The selectivity of coherent detection facilitates the use of wavelength-division multiplexed systems to exploit the bandwidth of monomode optical fibre. Optical amplifiers offer the possibility of gain in such systems without demultiplexing. The amplification of a pair of multiplexed signals with an antireflection-coated semiconductor laser has been demonstrated. It was found that the internal gain and resonant wavelengths varied with the power in the second channel, and that amplitude-modulated signals consequently caused crosstalk. From measurements of crosstalk against interference power, it was concluded that phase or frequency modulation will normally be preferable to amplitude modulation in multiplexed systems with amplifiers.
R. P. Webb,
"Evaluation Of A Semiconductor-Laser Amplifier For Multiplexed Coherent Systems", Proc. SPIE 0587, Fiber Optic Sources and Detectors, (9 July 1986); doi: 10.1117/12.951199; https://doi.org/10.1117/12.951199