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I read with great interest the article on optical communications by Gordon A. Thomas, David A. Ackerman, Paul R. Prucnal, and S. Lance Cooper (Physics Today, September 2000, page 30). A good portion of the text describes the nonlinear optical loop mirror (TOAD) device, a critical component for very high bit-rate modulation. The nonlinear element in the optical loop is a semiconductor optical amplifier (SOA). However, throughout the article and the figure captions, the authors refer to the SOA as a "silicon optical amplifier." This error is not just a matter of words. SOAs, for fundamental physical considerations based on conservation of energy and momentum, cannot be made from silicon. Furthermore, even if silicon could be used to make an SOA, it would operate in a wavelength range of little interest for optical communications.

Semiconductor optical amplifiers for optical fiber communications are made from gallium indium arsenide phosphide. This material is chosen because the energy and momentum of electrons can be conserved in optical interactions, thus permitting amplification to occur, and its bandgap energy can be adjusted to correspond to the region of photon energies for which the loss of optical fibers is a minimum.

Thomas replies: Tom Pearsall is right about our SOA error, and we're grateful to him. Based on the responses I've received, many teachers and lecturers have also read the article with "great interest," and have been passing on the (now corrected) optical communications information to their students.