Spectral Phase Shift Interferometry for Refractive Index Monitoring in Micro-Capillaries

Abstract

In this work, we demonstrate spectral phase-shift interferometry operating in the near-infrared wavelength range for refractive index (RI) monitoring of fluidic samples in micro-capillaries. A detailed theoretical model was developed to calculate the phase-sensitive spectral reflectivity when low-cost rectangular glass micro-capillaries, filled with samples with different refractive indices, are placed at the end of the measurment arm of a Michelson interferometer. From the phase-sensitive spectral reflectivity, we recovered the cosine-shaped interferometric signal as a function of the wavelength, as well as its dependence on the sample RI. Using the readout radiation provided by a 40-nm wideband light source with a flat emission spectrum centered at 1.55 µm and a 2 × 1 fiberoptic coupler on the common input-output optical path, experimental results were found to be in good agreement with the expected theoretical behavior. The shift of the micro-capillary optical resonances, induced by RI variations in the filling fluids (comparing saline solution with respect to distilled water, and isopropanol with respect to ethanol) were clearly detected by monitoring the positions of steep phase jumps in the cosine-shaped interferometric signal recorded as a function of the wavelength. By adding a few optical components to the instrumental configuration previously demonstrated for the spectral amplitude detection of resonances, we achieved phase-sensitive detection of the wavelength positions of the resonances as a function of the filling fluid RI. The main advantage consists of recovering RI variations by detecting the wavelength shift of “sharp peaks”, with any amplitude above a threshold in the interferometric signal derivative, instead of “wide minima” in the reflected power spectra, which are more easily affected by uncertainties due to amplitude fluctuations.

A NIR-Spectroscopy-Based Approach for Detection of Fluids in Rectangular Glass Micro-Capillaries

“…Furthermore, they strongly reduce the light scattering that arises when using round-section capillaries. In previous works, we exploited rectangular glass microcapillaries with channel depths of the order of few tenths of a nanometer to measure the real part of the refractive index (RI) of liquids and its variations with respect to a reference sample, either by exploiting the detection of the spectral resonances of these devices, which can be seen as optical resonators [14][15][16], or by inserting them into a Michelson scheme and performing interferometric measurements [17,18]. However, these kinds of analyses are non-specific since the same RI change can be produced by different substances present in the solution.…”

Periodic arrays of plasmonic crossed-bowtie nanostructures interspaced with plasmonic nanocrosses for highly sensitive LSPR based chemical and biological sensing

“…The sensitivity of the sensor is evaluated in terms of spectral shift in the plasmon resonance dip wavelength, Δ λ , with respect to a change in ambient refractive index, Δ n . 46 In this text, the sensitivity is reported from the slope of the linear fit trendline in the plots for shift in plasmon resonance dip wavelength, Δ λ , as a function of the ambient refractive index. The measure of performance of the sensor is also inversely proportional to the FWHM of the plasmon resonance dip because narrower dips are easier to detect.…”

Interferometric Probing of Physical and Chemical Properties of Solutions: Noncontact Investigation of Liquids

“…Microcapillary-based determination of refractive index can be performed for small sample volumes. Bello et al (81) measured the refractive index of liquid samples in a rectangular microcapillary mounted at the end of the measurement arm of a Michelson interferometer. These authors used a near-infrared light source with a full width at half-maximum of 40 nm, and an optical spectrum analyzer detected the interference signal.…”