Mirau interferometer

[1] The reference arm of a Mirau interferometer is located within a microscope objective assembly.

By changing the z position of the sample, interference images are acquired at a sequence of path (phase) differences: 0, λ/4, λ/2, and 3λ/4.

This orientation is often used in optical profilometers due to the increase in stability between the sample and reference path lengths.

Filling the microscope optics with medium restores the ideal interference conditions.

[3] In immersion Mirau interferometry, the interference patterns needed to reproduce the image are acquired at different times and vibrations, if they exist, prevent maintaining the desirable phase shifts between consecutive frames.

The figure shows the optical path of a Mirau-interferometer. Reference beam (5-4-6) and object beam (5-7-6) have identical optical path length and can thus cause white light interference. Parts of the Mirau interferometer: 1. Lens of the microscope, 2. Semitransparent mirror, 3. Object surface, 4. Reference mirror with reference beam, 5. First reflection of reference beam, 6. Third reflection of reference beam, 7. Reflection of object beam