Long-haul optics refers to the transmission of visible light signals over optical fiber cable for great distances, especially without or with minimal use of repeaters. Normally, repeaters are necessary at intervals in a length of fiber optic cable to keep the signal quality from deteriorating to the point of non-usability. In long-haul optical systems, the goal is to minimize the number of repeaters per unit distance, and ideally, to render repeaters unnecessary.
The main challenge facing developers of long-haul optics (and also the so-called ultra-long-haul or ULH optics) involves the loss inherent in the materials used in fiber optic cable. To some extent this loss can be overcome by increasing the brilliance of the optical signal at the input end, but this brute-force approach has limited practicality. Repeaters can be used to boost the signals at intervals along the transmission route, but this is costly, and the maintenance of such repeaters is difficult in underground or undersea fiber optic cable runs. This has resulted in a quest for the development of super-transparent substances that transmit energy at optical wavelengths with exceptionally low loss.
An indirect contributor to fiber optic cable loss is the fact that all transparent materials transmit energy at slightly different speeds, depending on the wavelength. This is the same effect that causes a prism to split white visible light into its constituent colors, and takes place because the index of refraction is dependent on the wavelength. This is observed as a "smearing-out-over-time" effect in long fiber optic cable runs, unless the energy for each signal can be kept within a narrow range of wavelengths. Engineers have developed schemes such as wave-division multiplexing (WDM) and dense wave-division multiplexing (DWDM) in an attempt to minimize this problem.
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