Development of a multi-sensor system for in-situ process monitoring of femtosecond laser micromachining

Femtosecond Laser micromachining (FLµM) is an effective method for fabricating micromechanical components, moulds, and medical devices with high precision and negligible thermal effects. However, FLµM remains challenging due to a large number of process variables and complex ablation dynamics. To meet the demand for high-quality products and low process cycle time in flexible production environments, in-process sensing of FLµM is desired. However, an automated in-situ quality diagnosis for FLμM remains challenging due to the need for high sensitivity to critical defects and adaptability to process changes. A multi-sensor approach using optical and acoustical systems is a promising strategy for monitoring the ablation regimes and evaluating the microscale structures. This paper describes the development of a multi-sensor system for monitoring FLµM using a structure-borne acoustic emission (AE) sensor together with an off-axis optical emission (OE) sensor. This system acquires sensor signals at a high sampling rate and employs an appropriate statistical data analysis methodology. Additionally, this system proposes a traditional machine vision-based focus detection, surface inspection before and after the process, and beam monitoring with complementary metal–oxide–semiconductor (CMOS) sensors for high precision and robustness of FLμM. The results highlight a significant correlation between the monitoring signals, process parameters, and the machined groove morphology. This work underlines the feasibility of AE- and OE-based sensors for online monitoring of laser-material removal during FLμM. The proposed monitoring technique has the potential to facilitate process control for laser micromachining, which will ultimately result in increased productivity and product quality.