Abstract:
This thesis describes the implementation of enhanced signal processing techniques in
electronic speckle shearing interferometry, including two-wavelength slope
measurement, phase stepping, and heterodyning and stroboscopic illumination in
vibration analysis. All the techniques were achieved using laser diode emission
wavelength modulation.
Slope measurement using two-wavelength illumination can generate slope fringes in a
mechanically passive manner and the fringe visibility is better compared to other
illumination-shifting and object-tilting methods. Three simple geometric objects were
measured using an x shear of 4 mm and AX ~ 0.45 nm. The results are in agreement
with a theoretical analysis. The measurement accuracy can be further improved by
calculating the simple equations of parameters in the fringe function.
A novel phase stepping technique has been demonstrated using laser diode injection
current modulation. An imbalanced Michelson-interferometer arrangement, with a
perspex block of 25 mm thickness inserted into the longer interferometer arm to
maintain equal image magnification for the two images, was used to obtain a 2n phase
shift for an optical frequency change of 7.25 GHz. The technique provides an additional
phase stepping method in shearography with the advantages of removing an active
phase-shifting component from the interferometer and a greater linearity in the phase
shifts through the diode wavelength modulation.
In vibration measurement, heterodyning and stroboscopic illumination have also been
successfully achieved in a mechanical passive manner. For shearing systems using a
Michelson interferometer, heterodyning was originally difficult to perform. With the
unbalanced optical configuration as used in the phase stepping work, heterodyning has
been demonstrated to measure vibration motion ~5.5 kHz and the diode optical
frequency modulation ~15 GHz. By pulsing the laser diode with an 11% duty cycle,
stroboscopic illumination was performed to obtain cosine fringes along with greatly
improved visibility. Phase stepping methods were then incorporated to automate the
fringe analysis.
