Apparatus and method for high-speed phase shifting for interferometric measurement systems
Described are a method and apparatus for high-speed phase shifting of an optical beam. A transparent plate having regions of different optical thickness is illuminated by an optical beam along a path of incidence that extends through the regions. The transparent plate can be moved or the optical beam can be steered to generate the path of incidence. The optical beam exiting the transparent plate has an instantaneous phase value according to the region in which the optical beam is incident. Advantageously, the phase values are repeatable and stable regardless of the location of incidence of the optical beam within the respective regions, and phase changes at high modulation rates are possible. The method and apparatus can be used to modulate a phase difference of a pair of coherent optical beams such as in an interferometric fringe projection system.
1. A method for modulating a phase difference of a pair of coherent optical beams, the method comprising:
illuminating a transparent plate at a first position on a linear path of incidence with a pair of coherent optical beams, the transparent plate having a pair of parallel surfaces, a first region having a first optical thickness and a second region having a second optical thickness, the linear path of incidence being parallel to the parallel surfaces, each of the coherent optical beams being incident at the transparent plate at one of the first and second regions wherein a phase difference of the pair of coherent optical beams exiting the transparent plate has a first value;
translating the transparent plate from the first position to a second position on the linear path of incidence;
illuminating the transparent plate at the second position with the pair of coherent optical beams so that one of the coherent optical beams is incident at the transparent plate in a region different from the respective region for the coherent optical beam for the first position, wherein the phase difference of the pair of coherent optical beams exiting the transparent plate has a second value;
translating the transparent plate from the second position to a third position on the linear path of incidence; and
illuminating the transparent plate at the third position with the pair of coherent optical beams so that one of the coherent optical beams is incident at the transparent plate in a region different from the respective region for the coherent optical beam for the second position, wherein the phase difference of the pair of coherent optical beams exiting the transparent plate has a third value.
2. The method of claim 1 wherein the illumination of the transparent plate at the first, second and third positions on the linear path of incidence is repeated periodically.
3. The method of claim 1 wherein the first, second and third values of the phase difference are −120°, 0° and 120°.
4. The method of claim 1 wherein the coherent optical beams are focused on a surface of the transparent plate.
5. The method of claim 1 wherein a physical thickness of the transparent plate in the first region is different from a physical thickness of the transparent plate in the second region.
6. The method of claim 1 wherein an index of refraction of the transparent plate in the first region is different from an index of refraction of the transparent plate in the second region.