOM also gains silicon germanium technology alongside its indium phosphide and gallium arsenide technologies. Silicon germanium has a lower cost structure once a design is being made in volume production, says Moroney, but the R&D and mask costs are generally higher. Silicon germanium also allows significant integration. "It is BiCMOS in nature," says Moroney. "You can integrate full CMOS functionality into a design too." For example digital control can be added alongside analogue functions. Moroney says the company will use silicon germanium for high-performance analogue designs like TIAs as well as high-frequency millimeter wave and microwave applications.
The company is considering its options regarding the future of the communications processors arm of Mindspeed's business. "MACOM is very much an analogue/ RF company," says Moroney. "It [communications processors] is certainly not seen as a core area of investment for MACOM."
u2t Photonics pushes balanced detectors to 70GHz
- u2t's 70GHz balanced detector supports 64Gbaud for test and measurement and R&D
- The company's gallium arsenide modulator and next-generation receiver will enable 100 Gigabit long-haul in a CFP2
"The performance [of gallium arsenide] is very similar to the lithium niobate modulator"
Jens Fiedler, u2t Photonics
u2t Photonics has announced a balanced detector that operates at 70GHz. Such a bandwidth supports 64 Gigabaud (Gbaud), twice the symbol rate of existing 100 Gigabit coherent optical transmission systems.
The German company announced a coherent photo-detector capable of 64Gbaud in 2012 but that had an operating bandwidth of 40GHz. The latest product uses two 70GHz photo-detectors and different packaging to meet the higher bandwidth requirements.
"The achieved performance is a result of R&D work using our experience with 100GHz single photo-detectors and balanced detector technology at a lower speed,” says Jens Fiedler, executive vice president sales and marketing at u2t Photonics.
The monolithically-integrated balanced detector has been sampling since March. The markets for the device are test and measurement systems and research and development (R&D). "It will enable engineers to work on higher-speed interface rates for system development," says Fiedler.
The balanced detector could be used in next-generation transmission systems operating at 64 Gbaud, doubling the current 100 Gigabit-per-second (Gbps) data rate while using the same dual-polarisation, quadrature phase-shift keying (DP-QPSK) architecture.
A 64Gbaud DP-QPSK coherent system would halve the number of super-channels needed for 400Gbps and 1 Terabit transmissions. In turn, using 16-QAM instead of QPSK would further halve the channel count - a single dual-polarisation, 16-QAM at 64Gbaud would deliver 400Gbps, while three channels would deliver 1.2Tbps.
However, for such a system to be deployed commercially the remaining components - the modulator, device drivers and the DSP-ASIC - would need to be able to operate at twice the 32Gbaud rate; something that is still several years out. That said, Fiedler points out that the industry is also investigating baud rates in between 32 Gig and 64 Gig.
Gallium arsenide modulator
u2t acquired gallium arsenide modulator technology in June 2009, enabling the company to offer coherent transmitter as well as receiver components.
At OFC/NFOEC 2013, u2t Photonics published a paper on its high-speed gallium arsenide coherent modulator. The company's design is based on the Mach-Zehnder modulator specification of the Optical Internetworking Forum (OIF) for 100 Gigabit DP-QPSK applications.
The DP-QPSK optical modulation includes a rotator on one arm and a polarisation beam combiner at the output. u2t has decided to support an OIF compatible design with a passive polarisation rotator and combiner which could also be integrated on chip. The resulting coherent modulator is now being tested before being integrated with the free space optics to create a working design.
"The performance [of gallium arsenide] is very similar to the lithium niobate modulator," says Fiedler. "Major system vendors have considered the technology for their use and that is still ongoing."
The gallium arsenide modulator is considerably smaller than the equivalent lithium niobate design. Indeed u2t expects the technology's power and size requirements, along with the company's coherent receiver, to fit within the CFP2 optical module. Such a pluggable 100 Gigabit coherent module would meet long-haul requirements, says Fiedler.
The gallium arsenide modulator can also be used within the existing line-side 100 Gigabit 5x7-inch MSA coherent transponder. Fiedler points out that by meeting the OIF specification, there is no space saving benefit using gallium arsenide since both modulator technologies fit within the same dimensioned package. However, the more integrated gallium arsenide modulator may deliver a cost advantage, he says.
Another benefit of using a gallium arsenide modulator is its optical performance stability with temperature. "It requires some [temperature] control but it is stable," says Fiedler.
Coherent receiver
u2t's current 100Gbps coherent receiver product uses two chips, each comprising the 90-degree hybrid and a balanced detector. "That is our current design and it is selling in volume," says Fiedler. "We are now working on the next version, according to the OIF specification, which is size-reduced."
The resulting single-chip design will cost less and fit within a CFP2 pluggable module.
The receiver might be small enough to fit within the even smaller CFP4 module, concludes Fiedler.
