Spectral purity of laser sources is typically investigated using phase or frequency noise measurements, which require extraction of the optical phase. This is a challenging task if the signal–to–noise–ratio (SNR) of the spectral line or the linewidth–to–noise–ratio (LNR) are not sufficiently high. In this letter, we present a statistically optimal method for optical phase noise measurement that relies on coherent detection and Bayesian filtering. The proposed method offers a record sensitivity, as the optical phase is measured at a signal power of −75 dBm (SNR of −11 dB in 1.1 GHz receiver bandwidth). Practically, this means that the phase noise measurements are, up to a high–degree, not limited by the measurement noise floor. This allows measurements down to −200 dB rad2/Hz and up to 10 GHz, which is useful when measuring the Schawlow–Townes (quantum noise limited) laser linewidth. Finally, the estimated optical phase is highly accurate allowing for quantum limited signal demodulation. The method thus holds the potential to become a reference measurement tool.

中文翻译：

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使用贝叶斯滤波的高灵敏度相位和频率噪声测量技术

通常使用相位或频率噪声测量来研究激光源的光谱纯度，这需要提取光学相位。如果谱线的信噪比 (SNR) 或线宽噪声比 (LNR) 不够高，这是一项具有挑战性的任务。在这封信中，我们提出了一种基于相干检测和贝叶斯滤波的光学相位噪声测量的统计优化方法。所提出的方法提供了创纪录的灵敏度，因为光相位是在 -75 dBm 的信号功率（1.1 GHz 接收器带宽中的 -11 dB SNR）下测量的。实际上，这意味着相位噪声测量在很大程度上不受测量本底噪声的限制。这允许测量低至 −200 dB rad2/Hz 和高达 10 GHz，这在测量 Schawlow-Townes（量子噪声受限）激光线宽时很有用。最后，估计的光相位高度准确，允许量子限制信号解调。因此，该方法具有成为参考测量工具的潜力。