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:
providing a 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 first and second optical thicknesses being different from each other;
illuminating the first region of the transparent plate with at least a first beam of a pair of coherent optical beams, wherein a phase difference of the pair of coherent optical beams has a first value;
moving the transparent plate from a first position with respect to the first beam to a second position with respect to the first beam; and
illuminating the second region of the transparent plate with at least the first beam, wherein the phase difference of the pair of coherent optical beams has a second value, the first and second values being different from each other.
2. The method of claim 1 wherein the illumination of the first and second regions of the transparent plate is repeated periodically.
3. The method of claim 1 , wherein the transparent plate further has a third region having a third optical thickness, the third optical thickness being different from at least one of the first and second optical thicknesses, the method further comprising:
moving the transparent plate from the second position to a third position with respect to the first beam; and
illuminating the third region of the transparent plate with at least the first beam, wherein the phase difference of the pair of coherent optical beams exiting the transparent plate has a third value, the third value being different from at least one of the first and second values.
4. The method of claim 3 wherein the illumination of the first, second, and third regions of the transparent plate is repeated periodically.
5. The method of claim 3 wherein the first, second and third values of the phase difference are −120°, 0° and 120°.
6. The method of claim 1 wherein the coherent optical beams are focused on a surface of the transparent plate.
7. 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.
8. 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.