A range of optical fibre-based sensors for the measurement of ethanol, primarily in aqueous solution, have been developed and are reviewed here. The sensing approaches can be classified into four groups according to the measurement techniques used, namely absorption (or absorbance), external interferometric, internal fibre grating and plasmonic sensing. The sensors within these groupings can be compared in terms of their characteristic performance indicators, which include sensitivity, resolution and measurement range. Here, particular attention is paid to the potential application areas of these sensors as ethanol production is globally viewed as an important industrial activity. Potential industrial applications are highlighted in the context of the emergence of the internet of things (IoT), which is driving widespread utilization of these sensors in the commercially significant industrial and medical sectors. The review concludes with a summary of the current status and future prospects of optical fibre ethanol sensors for industrial use.

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Few-mode fibers (FMFs) offer several distinct features from single mode fibers and multimode fibers, in terms of medium core size which is capable of accommodating several transverse modes (2-6 modes). In this work, fiber Bragg grating (FBG) has been inscribed in FMF which exhibits more than one resonant Bragg wavelengths. Few-mode fiber Bragg grating (FM-FBG) can be used as sensing device because of its modal sensitivity to the changes in an ambient environment, such as temperature and refractive index in terms of concentration of the solution. To increase its modal sensitivity for solution concentration, the cladding of FM-FBG is removed, and the Bragg wavelength difference of the modes increases during the etching process. It has been observed that by decreasing the core radius through chemical etching, the Bragg wavelength difference of respective modes increases whereas the power confinement ratio in the core for the respective modes decreases. Therefore we have optimized the core radius as at approximately 7 µm for the enhancement of refractive index modal sensitivities. The fabricated sensor has been characterized with the different concentrations of sodium chloride (NaCl(aq)) solution.

A unique side-polished balloon shaped heterocore structure plastic optical fibre (POF) sensor for real-time measurement of very low to high ethanol concentration in water is reported. The sensor is designed as a large core-small core-large core heterocore structure where small core fibre (SCF) acts as a sensing region, whereas large core fibre (LCFs) are used as input and output light waveguide s as well as to introduce the light leakage in the cladding of SCF at the heterocore structure’s input interface and hence generate the significant evanescent field. The principle of operation of the sensor is based on evanescent field interaction at the interface of modif ied SCF and the liquid boundary. The sensor is characterized for ethanol-water solutions in the ethanol concentration ranges of 20 %v/v to 80 %v/v, 1 %v/v to 10 %v/v, and 0.1 %v/v to 1 %v/v, demonstrating a maximum sensitivity of 54673 %/RIU. The experimentally evaluated high sensitivity of this sensor design for real-time measurement of ethanol concentration in water at different ranges makes it a potential candidate for implementation in the industry as a low-cost and real-time solution for ethanol sensing as well as other RI sensing applications.

A novel long period grating (LPG) inscribed balloon-shaped heterocore-structured plastic optical fibre (POF) sensor is described and experimentally demonstrated for real-time measurement of the ultra-low concentrations of ethanol in microalgal bioethanol production applications. The heterocore structure is established by coupling a 250 μm core diameter POF between two 1000 μm diameter POFs, thus representing a large core—small core—large core configuration. Before coupling as a heterocore structure, the sensing region or small core fibre (SCF; i.e., 250 μm POF) is modified by polishing, LPG inscription, and macro bending into a balloon shape to enhance the sensitivity of the sensor. The sensor was characterized for ethanol–water solutions in the ethanol concentration ranges of 20 to 80 %v/v, 1 to 10 %v/v, 0.1 to 1 %v/v, and 0.00633 to 0.0633 %v/v demonstrating a maximum sensitivity of 3 × 106 %/RIU, a resolution of 7.9 × 10−6 RIU, and a limit of detection (LOD) of 9.7 × 10−6 RIU. The experimental results are included for the intended application of bioethanol production using microalgae. The characterization was performed in the ultra-low-level ethanol concentration range, i.e., 0.00633 to 0.03165 %v/v, that is present in real culturing and production conditions, e.g., ethanol-producing blue-green microalgae mixtures. The sensor demonstrated a maximum sensitivity of 210,632.8 %T/%v/v (or 5 × 106 %/RIU as referenced from the RI values of ethanol–water solutions), resolution of 2 × 10−4%v/v (or 9.4 × 10−6 RIU), and LOD of 4.9 × 10−4%v/v (or 2.3 × 10−5 RIU). Additionally, the response and recovery times of the sensor were investigated in the case of measurement in the air and the ethanol-microalgae mixtures. The experimentally verified, extremely high sensitivity and resolution and very low LOD corresponding to the initial rate of bioethanol production using microalgae of this sensor design, combined with ease of fabrication, low cost, and wide measurement range, makes it a promising candidate to be incorporated into the bioethanol production industry as a real-time sensing solution as well as in other ethanol sensing and/or RI sensing applications.