11 February 2011 Inline coherent imaging of laser processing
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In applications ranging from noncontact microsurgery to semiconductor blind hole drilling, precise depth control of laser processing is essential. Even a priori characterization cannot compensate for material heterogeneity and stochasticity inherent to the material modification process. We image along the machining beam axis at high speeds (up to 312 kHz) to provide real-time feedback, even in high aspect ratio holes. The in situ metrology is based on broadband coherent imaging (similar to the medical imaging modality optical coherence tomography) and is practical for a wide-range of light sources and machining processes (e.g., thermal cutting using a quasi-continuous wave fiber laser, or nonlinear ablation achieved with ultrafast pulses). Coherent imaging has high dynamic range (> 60 dB) and strongly rejects incoherent signals allowing weak features to be observed in the presence of intense machining light and bright plasmas. High axial resolution (~5 μm) is achieved with broadband imaging light but center wavelength can be chosen appropriate to the application. Infrared (wavelength: 1320±35 nm) allow simultaneous monitoring of both surface and subsurface interfaces in nonabsorbing materials like tissue and semiconductors. Silicon based detector technology can be used with near infrared imaging light (804 ± 30 nm) enabling high speed acquisition (>300 kHz) or low cost implementation (total imaging system <10k$). Machining with an appropriate broadband ultrafast laser allows machining and imaging to be done with the same light source. Ultrafast technology also enables nonlinear optical processing of the imaging light, opening the door to improved imaging modalities.
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James M. Fraser, James M. Fraser, } "Inline coherent imaging of laser processing", Proc. SPIE 7925, Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XI, 792516 (11 February 2011); doi: 10.1117/12.877745; https://doi.org/10.1117/12.877745

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