Fibre Optic Crosstalk

♦ Where fibre optics is concerned, crosstalk refers to how well de-multiplexed channels are separated – the better the separation, the better the performance of the fibre network – that’s especially important in data heavy CCTV applications.

Each channel should only appear at its intended output port and not at any other. The crosstalk specification you read on equipment expresses how well a coupler maintains this port-to-port separation.

Such crosstalk specification might measure how much of an 820nm wavelength appears at the 1300nm port. For example, a crosstalk of 20dB means that 1 per cent of the signal appears at an unintended port.

Meanwhile, channel separation describes how well a coupler can distinguish wavelengths. In most couplers, the wavelengths must be widely separated – as wavelengths of 820nm and 1300nm, for example. Such a device will not distinguish between 1290 and 1310nm signals.

Wave division multiplexing allows the potential information carrying capacity of an optical fibre to be increased significantly as each wavelength (colour) transiting the fibre can be used to carry a different stream of information.

Something to remember that the bandwidth-length product used to specify the information-carrying capacity of a fibre applies only to a signal imposed on a single optical character.

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