Single-step interferometric radius-of-curvature measurements utilizing short-coherence sources
An interferometer includes a short-coherence source and an internal path-matching assembly contained within its housing. Because path matching occurs within the housing of the interferometer, it is removed from external environmental factors that affect measurements. Therefore, a single cateye measurement of an exemplary surface can be performed in advance and stored as a calibration for subsequent radius-of-curvature measurements. In one embodiment, a path-matching stage is incorporated into a dynamic interferometer where orthogonally polarized test and reference beams are fed to a dynamic imaging system. In another embodiment, orthogonal linearly polarized test and reference beams are injected into a remote dynamic interferometer by means of one single-mode polarization-maintaining optical fiber.
1. A method for measuring a radius of curvature of a test sample with a single confocal measurement, the method comprising the following steps:
providing an interferometer comprising a path-matching assembly for minimizing an optical path difference between short-coherence test and reference beams by shifting a mirror along an optical axis of the interferometer, said assembly being housed internally within the interferometer; and
an encoder in the path-matching assembly configured to detect a position of said mirror;
measuring an exemplary test surface at a cateye position;
determining a corresponding cateye position of the mirror in the path matching assembly;
storing said cateye position of the mirror in the path-matching assembly as a reference cateye position;
measuring a different test surface at a confocal position;
determining a corresponding confocal position of the mirror in the path-matching assembly; and
obtaining the radius of curvature of said different test surface from said reference cateye position and said confocal position of the mirror in the path-matching assembly.
2. The method of claim 1 , further including the step of repeating the steps of measuring a different test surface, determining a corresponding confocal position of the mirror, and obtaining the radius of curvature of said different test surface.
3. The method of claim 1 , wherein said interferometer further comprises:
a short-coherence light source producing said test and reference beams;
an interferometric setup adapted to receive the test and reference beams;
a light sensor detecting interferograms produced by the interferometric set up; and
a processor programmed to perform an interferometric measurement of a test surface based on said interferograms.
4. The method of claim 3 , wherein said test and reference beams are orthogonally polarized and said interferometric setup and light sensor are included in a dynamic imaging system for carrying out dynamic interferometry.
5. The method of claim 4 , wherein said interferometric setup is a Fizeau configuration, said test and reference beams are orthogonally polarized by a polarizing beam splitter and respective quarter-wave plates, and said dynamic imaging system includes a pixelated phase-mask sensor.
6. The method of claim 3 , wherein said interferometric setup and light sensor are included in a separate interferometric module, said test and reference beams are orthogonal and linearly polarized and are injected into the module via a single-mode polarization-maintaining optical fiber by aligning one of said beams with a slow-axis of the optical fiber and another of said beams with a fast-axis of the optical fiber.
7. The method of claim 6 , wherein said interferometric setup is a Fizeau configuration, said test and reference beams are orthogonal and linearly polarized by a polarizing beam splitter and respective quarter-wave plates, and said separate interferometric module includes a dynamic imaging system for carrying out dynamic interferometry.
8. The interferometer of claim 7 , wherein said dynamic imaging system includes a pixelated phase-mask sensor.
9. The method of claim 1 , wherein said interferometer is a Twyman-Green interferometer.