Spectral Optical Readout of Rectangular–Miniature Hollow Glass Tubing for Refractive Index Sensing

Abstract

For answering the growing demand of innovative micro-fluidic devices able to measure the refractive index of samples in extremely low volumes, this paper presents an overview of the performances of a micro-opto-fluidic sensing platform that employs rectangular, miniature hollow glass tubings. The operating principle is described by showing the analytical model of the tubing, obtained as superposition of different optical cavities, and the optical readout method based on spectral reflectivity detection. We have analyzed, in particular, the theoretical and experimental optical features of rectangular tubings with asymmetrical geometry, thus with channel depth larger than the thickness of the glass walls, though all of them in the range of a few tens of micrometers. The origins of the complex line-shape of the spectral response in reflection, due to the different cavities formed by the tubing flat walls and channel, have been investigated using a Fourier transform analysis. The implemented instrumental configuration, based on standard telecom fiberoptic components and a semiconductor broadband optical source emitting in the near infrared wavelength region centered at 1.55 µm, has allowed acquisition of reflectivity spectra for experimental verification of the expected theoretical behavior. We have achieved detection of refractive index variations related to the change of concentration of glucose-water solutions flowing through the tubing by monitoring the spectral shift of the optical resonances.

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

“…The capillary spectral reflectivity is characterized by minima at wavelengths corresponding to the optical resonances of the multilayer structure. The wavelength positions of resonances depend on the thickness of the layers and, in particular, on the RI of the liquid contained in the channel, as demonstrated in previous work [15][16][17]. Moreover, as shown in Figure 2a, the overall spectrum shifts towards higher wavelengths when the sample RI increases from water (black trace) to saline solution (red trace).…”

“…Considering a sensitivity of the order of 360 nm/RIU, previously calculated from the experimental results attained with water and saline solutions, when substituting ethanol with isopropanol we expect a spectral shift of the order of 5 nm-almost as large as the typical wavelength separation between consecutive minima relative to resonances of the same order. Note that it was already demonstrated in previous works [17] that the separation between consecutive minima differs from one to another. This second set of experiments was performed to demonstrate that phase-sensitive detection was feasible, and also suitable, for large RI variations, close to the limit of the unambiguous range.…”

“…Moreover, their symmetric structure and flat surface strongly reduce the issues related to light scattering that arise when using round-section capillaries. As reported in previous works [15][16][17], rectangular micro-capillaries can be envisioned as optical resonators: when they are illuminated by broadband light, the reflected power spectrum exhibits a sequence of minima characterized by a wavelength position that depends on the geometrical parameters of the micro-device and the material filling the channel. In particular, when the RI of the sample increases, the optical resonances shift towards higher wavelengths.…”

“…At each interface between layers with different refractive index, light is partially reflected and partially transmitted. In previous works [15][16][17], the overall theoretical spectral reflectivity R(λ) and transmissivity T(λ) as a function of the wavelength λ were retrieved by recursively applying the Fresnel formulas to calculate the electric field traveling back and forth and the spectral power density of reflected and transmitted signals. In particular, the spectral reflectivity R(λ) is given by:…”

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.…”