Absolute and relative surface profile interferometry using multiple frequency-scanned lasers

TL;DR

This paper demonstrates a surface profiling method using frequency-scanned lasers in interferometry, achieving nanometer precision and addressing challenges with steep slopes and phase ambiguity.

Contribution

It introduces a multi-wavelength frequency scanning interferometry technique that provides both absolute and relative surface measurements with high precision.

Findings

Achieved 2.7nm RMS surface measurement accuracy.

Successfully measured absolute phase over certain regions.

Identified systematic errors in steep slope areas.

Abstract

An interferometer has been used to measure the surface profile of generic object. Frequency scanning interferometry has been employed to provide unambiguous phase readings, to suppress etalon fringes, and to supersede phase-shifting. The frequency scan has been performed in three narrow wavelength bands, each generated by a temperature tuned laser diode. It is shown, that for certain portions of measured object, it was possible to get absolute phase measurement, counting all wave periods from the point of zero path difference, yielding precision of 2.7nm RMS over 11.75mm total path difference. For the other areas where steep slopes were present in object geometry, a relative measurement is still possible, at measured surface roughness comparable to that of machining process (the same 2.7nm RMS). It is concluded, that areas containing steep slopes exhibit systematic error, attributed to a combined factors of dispersion and retrace error.