Sometimes total electrical isolation is a good thing — and that’s the idea behind a power-over-fiber (PoF) communications cable being developed by Sandia engineers.
“It’s common to isolate communications between systems or devices by using fiber optic cables, but if power is required, then sending power down a copper wire can at times be a safety-critical issue, and substituting it with battery power may not be suitable or practical,” says Steve Sanderson (6623).
He, Titus Appel (6623), and Walter Wrye, a former intern, are co-inventors of a patent-pending hybrid cable design that uses fiber to send and regulate optical power to the communications electronics integral to the cable.
The cable ends resemble a typical copper electrical cable with pin and socket connectors. However, optical interface circuits integrated into the connector housing, or backshell, provide fiber optic transmission of both data communications and optical power. To conserve energy, optical power is delivered only on demand, Steve says.
“The key issue here is to maintain total electrical isolation from any stray electrical energy and high-voltage electrical surges caused by such things as lightning strikes,” he says.
The developers envision the PoF cable replacing existing copper cables in areas related to safety, such as security, explosives, explosion-proof environments, aviation, and medical devices.
“The first-generation PoF cable just delivers optical power to the cable’s internal electronics for data communication between devices. We are now adding the capability to deliver electrical power externally to a connected low-power device,” Steve says.
In the PoF cable’s current version, the backshell encapsulates circular stacked circuit boards with LEDs coupled to plastic optical fibers for communications, and a laser diode and miniaturized photovoltaic-type cell coupled to the ends of a single glass fiber for optical power delivery. The backshell is then filled with thermally conductive material to keep the laser diode cool during peak demands for power.
Working on Gen-2 version
In the next version, the team plans to use only glass fibers. “Although plastic fiber requires less preparation time than glass, it takes up more room,” Steve says. The team also is working with next-generation microcontrollers, new packaging layouts, and new optical devices to reduce the size.
The team recently tested a PoF low-energy detonator firing cable with fireset electronics built into the backshell. The optically powered fireset embeds a microcontroller that reports such things as detonator resistance, temperature, and charging voltages, and receives command messages to fire the detonator. When it’s idle or powered down, the circuitry is designed to short the detonator input leads to prevent unwanted electrical energy from reaching it.
Steve came up with the concept about three years ago after being given the challenge by a project needing total electrical isolation between communication devices to meet safety requirements.
The effort currently is being funded to develop a rugged PoF cable 3 to 4 meters long and production ready.
“Our customer requested this length and a data rate greater than 20 kbit per second, but you could go much greater distances at lower data rates without increasing power levels,” Steve says. “Or, by increasing the power levels, data rates could be the same or higher.”
The team built a prototype, fine-tuned the packaging, and began figuring out the process for mass production.
Now, they’re working to reduce the backshell’s length from 4 inches to 2.5 inches, as well as decrease the weight and lower costs.
“One of our ongoing objectives is to reduce the physical size so that it’s more widely used,” says Steve, who has seen technology packaging shrink dramatically in the 35 years he’s been with Sandia.
“The PoF cable has power limitations,” Steve says. “It’s not to be construed as a means to power your house, for example, or handle the high speeds of a computer network.
“But because there are growing needs of low-power sensor/control applications related to safety, having convenient optically generated power available is a tremendous benefit.”