Optimum Design for RF-to-Optical Up-Converter in Coherent Optical OFDM Systems

摘要

In this letter, we conduct analysis on the optimum design for RF-to-optical up-converter in coherent optical OFDM systems using an optical $I/Q$ modulator. We first derive closed-form expressions for the nonlinearity in the optical $I/Q$ modulator, represented by two-tone intermodulation products as a function of the bias point and modulation index. Additionally, we perform a numerical simulation to identify $Q$-penalty and the excess modulation insertion loss under various transmitter conditions. We find that in contrast to the direct-detected system, the optimal modulator bias point for the coherent system is $pi$, where the $Q$ -penalty and excess loss are minimized.