Refractive index profiling of direct laser written waveguides: tomographic phase imaging

Abstract

We present a technique to measure the refractive index profile of direct laser written waveguides. This method has the potential for straightforward implementation in an existing laser fabrication system. Quantitative phase microscopy, based on the Transfer of Intensity equation, is used to analyse waveguides fabricated with an ultrashort pulsed laser embedded several hundred micron below the surface of fused silica. It is shown that the cumulative phase change induced by the waveguide perpendicular to its axis may be monitored in real-time during the fabrication process. Results are verified through comparison with interferometry. Tomographic measurements using illumination from a high numerical aperture condenser lens are used to infer the waveguide cross-section. Results are compared with measurements of the waveguide cross-section from a third harmonic generation microscope.

Waveguide fabrication in KDP crystals with femtosecond laser pulses

“…4b, with a mean square error of 1.9 Â10 À3 from the measured field profile. These results are in good agreement with measurements based upon quantitative phase microscopy using a polarized source of illumination [24].…”

Low-cost, open-access refractive index mapping of algal cells using the transport of intensity equation

“…Conversely if the intensity is known along the propagation direction of the wave, then any phase objects which modulate the wave's intensity can be identified using the TIE and the phase information at the desired focal plane can be reconstructed. For sufficiently transparent samples only two images along the propagation direction of the wave are needed to obtain the intensity gradient and reconstruct the phase in the focal plane as noted in [19]. However, in this case, in line with [20] and for completeness, three images are used; one depicting the object at the focal plane of interest and two defocused images at symmetrical distances from the focal plane along the propagation path.…”

“…As noted by [19], the 'natural' quantity returned by the TIE is Optical Path Length (OPL). This can be obtained from the φ value obtained above by using Eq.…”

Feature issue introduction: Progress in Ultrafast Laser Modifications of Materials

“…A. Jesacher et. al [10] propose a quantitative phase microscopy, as an simple approach to gain indirect information about the waveguide. The technique requires no specialist equipment, can be easily performed at any point within a 3D waveguide network and is not computationally expensive.…”