Researchers at Lucent Technologies' Bell Labs (Holmdel, NJ) may have found a cheaper way of increasing the capacity of multimode fiber (MMF), the existing fiber of choice used inside buildings for high-speed internet connections. The new technique is based on recently developed ideas in the field of wireless communications, in which scattering is used to enhance the information capacity of a transmission system. The experiment demonstrated that the light from two lasers carrying independent bit streams could be combined into one fiber and separated at the receiver using two detectors and a 50-Mbit/s signal processor.
In the optical regime, modal dispersion produces effects analogous to multipath scattering in wireless. This concept can be applied to increase the effective data transmission of MMF, the very effect thought to decrease transmission. Because the new technique works with existing fiber (typically 50 to 60 µm in diameter), and uses lasers of any wavelength, the cost per increase in transmission rate may be less than that of wavelength-division multiplexing (WDM).
The achievable data rates of MMF have been severely limited due to the effects of modal dispersion, even though MMF theoretically has more spatial degrees of freedom than single-mode optical fiber, and therefore more information capacity. In practice, however, there is no simple way to address and detect the hundreds of individual modes of the fiber, nor to address the problem of mode-mixing during propagation.
In the wireless regime, the total received power increases as the number of receiving antennas increases. In an optical system, a fixed amount of optical power arrives at the end of the fiber, and this power is split between several detectors, reducing the average received signal-to-noise ratio. Previously, researchers at Bell Labs demonstrated that a rich multipath scattering environment in wireless systems provides a dramatic capacity enhancement when using multiple antennas at both the transmitter and receiver. The same concepts can be applied to MMF optical transmission systems using multiple lasers and multiple detectors, without wavelength multiplexing, to achieve a linear scaling of the capacity with the number of lasers.
The transmission system consists of N lasers coupled together onto a single multimode fiber and received at the end of this fiber link by M detectors. N independent bit streams are first modulated onto identical radio frequency (RF) carriers, and these electrical signals modulate the N lasers (see Figure). At the receiver, the optical power is split evenly between the M detectors, which convert the optical signals to electrical signals through direct detection. Each detector receives power from all N transmitting lasers. These signals are demodulated by coherent microwave detection, and the resulting M baseband signals (which contain a mix of all the data) are sent to a signal processing circuit that decodes the signal.
The system works because of a process referred to as "modal-coupling diversity." The effect is similar to WDM in that multiple lasers are used in the front end of the system, and multiple detectors on the back end. "Modal-coupling diversity means that each laser couples a bit differently into the fiber," said Bell Labs Scientist Howard R. Stuart. "This property enables multiple signals to be combined at the front of the system and separated at the receiver using signal processing."
"The next step," said Stuart, "is to build a faster signal processor for decoding at rates necessary for optical regimes." Stuart cautions that the exact cost of such a signal processor is unknown. "The cost of doing WDM is replaced by the cost of building the signal processor."
Valerie Coffey-Rosich | Contributing Editor
Valerie Coffey-Rosich is a freelance science and technology writer and editor and a contributing editor for Laser Focus World; she previously served as an Associate Technical Editor (2000-2003) and a Senior Technical Editor (2007-2008) for Laser Focus World.
Valerie holds a BS in physics from the University of Nevada, Reno, and an MA in astronomy from Boston University. She specializes in editing and writing about optics, photonics, astronomy, and physics in academic, reference, and business-to-business publications. In addition to Laser Focus World, her work has appeared online and in print for clients such as the American Institute of Physics, American Heritage Dictionary, BioPhotonics, Encyclopedia Britannica, EuroPhotonics, the Optical Society of America, Photonics Focus, Photonics Spectra, Sky & Telescope, and many others. She is based in Palm Springs, California.