New Approach To MWADI
A New Approach to Multiple Wavelength Absolute Distance Interferometry
Brief:
The main objective of this PhD programme is to find an optical measurement to locate the exact position of an object in 3D space using multiple wavelength interferometry.
Multiple-wavelength interferometry turns out to be of a great interest for large distance measurements with very great accuracy for industrial applications.
A first investigation of the different possible applications for this project has demonstrated that firms using CMM (Coordinate Measuring Machines) could have a lot of interest in it. CMM are really unwieldy and this idea could therefore help them to be more transportable and more widely used at a lower cost.
Figure 1: LH 65 Wenzel CMM
Deva Electronics Controls Ltd based in Birkenhead is collaborator for this project. Deva manufactures a range of CMM Controllers which provides a high performance and cost-effective solution for the OEM and retro-fit CMM market.
The aimed precision for this project is approximately of 100nm for a volume of 1m3.
Basic Principle:
The Michelson interferometer (Figure 2) is probably the most widely used instrument in interferometry and our project is based on its principle.
Figure 2: Michelson's Interferometer
It consists, in essential, of a half-silvered mirror, transmitting and reflecting equal amounts, which divides a beam into two parts of equal intensity. The first one reflects off the half-silvered mirror, goes to the top mirror and then reflects back, goes through the semi-transparent mirror, to the detector. The other first goes through the semi-transparent mirror, to the mirror on the right, reflects back to the semi-transparent mirror, then reflects from the half-silvered mirror into the detector.
Theory:
The proposal is to develop and implement a multiple scanning wavelengths Michelson's interferometer. This system would be based on the geometry of this interferometer and on using multiple wavelengths. The figure 3 shows the envisaged principle.
Figure 3: Multi-Wavelengths System Principle
The idea would be mainly based on working over some distance "d", such that the result is an infinite bright fringe.
An estimate of "d" with accuracy within 10μm is given by the CMM.
By changing the wavelength many times and by moving each time the mirror in order to reinstate the infinite fringe condition a set of equations will be obtained.
Then by using different methods of calculations and the estimate of "d" given by the CMM, the value of "d" should be found out with the aimed accuracy of 100nm.
Laetitia Tremblais.
