Measurement of sugar percentage or ºBrix (1 ºBrix=1% sugar mass percent) is crucial in many applications, such as fruit juice, wine making, carbonated beverage industry and the sugar industry among others.
Different ºBrix sensors have been proposed for use in several areas, including industrial process monitoring, quality control in the food industry or biomedical applications, which requires precise, fast, robust, portable, reusable and cost-effective devices.
The inherent characteristics of the fiber optic devices such as low weight, small size, electromagnetic immunity or easy wavelength multiplexing fulfil all the previously mentioned requirements [1–2]. Different interrogation techniques have been developed using fiber optic sensors for the measurement of sugar concentration, such as those based on interferometry  (visit our post about broadband light source interferometry), gratings  (see also our previous post for a description of these devices) or SPR .
A particular case of optical fiber refractometer for ºBrix measurement is also described in , which consists of indium tin Oxide (ITO) coated D–shaped optical fiber (Corning™ SMF-28 polished fiber) as it is represented in the detail of Figure 1.
In this application example D-shaped fiber was obtained from Phoenix Photonics™ with a polished length of 1.7cm and an attenuation peak of 15 dB at 1550 nm in high index oil (1.5). ITO thin-films were fabricated using a DC-Sputter deposition process (K675XD Sputter Coater from Quorum Technologies™) with a partial pressure of argon of 9×10-2 mbar and intensity of 150 mA. The ITO target 99.99% of purity was purchased from ZhongNuo Advanced Material Technology™.
The fabrication of the previously described structure (see detail of Figure 1) enables to obtain maximum attenuation bands at specific wavelengths, known as Lossy Mode Resonances (LMR). As it is mentioned in , the LMRs vary as a function of the thickness of the thin-film (ITO in this case) as well as the dielectric properties of both, the thin-film (ITO) and the surrounding medium.
Therefore, the refractive index of the surrounding media can be obtained as a function of the resonance wavelength for fixed thin-film thickness and material properties. Resonance wavelength can be obtained using a typical transmission interrogation setup (see Figure 1).
The interrogation setup consists of a broadband light source (see our previous post about SLED light sources) that covers the resonance wavelength working range connected to a polarization controller that permits to tune the LMR and followed by the sensitive fiber optic structure, which is finally connected to an optical interrogator.
Previous setup is used to determine the ºBrix when the sensitive structure is immersed in glucose–water solutions with different ºBrix: 5º, 10º, 15º, 20º, 25º and, 30º. The LMR wavelength shift of the transmittance peaks can be observed in Figure 2.
When the refractive index (sugar content) of the surrounding medium increases, the different transmittance peaks related to LMR shift to larger wavelengths. In particular, the LMR experiments a shift of 205 nm (from 1387 nm when the °Brix is 5°to 1592 nm when the °Brix is 30°). These values correspond to sensitivities of 8.2 nm/°Brix with an R2 factor of 0.9961 for a linear approximation.
These results, represented in Figure 2, confirm that LMRs generated by ITO coatings are highly sensitive to surrounding medium refractive index (SMRI) variations and enable an accurate determination of sugar concentration.
Written by J.J. Imas
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