Wang Ruolan, Chen Liao, Hu Hao, Zhao Yanjing, Zhang Chi, Zhang Wenfu, Zhang Xinliang
Opt Express. 2021 Jan 18;29(2):2153-2161. doi: 10.1364/OE.415933.
The radio frequency (RF) spectrum of microcombs can be used to evaluate its phase noise features and coherence between microcomb teeth. Since microcombs possess characteristics such as high repetition rate, narrow linewidth and ultrafast dynamical evolution, there exists strict requirement on the bandwidth, resolution and frame rate of RF measurement system. In this work, a scheme with 1.8-THz bandwidth, 7.5-MHz spectral resolution, and 100-Hz frame rate is presented for RF spectrum measurement of microcombs by using an all-optical RF spectrum analyzer based on cross-phase modulation and Fabry Perot (FP) spectrometer, namely FP-assisted light intensity spectrum analyzer (FP-assisted LISA). However, extra dispersion introduced by amplifying the microcombs will deteriorate the bandwidth performance of measured RF spectrum. After compensating the extra dispersion through monitoring the dispersion curves measured by FP-assisted LISA, the more precise RF spectra of microcombs are measured. Then, the system is used to measure the noise sidebands and line shape evolution of microcombs within 2s temporal window, in which dynamic RF combs variation at different harmonic frequencies up to 1.96 THz in modulation instability (MI) state and soliton state are recorded firstly. Therefore, the improved bandwidth and resolution of FP-assisted LISA enable more precise measurement of RF spectrum, paving a reliable way for researches on physical mechanism of microcombs.
微梳的射频（RF）频谱可用于评估其相位噪声特性以及微梳齿之间的相干性。由于微梳具有高重复率、窄线宽和超快动态演化等特性，对射频测量系统的带宽、分辨率和帧率有严格要求。在这项工作中，提出了一种具有1.8太赫兹带宽、7.5兆赫兹频谱分辨率和100赫兹帧率的方案，用于通过基于交叉相位调制和法布里 - 珀罗（FP）光谱仪的全光射频频谱分析仪（即FP辅助光强频谱分析仪，FP-assisted LISA）对微梳进行射频频谱测量。然而，放大微梳时引入的额外色散会恶化所测射频频谱的带宽性能。通过监测FP辅助LISA测量的色散曲线来补偿额外色散后，测量到了更精确的微梳射频频谱。然后，该系统用于在2秒的时间窗口内测量微梳的噪声边带和线形演化，其中首先记录了调制不稳定性（MI）状态和孤子状态下高达1.96太赫兹的不同谐波频率处的动态射频梳状变化。因此，FP辅助LISA改进后的带宽和分辨率能够更精确地测量射频频谱，为微梳物理机制的研究铺平了一条可靠的道路。
