When the F-35 Joint Strike Fighter becomes operational, it will be the world’s most advanced military aircraft with an extensive suite of avionics and weaponry. But the supersonic “network in the sky” does have an Achilles’ heel.
Beneath its stealthy skin, the fifth-generation fighter is dependent upon miles of copper wires that snake their way from the electronics bay in the fuselage into compartments found in the wings, nose and tail. Like veins, they connect subsystem components to computer processors to form an intricate communication network.
The problem is that the copper wires are heavy and susceptible to the extreme environmental conditions of flight operations. Moreover, they do not carry electrical signals efficiently and cannot readily accommodate the introduction of new equipment because of bandwidth limitations.
For those reasons, the Defense Department is slowly making the switch to fiber optics. In fiber, photons, or light energy, transmit the signals.
F-35 program managers at prime contractor Lockheed Martin Corp. are eyeing an emerging technology that would enable them to make the full transition from copper wires to fiber-optic cables.
Some subsystems aboard F-35 communicate via fiber optics already, but the fibers each carry only one digital signal. It would be far more efficient for the fibers to transmit multiple signals.
The Defense Advanced Research Projects Agency and Naval Air Systems Command are jointly funding a program to develop a network of fiber optics that can carry multiple digital and analog signals simultaneously. The development of transmitters and receivers capable of operating in that analog-digital fiber network is key to the effort, which is targeting the multi-billion dollar F-35 and other aircraft programs.
A single fiber can transmit multiple signals via a process called wavelength division multiplexing. By using different wavelengths, or colors, of a laser, any number of signals can travel inside one fiber, said Raj Dutt, chairman and chief executive officer of Culver City, Calif.-based Apic Corp., which is executing the program.
The firm has developed a computer chip that can transmit 32 digital signals simultaneously on a single fiber.
“The chip is what makes it viable for military aircraft,” said Dutt.
At Apic’s clean-room facility in Honolulu, Hawaii, scientists are fabricating silicon photonic components that integrate electronics on the same computer chip. In order for the analog signals to co-exist with digital signals, they developed a semiconductor laser with high power and relatively low intensity noise.
“That was the only way we could replicate what is in existence” in electronics currently, Dutt said.
Shifting to optics-based networks and components would save the Defense Department a significant amount of energy, industry officials said. In coaxial cable systems, the electricity that propagates the radio frequency signals also heats up the copper wires. In photonics, there is no heat generation in the fiber because the information travels by light energy.
“Using photons is an ideal green technology,” said Dutt.
The advent of photonics in military systems has many implications, including a potential savings of billions of dollars during the life cycle of a weapon system, officials said. Some speculate that engineers could replace hundreds or thousands of copper wires with a single thin fiber cable. That would reduce the weight on the aircraft and also diminish power and cooling requirements for onboard processors. Moreover, increasing bandwidth capacity would simply require more wavelengths to be propagated through the fiber rather than adding more cables.
“We’re trying to get photonics to become a staple diet for military systems,” said Dutt. “The technology is now reaching a level of maturity where we see some of these things transitioning into platforms.”
Apic’s prototype digital transmitters and receivers are on their way to Lockheed Martin’s Joint Strike Fighter facility in Fort Worth, Texas, for testing.
“Lockheed Martin is performing studies to use [highly integrated photonics] technology for a future F-35 upgrade. By leveraging the latest developments in waveform division multiplexing, we expect to be able to accommodate far more data on each optical fiber and provide a means to integrate new systems far more efficiently,” said David Jeffreys, senior manager for improvements and derivatives.
Apic at the end of the year will provide analog versions of the chips for system testing. If the technologies prove their muster, JSF program managers could elect to incorporate them into block-four upgrades, which may kick off as soon as 2013, said Jeffreys. Or engineers could include them in offerings for international customers who may want to integrate specific hardware onto the aircraft.
“We would very much like to be able to accommodate and integrate those specific customized subsystems without having to string a bunch of new wires and cables and spend the money to make those changes,” Jeffreys explained. “We’d like to have a [fiber-optic] system in place so that it’s fairly straightforward to add those systems.”
A spokesman for the Defense Department’s F-35 joint program office said that officials were unable to comment on future technologies.