Broadband communication services, both wired and wireless, are now widely available in much of the world. Long distance point-to-point delivery of these services is provided. almost universally, using optical fibers. A single fiber, a 100 microns in diameter, can now support data rates in the terabit/s (1012 bit/s) range over distances of 10, 000 km. Consequently fiber optic cables serve as the backbone for the internet and long-distance point-to-point communications. The penetration of fiber-to-the home is also increasing. Fiber has its own special set of channel impairments, such as polarization mode distortion, group velocity dispersion and four wave mixing. The use of different modulation formats to help combat these impairments is an area of active investigation. The implementation of error-correcting techniques that can operate at the high data rates supported by optical fibers is also an area of research, as is the development of new integrated and fiber optic devices.
Wireless communications, including cell-phone and satellite systems, operate primarily at radio or microwave frequencies. On earth, free-space optical communications occupy small niches, such as infrared-based, TV, remote controllers. The most important application of free-space optical communications is for space-based systems, including satellite-to-satellite relays and deep space (i.e., scientific satellite payloads)-to-earth data/telemetry links. Satellite-to-satellite optical relay links in earth orbit have already been demonstrated. In deep space, distances can be enormous (e.g., 132 million km from Earth to Mars) resulting in extremely high diffraction-induced propagation losses. Consequently at radio frequencies, reliable communication over such large distances is not possible at anything but very low data rates. For fixed size antennas these propagation losses, however, decrease as the square of the wavelength, thus making optical transmission the preferred technology. The deep space optical channel differs from the radio wave channel since the noise statistics are Poisson rather than Gaussian. Thus the analysis and design of free-space optical communications systems has its own set of problems distinct from those encountered by systems operating at non-optical frequencies. The development of powerful error-correcting techniques, especially those involving LDPC and Turbo codes, for deep space optical channels is an area of on-going research.
Islam, Mohammed N.
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