Photonic power seeks new frontiers

You might think that by Googling "photonic power", you d find references to exotic weaponry of the like found on board the USS Enterprise and fired at the command of Captain Kirk. You d be wrong, though. What Google actually throws up is a lot of links to details of a technology whose main application is far more prosaic.

Photonic power has been around for 10 years now, with more than 10,000 system deployments. Many of these are at electric utilities companies, where the technology provides a useful way to help measure the very high current delivered along power transmission lines.

The basic idea is to provide power to the measurement system photonically rather than electrically. Power can be delivered via an optical fiber instead of a regular copper cable. For utilities companies, the main advantage is a significant reduction in the weight and size of the measurement system.

Like any photonic system, it relies on three components. One to emit the light, a second to transport it over the required distance and a third to collect photons. In a photonic power system, the key element is the light collector and this is the III-V device that has been developed and tailored to a specific function. That s because the requirements are somewhat different from both conventional detector and solar-cell technology.

To be a useful detector, the photonic component only has to convert light into electrons to produce a measurable current. Because power also relies on voltage, the make-up of the light collector has to be adapted from the simple detector structure. And whereas a solar cell is designed to respond over a wide spectral range, a photonic power converter has to concentrate only on the very narrow part of the spectrum in which the laser or LED light source is operating.

The chief exponent of photonic power is Jan-Gustav Werthen, an engineer with a background in the development of multi-junction solar cells at Varian before it exited the semiconductor business. After that, Werthen set up his company Photonic Power – at the time a two-person operation involving just the engineer and his wife.

Last year, JDSU acquired Photonic Power. "I think that this is the best thing I ve ever done," Werthen said of the deal. "It s just terrific. We re a very innovative force within JDSU and we ve been well received. I can see a long list of good things going forward here."

Werthen explains that most of the systems already deployed are used to power sensors in environments that are either not suited to power delivery via copper cables, or where very bulky equipment would be required. "For an electric power utility application, you want to measure a current of maybe 1000–3000 A and a voltage of 100–500 kV," he said. "The normal way to do that would be to use an instrument transformer to go from a high current and high voltage to almost no voltage and a very low current."

"The alternative that we provide to systems integrators like Siemens is for this very-high-end application. The advantages are that they get a lightweight system of less than 50 kg replacing something very bulky, of the order of 2000 kg."

With a consumption of only 50–150 mW, sufficient power can be provided very comfortably by a 0.5 W laser and the converter, which has a typical efficiency of around 30%.

"Fiber-optic [power] installations happen in places like China, India and South America," said Werthen, explaining that it is the relatively new power transmission networks in the developing world that provide the biggest market opportunity because the lower transmission voltage of 100 kV is increasingly used in these geographies. He estimates the total available market to be in the region of hundreds of thousands of systems per year, which translates to an annual market value for systems worth more than a billion dollars.

Since the JDSU acquisition, the number of applications being targeted is increasing to include electromagnetic field sensing. "People do care about the electromagnetic fields that are emitted from a hairdryer, a radio next to your bed, or a cell phone," Werthen said. "These industrial sensors have hitherto been battery powered, but it can be a hassle to change the batteries."

That application might require as many as 50,000 units per year over the next four to five years, estimated Werthen. "It s considerably smaller than the utilities market, but on the other hand it is a sizeable market for anything involving lasers in the 0.5–1 W range."

The third market that has emerged over the last couple of years is in the field of medicine and has arisen because of the increasing use of MRI scanners. Because these scanners rely on very high magnetic fields of 3 Tesla or more, metal components such as copper wires are not welcome inside the imagers, so doctors need to find an alternative way to power sensors used to monitor patients. "The value proposition is that fiber cannot only power the sensors, but also send information back," Werthen said, "and you can add more channels to increase the resolution and sensitivity of the system."

The smallest of the three current commercial markets, photonic power systems could in theory be deployed in the installed global base of around 15,000 MRI scanners and expand as their use continues to grow.

It is the details of each specific application that determine the types of device and materials that Werthen and colleagues design into their components and systems.

Around 90% of deployed systems are designed to operate near 810 nm, because this is where sufficiently powerful lasers can be found at the cheapest prices. The power converter chip used for this wavelength is based on AlGaAs/GaAs. If the power requirement for a given application is higher than normal, the preferred solution shifts to a longer wavelength. "If you go to 940 nm then you have a plethora of lasers at 5 or 10 W available, so it becomes a question of power."

One such high-power application could be found in the wireless infrastructure sector. "Many of the existing antenna applications are pretty power hungry, so you might need a Watt or more out of the converter," explained Werthen. "This translates to 3 W or more on the laser side." Shifting to this longer wavelength demands a change in the converter chip material to an InGaP compound mismatched onto a GaAs substrate.

The third option is reserved for "long haul" applications, where the sensor is to be powered from a remote location. In these cases the best option is to use singlemode lasers in the 1310–1550 nm region, and a converter featuring InGaAs or InGaAlP on InP.

Whatever the specifics of the application, the physical appearance of the converter remains the same: "If you think of a pizza, that s what our converter looks like," said Werthen. "The pizza box is a 1 × 1 mm or 2 × 2 mm square, and the active area inside it is round and has been sliced up into segments."

Epitaxial growth is by MOCVD on a semi-insulating substrate and the segment boundaries are etched down to isolate each p-n junction. "Then you have to use proprietary technology to access the p- and n-side and re-connect them back in series. It s somewhere between an IC and a simple photodiode."

Mostly performed on 3 inch wafers, the process to manufacture the converter features around a dozen individual steps. It is well developed and not something that Werthen is particularly tempted to tamper with, mainly because the conservative electric utilities industry is focused largely on reliability. But work to improve conversion efficiency continues, and 50% has been demonstrated in experiments.

Converter chip fabrication has now switched to JDSU s headquarters in Milpitas, CA. Being under the JDSU umbrella also means that the parent company s lasers are used in the bulk of the systems. JDSU s acquisition suggests a belief that photonic power is an application that can drive plenty of volume through the parent company s III-V wafer fab in the future.

Werthen certainly envisages a couple of application areas that could deliver this. "If you think of a situation where everybody has a wireless [internet] connection, then eventually there s going to be a need for fiber going to the point of that wireless transmitter," he reckons. "At that point, needing to use both a copper wire for power and a fiber for data is cumbersome and impractical."

"You could have just one fiber going to the point of the transmitter to provide both the power and the data. There are indications that there will be such a scenario, because everybody is going to demand the high bandwidth," Werthen said.

The other possibility that Werthen foresees is power for an electro-mechanical switch that could be used to re-route access to a passive optical network. "If it gets cut then you want to be able to switch to another one quickly. You might want to have a switch out by the passive optical network, but where do you get the power to drive it?"

Werthen admits that this futuristic application is somewhat speculative, but dreaming up such ideas are all part of the evangelistic approach that he is adopting in a bid to educate both the market place and the technical community within the semiconductor business to the potential of photonic power.