Devices and materials for 4 μm-band fibre-optical communication

The possibilities of optical communication at 4 μm wavelength are discussed. New materials are proposed for optical fibres capable of very low loss at this wavelength. The possibility of fabricating not only glass but single-crystal optical fibres at acceptable growth rates is suggested. A novel heterojunction system is described for making the necessary detectors and high-radiance sources, with the possibility of room-temperature operation. Advantages of optical communication at 4 μm rather than at 1 μm include relaxed demands on the precision of optical alignment (particularly important for single-mode operation), greater tolerance of transition metal impurities in the fibre as well as reduced scattering loss, and, therefore, the promise of much lower fibre loss if the required purity can be achieved. Extremely long stage lengths, e.g. 1000 km, should be possible if, as seems likely, fibre losses approaching 10−3 dB/dm can be achieved. Other possible wavelengths for optical communications are also discussed.