Patent Application
20050179911
The accuracy to which a refractive optical element can be manufactured is fundamentally determined by how precisely the shape of the surface of the optical element can be measured. Physical measurement of a surface, such as using a profilometer, is very time consuming. Interferometry is used to measure the departure of a manufactured optical surface from an ideal optical surface. While interferometry allows straightforward testing of simple surfaces, such as flat surfaces and spherical surfaces, creating an ideal reference of more complicated surfaces with which to compare the manufactured surface is difficult. Further, the number of complicated reference surfaces required to measure a wide range of complicated surfaces is impractical.
Accurate interferometric metrology of aspheric surfaces continues to be complicated by several factors. These include decentration, tilt and aperture error between the optical probe wavefront and the surface under test. All of these factors create interactions between the ideally orthogonal Zemike polynomials. Use of a spherical wavefront to test an aspheric surface is another complicating factor in interferometric aspheric metrology. Even for a theoretical surface where the boundary and center are well defined and there are no coma, astigmatism or tilt aberrations, the radius of curvature (Rc) of the asphere cannot be accurately resolved. This is due to the fact that the merit function for the curve fitting algorithm will diverge as the spherical wavefront Rc approaches the base Rc of the asphere. The best fit Rc is considerably offset from the true Rc.
If an aspheric wavefront is used in the optical probe, the reduced uncertainty would improve the Rc measurement as well as reduce the sensitivity to decentration and aperture matching. Diffractive elements or computer generated holograms (CGH) have been used in conjunction with reference surfaces to extend the usefulness of interferometry for aspheres. However, these techniques still involve validation of many complicated reference surfaces.
The present invention is therefore directed to a method and system of interferometrically measuring optical surfaces using a diffractive reference that substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
It is a feature of the present invention to provide a diffractive reference at different locations throughout an interferometer used to measure an optical surface. It is another feature of the present invention to shape the wavefront to match an ideal surface of the lens under test. It is yet another feature of the present invention to provide a diffractive reference without altering the interferometer.
At least one of the above and other features may be realized by providing an interferometer including a light source outputting a beam, a detector, a stage for mounting a surface under test, a beam splitter creating a probe beam and a reference beam from the beam, the probe beam and the reference beam to interfere at the detector, and a diffractive optic providing a wavefront of an ideal surface of the surface under test.
At least one of the above and other features may be realized by providing a method for measuring an optical surface including: providing an interferometric system having a probe arm and a reference arm, and including a stage for mounting a surface under test; arranging a diffractive optic providing a wavefront of an ideal surface of the surface under test in the interferometric system, detecting an interference pattern including the wavefront, and using this interference pattern to measure the optical surface.
The diffractive optic may be placed between the light source and the beam splitter or in the path of just the reference beam. When the diffractive optic is placed on the stage, the interferogram of the diffractive optic serves as a calibration null for use with a surface under test. The diffractive optic may include a reflective surface.
These and other features of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
The foregoing and other features, aspects and advantages will be described with reference to the drawings, in which:
The present invention will be described in detail through embodiments with reference to accompanying drawings. However, the present invention is not limited to the following embodiments but may be implemented in various types. The embodiments are only provided to make the disclosure of the invention complete and make one having an ordinary skill in the art know the scope of the invention. Throughout the drawings, the same reference numerals denote the same elements.
A diffractive optic may be used to generate an accurate aspheric reference. Since validation of the diffracted wavefront can be accomplished by measurement of flat steps, i.e., the surface is discontinuous in z, the integrity of the diffracted wavefront can be established by a more conventional testing of the mechanical surface structure. Further, the diffractive optic may be used with a variety of aspheres. The diffractive optic may be placed in numerous locations in the interferometer, as illustrated below.
In the examples given below, the basic configuration of an interferometer 10, here shown as a Twyman-Green interferometer remains the same. The interferometer 10, shown in
Light from the light source 12 is directed onto the beam splitter 14, which splits the light into two beams. Typically, the two beams will have roughly the same intensity to provide maximum fringe contrast in the resulting interference pattern. A first beam proceeds to the lens 16, which focuses the beam onto the lens under test 40 and forms a probe beam. If the lens under test 40 is transparent, the first mirror 18 reflects the probe beam back through the lens under test 40 and the lens 16. Otherwise, the lens under test 40 reflects the probe beam back through the lens 16. The translation stage 24 controls the angular and positional adjustment of the surface under test in known manners. The probe beam is then directed back to the beam splitter 14, which directs the probe beam onto the detector 22. The second beam from the beam splitter 14 is directed to a second mirror 20, forming a reference beam. This reference beam is then directed back through the beam splitter 14 onto the detector 22, where it interferes with the probe beam. The detected interference pattern is output to the computer 26 for analysis and display.
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Although preferred embodiments of the present invention have been described in detail herein above, it should be clearly understood that many variations and/or modifications of the basic inventive concepts taught herein, which may appear to those skilled in the art, will still fall within the spirit and scope of the present invention as defined in the appended claims and their equivalents.
