Study On Acoustic Identification Method Of Optical Thin Film Damage Based On Frequency Characteristics

Laser induced damage threshold of optical thin film is always a critical parameter in measuring Laser induced damage. The key to testing damage threshold is judging the injury incident accurately. In this paper, the film damage judgment method based on inner relationship between acoustic frequency characteristics and damage is put forward. Compared with the traditional acoustic method, this method not only inherits the advantages of the previous photoacoustic one, but also reduces the false rate caused by amplitude characteristics, greatly improving the accuracy of film damage judgment.The advantages and disadvantages of common film damage judgment methods are first summarized. The sound mechanism of interaction between laser and optical films is analyzed. The increasing laser energy can bring about three mechanisms of heat expansion, gasification and optical puncture. The laser-induced damage threshold testing system and the acoustic acquisition are established. The single SiO2and HfO2film damage with different laser energy are studied. The frequency spectrum curves are obtained by the Fourier transform of in-time signal. In the experiment, by comparing and analysing the frequency spectrum curve between the damaged film and undamaged one, the film damage judgment based on frequency characteristics can be acquired.The experimental results show that SiO2film and HfO2film have similar characteristics of acoustic spectral curves before and after damage in the high spectral range (24KHz-40KHz). The spectrum curves of the sample are same before damaging, the spectrum curves of the undamaged sample tend to be same with increase of laser energy. The quantities of wave crest and wave trough become stable gradually, but the the spectrum curves are different before and after the damage. In the process of changing form the undamaged films to damaged one, there is a step point between24KHz and25KHz. The position of the point did not change obviously. When the film is absolutely damaged, the average spectrum curve is regarded as a reference spectrum curve. Compared with the high frequency spectrum of different laser energy, the similarity can be obtained. When the laser energy become larger, the more severe film damage is, the smaller similarity is. When SiO2film and HfO2film are damaged, the similarity range is respectively0～0.255and0～0.296. However, it is significantly greater than0.3, when the film is undamaged. The step point position is in the range of24.4KHz～24.5KHz. The measured result has a preferable consistency with microscopic method. Therefore, the acoustic frequency method can be easily efficient to judge accurately film damage.