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The operating principles and performance of waveguide Mach-Zehnder frequency translators suitable for use in coherent optical fibre systems are described. The performance of a coherent optical fibre system transmitting analogue colour video signals and employing a Mach-Zehnder electro-optic waveguide device as a double sideband suppressed carrier frequency modulator is reported. It is shown that the receiver sensitivity of such a system may be as much as 21 dB greater than that for an equivalent direct detection system. The use of electro-optic frequency translators in coherent optical networks to assemble a frequency multiplex of wideband signals is described. Particular attention is given to the linearity of devices, in systems employing up to four channels, and it is shown that unwanted products can be suppressed to 30 dB below the signal level. The use of this type of device as a frequency control element in optical multichannel networks is also considered.