Multiple wavelength image plane interferometry
First Claim
1. A system for interferometric inspection of an object, the system comprising:
- a tunable laser source for providing a coherent energy beam having a selectable variable wavelength;
a beam splitter for separating the energy beam into a reference beam and an object beam, directing the object beam toward the object for illumination thereof, combining energy reflected from the object and a reflected reference beam to form an image beam, and directing the image beam toward a detector;
a reference mirror for reflecting the reference beam back toward the beam splitter to generate the reflected reference beam, the mirror being positioned generally perpendicular to wavefronts of the reference beam and having an adjustable position in response to a command signal to vary phase of the reflected reference beam, the mirror being positioned such that optical path length of the reference beam differs from optical path length of the object beam;
an attenuator positioned between the reference mirror and the beam splitter at a nonperpendicular angle relative to the reference beam to reduce spurious reflections while attenuating the reference beam;
an aperture stop element having an aperture for controlling speckle size;
an optical element for focussing energy reflected from the object to a point within the aperture;
an imaging lens for focussing the image beam on the detector; and
a computer in communication with the tunable laser source, the reference mirror, and the detector, the computer determining range values for a plurality of points on the object.
10 Assignments
0 Petitions
Accused Products
Abstract
A system for interferometric inspection of an object includes a number of improvements to reduce spurious reflections and provide precision measurement of large objects. A neutral density filter of absorptive glass is used as an attenuator to reduce undesirable reflections which may otherwise result in detector saturation. A wedge-shaped beam splitter having at least one anti-reflective surface is also utilized to reduce unwanted reflections. The system uses multiple wavelength interferometry to provide range information for an object. Additional improvements in precision may be provided by using a wavelength calibration device such as an etalon, a wavemeter, or a reference cell having relatively narrow transmission peaks, to improve the accuracy in determining the laser wavelengths. The wavelength information may be used to more precisely determine range values for the object. The various improvements in precision and accuracy facilitate use of differing optical path lengths for the reference beam and object beam so that overall system size and complexity is reduced.
-
Citations
40 Claims
-
1. A system for interferometric inspection of an object, the system comprising:
-
a tunable laser source for providing a coherent energy beam having a selectable variable wavelength; a beam splitter for separating the energy beam into a reference beam and an object beam, directing the object beam toward the object for illumination thereof, combining energy reflected from the object and a reflected reference beam to form an image beam, and directing the image beam toward a detector; a reference mirror for reflecting the reference beam back toward the beam splitter to generate the reflected reference beam, the mirror being positioned generally perpendicular to wavefronts of the reference beam and having an adjustable position in response to a command signal to vary phase of the reflected reference beam, the mirror being positioned such that optical path length of the reference beam differs from optical path length of the object beam; an attenuator positioned between the reference mirror and the beam splitter at a nonperpendicular angle relative to the reference beam to reduce spurious reflections while attenuating the reference beam; an aperture stop element having an aperture for controlling speckle size; an optical element for focussing energy reflected from the object to a point within the aperture; an imaging lens for focussing the image beam on the detector; and a computer in communication with the tunable laser source, the reference mirror, and the detector, the computer determining range values for a plurality of points on the object. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28)
-
-
29. A multiple wavelength image plane interferometric inspection system for inspecting objects placed in an object space, the system comprising:
-
a tunable diode laser for providing a coherent energy beam at multiple selectable wavelengths; a first lens for focussing the energy beam to form a first point source; a beam splitter for separating the energy beam into an object beam and a reference beam, the beam splitter including at least one surface having an anti-reflective coating thereon; an attenuator positioned at an angle relative to a propagation axis of the reference beam to reduce spurious reflections; a spherical reference mirror for reflecting the reference beam from the attenuator back toward the beam splitter, the reference mirror having a radius of curvature which matches a radius of curvature of the reference beam, the reference mirror being translatable along the propagation axis of the reference beam to provide a selectable phase shift in the reference beam; a collimator interposed the beam splitter and the object space for collimating the object beam such that the object beam has substantially planar wavefronts, the collimator focussing energy reflected from the object space to a second point source on an opposite side of the beam splitter; an aperture stop having an aperture positioned at the second point source for reducing aberration; a detector array for detecting an interference pattern produced by a combination of the reference beam with the object beam; an imaging lens interposed the aperture stop and the detector array for forming an image of the interference pattern on the detector array; and control logic in communication with the tunable laser source, the reference mirror, and the detector array, the control logic operative to control selection of a series of wavelengths for the tunable laser source and generate, commands to position the reference mirror to induce a plurality of phase shifts in the reference beam with each phase shift based on a corresponding selected wavelength, the control logic further operative to process signals generated by the detector array to generate range information for points on an object. - View Dependent Claims (30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40)
-
Specification