AN ANALYTIC AND EXPERIMENTAL COMPARISON OF DIRECT AND EXTERNAL MODULATION IN ANALOG FIBEROPTIC LINKS

Analytic lumped-element small-signal models of directly anc externally modulated analog fiberoptic links have been derived. Incremental modulation efficiency is defined and used to compare the performance of these modulation techniques. In experiments to optimize link RF-to-RF gain and noise figure, the measurements obtained agreed with calculations to within ~ 1 dB. The externally modulated link was operated with two different impedance matching circuits. With a low-pass match the bandwidth was 150 MHz, and the link transducer gain was 1 dB; with a band-pass match the bandwidth was 22 MHz, the link transducer gain was 11 dB, and the noise figure was 6 dB. The directly modulated link was operated with a low-pass match. In this case, the bandwidth was 1 GHz, the link transducer gain was —14 dB, and the noise figure was 33 dB. These experimental results were achieved with no amplification and represent a significant improvement in performance over previously reported analog fiberoptic links. © 1990 IEEE