A. Mandelis et al., Novel lock-in waveform technique for signal-to-noise ratio and dynamic-range enhancement in highly noised photothermal experiments, ANAL SCI, 17, 2001, pp. S5-S8
Conventional frequency domain photothermal methods are basically single-end
ed techniques meaning that a 50% duty-cycle square wave, or a sinusoidal wa
ve is used for modulating the heating beam intensity along with a lock-in a
mplifier (LIA) for signal processing. The implications of single-ended dete
ction are that, if the signal contributions from sample inhomogeneities are
much smaller than that from the homogeneous bulk of the material (backgrou
nd signal), then they cannot be easily detected. In a single-ended techniqu
e the sensitivity of the experiment is determined by the magnitude of the b
ackground signal. Without further conditioning the signal level is simply t
oo high to probe variations of amplitude much smaller than this background.
In this work, these very small variations are called "contrast signals". W
e introduce a lock-in common-mode rejection demodulation signal methodology
as an alternative to the single-ended techniques. This signal generation s
cheme, when coupled to a photothermal detection system is, in principle, ca
pable of detecting very weak inhomogeneities in materials that are not poss
ible to detect with conventional techniques. Furthermore, some calibration
measurements obtained on a homogeneous Zr alloy sample will be presented. T
hese measurements will be further compared with that obtained by irradiatin
g the sample with the conventional 50% duty-cycle square wave, in order to
compare their noise characteristics. Finally, some preliminary measurements
on Zr-2.5Nb shot-peened samples will be presented as a case study of weakl
y inhomogeneous solids and for comparison with that obtained with the conve
ntional frequency scan.(1)