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Frequency noise is a key limitation factor for frequency sensitive ultra-high precision measurement. An excellent methodology for its calibration is the error signal extracting from a cavity length locking process via Pound–Drever–Hall (PDH) technique, and also can be applied as a noise discriminator for active feedback control. However, the stabilized laser frequency noise performance was poor to meet the applications for ultra-high precision measurement, and no literature has analyzed the cause in detail. In this Letter, we found that various of noise floors in the in-loop sensing and modulation depth in the PDH error signal should be responsible for the poor noise performance. Meanwhile, differing from the theoretical prediction (1.08), the modulation depth in frequency noise feedback controlling should be optimized by simultaneously considering the detector's gain and saturation power, cavity impedance matching, and control loop gain. In our case, it was fixed to be 0.17. The experimental results confirm our theoretical analysis well. Finally, the stabilized laser frequency noises are reduced to 10 − 1 Hz / Hz for out-of-loop and 3 × 10 − 2 Hz / Hz for in-loop. The results demonstrate an efficient settlement solution for active frequency noise feedback control. • Optimizing modulation depth for frequency noise suppression is modelled. • Difference in frequency noise between in-loop and out-of-loop is demonstrated. • In-loop noise is observed on-line with the performance of out-of-loop. • Under optimal parameters, a minimum out-of-loop noise of 10 − 1 Hz / Hz so far is demonstrated. [ABSTRACT FROM AUTHOR]

• Classical phase noise is the main limited factor for phase sensitive precision measurement. • Mode cleaner plays a low-pass filtering role not only for amplitude quadrature but also for phase one. • The noise suppression process for one quadrature noise is accomplished by a noise intercoupling between the two quadratures. • A complete experimental scheme for a full phase to amplitude noise conversion has been designed for phase noise characterization. Suppression phase noise of a laser is a great practical significance in applications of phase-sensitive precision measurement. To suppress the phase noise, an ultra-stable cavity incorporating with an electrical feedback control loop is always the popular choice, in which the linewidth of the cavity should be narrower than that of the laser to provide a stable reference standard. Here, we experimentally verify that mode cleaner plays a noise suppression role for phase quadrature in the whole frequency range of the photoelectric detectors, even though its linewidth is 10 3 larger than the laser one. Meanwhile, we found that the noise suppression for one quadrature is codetermined by the noise of the two quadratures of the input field. Our demonstration provides a new and in-depth interpretation for phase noise suppression in a mode cleaner with larger linewidth than the laser one that is important in phase-sensitive sensing application. [ABSTRACT FROM AUTHOR]