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Abstract

Laser treatment has emerged into a cornerstone of industrial manufacturing. This creates the demand to understand in detail the involved physical processes and their dynamics, to control the process, and to keep guard in-situ the laser manufacturing steps. Given the contactless interaction of light with matter, particularly optical methods appear most suitable for probing laser processing. This chapter reviews the current scientific and technological state of analyzing laser-based material processing by short and ultrashort optical pulses. For that, sequential passive interrogation of material properties (via reflection, absorption, scattering, diffraction, interference, or specific nonlinear responses) can be employed through so-called pump-probe techniques (where the time-resolution is not given by the detector response time but by the duration of an electromagnetic probe pulse that is interrogating the transient scenery at a controlled delay (typ. fs to \mu s) with respect to the arrival of the pump laser pulse). These approaches are capable to analyze on microscopic to macroscopic scales a plethora of different processes involved in laser processing, such as heating, melting, ablation, plasma- and shockwave-formation, optical breakdown in surrounding liquids and gases, shielding and accumulation effects, surface solidification and restructuring, etc. A special focus is set on the variety of ultrafast optical imaging schemes for visualizing the dynamics of laser processing at the surface and in the bulk of the irradiated materials, including photography and microscopy, holography, tomography, ptychography, and coherent diffractive imaging. The chapter provides a historic survey, highlights important scientific and technological developments and breakthroughs, discusses selected applications, explores the requirements and current limitations, and sheds light on emerging future trends.