Monitoring of Femtosecond Laser Micromachining Using Ultra-high Speed Photodiodes: The Effect of Feature Depth on the Optical Process Emission

The femtosecond (fs) pulsed laser is a versatile tool to produce microstructures down to a few microns on all kinds of materials including metals, ceramics and polymers. Generally, the topog-raphy of the fabricated structures is characterized using ex-situ measurement techniques such as confocal microscopy or white light interferometry, which significantly increases the process cycle time. Besides, it is difficult to control the laser process parameters in real-time with offline char-acterization methods. To closed loop the fs laser micromachining (FLµM) process, an in-process sensing strategy is required. However, unlike for laser processing with continuous and short-pulsed lasers, the process monitoring for FLµM has not been fully explored. In this work, a monitoring system based on off-axis ultra-high speed photodiodes was integrated and applied to the detection of optical process emissions during FLµM of stainless steel. Three photodiodes, whose specifica-tions are described hereafter, were used. The reflected process radiation passes through a filter, with a specific wavelength range of visible (500-900 nm), laser beam reflection (1030 nm) and in-frared (1100-1700 nm), focused onto the photodiode with an amplifier circuit. FLµM of a single line ablation at different pulse energies and scanning passes were performed to study the effect of the feature depth on the off-axis photodiode-based monitoring. In addition, a high-speed spectrom-eter was implemented in the FLµM working station to capture the spectral distribution of the plasma emission. The characteristics of process optical emission, temporal evolution of the spec-trometer and photodiode signals, and the effects from laser machining factors were discussed. The proposed monitoring technique using optical emission-based sensor has the potential to facilitate the process control for femtosecond laser micromachining, which will ultimately result in in-creased productivity and quality.