双音调制下星上微波光子系统的交调失真分析

马赫-曾德尔调制器的非线性会严重恶化星上微波光子系统的性能。建立了包括光源、马赫-曾德尔调制器和光电探测器的双音调制理论模型,利用傅里叶级数展开、傅里叶变换和Gegenbauer加法定理,推导出了调制器非线性失真的严格通用解析解。根据该解析解,可精确地预计不同调制方式下星上微波光子系统的非线性失真,优化系统性能。分析结果表明,双边带和单边带调制条件下,将调制器输出信号保留至二次谐波能较好地近似计算三阶交调失真比和三阶交调截点,三阶交调失真比随调制系数的增大先增大后减小。当调制系数小于1.4674时,单边带调制三阶交调失真比至少比双边带调制小6dB,适合于星上微波光子系统的应用。

Inter-Modulation Distortion Analysis for On-Board Microwave Photonics System under Dual-Tone Modulation

The nonlinearity of Mach-Zehnder modulator greatly deteriorates the performance of on-board microwave photonics system. A dual-tone modulation model is presented including optical source, Mach-Zehnder modulator and photodiode, and an exact analytical solution is derived with the method of Fourier expansion, Fourier transform and Gegenbauer addition theory. According to the analytical expression, it can precisely predict the nonlinearity characteristics of the on-board microwave photonics system under different modulation and optimize the system performance. Analytical results indicate that the third-order inter-modulation distortion ratio and the third-order inter-modulation intercept can be approximately calculated with the case when the second-order terms of modulator output signal are included, and the third-order inter-modulation distortion ratio increases and then decreases as the modulation index increases. When the modulation index is less than 1.4674, the third-order inter-modulation distortion ratio of single sideband modulation is at least 6 dB less than that of double sideband modulation and it is suitable for the application of on-board microwave photonics system.