Yasuyuki Okamura

Optical circulator for an in-line-type compact lidar

The application of an optical circulator is demonstrated for an in-line-type lidar. The lidars transmitter and receiver are installed in a telescope. The optical circulator of interest here can separate the transmitting laser beam and the echo lights on the same optical axis. It can also divide the echo lights simultaneously into orthogonally polarized components. An insertion loss of 2.2 dB and isolation of >60 dB for the developed optical circulator are obtained in a laser-transmitting situation. This optical circulator makes it possible to measure the polarization ratio caused by cloud phases with a narrow field of view in an in-line-type lidar operation.

Long-optical-path scanning mechanism for optical coherence tomography.

A new scanning mechanism for changing long optical paths is proposed. This mechanism consists of corner reflectors arranged equally upon a disk and an outer mirror. Rotating the 120-mmϕ disk cases a long-optical-path change in each reflector with a near linearity of more than 40 mm. An optical coherence tomography system is described that confirms the usefulness of the proposed mechanism. Its operating characteristics and accuracy are evaluated by analysis and experiment. The deviation of the optical-path change is less than 1.52% at a reflector rotation angle of ±10°. A high-speed lock-in amplifier is utilized for fundamental measurements of glass samples.

Characteristics of modulated white-LED and their application to electrically controlled spectroscopy

We have investigated characteristics of a commercially available one-chip white-LED based on indium-doped gallium nitride when modulating with an electrical pulse. The observation of a spectrum of the white-LED revealed that it contained four large peaks at wavelengths of 390nm (ultra-violet), 430nm (blue), 510nm (green), and 620nm (red), respectively. When the LED was modulated by use of a squared pulse signal with a 16kHz repetition frequency and an 85% duty cycle, each color exhibited different electric characteristics: ultra-violet and blue colors showed the similar electrical spectrum of the original input signal having three frequency bands (1st band: 16~108kHz, 2nd band: 108~208kHz, 3rd band: 208~320kHz), meanwhile a green color was presented in the two bands and a red component did not manifested in any bands. The measured result can be explained in terms of different response of the fluorescent materials responsible for the light emission. Use of electrical band filters enables to separate a white light into desired one.

Nearly nondiffractive beam for lidar application

This study reports the analysis of the long distance propagation characteristics of the annular beam and its application to lidar. In our analysis, the annular beam is formed by a couple of axicon prisms. The waveform of the annular beam is transformed in to the nearly nondiffractive beam through the propagation. The propagation characteristics can be easily controlled by the waveform of the outgoing annular beam. The center peak intensity, FWHM, and the intensity ratio of the center peak intensity of the transformed nondiffractive beam to the whole beam intensity were examined in the various viewpoints. We also considered the spread and focusing angle of the annular beam, and obtained the critical angle to transform the nondiffractive beam. We confirmed that the permissible error of the optical alignment has an enough margin with the desired beam divergence. The annular beam proves its merit on a co-linear type lidar because of utilization of a large reflecting telescope with high transmitting efficiency and near distance measurement with a narrow FOV.

Modulated white-LED interferometer

ZnSe and InGaN based white-LEDs have been utilized for an interferometer. These white-LED consist of blue-LED light and yellowish green one. When these white-LED were modulated by rectangular wave, there were differences in response speed between blue-LED light and yellowish light. The response speed of blue-LED light of ZnSe type white-LED was 50ns, while that of yellowish light was 5us. The spectral bandwidths of the blue-LED light and the yellowish light were 10nm and 100nm, respectively. Coherence lengths of these lights were 10um and 2um, respectively. Combining the blue-LED light with the yellowish light, we observe an unique interference when scanning the optical path of the low coherence interferometer. We also propose a method for a color separation of an interference in a low coherence interferometer with the ZnSe white-LED modulation. The ZnSe white-LED was modulated with much higher frequency (100kHz) than the Doppler frequency of the above interference. The interference fringe of white light appeared at the upper side on the rectangular modulated light emission, while that of the residual yellowish light was presented at the bottom. The interference fringe of the blue light was derived by subtracting the yellowish light interference from the white light one.

Long-optical-path scanning mechanism for optical coherence tomography

A new scanning mechanism for a long optical path change in an interferometer is proposed. It consists of corner reflectors, arranged on a rotating disk at regular intervals, and an outer mirror. A reference beam in the interferometer is reflected on the corner reflector and the mirror, and then comes back to the same way whenever the reflector moves along to the disk rotation. This action makes it possible to obtain a long path change with nearly linear motion. An optical path length change, a scanning speed, and a repetition rate can be designed suitably. The rotating disk of the diameter 120mm gives the optical path change of more than 40mm. The deviation of the optical path change against the linear motion is less than 0.3%. An optical coherence tomography system with the proposed long path optical scanner has been developed and evaluated the motion characteristics. With the disk size mentioned above, the maximum repetition rate was designed as 15scans/s at the disk rotation of 60rpm. We have demonstrated the fundamental experiment of samples such as stacked slide glasses and a mirror separated from a slide glass. The experiment was in good agreement with the prediction.