Silicon photonics: "The excitement has gone"

The opinion of industry analysts regarding silicon photonics is mixed at best. More silicon photonics products are shipping but challenges remain.

Part 1: An analyst perspective

"The excitement has gone,” says Vladimir Kozlov, CEO of LightCounting Market Research. “Now it is the long hard work to deliver products.” 

Dale Murray, LightCounting

However, he is less concerned about recent setbacks and slippages for companies such as Intel that are developing silicon photonics products. This is to be expected, he says, as happens with all emerging technologies.

Mark Lutkowitz, principal at consultancy fibeReality, is more circumspect. “As a general rule, the more that reality sets in, the less impressive silicon photonics gets to be,” he says. “The physics is just hard; light is not naturally inclined to work on the silicon the way electronics does.”

LightCounting, which tracks optical component and modules, says silicon photonics product shipments in volume are happening. The market research firm cites Cisco’s CPAK transceivers, and 40 gigabit PSM4 modules shipping in excess of 100,000 units as examples. Six companies now offer 40 gigabit PSM4 products with Luxtera, a silicon photonics player, having a healthy start on the other five.

Indium phosphide and other technologies will not step back and give silicon photonics a free ride

LightCounting also cites Acacia with its silicon photonics-based low-power 100 and 400 gigabit coherent modules. “At OFC, Acacia made a fairly compelling case, but how much of its modules’ optical performance is down to silicon photonics and how much is down to its advanced coherent DSP chip is unclear,” says Dale Murray, principal analyst at LightCounting. Silicon photonics has not shown itself to be the overwhelming solution for metro/ regional and long-haul networks to date but that could change, he says.

Another trend LightCounting notes is how PAM-4 modulation is becoming adopted within standards. PAM-4 modulates two bits of data per symbol and has been adopted for the emerging 400 Gigabit Ethernet standard. Silicon photonics modulators work really well with PAM-4 and getting it into standards benefits the technology, says LightCounting. “All standards were developed around indium phosphide and gallium arsenide technologies until now,” says Kozlov.

You would be hard pressed to find a lot of OEMs or systems integrators that talk about silicon photonics and what impact it is going to have 

Silicon photonics has been tainted due to the amount of hype it has received in recent years, says Murray. Especially the claim that optical products made in a CMOS fabrication plant will be significantly cheaper compared to traditional III-V-based optical components. 

First, Murray highlights that no CMOS production line can make photonic devices without adaptation. “And how many wafers starts are there for the whole industry? How much does a [CMOS] wafer cost?” he says. 

“You would be hard pressed to find a lot of OEMs or systems integrators that talk about silicon photonics and what impact it is going to have,” says Lutkowitz. “To me, that has always said everything.”  

Mark Lutkowitz, fibeReality LightCounting highlights heterogeneous integration as one promising avenue for silicon photonics. Heterogeneous integration involves bonding III-V and silicon wafers before processing the two.

This hybrid approach uses the III-V materials for the active components while benefitting from silicon’s larger (300 mm) wafer sizes and advanced manufacturing techniques.

Such an approach avoids the need to attach and align an external discrete laser. “If that can be integrated into a WDM design, then you have got the potential to realise the dream of silicon photonics,” says Murray. “But it’s not quite there yet.”

This poses a real challenge for silicon photonics: it will only achieve low cost if there are sufficient volumes, but without such volumes it will not achieve a cost differential

Murray says over 30 vendors now make modules at 40 gigabit and above: “There are numerous module types and more are being added all the time.” Then there is silicon photonics which has its own product pie split. This poses a real challenge for silicon photonics: it will only achieve low cost if there are sufficient volumes, but without such volumes it will not achieve a cost differential.

“Indium phosphide and other technologies will not step back and give silicon photonics a free ride, and are going to fight it,” says Kozlov. Nor is it just VCSELs that are made in high volumes.

LightCounting expects over 100 million indium phosphide transceivers to ship this year. Many of these transceivers use distributed feedback (DFB) lasers and many are at 10 gigabit and are inexpensive, says Kozlov. 

For FTTx and GPON, bi-directional optical subassemblies (BOSAs) now cost $9, he says: “How much lower cost can you get?”  

Comments

[John P]

An interesting thought about heterogeneous integration. However, no CMOS fab would ever process wafers with III-V materials bonded to the surface due to lack of compatibility and contamination issues. Thus, you are then forced to use a 3-5 fab for this SiP approach. All cost and other benefits of SiP fall through. So, how does this make sense? At least with pure SiP, light sources can be kept far away, or pick and placed.

[Roy Rubenstein]

Having spoken to heterogenous players, they say III-V processing is being done by several silicon fabs. Not all foundries accept III-V materials but several do.

There is also a suggestion that leading chip companies are looking at InGaAs for their sub-7nm CMOS node processes. If the reports are accurate, silicon fab using III-V materials may be more common in future.