The sources of optical radiation between approximately 0.1 μm and 1 μm wavelength (1 μm = 10−6 m) may be classed into two types according to their spectral line width. The communication engineer is aware of the desirability of a signal source which is monochromatic. Such a single frequency source may be modulated and can act as the carrier of a high concentration of information. On the other hand, a wide bandwidth source would not be suitable for communication applications, since the modulation signal would become irretrievably scrambled in the transmission and detection process. Thus, the most important question we may ask about an optical source is whether or not its emission has a narrow spectral width.* The coherent sources, the gas, liquid or solid state lasers, have a spectral width of the order of 0.01 to 0.1 nm giving a relative bandwidth of the order of 10−5 to 10−4 (the line width of a He-Ne laser emitting red light at λ0 = 632.8 nm is Δf = 7.5 GHz). The incoherent sources, the electroluminescent devices such as the light emitting diode, have a spectral width of the order of 10 nm with a relative bandwidth of the order of 10−2. The applications of electroluminscent devices are mainly in optical coupling, and optical display and illumination.
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