脉宽可调钬激光诱导水下声波信号特性实验研究

Experimental Research of Acoustic Transients Induced by Holmium:YAG Lasers with Tunable Pulse Duration Underwater

800 mm芯径低氢氧根光纤传输自由运转钬激光脉冲爆炸式汽化光纤端面的水形成汽化泡,汽化泡闭合时会辐射振荡波信号。不同激光参数条件下汽化泡形貌和运动状态有差异,导致辐射振荡声波的个数、强度、谐振周期、声学频率等特性参数不同。为研究不同脉宽对声信号的影响,搭建了声信号测量系统,通过示波器分析计算振荡波声学信号特征参数。结果表明电源电压为1000 V、频率为5 Hz、电源脉宽为0.7--1.6 ms的条件下,随着脉宽增加,声压值整体上先升高但存在多个拐点,达到峰值1.01 MPa后呈现下降趋势,但第一个声学信号频率总体呈下降趋势且最大值为400 Hz。高能量、短脉宽的钬激光脉冲能诱导高强度、多个数、短周期、高频率的振荡波信号。

Free-running holmium： YAG lasers transmitting in a fiber with core diameter of 800 pm can induce vaporization bubble explosively at the end of fiber underwater. Shock waves will be produced upon the vaporization bubble collapse. The shape and dynamic state of vaporization bubble under different laser parameters, can result in variable parameters of the number, intensity, oscillation period, acoustic frequency, and so on, of shock waves. An acoustic measurement system has been built to investigate the influence of pulse duration on acoustic transients, and the characteristic parameters of the first acoustic transient can be recorded by an oscillograph. The experimental results indicate that under the condition of 1000 V voltage, 5 Hz frequency, 0.7-1.6 ms pulse duration of pump power, resonance will happen for the intensity of acoustic transients at the starting period. Finally the intensity will decrease after reaching the peak value of 1.01 MPa. The frequency of acoustic transients will always decrease gradually as the pulse duration of lasers increases, and its peak value can reach 400 Hz. Holmium ： YAG lasers with higher energy and shorter pulse duration can induce acoustic transients with higher intensity, more number, shorter period, and higher frequency.