Speckle interferometry

Description

In holographic interferometry, invented by Stetson and Powell in 1965, two holographic recordings were made on the same photographic plate, one obtained before object deformation and one after. In the 1970's, electronic recording media started to replace photographic plates; today, the CCD-detector is in common use. The spatial resolution of electronic recording media was, however, not nearly as high as for holographic plates.

The observed speckle pattern could be thought of as a "fingerprint" of the illuminated area in the sense that the observed pattern is unique for the microstructure of the specific surface area. Another area will give rise to a totally different random speckle pattern. When the surface area is moved or deformed, the observed speckles in the image plane will also move accordingly.

In speckle interferometry the phase change of each speckle before and after deformation is recorded, instead of the position of the speckle as in speckle correlation. The size of the speckles will affect how sensitive the system is to decorrelation due to large movements of the object. If the aperture is circular, the speckle size in the x- and y- directions (perpendicular to the propagation direction), is given by: σx, y = 1.22 λ f# (where f# is the effective f-number of the imaging system). The speckle size on the propagation direction is given by: σz = 8 λ f#2. This means the speckle has the shape of a cigar and it is much larger in the z-direction than in the x , y-directions.

Results

We built a Mach-Zender like interferometer setup (see figure bellow) in order to imagine and characterize small vibrations of a square steel plate, 200x200x1 mm. The vibrations were induce to the plate with a shaker and the plate was attached by the center.


The vibration modes of the plate for various frequencies [0.51, 1.55, 2.5, 3.05, 4.18, 4.53, 4.56, and 0.22 kHz] are shown bellow. Note that the amplitude of the vibrations was on the order of three fringes (949.2 nm @ the He-Ne laser wavelength of 632.8nm).