ADUAL-CHANNEL PHOTOACOUSTIC MICROSCOPE FOR NONDESTRUCTIVE EVALUATION OF SOLID SAMPLES

摘要

Photoacoustic microscopy (PAM) is an emerging technique for nondestructive evaluation of special solid samples. This present paper describes the design and development of a dual-channel photoacoustic microscope for the detection and evaluation of surface and subsurface defects in solid samples. Monochromatic light from a laser is intensity modulated with a mechanical chopper and guided through an optic fiber on to the surface of a solid sample kept in the photoacoustic cell. We have used a dual-channel cell that has high signal to noise ratio than conventional single microphone cells. The signals from the two microphones are added after a stage of pre-amplification. The resultant signal is fed to a lock-in amplifier for measurement. The laser beam is made to scan the entire sample surface in a step-by-step manner using X-Y translation stages driven by stepper motors. The signals received from each position of the beam are recorded and a surface plot showing the variation of photoacoustic amplitude or phase with position of the beam on the sample is generated. The variation of the signal clearly reflects the presence of surface and/or subsurface defects, change of material, differences in optical absorption etc. The whole setup is automated and controlled by a user friendly software developed in Visual Basic. Results on selected samples are presented here.