Freeform characterization based on nanostructured diffraction gratings

Area of Science:

• Optical engineering
• Metrology
• Nanotechnology

Background:

• In-line characterization of freeform optical elements during production is difficult.
• Previous work presented a sensor for transmission wavefront characterization using a common-path interferometer.

Purpose of the Study:

• To propose and demonstrate an advanced sensor setup for measuring wavefronts generated by freeform elements in reflection.
• To overcome challenges associated with uni-axial illumination, stray light, and back reflections.

Main Methods:

• Development of a novel common-path interferometer for reflected wavefront measurement.
• Implementation of uni-axial illumination to simplify the optical setup.
• Utilization of a highly absorbing amplitude grating based on nanostructured silicon to mitigate stray light and back reflections.
• Adaptation of diffractive components and Fourier filtering for numerical post-processing.

Main Results:

• Demonstration of an experimentally realized measurement system for reflected wavefronts.
• Successful mitigation of stray light and back reflections using the nanostructured silicon grating.
• Comparison of the developed sensor's performance against a commercial Shack-Hartmann sensor.

Conclusions:

• The proposed advanced sensor setup enables effective in-line characterization of freeform optical elements in reflection.
• The use of nanostructured silicon gratings is crucial for maintaining signal-to-noise ratio in uni-axial reflection measurements.
• The developed system offers a viable alternative to existing metrology solutions for freeform optics.