Performance evaluation of frequency modulation and slope detection in analog optical links

One unsolved challenge in the application of analog optical links is the limited dynamic range. It is desirable to have a scheme which offers low noise and high linearity. In other words, it is required to have a scheme employing Class-B operation in analog optical links. In a Class-B optical link all received optical power contributes to the radio frequency (RF)-signal power. Since the noise is dependent on the average optical power, the noise in a Class-B optical link depends on the signal power. A good candidate for the Class-B scheme is the combination of frequency modulation and slope detection. The scheme consists of a directly modulated laser and an inte- grated optical filter. The directly modulated laser generates the frequency modulated signal. The optical filter converts the signal into two intensity modulated signals. At the receiver end, these signals are detected by a balanced photodetector. It restores the original modulating signal. The filter performing the slope detection consists of two-coupler rings. It is designed in such a way, that the conversion from frequency modulation to intensity modulation will yield two optical signals that comprise the complementary half-wave rectified versions of the original modulating signal. This half-wave rectification is the key element for realizing the Class-B operation in analog optical links. The report gives an overview of the filter design. Furthermore, the feasibility of frequency modulation by the laser and the feasibility of half-wave rectification based on optical frequency modulation are experimentally shown.