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Depth monitoring system during laser processing using KTN-based wavelength-swept light source of 1.3-μm wavelength band

Structural monitoring during laser processing has recently been achieved with optical coherence tomography (OCT). Although a couple of industrial OCT monitoring systems have appeared, most are based on spectral-domain OCT, which uses an 800-nm band light source. In some applications, a longer wavelength light is preferable because of its transmission property. Hence, we constructed a swept-source OCT system using an industrial wavelength-swept light source that outputs a 1.3-μm band light. Since the light source employs an electro-optic crystal of potassium tantalate niobate (KTa1-xNbxO3, KTN) as a driver for sweeping the output wavelength, it has no mechanically moving components and can stably be used in industrial applications that require quantitative analyses. It produces a wavelength-swept light of 6 mW in average power, 1310 nm in central wavelength, and 80 nm in bandwidth with a repetition rate of 20 kHz. The light is derived to an interferometer module, which is composed of fiber components, a reference reflector, and a balanced photo-detector. The module is connected to a probe head, which includes a beam scanner for observing the profile around the laser processing point. The beam scanner is controlled by a voltage waveform that arises from a multifunctional board, which also converts the interference signal to digital data. The obtained data are numerically processed in real time and converted to tomographic images. As an example, we applied our system to the in-process monitor of laser plastic welding, where plates of acrylonitrile butadiene styrene were used as samples.