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为压缩激光声信号的带宽,基于热膨胀激光致声理论提出了一种采用高重频激光脉冲串产生窄带声信号的方法(重频法),推导了最优重复频率的计算过程,并通过实验测量对重频法进行了验证。以声信号在中心频率处的功率谱密度为参考标准,将重频法与长脉冲法产生窄带声信号的性能进行了比较。理论分析与实验结果表明,激光器在最优重复频率下产生的声信号窄带特性最好;最优重复频率与激光波长、光束半径以及观测角有关;激光脉冲数大于10时,重频法的性能优于长脉冲法;脉冲数为100时,在中心频率处重频法的功率谱密度比长脉冲法高9.3dB。
In order to compress the bandwidth of laser acoustic signal, a method of generating narrowband acoustic signal by using high-repetition laser pulse train (repetition method) is proposed based on the theory of thermal expansion laser induced sound. The calculation process of optimal repetition frequency is deduced. Measurements have been verified by the re-frequency method. Taking the power spectral density of the acoustic signal at the center frequency as the reference standard, the performance of the narrow-band acoustic signal generated by the repetition frequency method and the long pulse method is compared. The theoretical analysis and experimental results show that the laser produces the best narrow-band characteristic of the acoustic signal at the optimal repetition frequency. The optimal repetition frequency is related to the laser wavelength, beam radius and observation angle. When the laser pulse number is more than 10, Which is better than the long pulse method. When the pulse number is 100, the power spectral density of the repetition frequency method is 9.3 dB higher than that of the long pulse method at the center frequency.