Phase modulation to amplitude modulation conversion of CW laser light in optical fibres

We have measured the induced amplitude modulation of sinusoidally phase-modulated CW 1.5 ?m laser light propagating in a single-mode fibre. Amplitude modulation approaching 100% was observed for light phase-modulated at 4 GHz and transmitted through 50 km of fibre. PM-AM conversion produces a power penalty in coherent transmission systems.