Method and apparatus for automated focusing of an interferometric optical system

An interferometric apparatus for automated focusing of an optical system on a test surface disposed for relative axial movement between the test surface and optical system and having a detector upon which a cyclically-varying interference beam impinges, comprising:

• means connected to the detector for providing a first signal indicative of distributive position of the impinging interference beam relative to an electro-optical center of the detector;
• first means for determining a first time instant when a first point occurs in each cycle of the interference beam;
• second means for determining a second time instant when a second point, 180 degrees out of phase with said first point, occurs in each cycle of the interference beam;
• means for sampling the first signal at each of said first and second time instants to provide a respective first and second sample of the first signal for each said cycle of the interference beam;
• means for adding the first and second sample for each said cycle of the interference beam to provide a focus error signal;
• processing means for determining a required direction of axial movement toward focus of the test sample relative to the optical system as a function of the focus error signal such that a positive state of the focus error signal relative to a predetermined value indicates required relative movement of the test surface in a first axial direction, a negative state of the focus error signal relative to said predetermined value indicates required relative movement of the test surface in a second axial direction opposite the first direction, and equality of the focus error signal with said predetermined value indicates that the test surface is in focus so that no relative movement of the test surface is required;
• and
• means for axially moving the test sample relative to the optical system.

A method for automated focusing of an optical system on a test surface disposed for relative axial movement between the test surface and optical system in an interferometric apparatus in which a cyclically-varying interference beam impinges on a detector, comprising the steps of:

• providing a first signal indicative of distributive position of the impinging interference beam relative to an electro-optical center of the detector;
• determining a first time instant when a first predetermined point occurs in each cycle of the interference beam;
• sampling the first signal at each said first time instant to provide a first sample of the first signal for each said cycle of the interference beam, the plural first samples defining a focus error signal;
• determining a required direction of axial movement toward focus of the test sample relative to the optical system as a function of the focus error signal such that a positive state of the focus error signal relative to a predetermined value indicates required relative movement of the test surface in a first axial direction, a negative state of the focus error signal relative to said predetermined value indicates required relative movement of the test surface in a second axial direction opposite the first direction, and equality of the focus error signal with said predetermined value indicates that the test surface is in focus so that no relative movement of the test surface is required;
• and
• moving the test sample relative to the optical system in said required direction toward focus as indicated by the state of the focus error signal.