雷射超音波技術是一種非接觸式的非破壞性檢測方法，發展至今已有一段時間，主要是藉由量測聲波波速來檢測材料性質。

　　在本研究中，主要利用光柵投射式的雷射超音波產生窄頻的超音波波速量測系統。簡單地說，是利用一個光學的成像系統，讓光通過光罩在試件表面上形成週期性的雷射圖案而激發超音波。與一般使用雙光束干涉的雷射引致光柵法比較，本方法只需更換光柵週期就可變換雷射激發超音波的頻率與波長，也可改變光罩樣式，在試件上形成不同的光圖案，相對而言運用上較為簡便與彈性。

　　實驗中，將以脈衝時間5～6 ns的Nd: YAG雷射當成激發超音波的雷射，而利用刀緣檢測法以氦氖雷射為偵測光源來進行聲波量測。因此，透過更換光柵的週期並透過改善訊噪比就可獲得窄頻的聲波訊號，若將其使用在一個頻寬內掃瞄得到波速。應用在量測層狀結構的表面波與藍姆波波速，準確獲得其波速與頻率關係，求出頻散曲線。
此外，在實驗中可得到一個相對於主要量測聲波較慢的波形，此波形很清楚但較少出現在聲波量測系統的文獻中。經由本文實驗可以了解此波形是由脈衝雷射加熱而成的震波，並描述其特性。

　Laser ultrasound technique for measuring acoustic waves has been developed for a long time as a non-contact and nondestructive method for material property characterization. In this work, a narrowband laser ultrasound measurement system
based on a mask projection grating (MPG) method is developed. In short, an imaging optical system is used to project the pattern of a photo-mask onto a sample surface for wave excitation. As compared to conventional two-beam interference grating, the MPG method is much easier to implement and can flexibly obtain all kinds of different projected patterns. Periodic linear gratings with various periods are used for
exciting acoustic waves of different wavelengths and frequencies. An Nd: YAG pulse laser with a pulse duration of 5~6 ns is used for laser ultrasound generation and a He-Ne laser with a knife-edge measurement system is for detection. By varying the period of grating, wave frequency ranging from a few MHz to roughly 30 MHz can be
excited and detected. Therefore this MPG laser ultrasound system is capable of narrowband detection with improved signal/noise and wide range frequency scanning by varying the photo-mask patterns. A number of acoustic waves and sample configurations, including surface acoustic waves and Lamb waves, on isotropic and layered samples, are experimentally tested. The wave velocities and dispersion curves over a wide frequency range are accurately obtained.

　Besides, an unexpected wave has been detected by the laser ultrasound system which has not been reported in the literature before. This unknown but clear wave signal appears all the time with a relatively slow velocity. Through careful examinations, we conclude it is a shock wave in the air created by pulsed laser heating. The
details characteristics of this wave signals will be addressed.

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