Application of Self-Mixing Interferometry for Depth Monitoring in the Ablation of TiN Coatings

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 and 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 fibre 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.