Opt Lett. 2018 Jul 1;43(13):3108-3111. doi: 10.1364/OL.43.003108.
Broadband characterization of the carrier-envelope phase (CEP) noise spectral density of free-running mode-locked lasers is essential for advanced low-noise optical frequency comb designs. Here we present a direct method that utilizes an optical heterodyne beat between a pair of repetition-rate-locked mode-locked lasers for CEP noise characterization, without requiring an f-2f interferometer or nonlinear optical conversion steps. A proof-of-principle experiment in a femtosecond Yb-fiber laser achieves CEP noise spectral density characterization with >270 dB dynamic range over a Fourier frequency range from 5 mHz to 8 MHz. The measurement noise floor is well below 1 μrad/√Hz, enabling dependable detection down to a quantum-limited noise floor. The method can resolve various noise mechanisms that cause specific CEP noise spectral shapes. The underlying mechanisms are further analyzed in terms of spurious temporal correlation to distinguish between technical and stochastic noise signatures. Moreover, a Hadamard deviation analysis reveals a varying degree of frequency stability in the measured CEP time series.
自由运转锁模激光器载波包络相位（CEP）噪声谱密度的宽带特性是先进的低噪声光频梳设计的关键。在这里，我们提出了一种直接方法，利用一对重复频率锁定锁模激光器之间的光外差拍频来进行 CEP 噪声特性描述，而无需使用 f-2f 干涉仪或非线性光学转换步骤。在飞秒 Yb 光纤激光器中的原理验证实验实现了 CEP 噪声谱密度特征描述，在傅里叶频率范围从 5 mHz 到 8 MHz 内具有超过 270 dB 的动态范围。测量噪声基底远低于 1 μrad/√Hz，能够可靠地检测到量子极限噪声基底。该方法可以分辨出引起特定 CEP 噪声谱形状的各种噪声机制。进一步根据杂散时间相关分析来区分技术噪声和随机噪声特征，以分析其潜在机制。此外，Hadamard 偏差分析揭示了测量的 CEP 时序列中频率稳定性的不同程度。
