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. 2024 Apr 3;15(5):2767-2779.
doi: 10.1364/BOE.520195. eCollection 2024 May 1.

Interferometer-based chemical sensor on chip with enhanced responsivity and low-cost interrogation

Flaminia Piretta  1   2 , Francesca Samà  1 , Francesca Bontempi  3 , Javier Elaskar  1 , Debora Angeloni  2 , Claudio J Oton  1
Affiliations

Interferometer-based chemical sensor on chip with enhanced responsivity and low-cost interrogation

Flaminia Piretta et al. Biomed Opt Express. .

Abstract

We report experimental results of an interferometric chemical sensor integrated on a silicon chip. The sensor measures refractive index variations of the liquid that contacts exposed spiraled silicon waveguides on one branch of a Mach-Zehnder interferometer. The system requires neither laser tuning nor spectral analysis, but a laser at a fixed wavelength, and a demodulation architecture that includes an internal phase modulator and a real-time processing algorithm based on multitone mixing. Two devices are compared in terms of sensitivity and noise, one at 1550 nm wavelength and TE polarization, and an optimized device at 1310 nm and TM polarization, which shows 3 times higher sensitivity and a limit of detection of 2.24·10-7 RIU.

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Conflict of interest statement

The authors declare that there are no conflicts of interest related to this article.

Figures

Fig. 1.
Fig. 1.
Intensity of the electric field for the reference (a) and improved (b) waveguide designs, in presence of water as top cladding material. (a) TE mode at 1550 nm wavelength, (b) TM mode at 1310 nm wavelength.
Fig. 2.
Fig. 2.
Optical microscope image of the photonic chip. The central part shows magnified areas of the TM grating couplers, TE spirals and the heaters.
Fig. 3.
Fig. 3.
SEM images. In (a) covered spirals and in (b) exposed spirals are shown. The difference between the two branches of the MZI is clear: on the right side spiral the top cladding is removed. The difference is enhanced in the lower pictures, where an over etching is visible in the exposed spiral.
Fig. 4.
Fig. 4.
Sensing system schematic design. The figure describes the light path, starting from the laser source, passing through the chip, where its phase varies with respect to the liquid sliding in the sensing window, being detected by the acquisition board through the photodiode and finally being processed in the PC.
Fig. 5.
Fig. 5.
Fluidic system design. The metal holder is necessary to make the PMMA plate fixed to the chip by using two screws.
Fig. 6.
Fig. 6.
In red the MZI spectra from fiber array alignment in TE polarization (a) and TM polarization (b); in blue the reference signal from the input and output grating couplers.
Fig. 7.
Fig. 7.
Signal detected from TE and TM interferometers during fluidics measurements. The expressed percentages are the glycerol volume concentrations.
Fig. 8.
Fig. 8.
Phase Variation versus glycerol volume concentration for the TE (blue) and TM (red) MZIs.
Fig. 9.
Fig. 9.
Plot showing for TE and TM the time variation of the refractive index of the liquid flowing on the surface of the chip. The expressed percentages are the glycerol volume concentrations.
Fig. 10.
Fig. 10.
Zoom in the time variation of the refractive index, to show the noise level in the measurements for both TE and TM.
See this image and copyright information in PMC

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