Optoelectronic speckle shearing interferometry

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.