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How does the spectral line-width of light source (let that be ordinary light, LED, LASER etc.) decides for which communication purpose it will be used for ? Is it like spectral line-width effects the SNR or Bandwidth ? Thanks in advance !

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In fiber optic communications, the linewidth of the source often affects the bandwidth of the transmission medium (fiber). This is because the glass of the fiber is at least somewhat dispersive, meaning that different wavelengths of light propagate at different speeds along the fiber. Because of this, if you launch a narrow pulse of light (representing a single bit of your message, for example) into a fiber, it will spread as it travels, and eventually be indistinguishable from the other pulses you launched to represent other bits in your signal.

This effect generally leads us to require a narrower linewidth source for longer-distance communications links. And indeed intercontinental links typically use the narrowest linewdith lasers available (which are relatively expensive), and further use temperature controls and other measures to maintain that narrow linewidth (as well as stabilize the center wavelength so that multiple signals can share the same fiber using wavelength division multiplexing).

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