采用波长为1 064 nm/532 nm、脉宽6 ns(FWHM)的高重复频率调Q激光, 研究了磷酸氧钛钾(KTP)晶体中灰迹的产生机理, 以及色心密度对灰迹的影响。晶体透过率表征了色心密度, 根据透过率与色心密度的关系以及色心密度对灰迹产生的决定作用, 定义临界灰迹密度, 当晶体透过率高于此值时可安全运行, 而低于此值时, 为避免晶体发生灾难性损伤应立即停止运行。实验结果表明：灰迹不仅大量吸收紫外及可见光能量, 而且大量吸收近红外波段能量, 这为灰迹的在线监测提供了一种监测方法。

The mechanism of gray-tracking and the influence of color center on gray-tracking are studied for KTP crystal, with a Q-switched laser whose wavelength and pulse width are 1 064 nm/532 nm and 6 ns respectively. The transmittance of crystal represents color center density. According to the relation between the transmittance and color center and the decisive role of color center density on gray-tracking, the critical gray-tracking density is defined, above which safe operation can be ensured. The experimental results show that gray-tracking absorbs not only the energy of ultraviolet and visible lights, but also that of near infrared light, giving a method for monitoring gray-tracking on line.