对稳频半导体激光器的频率进行在线实时监测的需求量和迫切性一直在不断增长。特别是近几年发展起来的连续波激光雷达通常以单频半导体激光器为种子源，并通过相干检测方式获得雷达信号的频率，从而获得目标物的距离信息，这就使得种子光源的频率精度直接与测距精度密切相关，因此，对光源的频率稳定性的表征也提出了新的要求：更关注短期（在相干时间内，亚微秒~数毫秒）的频率变化模式，对长时段内（数分钟~24 h）的绝对频率高精度监测的需求减弱；同时要求频率监测系统具有在线实时监测能力。针对这些需求，基于延时自外差原理，提出了一种表征稳频激光器的频率变化的方法，经过严谨的原理推导和算法编程，使得监测系统不仅结构简单，还实现了在线实时监测功能，并测量了一台利用氰化氢（H¹³C¹⁴N）气体吸收谱线基于边频锁定技术的稳频分布反馈式半导体激光器（DFB-LD）频率变化曲线。测量结果是：在10 ms内稳频激光器的最大频率变化约为25 MHz，并且清楚地观察到激光器的频率变化不是单向的漂移模式。为了进一步验证该方法的精度，采用主流的飞秒光频梳拍频法离线测量了同一台稳频DFB-LD的频率变化，实验结果是：在50 min内频率变化约为30 MHz。两种测量方法的测量结果均在相同的MHz量级，证明了该方法是一个快速可靠的光频率分析手段，可应用于实时调节稳频激光器的伺服回路系统。

The phenomenon of branched flow has attracted researchers since its inception, with recent observations of the light branching on soap bubbles. However, previous studies have primarily focused on the flat spacetime, overlooking the effect of surface curvature on branched flows. In this paper, we explore the branched flow phenomenon of light on a rough curved surface called constant Gaussian curvature surfaces (CGCSs). Compared with flat space, a CGCS demonstrates that the first branching point advances due to the focusing effect of the positive curvature of the surface. Furthermore, unlike on flat space, optical branches on curved surfaces do not consistently become chaotic during its transmission in a random potential field. On the contrary, the “entropy” decreases at specific positions, which reveals a sink flow phenomenon following the generation of branched flows. This result highlights the time inversion characteristics of CGCSs. Lastly, we demonstrated that the anomalous entropy reduction is related to the transverse and longitudinal coherence transformations of light. We suppose these efforts would fuel further investigation of the thermodynamic evolution and spatiotemporal inversion of random caustics, as well as their future application in the information transmission of random potentials in curved spacetime.

For most atom interferometers, the vibration isolation unit is applied to reduce vibration noise. In our experiment, instead of isolation, the vibration signals are monitored, and combining with the sensitive function, the compensation phase shift for the atom interferometer is obtained. We focus on the correction over a wide spectrum rather than on “monochromatic” frequencies. The sensitivity of the atom gravimeter can be upgraded by a factor of more than two. Furthermore, we demonstrate that the atom interferometer can still produce a good measurement result without passive vibration isolation in extremely noisy environments by using vibration compensation.

The first Asia-Pacific Comparison of Absolute Gravimeters (APMP.M.G-K1) was organized by the National Institute of Metrology (NIM) of China from December 21, 2015 to March 25, 2016 in Changping, Beijing. Our compact cold atom gravimeter (CCAG) was transported from Hangzhou to Beijing with a long distance of about 1200 km to participate in this comparison. The CCAG is the only one, to the best of our knowledge, that is based on the principle of atom interferometry among all the instruments. Absolute gravity in the indicated three test sites has been measured as requested by the organizer. The sensitivity of our CCAG is estimated to be 90 μGal/Hz, even when the measurements are carried out without any vibration isolation. Besides, the accuracy of this gravimeter has been evaluated to be about 19 μGal by considering the significant system errors. Our results show a good agreement with the given reference value.