Partial coherence interferometer with measurement ambiguity resolution
AssigneeQuality Vision International, Inc.
InventorDavid B. Kay
Life of this patent
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Abstract
A partial coherence interferometer incorporates a focusing system for resolving measurement ambiguities. A focus-sensing beam is directed through a common objective with the measurement beam of the interferometer for conveying the beams to and from a test surface. An unambiguous measuring range is equated to a predetermined range of focusing errors.
Claims24
A partial coherence interferometer of a type that employs a measuring beam having a given spectral bandwidth comprising
- a test arm for conveying a test beam portion of the measuring beam along a pathway encountering a test surface,
- a reference arm for conveying a reference beam portion of the measuring beam along a pathway encountering a reference surface,
- the test arm including a focusing optic for focusing the test beam onto the test surface and for collecting a reflected portion of the test beam from the test surface,
- a spectrally sensitive detector system coupled to the test and reference arms that monitors interference as a function of wavelength throughout a range of the measuring beam spectral bandwidth as a measure of unsigned optical path length differences between the test and reference arms, and
- a focus detector system calibrated with the interferometer for distinguishing optical path length differences between the test and reference arms that are significantly greater than zero from optical path length differences between the test and reference arms that are significantly less than zero for resolving ambiguities associated with the measure of unsigned optical path length differences between the test and reference arms.
A partial coherence interferometer that measures optical path length differences traversed by test and reference beam portions of a measuring beam by monitoring interference variation over a range of wavelengths of the combined beams comprising:
- test and reference arms for conveying the test and reference beam portions of the measuring beam from a light source,
- the test arm including a focusing optic for focusing the test beam onto the test surface and for collecting a reflected portion of the test beam from the test surface,
- a focus detector system including a focus arm that conveys light reflected from the test surface through the focusing optic to a focus detector, and
- a focus of the focusing optic being related to a given optical path length difference between the test and reference arms so that the focus detector system can distinguish between optical path lengths of the test arm that are significantly shorter than or longer than the optical path length of the reference arm.
A method of resolving a measurement ambiguity of partial coherence interferometric measurements based on unsigned optical path length differences between test and reference beams comprising steps of
- generating a measuring beam having a given spectral bandwidth,
- dividing the measuring beam into a test beam propagating along a test arm and a reference beam propagating along a reference arm,
- directing the test beam along the test arm through a focusing optic to and from a test surface,
- recombining the test and reference beams,
- monitoring rates of interference variation over a range of wavelengths of the recombined test and reference beams as a measure of unsigned optical path length differences between the test and reference arms,
- conveying light reflected from the test surface through the focusing optic to a focus detector for measuring focusing errors, and
- distinguishing between optical path lengths of the test arm that are significantly shorter than or significantly longer than the optical path length of the reference arm based on the measured focusing errors.
Dependencies are inferred from the claim text.