Application of passive quadrature phase demodulation for the detection of laser ultrasound

In this paper we describe a laser ultrasound detection scheme based on a new passive phase demodulation technique. Typical laser interferometers suffer from the loss of sensitivity because of phase drifts resulting from the mechanical movements of optical components and normally require closed loop stabilisation control. Through the use of a custom-designed grating mask, the present design completely eliminates the need for any feedback control by simultaneously sampling the interferometer output in the four quadrants of the optical phase circle. The signals are then processed to reproduce the actual waveform of the ultrasound pulse. The operation of the proposed technique is demonstrated through the detection of laser-generated ultrasound waves propagating inside an aluminium block.