Phase and fringe order determination in wavelength scanning interferometry

Area of Science:

• Optical Metrology
• Surface Characterization
• Interferometry

Background:

• Wavelength scanning interferometry (WSI) traditionally faces challenges with fringe order ambiguity.
• Phase shifting interferometry (PSI) offers high accuracy but can be complex.
• Existing WSI methods often lack the repeatability required for demanding applications.

Purpose of the Study:

• To develop an improved WSI method for unambiguous surface height measurements.
• To enhance measurement repeatability and reduce z-axis non-linearity.
• To achieve performance comparable to phase shifting interferometry.

Main Methods:

• A novel WSI approach combining fringe frequency and fringe phase information.
• Resolving fringe order ambiguity using frequency data.
• Integrating frequency-derived and phase-derived height measurements.
• Developing a theoretical model to assess performance under additive noise.

Main Results:

• Achieved sub-nanometre measurement repeatability, a tenfold improvement over frequency-only WSI.
• Reduced z-axis non-linearity (bleed-through/ripple error) by a factor of ten.
• Demonstrated experimental validation of the theoretical model.
• Showcased practical measurement examples illustrating performance gains.

Conclusions:

• The developed WSI method provides unambiguous and highly repeatable surface height measurements.
• The technique offers accuracy and repeatability comparable to phase shifting interferometry.
• This advancement is crucial for high-precision metrology and surface analysis.