High-spatial resolution second-harmonic interferometry: A robust method towards quantitative phase imaging of transparent dispersive materials

Chromatic dispersion is a fundamental optical property of materials related to physical quantities such ad density and chemical composition. The dispersion can be measured by multi-wavelength interferometry which is more robust compared with single wavelength interferometry since common-phase noise, e.g., optical path length fluctuations, are efficiently compensated. Therefore, a sensitive and fast method to measure dispersion with high spatial resolution would be a very efficient and robust tool in metrology, diagnostics, and industrial inspection. A simple, yet very effective and sensitive, method to exploit dispersion is the use of the so-called second-harmonic interferometer (SHI), which is a fully common-path two-color interferometer [1]. In a SHI the fundamental and second-harmonic beams co-propagate through the sample, then the fundamental beam is frequency doubled again, and finally the two second-harmonic beams interfere. The phase shift measured by the SHI is 4π/λ ∫L Δn dl, where Δn = n(λ/2) - n(λ), and L is the geometrical path between the two harmonic converters. The use of a single laser source greatly simplifies the set-up and the optical design compared to typical multi-wavelength interferometers, for an easy implementation also in industrial environments. Recently, high sensitivity SHI have been used to measure electron density in large plasma machines [2], and gas density in pulsed jet [3].