We report on the fabrication and testing of a fiber optics sensor based on multimodal interference effects, which aims at the detection of different types of sweeteners dissolved in water. The device, which has a simple structure, commonly known as the SMS configuration, is built by splicing a segment of commercial-grade, coreless multimode fiber (NC-MMF) between two standard single-mode fibers (SMFs). In this configuration, the evanescent field traveling outside the core of the NC-MMF allows the sensing of the refractive index of the surrounding media, making it possible to detect different levels of sugar concentration. The optical sensor was tested with aqueous solutions of glucose, fructose, and sucrose in the concentration range from 0 wt% to 20 wt% at room temperature. The proposed device exhibits a linear response with a sensitivity of 0.1835 nm/wt% for sucrose, 0.1687 nm/wt% for fructose, and 0.1694 nm/wt% for glucose, respectively, with a sensing resolution of around 0.5 wt%. Finally, we show that, despite having similar concentration behavior, some degree of discrimination between the different sugars can be achieved by assessing their thermo-optical response.

Fiber optics sensors based on multimodal interference (MMI) have proven effective for refractometry of liquid samples. Here, we extend these capabilities to demonstrate that aqueous solutions with a similar refractive index (RI), which at room temperature are indistinguishable at the same concentration, can be discriminated against based on their thermo-optical response. We used an MMI sensor with the standard singlemode–multimode–singlemode architecture, where a section of no-core multimode fiber provides environmental sensitivity to the fiber surroundings. The proposed idea has been tested on aqueous solutions of tris and fructose, whose RI has a similar dependence on concentration. Indeed, we verified that they produce indistinguishable wavelength shifts as a function of concentration, measuring 0.2179 nm/% for tris and 0.2264 nm/% for fructose. Then, by varying the temperature in a controlled manner, from 25 °C to 45 °C in 2.5 °C increments, the distinct thermo-optic response can be unveiled for the two samples, which now permits differentiating them. Thermal sensitivities of 0.14433 nm/°C for tris and 0.1852 nm/°C for fructose were observed. This optical sensor requires no specific preparation or specialized equipment because the temperature range needed to achieve thermo-optical discrimination is accessible. Therefore, the measurement protocol can be incorporated into commercial refractometers equipped with temperature control.

In this work, we report on the fabrication and testing of a fiber optics sensor based on multimodal interference (MMI) that is capable of measuring sugar concentration in aqueous solutions. The sensor has a simple structure, built by splicing a segment of multimode fiber (MMF) between two single-mode fibers (SMS architecture). The sensor’s operating mechanism is based on the spectral shift due to changes in the effective refractive index (RI) of the media surrounding the MMF. The optical sensor was tested with fructose and sucrose, which were diluted in distilled water in a concentration range from 0% to 18.5%. The proposed device exhibits a linear response with a sensitivity of around 0.17524 nm/% and 0.16321 nm/% for sucrose and fructose, respectively. In addition, this optical sensor shows the advantage of simple construction, low cost, and linear response, it does not require additional processes or coatings on the optical fiber.