Trying to quantify the data flow is hard; industry analyst IDC has predicted the amount of digital information produced in 2020 will be 35,000 exabytes (1018) – 44 times as much as in 2009. Put another way, that’s close to 100Ebyte of data per day.
How to handle all this data is a challenge being addressed by a range of companies and organisations. Fibre optics is the obvious solution. Originally used for backbone communications, fibre is getting ever closer to the end user and some can now take advantage of fibre to the home to access very high speed internet access. But one of the reasons why fibre often remains ‘just out of reach’ is its cost.
However, a US company believes it has developed a way to create the optical components needed in these systems at lower cost. California based Cosemi says the combination of its innovative technology and a fabless manufacturing approach can deliver cost and performance improvements that will help to make fibre optics more widely available. In fact, the company claims it is ‘on a mission’.
Chief executive Nguyen X Nguyen said the company has been working toward this goal since its inception in 2006. “Our goal was always to expand the applications that can take advantage of fibre optics. By leveraging our line up of high speed photo detectors and innovative packaging capabilities, we have found a way to do that. Cosemi is focused on making this technology available to a wider audience, while still serving high end markets such as datacom/telecom – and further expanding into data centres.”
Nguyen did a PhD at the University of California, Santa Barbara, working on high speed semiconductors, including GaN power amps for mobile phones. He said a lot of optical products have spawned from research at places like Bell Labs. “Optoelectronics is one of the younger technologies in the electronics industry,” he contended, “and, over the years, was going to be the ‘next big thing’. But a lot of ideas were good in the lab, but not suitable for high volume production.”
And it is this latter goal which Cosemi has in its sights. “Gigabit I/O is the next big thing,” Nguyen continued, “but how can we do it at low cost?”
Cosemi started by looking at components where it believed it could add value. “We want to make optical components in a way more like electronic devices are made and one of the first things we addressed was an optical receiver.”
When Cosemi started to sell products, it faced questions such as ‘can your products really do what you say?’ and ‘don’t you need materials processing?’. “Once we got over those questions,” Nguyen said, “things started to move in our favour because we were building products on 4in III-V wafers. We got economy of scale because one 4in wafer could yield 100,000 photodetectors.”
These photodiodes are said to be suited to high bandwidth applications using long and short wavelengths, featuring high responsivity and low dark current. The devices are supplied as known good die – all parts are tested and guaranteed – allowing the customer to package them appropriately. To date, more than 30million of these have been shipped.
“We went to volume production in competition with companies like Hamamatsu,” Nguyen continued. “We also talk with companies like Finisar, who are involved in make versus buy decisions.”
In his view, packaging is one of the bigger problems facing those in the optoelectronics sector. This not only addresses issues such as component size, but also usability and cost. “We’re looking to bring optical to the consumer segment and our customers are looking for low cost,” he pointed out. “We had to work out how to provide something new and the way forward was to move to a MEMS based solution, combining this with wafer level packaging.”
He noted that some potential data centre customers can’t always handle chip level solutions. “That means they need a packaged product, so we are also working on complete ‘plug and play’ solutions for these applications. Optical sub assemblies need the right semiconductor and and the right package; something I think the industry hasn’t been able to do.”
Looking to meet various form factor requirements, Cosemi offers transmitter, receiver and bidirectional optical sub assemblies – TOSA, ROSA and BOSA, respectively.
Amongst these products is a BOSA for gigabit passive optical network (GPON) applications. The device integrates a receiver featuring a 2.5Gbit/s InGaAs PIN photodiode and an auto gain control transimpedance amplifier (TIA) as the receiver and a 1.25Gbit/s 1310nm distributed feedback laser as the transmitter. According to the company, by packaging the PIN photodiode with the TIA, it can offer a cost effective, high performance device.
Nguyen also noted that, because Cosemi has a fabless business model, it is looking to develop products with volume potential. “Customers are looking for low cost, which drives them towards us,” he asserted.
MEMS is being used at the wafer level, although it’s not MEMS as it’s generally accepted. “We create a piece of silicon with a through silicon via,” he said. “The chip is flipped to present a photodiode area on the back. The chip is mounted on a PCB and the optical fibre is then pushed through holes in the board which allow both elements to align precisely. It’s a light engine,” he claimed.
The advantage of this approach is to reduce packaging volume. “Four fibre pairs would make things bulky and you’d need to align four fibres precisely with four photodiodes. This approach can be used to replace the TOSA/ROSA/BOSA package with something smaller.”
Cosemi has recently received a US Patent which, it says, will be the foundation of a platform intended to provide high speed connectivity for consumer applications ranging from 4K ultra high definition television to Gigabit I/O communications between mobile devices. These products will be available at a ‘significantly lower’ cost than competitive approaches, the company claims.
“This patent places Cosemi in a leading position to provide a variety of products, such as active HDMI cables, which are increasingly needed for HD/UHD video connections and especially for long distances and high performance resolutions, like 4K and 8K,” Nguyen said. “The hybrid nature of this technology platform enables Cosemi to produce very high speed active cables without external power.”
The architecture uses fibre optic cables to transmit data at multi gigabit rates, but there are also copper wires to carry power and perform other functions.
“When we look to integrate,” he continued, “we need to power the chip and therefore need something to carry the power. You can’t do that over fibre, so copper not only carries the power, but also provides mechanical stability.”
Now, Cosemi is looking at more aggressively packaged products. “We need to explore 3D wafer packaging,” Nguyen concluded, “and we are working on a 16 x 25 array capable of handling 400Gbit/s. Beyond that, we want to apply the same approach to single mode fibre to support longer distance connectivity.”
