Distortion Analysis for RoF System Based on Optical Carrier Suppression Modulation

The Mach-Zehnder electro-optical modulator is widely used in RoF system. The nonlinearity of the light to current(L-I) conversion curve of an Mach-Zehnder electro-optical modulator is an important cause of intra-band, inter-band Harmonic Distortions(HD) and inter-modulation distortions(IMD). This nonlinearity of Mach-Zehnder modulator greatly deteriorates the performance of RoF system based on optical carrier suppression modulation. An exact analytical solution for arbitrary order inter-modulator distortion(IMD) and harmonic distortion of the modulator driven by a two-tone signal is derived. Analytical results indicate that the third-order IMD of the system increases with the increasing modulation index of the RF signal and is independent of the offset phase shift of the modulator. In addition, when the offset phase shift equals π/2 there are only odd-order harmonics. And there are only even-order harmonics, when the offset phase shift equals 0(or π). These results are then confirmed by simulation based on Optisystem. With the presented analytical expression, it is convenient to design an analog optical communication system, precisely predict the nonlinear characteristics of the external modulator and optimize the system´s performance.