Nonlinear Distortions Induced by Coherent Combinations in Microwave Photonic Links

In this paper, the impact of nonlinear distortions created by coherent combination on the spur-free dynamic range (SFDR) and response stability of microwave photonic (MWP) systems is studied. High SFDR is one of the key requirements in demanding modern communication networks which is usually limited by the nonlinearity of active devices. However, we demonstrate that even purely passive devices can generate nonlinear distortion when modulated optical signals coherently combine on photonic integrated circuits (PIC) or in fiber-based systems. This type of combination is more frequent in PICs since the total signal path on the chip is usually shorter than the coherence length of the laser source. Two different major types of coherent interference, Fabry-Perot (FP) and Mach-Zehnder (MZ), are considered and the induced nonlinearity is simulated for a directly intensity modulated optical signal. Two-tone method is employed to characterize intermodulation (IMD) products added to the signal. Based on the simulation results in the case of FP interference, the dynamic range of a link with 30% optical modulation index (OMI) and reflections as low as - 20 dB can be limited to 90.32 dB.Hz1/2 or 118.89 dB.Hz2/3, respectively, by SFDR2 or SDFR3 rather than the distortions of active devices. Also, the output signal can be dominated by nonlinear distortions even higher than the fundamental signal when MZ interference happens. Finally, the response instability and environmental dependency of the link is discussed based on the simulation results.