Resins chemical transformations are a key issue in composite manufacturing due to the influence in the mechanical behaviour of the composite material. Even if it is possible to find different techniques to monitor the degree of curing of thermoset resins based on the changes on the electrical properties, is also possible to monitor the degree of curing by the changes on the refraction index. Accurately measuring the surrounding refractive index (SRI) is therefore essential for predicting curing depth and conversion profiles. A range of optical techniques has been employed to monitor SRI variations, including Abbe refractometers, low-coherence interferometry (LCI), Fabry-Perot interferometers, long-period fiber gratings (LPFG), and Tilted Fiber Bragg gratings (TFBG). These approaches rely on fundamental principles such as Snell’s law, interferometry, and mode coupling. In this work it is proposed to monitor the refractive index of photocurable thermoset resins. This parameter depends on both their density and molecular polarizability. During photopolymerization, volume shrinkage increases density while reducing polarizability. Experimental evidence indicates that density-related effects dominate, influencing the overall SRI during curing. In photocurable resins, curing depth is limited by the attenuation of light due to absorption by monomers and photoinitiators, as well as scattering and refraction at filler interfaces. A Mach-Zehnder Interferometer (MZI) integrated on a photonic integrated chip (PIC) is proposed to monitor real-time and high-sensitivity changes of SRI during resin curing. This on-chip approach, unlike conventional refractometric techniques, offers compact, inline measurements suitable for industrial applications. The MZI comprises a fully coated reference arm and a partially coated sensing arm in direct contact with the resin. As the resin’s SRI evolves, light coupling between guided and radiated modes alters the effective index, the associated phase differences, and further shifts the interference pattern. These changes, quantified via Optical Backscatter Reflectometry (OBR), yield deeper insights into curing kinetics and support improved process control in composite manufacturing.