AM/PM compensation improves performances of microwave link using a multimode optical fibre and a multimode VCSEL (patent pending)

The aim of this paper is to present an improvement in analog RF, up to 3 GHz, low-cost multimode optical fibre links with directly modulated multimode VCSEL. The performances of such links suffer from nonlinear distortiom due to fibre dispersion, which generates spurious third order intermodulation distortion on the received RF modulation signal. The distortion increases with the length of the multimode fibre and if the fibre is longer than 100-200 metres it is unlikely to use it. We measnred the distortion generated by the multimode fibre on the RF signal: at low level the residual distortion is mainly phase distortion, and it is possible to reduce it with a simple and low-cost RF AM-PM distortion circuit. In this way it is possible to achieve good performances over a transmission distance up to 500 metres.