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.