在传统的高倍频光毫米波系统中,当边带间的抑制比较大时,高阶边带的存在可直接忽略。当边带抑制比很小时,高阶边带的功率较高,将对光毫米波产生影响。为研究高阶边带对生成高倍频光毫米波的影响,利用级联马赫-曾德尔调制器(MZM)的抑制偶数阶边带方式产生8倍频光毫米波。在实际的光毫米波产生系统中,高阶边带经过调制后会生成冗余的低阶边带,使得边带功率累加,相位不相干,从而造成系统无法解调。为此,详细讨论了不同调制深度下调制器输出的高阶边带对系统的影响。通过对高阶边带的分析可知,当射频信号满足最优的相位关系式时,由第1个MZM输出±1,±3,±5,±7阶平坦边带后,无需光滤波器,即可直接经过第2个MZM调制得到4阶边带的8倍频光毫米波。利用该方法,不需要大幅度的射频信号就可得到纯净的8倍频毫米波。该方案结构简单,频谱纯度高。

In the conventional high-frequency millimeter wave system, the high-order sidebands can be directly ignored when the sideband suppression ratio is relatively large. However, for a small sideband suppression ratio, the big power of a high-order sideband obviously influences the generation of millimeter waves. The effects of high-order sidebands on the generated high-frequency millimeter waves are mainly demonstrated in this paper. In addition, an eightfold-frequency millimeter wave is obtained based on even-order sideband suppression by the cascaded Mach-Zehnder modulators (MZMs). In an actual optical millimeter wave system, the redundant low-order sidebands usually occur after the modulation of high-order sidebands, which makes the power of low-order sidebands accumulated. However the phases are incoherent, and thus the system is unable to demodulate. Therefore, the influences of high-order sidebands outputted from the modulators with different modulation depths are discussed in detail. From the analysis of high-order sidebands, one can know that when the radio-frequency signals satisfy the optimal phase formula, the ±1 st, ±3 rd, ±5 th, ±7 th order flat sidebands are generated by the first MZM, and the eightfold-frequency millimeter wave with a 4 th order sideband can thus be obtained through the second MZM without a filter. With this scheme, a pure eightfold-frequency millimeter wave can be obtained without large radio-frequency signals. For this scheme, not only the design structure is simple, but also the spectral purity is high.