twowavelength

Two-wavelength interferometry, commonly abbreviated as twowavelength, is a measurement approach that uses two distinct optical wavelengths to extract phase information and extend the non-ambiguous measurement range beyond what a single wavelength provides. This technique is widely used in optical metrology and precision sensing to improve range and accuracy.

The core idea is to measure the phase at each of the two wavelengths and combine them to obtain a synthetic wavelength that is larger than either component, enabling absolute distance or surface height measurements over a greater span. The effective synthetic wavelength, often denoted as lambda_s, is approximately lambda_s = (lambda1 * lambda2) / |lambda1 - lambda2|. By obtaining the phases phi1 and phi2 from the two wavelengths, one can compute a coarse distance from the shorter scale and then resolve the 2π ambiguities using the second wavelength, yielding an unambiguous result.

Common implementations include dual-wavelength interferometers that use two diode lasers or two spectral lines from a tunable source, paired with heterodyne or homodyne detection. Phase stepping or other phase extraction techniques provide phi1 and phi2, and specialized phase unwrapping algorithms combine the information to determine the absolute height or surface profile. The approach often relies on precise wavelength control and calibration to maintain accuracy.

Applications span long-range surface profilometry, precision manufacturing metrology, fiber-optic sensing, and geodesy. Advantages include an extended unambiguous range and improved robustness against noise at long distances. Limitations involve sensitivity to refractive index variations between wavelengths, dispersion, and the need for stable, well-characterized light sources and careful calibration.

Twowavelength is related to concepts such as spectral interferometry and phase-shifting methods and is used to overcome the fundamental range limits of single-wavelength techniques.