亚 赫 兹 线 宽 稳 频 激 光 与 光 学 分 频（特 邀）

Significance Narrow-linewidth ultra-stable lasers constitute the cornerstone of modern precision measurement systems, underpinning breakthroughs in optical atomic clocks, gravitational wave astronomy, and quantum information processing. Their unparalleled spectral purity—characterized by sub-hertz linewidths and fractional frequency stabilities—directly determines the resolution and accuracy of fundamental measurements. The current gold standard for generating such lasers involves locking a single-frequency source to the resonance of ultra-stable optical cavities using the Pound-Drever-Hall (PDH) technique, which employs optical heterodyne detection to achieve shot-noise-limited frequency stabilization. Optical frequency division (OFD) extends this capability by enabling coherent wavelength conversion across the entire optical spectrum while preserving the original laser’s coherence properties. Through photonic division of high-stability optical references, OFD systems can generate tunable outputs at any desired wavelength with additional frequency noise far below that of the optical frequency references, ensuring that the coherence and accuracy of the references are fully preserved throughout the conversion process. These advancements collectively remove critical barriers to the widespread deployment of sub-hertz linewidth lasers in real-world applications. By enabling sub-hertz linewidth laser operation at arbitrary wavelengths, OFD technologies are now facilitating next-generation primary frequency standard and ultra-precise tests of fundamental physics—ushering in a new era of measurement science with unprecedented sensitivity and reliability. Progress This paper first provides an overview of the development history of sub-hertz linewidth stabilized lasers, including how scientists have improved laser frequency stability by reducing the sensitivity of reference cavities to vibration noise and by developing various methods to reduce the thermal noise of reference cavities. It also introduces the domestic development in this field. Subsequently, the paper introduces the research background and advancements of high-precision, low-noise optical frequency division. In the second part of this paper, the principle of the PDH laser frequency stabilization technique is introduced, along with the impact of residual amplitude modulation on laser frequency stabilization and methods to suppress it. Then, the paper focuses on the state-of-the-art laser systems with the highest frequency stabilities and a linewidth at the millihertz level, which are realized by frequency stabilizing to the resonance of cryogenic cavities. It mainly discusses the impact of temperature stability and vibration sensitivity, the two main factors affecting laser frequency stability. Moreover, the authors introduce two main methods for achieving cryogenic temperature control. Given the application background of sub-hertz linewidth stabilized lasers outside the laboratory, the paper also introduces transportable sub-hertz linewidth frequency-stabilized laser systems, focusing on how to balance transportability and vibration insensitivity. It describes the performance of such portable sub-hertz linewidth laser systems and the design scheme of transportable, highly stable optical reference cavities. At the end of the second part, the paper looks ahead to the development trends of sub-hertz linewidth stabilized lasers, including how to further improve laser frequency stability to the 10^-18 level and the laser linewidth to below 5 mHz. The authors also prospect the development trends of transportable sub-hertz linewidth frequency-stabilized laser systems. In the third part of this paper, the principle of optical frequency division based on narrow-linewidth optical frequency combs is introduced, including methods for dividing an optical frequency reference to other optical wavelengths and to the microwave band. Subsequently, the paper introduces the principles of optical frequency division based on the technique of comb frequency noise immunity. The paper introduces the performance of existing optical frequency dividers, including division noise and uncertainty in the optical-to-optical frequency division for each method. The paper also discusses the advantages and disadvantages of each method. Additionally, the paper briefly introduces the division noise and uncertainty in optical-to-microwave frequency division. At the end of the third part, the paper looks ahead to the development trends of high-precision optical frequency division, including expanding the spectral range of target frequency division, frequency tunability, and automatic control based on existing optical frequency division techniques, as well as the feasibility of achieving high-precision optical frequency division using chip-scale microcavity-based optical frequency combs. Conclusions and Prospects In this paper, we provide a comprehensive review of the recent advancements in sub-hertz-linewidth frequency-stabilized lasers and low-noise optical frequency division. We systematically analyze the fundamental operating principles and performance characteristics of cutting-edge laser systems, including ultra-stable lasers with millihertz linewidth, compact ultra-stable portable lasers for field deployments, and advanced optical frequency dividers utilizing both narrow-linewidth optical frequency combs and innovative comb-frequency-noise suppression techniques. Through detailed comparisons of experimental results and theoretical models, we elucidate the critical parameters influencing system performance. Looking forward, we discuss prospective research directions aimed at achieving even lower frequency noise laser stabilization and optical frequency division, compact size and robust operation. The optical frequency division derived from sub-hertz-linewidth lasers or optical atomic clocks will become an indispensable and highly convenient toolset, enabling unprecedented precision in spectroscopy, gravitational wave detection, and fundamental physics tests by providing coherent laser light at any desired wavelength.