Non-contact optical metrology system to measure simultaneously the relative piston and the relative inclination in two axes (tip and tilt) between two reflective surfaces

A non-contact optical metrology system to measure simultaneously a relative piston and a relative inclination in two axes between two reflective surfaces based on a use of a modified Mach-Zehnder interferometer pointing to two adjacent almost coplanar mirrors, with two light beams traveling along two different optical paths until converging in an interferogram image plane to form an interference pattern, the non-contact optical metrology system comprising:

• a partially coherent light beam that forms two light beams going in two different paths after hitting a first beam splitter:
• a first light beam that is transmitted towards a second beam splitter, aligned with the first beam splitter that illuminates an area of interest of a reference segment, the light is reflected in said segment and travels back to said second beam splitter, there it is reflected towards a third Beam Splitter where after reflection is transmitted through a fourth beam splitter to the image capture system, and
• a second light beam which is reflected by the beam splitter towards the fourth beam splitter and from this it is transmitted by the third beam splitter to illuminate the target mirror segment, which in turn reflects and transmits it through the beam splitters and to the image capture system that captures an interference between the second light beam and the first light beam;
• wherein:
• an optical component is interposed in a target arm through which the second light beam passes, which creates a linear optical path difference between two arms of the interferometer;
• and
• a compensation plate is interposed in a reference arm followed by the first light beam so that the phase gradient in the light is due solely to the change in the optical path introduced by the optical component;
• mechanisms that generate a fixed interference pattern modulated by an amplitude envelope, in which only fringes close to a region in which the optical path difference in the two arms of the interferometer is zero, will be visible;
• when projecting said interference pattern in a camera, three magnitudes measured are decoupled, so the three magnitudes measured are extracted with simple morphological operations and represented by:
• the relative piston by the position of the envelope of the interference pattern,
• the inclination in one axis by the thickness of the fringes and can therefore be measured from the width of a modulation pattern, and
• the inclination in another axis by the inclination of the fringes.