OIF promotes uni-fabric switches & 100G transmitter
The OIF's OTN implementation agreement (IA) allows a packet fabric to also switch OTN traffic. The result is that operators can now use one switch for both traffic types, aiding IP/ Ethernet and OTN convergence. Source: OIF
Improving the switching capabilities of telecom platforms without redesigning the switch as well as tinier 100 Gigabit transmitters are just two recent initiatives of the Optical Internetworking Forum (OIF).
The OIF, the industry body tackling design issues not addressed by the IEEE and International Telecommunication Union (ITU) standards bodies, has just completed its OTN-over-Packet-Fabric protocol that enables optical transport network (OTN) traffic to be carried over a packet switch. The protocol works by modifying the line cards at the switch's input and output, leaving the switch itself untouched (see diagram above).
In contrast, the OIF is starting a 100 Gigabit-per-second (Gbps) transmitter design project dubbed the integrated dual-polarisation quadrature modulated transmitter assembly (ITXA). The Working Group aims to expand the 100Gbps applications with a transmitter design half the size of the OIF's existing 100Gbps transmitter module.
The Working Group also wants greater involvement from the system vendors to ensure the resulting 100 Gig design is not conservative. "We joke about three types of people that attend these [working group] meetings," says Karl Gass, the OIF’s Physical and Link Layer Working Group vice-chair. "The first group has something they want to get done, the second group has something already and they don't want something to get done, and the third group want to watch." Quite often it is the system vendors that fall into this third group, he says.
OTN-over-Packet-Fabric protocol
The OTN protocol enable a single switch fabric to be used for both traffic types - packets and time-division multiplexed (TDM) OTN - to save cost for the operators.
"OTN is out there while Ethernet is prevalent," says Winston Mok, technical author of the OTN implementation agreement. "What we would like to do is enable boxes to be built that can do both economically."
The existing arrangement where separate packet and OTN time-division multiplexing (TDM) switches are required. Source: OIF
Platforms using the protocol are coming to market. ECI Telecom says its recently announced Apollo family is one of the first OTN platforms to use the technique.
The protocol works by segmenting OTN traffic into a packet format that is then switched before being reconstructed at the output line card. To do this, the constant bit-rate OTN traffic is chopped up so that it can easily go through the switch as a packet. "We want to keep the [switch] fabric agnostic to this operation," says Mok. "Only the line cards need to do the adaptations."
The OTN traffic also has timing information which the protocol must convey as it passes through the switch. The OIF's solution is to vary the size of the chopped-up OTN packets. The packet is nominally 128-bytes long. But the size will occasionally be varied to 127 and 126 bytes as required. These sequences are interpreted at the output of the switch as rate information and used to control a phase-locked loop.
Mok says the implementation agreement document that describes the protocol is now available. The protocol does not define the physical layer interface connecting the line card to the switch, however. "Most people have their own physical layer," says Mok.
100 Gig ITXA
The ITXA project will add to the OIF's existing integrated transmitter document. The original document addresses the 100 Gigabit transmitter for dual-polarisation, quadrature phase-shift keying (DP-QPSK) for long-haul optical transmission. The OIF has also defined 100Gbps receiver assembly and tunable laser documents.
The latest ITXA Working Group has two goals: to shrink the size of the assembly to lower cost and increase the number of 100Gbps interfaces on a line card, and to expand the applications to include metro. The ITXA will still address 100Gbps coherent designs but will not be confined to DP-QPSK, says Gass.
"We started out with a 7x5-inch module and now there is interest from system vendors and module makers to go to smaller [optical module] form factors," says Gass. "There is also interest from other modulator vendors that want in on the game."
The reduce size, the ITXA will support other modulator technologies besides lithium niobate that is used for long-haul. These include indium phosphide, gallium arsenide and polymer-based modulators.
Gass stresses that the ITXA is not a replacement for the current transmitter implementation. "We are not going to get the 'quality' that we need for long-haul applications out of other modulator technologies," he says. "This is not a Gen II [design].
The Working Group's aim is to determine the 'greatest common denominator' for this component. "We are trying to get the smallest form factor possible that several vendors can agree on," says Gass. "To come out with a common pin out, common control, common RF (radio frequency) interface, things like that."
Gass says the work directions are still open for consideration. For example, adding the laser with the modulator. "We can come up with a higher level of integration if we consider adding the laser, to have a more integrated transmitter module," says Gass.
As for wanting great system-vendor input, the Working Group wants more of their system-requirement insights to avoid the design becoming too restrictive.
"You end up with component vendors that do all the work and they want to be conservative," says Gass. "The component vendors don't want to push the boundaries as they want to hit the widest possible customer base."
Gass expects the ITXA work to take a year, with system demonstrations starting around mid-2013.