Application of self-mixing interferometry for depth monitoring in the ablation of TiN coatings

Abstract

Among possible monitoring techniques, self-mixing interferometry stands out as an appealing option for online ablation depth measurements. The method uses a simple laser diode, interference is detected inside the diode cavity and measured as the optical power fluctuation by the photodiode encased in the laser diode itself. This way, self-mixing interferometry combines the advantages of a high resolution point displacement measurement technique, with high compactness and easiness of operation. For a proper adaptation of self-mixing interferometry use in laser micromachining to monitor ablation depth, certain optical, electronical, and mechanical limitations need to be overcome. In laser surface texturing of thin ceramic coatings, the ablation depth control is critically important to avoid damage by substrate contamination. In this work, self-mixing interferometry was applied in the laser percussion drilling of TiN coatings. The ∼4 μm thick TiN coatings were drilled with a 1 ns green fiber laser, while the self-mixing monitoring was applied with a 785 nm laser diode. The limitations regarding the presence of process plasma are discussed. The design criteria for the monitoring device are explained. Finally, the self-mixing measurements were compared to a conventional optical measurement device. The concept was validated as the measurements were statistically the same

Evaluation of Self-Mixing Interferometry Performance in the Measurement of Ablation Depth

“…The peak-to-peak fringe amplitude in this case is around 50 mV. The present fringe amplitude during laser ablation is about five times higher compared with [23], where higher BW (35 kHz) was employed. The results demonstrate that 1-MHz BW level is more suitable as dynamic features of the signal are not compromised with higher signal-to-noise ratio.…”

“…Speckle formation is also expected due to the changes in the surface morphology [32]. The use of side gas has been found to overcome the problems related to plume generation [23]. On the other hand, speckle effects are intrinsically difficult to tackle due to the fact that the measurement beam should pass through the realized hole as it is formed.…”

“…The design criteria are explained in detail in [23]. A GaAlAs LD with a multiquantum well structure (HL7851G from Hitachi, Ibaraki, Japan) was used as the light source.…”

“…The measured average and estimated maximum peak powers are 6 W and 16 kW, respectively. The processing laser is characterized by a delay and ramped emission in the initial phase of the emission [23]. In order to control the number of pulses emitted, the emission delay was measured for all experimented conditions and was added to the effective drilling time for piloting the laser with the correct modulation duration.…”

“…The system also flexibly allows for increasing the temporal resolution, as megahertz level electronic bandwidths (BWs) are achievable. Previous work [23] showed that stable SMI measurements in ceramic coatings ablation were possible in the presence of a side gas jet. Although its potential use in ablation depth monitoring has been proven, the measurement performance of SMI in microdrilling depth measurement has not been studied 0018-9456 © 2016 IEEE.…”