Sharp reader catches inconsistencies
I wish to point out several errors in the July 2000 issue of Laser Focus World.
In "Expanding technologies boost available bandwidth," Stephen C. Bennett says, "Most single-mode fiber used in communications networks is dispersion-shifted fiber designed for use at 1550 nm." In Jeff Hecht's article on page 107, he contradicts this by saying, "Zero-dispersion-shifted fibers found only limited applications..." I agree with Mr. Hecht. Conventional dispersion-unshifted (ITU G.652) fiber remains the most widely used single-mode fiber.
Also, in Stephen Bennett's article, he says, "However, by making the zero-dispersion wavelength 1550 nm (that is, nonzero-dispersion-shifted) undesirable nonlinear effects within the fiber...are generated.'' Fibers with zero dispersion at 1550 nm are called dispersion-shifted (G.653) fiber—not "nonzero-dispersion-shifted." The latter (G.655) fibers have zero dispersion wavelength either shorter than or long than 1550 nm as shown in Figure 1 of Jeff Hecht's article.
In Jeff Hecht's article, "Dispersion control boosts high-speed transmission," he says, "For a typical fiber type, that slope is about 0.08 ps/nm2-km near 1550 nm." Although it's not clear what is meant by "typical fiber type," it becomes clear on p. 110, right column, bottom (". . . 0.08 ps/nm2-km of standard fibers. . .") that he is referring to the standard fiber whose dispersion curve is shown in Figure 1. For that fiber, the dispersion slope at 1550 nm is about 0.056 ps/nm2-km—not 0.08! 0.08 is the approximate slope at 1310 nm.
Mr. Hecht also states, "In normal usage, chromatic dispersion is positive when long wavelengths travel faster than shorter ones." Positive chromatic dispersion occurs at wavelengths longer than the fiber's zero dispersion wavelength—exactly as shown in Figure 1. In this region, propagation delay increases as wavelength increases, and long wavelengths travel slower than shorter ones—not faster.
James J. Refi
Distinguished Member of Technical Staff
Norcross, GA 30071
The editor replies:
Thank you for your comments. Jeff Hecht has acknowledged his calculation error and a correction appeared in Laser Focus World, October 2000, p. 36. He also clarified that "typical fiber type" referred to nonzero-dispersion-shifted fiber.
Another reason to leave articles
I just wanted to make a comment concerning Milton Chang's Business Forum article concerning employees taking journal articles with them when they leave an employer (Laser Focus World, Sept. 2000, p. 84).
In addition to the reasons cited by Mr. Chang for leaving the journal articles behind, the articles were probably copied under a CCL (Copyright Clearing House) license, which only covers internal use. The minute the person is no longer a company employee, it would be a violation of copyright law for that person to take the journal articles.
A. Cynthia Weber
Specialty Minerals Inc.
Bethlehem, PA 18017
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