A New Miniaturised Optical System for Chemical Species Spectroscopic Detection Based on a Scanning Integrated Mach–Zehnder Microinterferometer on LiNbO3

Absorption or emission spectroscopy is a powerful tool for detecting chemical compounds, diluted in fluid media: the sensitivity of this technique depends on the optical path of the source radiation, on the spectral window used for analysis and on the spectrometer performances. In this view, we designed and produced the first prototypes of an integrated scanning Fourier Transform Microinterferometer with Mach–Zehnder geometry, by using MEOS (Micro Electro Optical Systems) technologies. The microdevice, obtained by fabricating integrated optical waveguides on LiNbO3 (LN) crystals, is electrically driven, without moving parts, by exploiting the electrooptical properties of the material. The microdevice operates the Fourier Transform of the input radiation spectral distribution, which can be reconstructed starting from the output signal by means of Fast Fourier Transform (FFT) techniques. The microinterferometer weights few grams, the power consumption is of a few mW and, in principle, can operate in the LN transmittance range (0.36–4.5 μm). The microinterferometer performances were preliminary tested in the (0.4–1.7 μm) spectral window. In the Visible region (0.4–0.7 μm) this microsystem demonstrated a spectral resolution suitable for detecting the characteristic lines of the solar spectrum together with the absorption bands of common gases present in Earth’s atmosphere. In a further experiment we tested its performances for gas trace detection by using a calibrated NO2 optical gas cell, showing the possibility to reveal up to 10 ppb, when suitable optical paths are used. Finally, colorimetry tests for the titration of an organic dye (E131) in alcohol solution are presented.