Optimisation of carrier-to-sideband ratio by triple-arm Mach–Zehnder modulators in radio-over-fibre links

Abstract

A novel Mach–Zehnder modulation technique based on a 1×3 multimode interference coupler and three optical phase-modulator waveguides for the generation of optical single sideband (SSB) signals in the radio-over-fibre millimeter-wave system is proposed. Two of the three modulator arms are biased by the direct current (DC) voltage and modulated by the radio frequency (RF) signal in the same way as conventional SSB modulation method to produce optical RF signals, but the third one which can be named as ‘dc arm’ is only biased by DC to change the path of the optical carrier passing through it. The interference at the output of the triple-arm modulator happens at the optical carrier frequency, which makes the power of the optical carrier in the SSB signal controllable. Optimised modulation depth between the sideband component and the carrier of the generated SSB signal could be obtained by adjusting the bias voltage applied to the ‘dc arm’. It is proposed and analysed that the novel technique could improve the sensitivity of the optical receiver significantly compared with the conventional SSB modulation scheme.