载波调制水下探测激光雷达利用散射光和信号光在频域特性上的差异来抑制杂散光，而不同水质中的散射光的截止频率是载波调制水下探测激光雷达设计中的重要参数。测量散射杂波的频率响应需要将目标回波与散射杂波分开，尤其是前向散射，与目标回波空间上重合，不容易被分开。文中提出了一种在载波强度调制激光雷达中分别测量水下目标和散射杂波的频率响应方法，利用散射光和目标反射的信号光在空间相干性上的差异，在回波光路上使用螺旋相位板将其分开并分别采集，再通过傅里叶变换得到它们的频谱。实验结果表明，前向散射杂波具有低通特性，而目标回波的响应是平坦的。将信号与散射在频谱中调制频率处的幅度之比定义为“信杂比”，并且用其来表示探测的有效性：当信杂比大于1时，认为探测到了目标。在较浑浊的水中，为了获得大于1的信杂比，载波的调制频率需要更高，基于蒙特卡洛方法对光子在海水中的传播进行了仿真模拟，结果与实验相符。测距实验也证明了，提高调制频率可以减小浑浊水中的测距误差。当信杂比小于1时，提高调制频率，测距误差降低明显。

We report on the observation of the highly forbidden S10–P30 optical clock transition in laser-cooled Hg199 atoms. More than 95% depletion of cold Hg199 atoms is detected in the magneto-optical trap. Using the free-of-field detection method, the AC Stark shift from the cooling laser is removed from the in-field spectroscopy. At low-power clock laser pumping, the linewidth of the clock spectroscopy is approximately 450 kHz (full width at half-maximum), which corresponds to a Doppler broadening at the atom temperature of 60 μK. We determine the S10–P30 transition frequency to be 1,128,575,290.819(14) MHz by referencing with a hydrogen maser and measuring with a fiber optical frequency comb. Moreover, a weak Doppler-free signal is observed.

We experimentally observe polarization spectroscopy (PS) of the S01-P31 transition of mercury atom gases at 253.7 nm. The PS signal can be observed in all six richly abundant isotopes and the PS signal of six transitions for laser cooling are all clear and of a dispersive line shape. The optimized pump power and probe power are found for the PS of Hg202. We find the linearly polarized component in the pump beam will distort the original PS signal due to the use of linear PS. Consequently, the purity of the pump beam is crucial to laser frequency stabilization by PS.