By Karl Hill
Researchers solve key problem in laser communication technology
New Mexico State University researchers have developed a new and effective method for overcoming one of the major obstacles encountered by free-space laser communication systems – the signal interference caused by atmospheric turbulence.
Free-space laser communication can be thought of as “wireless” optical communication, said Michael Giles, professor emeritus of electrical engineering. Unlike fiber-optic systems, in which an optical signal travels along a fiber from a transmitter to a receiver, free-space laser communication uses a laser beam to transmit the signal through the atmosphere.
The technology’s advantages over radiofrequency communication, including high data rates and security, make it attractive for purposes such as sending live video signals from an unmanned aircraft to a station on the ground, among other applications. But a number of practical problems need to be solved, including the signal loss caused by atmospheric turbulence.
With funding from the U.S. Air Force Office of Scientific Research, a group of NMSU researchers is working on several fronts to solve those problems. Giles and Qingsong Wang, a graduate student who just completed his doctoral degree in electrical engineering, have devised a method for altering a laser beam in a way that greatly reduces the interference that occurs when the beam passes through turbulence.
Their solution involves reducing the coherence of the laser beam by transmitting the beam through a combination of a singlemode fiber followed by a multimode fiber bundle, Wang said.
The method, protected by a patent application, dramatically reduces signal error rates in laboratory tests in which a laser beam is transmitted through artificial turbulence, Giles said. For the next stage of the project, researchers have built a transmitter and receiver for testing the performance when the beam is transmitted a few hundred yards through actual atmospheric conditions.
“We have no reason to believe it will not work as well transmitting through the real atmosphere,” Giles said. “Then we will move on to longer paths, and to incorporating the fiber bundle into the transmitter system.”
The method overcomes a basic problem in free-space laser communication, caused when atmospheric turbulence accentuates the speckle pattern that occurs in highly coherent laser beams, he said.
“Normally in a laser with high coherence, parts of the beam will interfere with each other and create a speckle pattern,” he said. “If you transmit through an atmospheric path, you are going to accentuate the speckle pattern and you will have areas of darkness and areas of brightness that move around due to the turbulence.”
That would not be a problem if the signal receiver could collect the whole beam, he said. But because the beam expands over distance, to a meter or more in diameter, the wandering dark spots in the beam can cause unacceptable signal dropouts.
“It’s the same effect you see when you look at a star and it twinkles,” Giles said. “A star is a coherent source of light because it is so far away. As the light passes through atmospheric turbulence, it tends to interfere with itself, and that’s why it will go darker and lighter, darker and lighter.”
For high-speed data transmission by laser beam, the “twinkle” means loss of signal. Researchers have known that a partially coherent laser beam performs better through turbulence by reducing this interference. But previous methods of generating a partially coherent beam have had serious drawbacks for transmitting data.
“They’re not quick enough,” Giles said. For instance, one method uses a rotating diffuser to reduce the beam’s coherence, but it can’t rotate fast enough to allow for high-speed communications.
“For this kind of communication, a bit of information lasts for a billionth of a second,” he said.
“The Air Force tasked us to try to find something new, and that’s what Qingsong did,” Giles said. “He went right to this winning combination, the single-mode fiber and then the bundle. No one else has used the fiber in this way.”
The NMSU research, funded for a total of $1 million by the Air Force, is in the third year of a five-year contract.
Although the NMSU group has discovered a promising way to reduce the coherence of the transmitted laser beam and improve the performance of the link, one of the important remaining questions is: What is the best or optimum beam for a given situation? A link through a turbulent atmosphere may do better with a beam with less coherence while a link through a quiet atmosphere may do better with more coherence.
Adjusting the beam parameters to achieve the best throughput is a possibility that an NMSU group led by David Voelz of the Klipsch School of Electrical and Computer Engineering is studying. Their goals: knowing the optimal coherence parameters and being able to control the beam to reach them.
In addition to the military applications the Air Force is interested in, free-space laser communication has potential applications in medicine and other fields, Giles and Wang said.
“In medicine, sometimes they need to transmit beams through turbid media or even through tissue,” Giles said. “This same kind of effect happens if you are trying to image through tissue.”
For military communications, free-space laser systems offer the advantages of high bandwidth and greater security, Giles said.
“It’s more highly directional than radiofrequency communication, which sends the signal in all directions,” he said. “Not everybody is going to be able to receive it – only the one you’re pointing to.”