Lock-in common-mode rejection demodulation: Measurement technique and applications to thermal-wave detection. Experimental

A new signal generation methodology based on lock-in amplifier common mode rejection demodulation for materials nondestructive evaluation has been imp lemented experimentally with thermal waves in a photothermal radiometric ap paratus. A procedure to calibrate the experiment is described in order to t ake into account instrumental time delay phase shifts between the reference and the optical excitation wave form. Some preliminary results obtained on Zr-2.5Nb shot peened samples are presented and compared to those obtained by temporally modulating the pump intensity as a 50% duty cycle square wave . The comparison shows the new measurement methodology to be a very promisi ng technique for nondestructive evaluation and depth profiling applications , featuring high detectivity for low-dynamic-range and poor-signal to noise ratio signals, such as those obtained with thermal-wave diagnostics. (C) 2 000 American Institute of Physics. [S0034-6748(00)03206-8].