Optical frequency control for wavelength multiplexed systems

The capacity of a fiber-optic transmission system can be increased dramatically by optically combining light at different wavelengths from several sources onto a single fiber. The technique is analogous to the current practice of frequency division multiplexing in electrical transmission systems. Unfortunately, while the linewidth of semiconductor laser sources is very small, uncertainties resulting from thermal drift, manufacturing variation, and other effects require a large window for each wavelength. Thus, existing technology allows only relatively poor use of the available fiber transmission bandwidth. This paper will describe limitations on wave-length-multiplexed system channel capacity, which result from instabilities of currently available sources and characteristics of optical filters and gratings. Finally, the requirements for improved source stability and multiplexer performance will be outlined and the resulting opportunities for large-capacity systems will be described.