Papers by Author: Yoshitaka Takahashi

Abstract: Optical sensors are commonly used and most of them detect the variation of light intensity and then if there is the uctuation of light source intensity or change in propagation loss it causes measurement errors. A ring laser sensor is one of the potential candidates which are insensitive to such uctuation, but it has mainly two problems. One is lock-in effect and the other is insensitiveness to detect the sign (polarity) of the sensing signal. To solve these problems a new phase control system has been proposed. It consists of two polarizers, two quarter-wave plates, and one Faraday rotator. Using this system the phase is controlled by the simultaneous rotation of two quarter-wave plates.

Abstract: Digital holography can record 3-D image of the object by using one CCD camera. Surface shape can be measured by changing angles of the object-illuminating mirror. But the measured signals are wrapped because of the periodicity of a sinusoidal function, and then phase unwrapping is needed. We have developed a new modified phase unwrapping method and applied it to surface measurement. Unwrapping errors were significantly reduced in comparison to application of the previous method.

Abstract: In order to develop a new orthogonal dual-frequency laser, we studied a fiber ring laser using a SOA (semiconductor optical amplifier) and aim to apply it to a novel light source for optical heterodyne interferometry. The frequency difference was generated by a linear or circular birefringent medium inserted into the ring cavity. To obtain heterodyne signal optical and electrical filters were introduced.

Abstract: In recent years optical disks have been very popular and in manufacturing process error detection in optical disks becomes very important, especially for the defects whose size of a few millimeters in length and several hundred nanometers in depth. The authors have developed a new system to measure the defects of disk substrates using a Mach-Zehnder interferometer with a phase shifting method. But it was found that optical disks had undulation of the optical thickness and it disturbed the detection of defects. To solve the problem moving-average method was developed and introduced in data processing to distinguish the defect from the undulation, and with modified unwrapping program the defect was visualized by binary image processing.

Abstract: A wavelength-tunable laser is a powerful tool as light source for sensing and its research and development has been studied so far. In order to obtain a new tunable laser the authors have developed a tunable Er³⁺-doped fiber laser in Littman/Metcalf configuration, and incorporating a Galvano mirror, scanning of the lasing wavelength is demonstrated. For the emission range that a semiconductor-based light source hardly covers, a tunable Tm³⁺-Ho³⁺ fluoride fiber laser is also demonstrated.

Abstract: We have studied a SOA-fiber ring laser and demonstrated its application to a fiber ring laser sensor so far. But in our previous study its accuracy and stability was found to be insufficient. Further investigation by numerical analysis suggested that polarization of the propagating light should be stabilized in the fiber ring laser cavity. In order to control the polarization in the cavity, we have improved the laser. The lasing linewidth has reduced significantly, which shows that the polarization is controlled and stabilized well.