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

The ability of conventional single-ended photothermal techniques to detect weak inhomogeneities in a given material is mainly limited by two instrumen tal factors: the signal-to-noise ratio (SNR) and the amplitude dynamic rang e. The amplitude level is limited by the output signal baseline, and may be too high to monitor relatively small variations introduced by the presence of weak inhomogeneities. The purpose of this work is to introduce a novel photothermal signal generation methodology, the principle of which can be b roadly applied to any technique utilizing a lock-in analyzer demodulation s cheme of periodic signal wave forms. Unlike the conventional single-ended p eriodic excitation wave form, which uses a 50% duty-cycle square wave or si nusoidal modulation of the pump laser heating beam, a more complicated peri odic modulation wave form is employed, resulting in the equivalent of diffe rential-signal demodulation. The new wave form takes advantage of the real- time differential action performed by the lock-in amplifier weighing functi on over the two half periods of the modulated signal. This results in enhan ced signal dynamic range due to the efficient suppression of the baseline a nd a substantial improvement in the SNR. The main features of this techniqu e are investigated with a theoretical model for an arbitrary repetitive sig nal wave form and, in particular, for a photothermal signal. The dependence of the signal on the wave form parameters is also discussed. (C) 2000 Amer ican Institute of Physics. [S0034-6748(00)03106-3].