Adaptive pinhole point diffraction interferometer based on a liquid crystal on silicon spatial light modulator

Area of Science:

• Optical Metrology
• Adaptive Optics
• Interferometry

Background:

• Conventional adaptive optics (AO) systems, often paired with common-path interferometers, struggle to measure large-asphericity surfaces due to aperture and resolution constraints.
• Existing AO systems are limited to measuring surfaces comparable to the beam spot size (10-20 mm) and employ deformable mirrors (DM) with limited lateral resolution.
• The need for precise metrology of complex aspheric surfaces in advanced manufacturing, like EUV lithography, necessitates improved measurement techniques.

Purpose of the Study:

• To introduce an adaptive pinhole point diffraction interferometer (PPDI) integrated with a liquid crystal on silicon spatial light modulator (LCOS-SLM).
• To overcome the limitations of conventional AO systems in measuring large-asphericity and high-order aspheric surfaces.
• To enable scalable and high-resolution metrology for aspheric surfaces in precision optical manufacturing.

Main Methods:

• Integration of a PPDI with a LCOS-SLM to dynamically modify diffracted wavefronts and compensate for high-order aspheric aberrations.
• Utilizing the LCOS-SLM's high-definition phase modulation for synthesizing aspheric wavefronts via Zernike polynomial-based patterns (e.g., Z4 defocus term).
• Employing phase-shifting interferometry for precise surface profile retrieval and experimental validation on parabolic and 6th-order aspheric mirrors.

Main Results:

• Experimental validation demonstrated measurement accuracy with root-mean-square (RMS) deviations below λ/25 for both parabolic and aspheric mirrors.
• The LCOS-SLM-driven wavefront compensation effectively addressed high-order aberrations and expanded the lateral measurement range.
• The integrated PPDI and LCOS-SLM system eliminated aperture constraints and demonstrated scalability for larger apertures.

Conclusions:

• The proposed adaptive PPDI with LCOS-SLM offers a scalable solution for rotationally symmetric aspheric surface metrology.
• LCOS-SLM overcomes the low resolution limitations of traditional deformable mirrors, enabling high-order aberration modulation.
• This technique is crucial for applications requiring precise metrology of complex optical surfaces, such as extreme ultraviolet lithography and precision manufacturing.