Figure 2
From: Focus Tracking System for Femtosecond Laser Machining using Low Coherence Interferometry
(a) The experimental setup: femtosecond laser (FSL) pulses are transmitted through the polarized beam splitter (PBS) and then divided by a 50:50 beam splitter (BS). Part of the FSL radiation is directed to a reference arm after passing a neutral density filter (NDF), and is reflected by the mirror (M2) towards the spectrometer. The other portion of the FSL is focused on the sample’s surface, performing the ablation, and a fraction is backscattered to the BS and spectrometer. The back reflected light also reaches the PBS and, after going through a bandpass filter (BPF), is imaged through a CCD camera for visual inspection. The sample is positioned on a XYZ translation stage. (b) The processing diagram for the system, where a “goal” OPD position - which coincides with the focus of the system - is defined. During machining, the software continuously analyzes the OPD deviation from the “goal”, and decides whether an action on the XYZ translation stage should be carried out.
