Wavelength division multiplexing (WDM) offers a promising technology to enhance the transmission capacity of fiber-optic communication systems. The technology can be applied not only to wide band trunk systems but also to broadband local networks. In either case, single-mode fibers are expected to be used more than multimode ones, primarily because the single-mode fiber is wide band and becoming more economical. As shown in Fig. 1, both wideband and low-loss characteristics are available only in a limited spectrum range even for a single-mode fiber, and, therefore, it is necessary to multiplex closely spaced lightwaves at the spectrum region where fiber characteristics are optimum. To implement closely spaced WDM, introduction of integration and guided-wave technologies is requisite from the viewpoints of producibility and performance stability. Since modal behaviors in a single-mode fiber are much more well-defined compared with a multimode fiber, integration and guided-wave technologies have affinity with a single-mode fiber.

© 1987 Optical Society of America

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