Design of a plane-mirror displacement measuring interferometer with enlarged angle acceptance.

In Displacement Measuring Interferometry (DMI) a plane-mirror target allows motion perpendicular to the laser beam, which is a desirable property for motion platforms like XY-stages. However, tip and tilt acceptance of the target mirror for a typical system (double pass optics with a beam diameter of 6 mm and a range of 0.5 m) is limited to about +/-1.5 mRad. Enlarging the angle acceptance to +/- 20mRad increases the applicability of plane mirror DMI systems and enables use in new applications. This presentation introduces a DMI measurement head with active beam steering, achieving an angle acceptance of +/- 20 mRad without increasing measurement head size. The opto-mechatronic design of this measurement head is discussed. Measurement uncertainty due to geometrical effects is shown to be <10nm(3σ) for certain use cases. By employing a lock-in technique, active beam steering is achieved using only a steering mirror and a single photodiode. Measurements demonstrate the capability to track a mirror with <25 urad orthogonality error using this method. Based on a tolerance analysis, angle-dependent measurement error is expected to be <20 nm across the entire +/- 20 mRad range.