Peng Huanfa, Liu Naijin, Xie Xiaopeng, Chen Zhangyuan
Opt Express. 2021 Feb 15;29(4):6220-6235. doi: 10.1364/OE.416336.
Highly stable, low phase noise microwave oscillators are essential for various applications. An optoelectronic oscillator (OEO) can overcome the short-term phase noise limitation of pure electronic oscillators at high oscillation frequency. Nonetheless, the long-term frequency stability should be addressed. To stabilize the frequency of OEO, a phase-locked loop (PLL) is widely used to synchronize the OEO to a stable reference. However, due to the narrow free-spectral-range (FSR) of the oscillation cavity of the OEO, the pull-in range of the PLL is limited. It is challenging to acquire phase-locking at startup and phase-relocking when mode-hopping of OEO occurs. Here, by using an automatic frequency calibration (AFC) assisted PLL, we attain a highly stable 10 GHz phase-locked OEO with robust phase-locking at startup and phase-relocking when mode-hopping of OEO occurs, for the first time. With the use of a fast digitally-controlled frequency shifter and a real-time frequency error detection unit in the AFC loop, the phase-locking and phase-relocking time are below 120 ms. Furthermore, it shows the phase noise of -135 dBc/Hz at 10 kHz offset, side-mode suppression ratio (SMSR) of 128 dBc, and Allan deviation of 4.8×10 at 5000 s for the phase-locked OEO. We thoroughly investigate the dynamics of the automatic frequency calibration, the phase-locking process, the phase-relocking after OEO mode-hopping, the system under vibration, and the frequency switching. Our approach is promising to generate a highly stable, low phase noise, and determinate frequency microwave signal, which can be used as a low phase noise reference for a microwave frequency synthesizer and high performance sampling clock for a data conversion system.
高稳定性、低相位噪声的微波振荡器对各种应用至关重要。光电子振荡器（OEO）可以克服纯电子振荡器在高振荡频率下的短期相位噪声限制。尽管如此，仍需解决长期频率稳定性问题。为了稳定OEO的频率，锁相环（PLL）被广泛用于将OEO与稳定参考源同步。然而，由于OEO振荡腔的自由光谱范围（FSR）较窄，PLL的捕捉范围有限。在OEO启动时实现锁相以及在OEO发生模式跳变时进行相位重新锁定具有挑战性。在此，通过使用自动频率校准（AFC）辅助的PLL，我们首次实现了一个高度稳定的10 GHz锁相OEO，在OEO启动和模式跳变时进行相位重新锁定时具有稳健的锁相性能。通过在AFC环路中使用快速数控移频器和实时频率误差检测单元，锁相和相位重新锁定时间低于120 ms。此外，对于锁相OEO，其在10 kHz频偏处的相位噪声为-135 dBc/Hz，边模抑制比（SMSR）为128 dBc，在5000 s时的阿伦偏差为4.8×10。我们深入研究了自动频率校准的动力学、锁相过程、OEO模式跳变后的相位重新锁定、振动下的系统以及频率切换。我们的方法有望产生一个高度稳定、低相位噪声且频率确定的微波信号，可作为微波频率合成器的低相位噪声参考源以及数据转换系统的高性能采样时钟。
