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.
Using light-beam scanning technology based on a potassium tantalate niobate (KTa1-xNbxO3, KTN) single crystal, we constructed a wavelength-swept light source for industrial applications. The KTN crystal is placed in an external cavity as an electro-optic deflector for wavelength scanning without any mechanical operation. Cavity arrangement and mechanism elements are specially designed for long-term stability and environmental robustness. In addition, we updated the handling of the KTN crystal. We used a pair of thermistors for accurate temperature monitoring, and weakly irradiated the crystal with a 405-nm light during operation to achieve drift suppression. We selected a moderate repetition rate of 20 kHz to suit the practical application. The output of the light source was 6.2 mW in average power, 1314.5 nm in central wavelength, and 83.3 nm in bandwidth. The interference fringes of the light enable us to specify the thickness of a wafer sample by the peak positions of the point spread functions. We measured the thickness of a silicon wafer as 3651 μm in the optical path length using a reference quartz plate. The distribution of the obtained values is about 0.1 μm (standard deviation). We experimentally confirmed that this property persists continuously at least over 153 days. Our light source has a remarkable feature: extremely low timing jitter of the sweep. Thus, we can easily reduce the noise level by averaging several fringes, if necessary.
We have developed a highly stable electro-optic KTa1-xNbxO3 (KTN) deflector by enhancing electron transportation through KTN crystal. The amount of current is increased with 405-nm light irradiation to rapidly generate a stable refractive-index change, which induces deflection. The deflection angle is set at 160 mrad within tens of seconds and is kept at that angle for 3,000 hours. The developed deflector has been applied to a wavelength-swept light source to measure the thickness of Si wafers with a 3.6-mm optical length. The precision of 0.1-μm has been continuously achieved corresponding to the stability of the KTN deflector.
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