在光外差测量中,信号光与本振光的光程差、探测器的相对位置、探测器光敏面相对于激光光斑的大小和聚焦透镜的焦距,对系统的外差效率都有重要影响。针对光学外差无损探伤应用,计算分析了这4个因素对外差效率的影响。对外差效率的数值计算表明:系统外差效率随着光程差的增大而降低,探测器位置不同,此下降趋势不同；探测器相对位置和探测器光敏面大小则具有最佳值；聚焦透镜对外差效率有很大的影响,其焦距只能在一个很窄的范围内选择,被测目标物体与测量系统中透镜的距离越近,此选择范围越窄。根据理论计算所设计的超声外差探伤系统,外差效率可达0.965。系统用于由脉冲激励超声工件的内部探伤,观察到距离1555 mm处钢管微小裂缝的超声反射信号。

In optical heterodyne technique, optical path difference between the signal and local beams, the relative position of detector, ratio of the area of the photo-sensitive surface to the laser spot, and focal length of focused lens, have important effects on the heterodyne efficiency. The effects of these factors on the heterodyne efficiency for a nondestructive evaluation system employing optical heterodyne technique are analyzed in detail. The numerical calculations of heterodyne efficiency show that: 1) the efficiency is reduced as the optical path difference increases, 2) the relative position of detector and the area of the photo-sensitive surface for the detector have optimum values, and 3) focused lens in the optical system has crucial effects on the heterodyne efficiency, which leads to a narrow range of the focal length allowed. The heterodyne efficiency of the nondestructive evaluation system designed according to the theoretical calculations can be increased up to 0.965. The system is employed for interiorly nondestructive evaluation of work pieces in which ultrasonic is motivated by pulse. A ultrasonic signal reflected by a mini split within a steel pipe at a distance of 1555 mm is received with this heterodyne system.