The invention relates to an optical device and to a process of in-situ monitoring and controlling the composition of molten steel from degassing installations, in real time, applied to the aerosols extracted from the melt. According to the invention, the optical system comprises: a laser source Nd:YAG which emits pulses of the wavelengths of 1064 nm, 532 nm, 355 nm and 266 nm and energy of up to 800 mJ, an optical system for transporting the laser beam and focusing it in the aerosol flow, an optical system for collecting the radiation emitted by the plasma generated on the aerosols, a spectroscopic device for the analysis of the emitted optical signal and a computer for process control and data analysis, allowing identification of spectral lines of interest, measurement of their intensity and determination of plasma parameters and aerosol composition. The process, as claimed, comprises the analysis of the aerosols extracted from the furnace and entrained by the gas flow towards the vacuum pump, throughout the steel degassing process, while continuously correlating and comparing the measured aerosol composition with the composition standards, as recorded after previous measurements performed on steel samples of certified composition.