Remote ultrasound detection with a quasi-balanced confocal Fabry–Perot interferometer

Abstract

In this article, we show the benefits of a quasi-balanced fringe hopping confocal Fabry–Perot interferometer (CFPI) with broadband common mode rejection ratio (CMRR) for remote ultrasound detection. In laser ultrasound, the ultrasonic information, in general, lies in the phase modulation of laser light which in this case is demodulated using the CFPI at a certain working point on a fringe. By hopping from the positive to the negative slope on the same fringe, the detected ultrasonic signals are inverted. In contrary, interference signals – such crosstalk from the generation, ghosts or noise correlated to pulse laser excitation – are not influenced and hence get rejected by subtracting the signals measured at both slopes. Hence, a minimum of two measurements is needed for common mode rejection. The fringe hopping from the positive to the negative slope is done by changing the distance of the CFPI mirrors with a precise piezoelectric-stack and a fast high-resolution digital controller. As only one photodetector with a transimpedance amplifier is needed, a high CMRR can be accomplished. The CMRR is not affected by the symmetry of the fringe but only by pulse-to-pulse energy fluctuations of the generation laser. We show that with fringe hopping and averaging the signal-to-noise ratio increases much faster than with averaging without fringe hopping. This is due to the correlation of the quasi-noise with the generation cycle.

Keywords:

• balanced Fabry–Perot interferometer
• laser ultrasound
• signal to noise ratio
• common mode rejection

Acknowledgements

This work has been supported by the European Regional Development Fund (EFRE) in the framework of the EU-program Regio 13, by the Christian Doppler Research Association, by the Federal Ministry of Economy, Family and Youth, and by the federal state Upper Austria.