Relative optical wavefront measurement in displacement measuring interferometer systems with sub-nm precision

Area of Science:

• Metrology
• Optical Engineering
• Laser Interferometry

Background:

• Displacement measuring interferometer systems are susceptible to various error sources.
• In heterodyne interferometry, non-ideal wavefronts and detector walk-off of laser sources increase measurement uncertainty.
• Accurate wavefront characterization is crucial for precise displacement metrology.

Purpose of the Study:

• To introduce a novel wavefront measurement tool for coaxial heterodyne laser sources.
• To quantify the relative phase difference between two laser source frequencies in commercial interferometer systems.
• To enhance the accuracy of displacement measurements by addressing wavefront errors.

Main Methods:

• Utilized standard commercial optics and existing phase measurement equipment.
• Employed a multimode fiber tip as a local sampling aperture for wavefront scanning.
• Reconstructed wavefronts by analyzing the phase difference between a scanning fiber's beat frequency and a reference beat frequency.

Main Results:

• Achieved a phase resolution of approximately 25 picometers (λ/25000 at 632.8 nm).
• Demonstrated a spatial resolution of approximately 60 micrometers.
• Exhibited repeatability better than 1 nanometer over a one-week period.

Conclusions:

• The developed method effectively measures wavefronts in heterodyne interferometer systems.
• This technique offers high resolution and repeatability, crucial for accurate displacement metrology.
• The tool enables precise characterization of laser source wavefronts, mitigating a key source of measurement uncertainty.