A comparative study of quantitative phase imaging techniques for refractometry of optical waveguides is presented. Three techniques were examined: a method based on the transport-of-intensity equation, quadri-wave lateral shearing interferometry and digital holographic microscopy. The refractive index profile of a SMF-28 optical fiber was thoroughly characterized and served as a gold standard to assess the accuracy and precision of the phase imaging methods. Optical waveguides were inscribed in an Eagle2000 glass chip using a femtosecond laser and used to evaluate the sensitivity limit of these phase imaging approaches. It is shown that all three techniques provide accurate, repeatable and sensitive refractive index measurements. Using these phase imaging methods, we report a comprehensive map of the photosensitivity to femtosecond pulses of Eagle2000 glass. Finally, the reported data suggests that the phase imaging techniques are suited to be used as precise and non-destructive refractive index shift measuring tools to study and control the inscription process of optical waveguides.

This Letter demonstrates a method to simultaneously measure the quantitative-phase signal (QPS) of the observed specimen and the refractive index of its surrounding medium ( n m ) in a time-resolved manner using a micro-structured coverslip. Such coverslips, easily integrated into perfused live-cell imaging chambers, allow to use various quantitative-phase imaging techniques to achieve this dual measurement. Since QPS is crucially dependent on n m , the measurement of the latter paves the way for its manipulation in a controlled manner leading to a QPS contrast modulation for appealing applications, including visualizing the interior of cells.

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