Nano-Scale Full-Field 3-D Surface Displacement Measurements Using ESPI


Student: Filipe Zannini Broetto
Supervisor: Gary S. Schajer

Renewable Resources Laboratory
Dept. Mechanical Engineering
University of British Columbia
Vancouver BC, Canada V6T 1Z4


Electronic Speckle Pattern Interferometry (ESPI) provides a valuable means to measure full-field nanometre-scale displacements of active surfaces. This type of measurement provides an effective and very revealing measurement tool for non-destructive testing and for a wide range of scientific applications. Typical ESPI instruments make measurements in a specific direction, called the sensitivity direction. Depending on the optical configuration used, this direction could be within plane, out of plane, or some combination. In many practical applications, it would be very valuable to have a full 3-dimensional displacement measurement. A few such instruments presently exist, but they are very complex because they are essentially just three 1-dimensional interferometers combined together. To make things work, many complex optical components are needed, notably including a very high quality (and rather costly) laser light source.


The objective in this project is to design, build and demonstrate a simplified 3-dimensional ESPI interferometer that has a simple geometric design that requires a minimum number of optical components and that can operate using a conventional laser diode as the light source.


The proposed ESPI interferometer design uses a double diffraction grating to divide the light from the laser souces into separate paths to create the interferometer function. In this way, a parallelogram optical geometry is created that causes all optical path lengths to be uniform throughout the measured image field. Ths path length uniformity greatly relaxes the requirement for laser source coherence (a measure of spectral purity), so allowing a conventional laser diode to be used as the light source. This use of a laser diode enables substantial reductions in both physical size and cost of the apparatus.

The proposed design uses two in-plane interferometers with perpendicular sensitivity directions. They are deliberately designed to have different incident angles so that when combined together, they are also sensitive to out-of-plane displacements. Thus, by selecting the measurement axes in different combinations, it is possible to separate a full 3-dimensional surface displacement measurement with nanometre sensitivity.


A prototype interferometer has been designed and built. All work was done in-house by lab students using the UBC Department of Mechanical Engineering machine shop facilities. They have conducted an extensive series of experiments with the equipment to verify its operation. A journal paper is presently in preparation.