Low Coherence Distance Measuring Interferometer

Low Coherence Distance Measuring Interferometer
Photograph of the Low Coherence Distance Measuring Interferometer.

The initial motivation of this project was to perform non-contact measurements of the absolute thickness of the gradient index (GRIN) optical materials, without knowing the refractive index of the materials. The key to this project is to retrieve the thickness information only based on the reflection signal. As such, this system can be potentially used to measure the thickness dimension for all plane-parallel reflective samples, including transparent, opaque, inhomogeneous materials.

The interferometer takes advantages of the short coherence length of the broadband light and measures the surface positions of the sample using low-coherence interferometry. By using the Twyman-Green configuration, a 1-inch aperture was measured. The scanning reference arm can probe the surface with a resolution as small as 20 nm, and the overall thickness can be measured. with a repeatability less than 150 nm.