Masao Takabe

Nondiffracting narrow light beam with small atmospheric turbulence-influenced propagation

A narrow light beam that propagates in the atmosphere with less disturbance than conventional light beams is introduced. The operating method and features of the newly proposed long-range nondiffracting beam (LRNB) are briefly demonstrated. Some experimental results of the atmospheric propagation of this beam at a distance of 500 m are shown in comparison with a conventional collimated beam and a focused beam. The results and related analyses show that the LRNB is much less influenced by atmospheric turbulence than other beams and suggest that the LRNB can apply to many fields.

Application of an exclusively binarized correlation-calculation method to wind velocity measurement by use of stellar scintillation patterns

An exclusively binarized method for the correlation calculation of image processing is presented with application to velocity measurements. The exclusively binarized correlation-calculation method is capable of determining the peak position of the correlation even with a small number of random-pattern images. Because the velocity is deduced from the movement of the correlation-peak position within a time interval, the method is suitable for the velocity measurement of random patterns. The method is applied to the measurement of the upper-atmospheric wind velocity by use of stellar scintillation patterns.

A new data acquisition system for measuring the movement of atmospheric speckle patterns

To measure the movement of atmospheric speckle patterns, we establish a new data acquisition system that consists of a 50 cm telescope, an intensified charge-coupled device (ICCD) camera, a photomultiplier with a pinhole, and two personal computers (PCs) including data acquisition hardware. This system is easy to establish and allows the capture of a high-resolution image. A simple positioning method of the pinhole in front of the photomultiplier is introduced. With 1500 pairs of the measurement data samples, a clear correlation peak can be found by processing them according to the general definition of correlation. Some measurement and analysis results are given.

Laser-ranging scanning system to observe topographical deformations of volcanoes.

We have developed a laser-ranging system to observe the topographical structure of volcanoes. This system can be used to measure the distance to a target by a laser and shows the three-dimensional topographical structure of a volcano with an accuracy of 30 cm. This accuracy is greater than that of a typical laser-ranging system that uses a corner-cube reflector as a target because the reflected light jitters as a result of inclination and unevenness of the target ground surface. However, this laser-ranging system is useful for detecting deformations of topographical features in which placement of a reflector is difficult, such as in volcanic regions.

Atmospheric turbulence measurements through stellar observations

The design of any optical system operating in the atmosphere requires previous investigations of atmospheric turbulence conditions at the systems location. In addition, if outdoor optical measurements are to be useful in assessing the performance of the optical system, or in checking propagation theory, they must be supported by simultaneous direct measurements of the atmospheric turbulence. Stellar observations provide a useful and convenient means for these purposes in the case of ground-satellite optical communications. We first review the refractive index structure parameter profiling techniques based on stellar observations. For implementation we have selected the technique based on spatiotemporal analysis of captured speckle patterns. There is ample evidence that turbulence in the free atmosphere confined to thin, horizontal layers separated by nonturbulent regions. The lifetime of such layers are of the order of several hours. The technique allows us to detect these layers, giving simultaneously the altitude, horizontal mean wind velocity and integrated refractive index value for each 1 km interval. We briefly describe the experimental setup and the measurement technique. We then present some sample results. Initial results show that the current state of the experimental setup and the processing algorithm can detect only the most prominent layers, and altitude accuracy is less than expected. We are currently looking for ways to improve the technique to allow detection of more layers.

Plan for the experiment with the retroreflector in space (RIS) on ADEOS

Experiments on the earth-satellite-earth laser long-path absorption measurements of atmospheric trace species will be carried out with the retroreflector in space (RIS) for the Advanced Earth Observing Satellite (ADEOS). The RIS is a single-element hollow retroreflector with an effective diameter of 0.5 m, which was designed for spectroscopic measurement in the infrared region. The ground system for the experiment employs two single-longitudinal-mode pulsed CO2 lasers. High-resolution atmospheric absorption spectra are measured by using the Doppler shift of the return beam caused by the satellite movement. Vertical profiles of O3 and CH4, and column contents of CFC12, HNO3, CO, N2O, etc. will be obtained from the measured spectra.

Upper atmospheric wind monitoring using correlation of speckle patterns

Observation of stellar speckle-patterns is a useful optical remote-sensing technique of terrestrial atmospheric turbulence. The wind velocity (magnitude and direction) at the turbulence layer is determined by movement of the speckle patterns. The altitude and refractive index structure constant of the turbulence layer are also deduced from the peak height and width of speckle-pattern correlation-peak. A new simple system, which consists of an image intensified CCD and a photomultiplier tube attached on a 50 cm telescope, has been developed for observing the speckle-patterns. The system is utilized to continuously monitor the parameters of atmospheric turbulence ranging from 2 km to 20 km. Results of the monitoring measurement over a year are presented.

Diffraction properties of image-bearing beam from a fixed index grating in a photorefractive medium

We numerically investigated the diffraction properties of an image-bearing beam from a fixed index grating in a photorefractive medium at steady-state, in which the field of the image-bearing beam is decomposed into a combination of Fourier plane-wave components, and the newly induced photorefractive gratings written by the pairs of the incident and its diffracted plane-wave components are taken into account. The numerical results are presented and discussed.