 second, or 0.001 petabits per second). First, there are several limitations such as source spectra, multiplexer filter technique, modulation- induced effects such as frequency chirp and mode-partition noise, and fiber dispersion characteristics.1 Secondly, the available fiber bandwidth can be estimated from looking at wavelength regions having low loss (less than 0.5 dB/km or so), such as from 1.27 to 1.35 um and from 1.48 to 1.60 um. These regions provide a total wavelength band of 200 nm and correspond to a fre- quency bandwidth of about 29 terahertz yet it is used to support the few (dozen) noncoherent channels. Coherent transmission can provide many more channels. Assuming a 10-bits-per-hertz modulation scheme, the 29 terahertz could support 0.3 petabits per second (0.3x1015 bps). 2.4.2 Coherent Transmission The available fiber bandwidth could be used more efficiently if coherent transmission (full-frequency modulation) is used. The optical frequencies could be down-converted to lower radio frequencies and processed electron- ically with existing technology for improved sensitivity, filtering, and equal- ization. This, however, requires several stringent design considerations: a laser with a very narrow line width or single-frequency spectrum and single longitudinal mode, very high frequency and phase stability, automatic polar- ization correction, automatic frequency control of the source and of the local receiver laser, and SO on. Because of these stringent design requirements, coherent transmission methods have not yet seen commercial deployment. Optics Primer 2.5 Design Parameters This section summarizes some design parameters for typical telecommuni- cation applications.10 2.5.1 Attenuation Coefficient Versus Wavelength For single-mode fibers, the coefficient between 1.285 and 1.330 um should not exceed that at 1.31 um by more than 0.1 dB/km. This is to provide a low- loss transmission window. 2.5.2 Water Peak Attenuation The coefficient at 1.38 um should not exceed an objective of 3 dB/km to con- trol 