A Preliminary Study on The Thought of Maritime Defence in Modern China and Japan: with special reference to Wei Yuan [1794-1857], Xu Jiyu [1795-1873], Hayashi Shihei [1738-1793] and Sakuma Shozan [1811-1864]

Lasers offer a non-contact method for generating ultrasound. The main advantages are that specimen can be scanned more easily and the complications of a coupling layer or liquid are eliminated. Also, more accurate velocity measurements can be obtained using lasers and for holographic generation, higher frequency coherent waves can be obtained. Lasers have been used to generate ultrasonic waves in many configurations, including the generation of bulk longitudinal and shear waves, Rayleigh waves along solid surfaces, and Lamb modes in solid layers. General reviews of laser generation techniques can be found in [1] and [2]