Naohiko Goto

TRANSFORMATION OF A LASER BEAM INTENSITY PROFILE BY A DEFORMABLE MIRROR

We developed a deformable mirror with nine actuators for transformation of a laser beam intensity profile. A circular-cross-section Gaussian beam was successfully transformed into a rectangular-cross-section beam by the deformable mirror with conservation of spatial coherency.

Lidar measurement of constituents of microparticles in air by laser-induced breakdown spectroscopy using femtosecond terawatt laser pulses

We experimentally demonstrated remote sensing of the constituents of microparticles in air by combining laser-induced breakdown spectroscopy (LIBS) and lidar, using femtosecond terawatt laser pulses. Laser pulses of 70 fs duration and 130 mJ energy generated filaments when focused at a focal length of 20 m and the pulses irradiated artificial saltwater aerosols in air at a 10 Hz pulse repetition rate. Na fluorescence was observed remotely at a distance of 16 m using a 318 mm diameter Newtonian telescope, a spectrometer, and an intensified CCD camera. These results show the possibility of remote measurement of the constituents of atmospheric particles, such as aerosols, clouds, and toxic materials, by LIBS-lidar using femtosecond terawatt laser pulses.

Laser beam intensity profile transformation with a fabricated mirror.

A nonaxisymmetric mirror is designed by the same method as a computer-generated hologram for laser beam intensity profile transformation and is fabricated by plasma chemical vaporization machining. We successfully transformed a circular Gaussian beam of a He-Ne laser into a rectangular uniform beam maintaining spatial coherence and using a nonaxisymmetric surface profile mirror. There are ripples in the intensity profile of the transformed rectangular beam. These ripples in the intensity profile result from small ripples on the mirror surface. These results show that we can perform coordinate transformation using these fabricated mirrors, which has so far been possible only by using computer-generated holograms.

Leader effects on femtosecond-laser-filament-triggered discharges

Dynamics of laser filaments in strong nonuniform electric fields is studied with high temporal and spatial resolution. Considerable reduction of the breakdown potential is found and is attributed to a filament-induced leader. Two breakdown modes, fast and slow, are found in 0.4MV positive dc-voltage discharges activated by filaments that are induced by 65fs, 170mJ laser pulses. In the fast mode with duration order of a few microseconds, the filament may acquire the electrode potential and temporarily maintain it, becoming a leader. This gives rise to an average electric field over the attachment instability threshold between a leader head and cathode. Ionization waves precede the breakdown with maximal voltage reduction up to 40% for this mode. The slow mode with its duration order of 1ms appears with a considerably smaller voltage reduction when the leader decays before the secondary streamer; the breakdown delay depends on negative and positive ion mobilities in this case.

Error analysis of SO2 measurement by multiwavelength differential absorption lidar

A multiwavelength method to improve the accuracy of a dif- ferential absorption lidar (DIAL) for measuring SO2 in the lower atmo- sphere is presented. A dual-DIAL method consisting of a combination of two DIAL pairs (three or four measurement wavelengths) is proposed, and the measurement error is estimated from the lidar equation. By the selection of appropriate measurement wavelengths, the effects of ozone and aerosols can be suppressed. In principle, the accuracy can be im- proved to below 1 ppb for 300-m range resolution.

Laser beam-forming by deformable mirror

A rectangular laser beam of uniform intensity is very suitable for laser photochemistry. In this paper, we propose a beam-forming system that consists of two deformable mirrors. One of the mirrors changes the beam intensity and the other compensates for phase distortion. We simulate the beam-forming property using a Fresnel equation solved by a Fourier transformation. We reshaped a Gaussian-like He-Ne laser beam into a beam with a more uniform intensity profile by a simple deformable mirror.

Estimation of differential absorption lidar measurement error for NO2 profiling in the lower troposphere

Measurement error for NO2 profiling in the lower troposphere, including the effects of aerosol backscatter, molecular backscatter, and extinction due to inhomogeneous aerosol distribution, is estimated for vertical NO2 concentration profiles measured by differential absorption lidar (DIAL). Vertical NO2 concentration profiles of 0 to 40 ppb and null profiles are obtained for an altitude of 900 to 2250 m with a 150-m range resolution. The deviation of the null profiles from zero, which provides a measure of how precisely the laser beams were aligned, was ,2.0 ppb. The mean statistical error, systematic error from aerosol, and absorption cross section error were 3.4, ,3.0, and ,1.2 ppb, respectively.

NO2 vertical concentration monitoring by DIAL with high accuracy

Vertical NO2 concentration profiles, which are important in studies of atmospheric chemistry and urban pollution, were measured by a differential absorption lidar (DIAL) based on a pair ofNd: YAG pumped dye lasers. In the experiment, 448.1 nm and 446.8 am were used for the on and off wavelengths, respectively. NO2 concentration profiles of 0-40 ppb were obtained for altitude 900 —2250 m with 150 m range resolution. Null error, which was estimated by the deviation ofthe null profile from zero, was <2ppb. The statistical error, systematic error from aerosols, and error due to uncertainty of absorbtion cross section were ?3.35 ppb, <3 ppb and <1 ppb, respectively. The total error was about 5 ppb. The estimation of aerosol backscatter and extinction error in NO2 measurement due to inhomogeneous aerosol distribution is treated in detail.

Evaluation of differential absorption lidar (DIAL) measurement error by simultaneous DIAL and null profiling

A differential absorption lidar system for measurement of trace atmospheric substances in the visible and UV regions is developed. The system is based on a pair of Nd:YAG pumped dye lasers, each capable of emitting two wavelengths on alternate shots. The measurement error of the system was evaluated by measurement of tropospheric O3 using two identical differential absorption lidar (DIAL) pairs, from which two null profiles and four DIAL profiles are obtained simultaneously. The null profiles, obtained for both the on and off wavelengths used in DIAL measurement, are useful to evaluate systematic DIAL measurement error due to beam misalignment. The null profiles show that this systematic error is at most 1.3%. The total DIAL measurement error is about 5%, most of which can be attributed to statistical error.

Plasma density control in a low-pressure RF resonant field

A theoretical study of plasma density control in a RF resonant field has been carried out. The equation expressing the RF resonant field is derived from the dispersion relation for electrostatic electron waves propagating at an angle relative to the magnetic field. Calculations were executed by varying the magnetic flux density at 0.133 Pa and demonstrate the RF resonant field in the condition under which parameters satisfy the equation. The RF resonant field allows an electric field to be induced in bulk plasma, which generates ions in the bulk as a result of electron impact ionization. The low magnetic flux density, 5 mT, facilitates control of the plasma density in the bulk by allowing adjustment of the extent of the ionization even at low pressures such as 0.133 Pa.