Laser Ultrasonic Testing System for Remote Inspection

The ultrasonic testing method has been used in the fields of Non-Destructive Testing (NDT) for many decades. The conventional transducers are mainly used to generate and detect an ultrasonic signal. But in the case of a specimen with a complex geometry and a narrow space, it is difficult to detect cracks by using the ultrasonic inspection method using conventional transducers. And the contacting transducer method can not be applied to the inspection of some irradiated structures, such as nuclear reactor pressured vessels and internals. The laser ultrasonic method that uses laser beams for the generation and detection of an ultrasonic signal is applicable to the inspection of these parts since it is inherently a non-contact inspection method and broadband. Nowadays laser ultrasonic testing is a widely used non-destructive inspection technique for many industrial fields. The laser ultrasonic techniques consist of a pulsed laser for an ultrasonic wave generation and a laser interferometer for the detection of an ultrasonic signal. There are many kinds of interferometers to detect an ultrasonic signal, such as the Michelson or Mach- Zehnder interferometer, Fiber-optic interferometer, Wave-mixing interferometer, Confocal Fabry- Perot interferometer, etc.. In many industrial applications, a sample surface may not be optically flat and some environmental vibrations can exist. In these industrial applications, a confocal Fabry-Perot interferometer is more useful than other plane interferometers because the confocal interferometer can receive many speckles scattered over a wide angle