Cheol-Jung Kim

A study on a fast measuring technique of wavefront using a Shack-Hartmann sensor

Abstract The measuring resolution and speed for wavefront are important to improve the performance of an adaptive optics system. In this paper, we propose a fast measuring technique with high resolution in the Shack–Hartmann wavefront sensor. The proposed measuring technique of wavefront combines the conventional center of mass algorithm with an estimated weighting factor. The estimated weighting factor is a real value anticipating the real center of mass in a wavefront spot image. This estimated weighting factor can be calculated at the initialization step of an adaptive optics system. We designed a robust adaptive optics system with this proposed measuring algorithm. The measuring accuracy and speed are investigated by using the proposed algorithm compared with the conventional center of mass algorithm in experiments.

Two-channel spatial phase shifting electronic speckle pattern interferometer

A two-channel spatial phase shifting (SPS) electronic speckle pattern interferometer for the deformation measurement of a transient process was developed. The system was composed of two cameras for the simultaneous acquisition of two phase shifted speckle fringe patterns, and several polarization components for SPS. The phase shifted speckle fringe patterns, captured by two cameras, were analyzed by two kinds of phase change calculation algorithms. Applications of the system to measurement of the object deformation and the analysis of the fringe patterns are described.

Auto-focus control and weld process monitoring of laser welding using chromatic filtering of thermal radiation

The thermal radiation from a weld pool is focused on an aperture and the transmitted thermal radiation is monitored at two wavelengths with high-speed single-element detectors. Due to the chromatic aberration introduced in the focusing optics, the transmittance curve of thermal radiation varies by the wavelength. Likewise, the detector field of view varies by the wavelength. Owing to this difference in the transmittance and in the field of view, the local variation in a weld pool can be monitored by processing the two spectroscopic signals from two detectors. In this paper, the algorithms to monitor the weld pool size and the focus shift are presented and the performances of weld pool size monitoring and auto-focus control are shown for a pulsed Nd:YAG laser welding. The size variation monitoring has been applied to the weld depth and weld defects monitoring. The effects of laser power change and weld defects on the weld pool size variation are also shown.

Microroughness Reduction of Tungsten Films by Laser Polishing Technology with a Line Beam

An optical polishing technology utilizing a line shaped excimer laser beam was introduced to reduce the microroughness of the tungsten films deposited onto the Si wafer surface. The results of the microroughness reduction and the comparisons were made before and after the line beam irradiations for as-grown and post-chemical mechanical planarization (CMP) tungsten films as a function of irradiated laser wavelength, in 248 nm and 308 nm, laser fluence, and the irradiated number of pulses. Characterizations of the microroughness were performed by atomic forced microscopy. The root-mean-square (RMS) roughness and peak-to-valley, Rp-v, roughness of the as-grown tungsten films were decreased by about 30%–40% compared with the initial values. The RMS roughnesses of the post-CMP tungsten films were 25–30 A for a 5 µm×5 µm measurement. Fluctuations of the reduced microroughness were mainly caused by the beam inhomogeneity and local distributions of the hot (peak energy) spots along or across the line beam. The lowest RMS roughness of 8.0–8.4 A was obtained for the post-CMP tungsten films by the irradiation of 30 pulses of 308 nm with a fluence of 100 mJ/cm2. Irradiation of the 248 nm and 308 nm homogenized flat-top line beam reduced the peak-to-valley roughness, Rp-v, of the post-CMP tungsten films down to 1/3 of its initial value. It was found that the optimum number of pulses for microroughness diminution of the post-CMP tungsten film is 50 pulses with a fluence of 150 mJ/cm2. The wavelength dependency of the microroughness diminution was not surprising, but 308 nm irradiation showed a slightly better performance with consistent results than the 248 nm irradiation did.

Process monitoring of laser welding using chromatic filtering of thermal radiation

An innovative real-time weld monitoring technique using chromatic filtering of the thermal radiation from a weld pool is developed. The thermal radiation from the weld pool is focused onto an aperture and the transmitted thermal radiation is monitored at two wavelengths with high-speed single-element detectors. Owing to the chromatic aberration introduced in the focusing optics, the transmittance curve of thermal radiation varies according to the wavelength. Owing to this difference in the transmittance, the local variation of thermal radiation from the weld pool can be monitored by recording the two spectral signals from the two detectors. In this paper, the algorithms used to monitor the laser power on the weld specimen and the focus shift of the focusing optics are investigated and the performance of laser power and focus monitoring for pulsed Nd:YAG laser welding is shown.

Measurement of the populations of metastable levels in gadolinium vapor by diode laser-based UV and near-IR absorption spectroscopy

Abstract The metastable populations of gadolinium atoms, which were generated by an electron beam gun, were measured by diode laser-based absorption spectroscopy. Transitions originating out of the ground and low-lying metastable levels were investigated in the spectral range of 390–394 nm. A tapered semiconductor power-amplifier which was seeded by a single-mode external-cavity diode laser was used as a fundamental light source for the second-harmonic generation in the ultraviolet wavelength range. The population appears to be frozen at an atomic excitation temperature of ∼600 K, which is about 1/4 of the heated surface temperature. The results obtained from the ultraviolet absorption spectra are compared to those obtained from the near-IR absorption spectra with wavelength ranges of 673–692 and 773–775 nm.

New centroid detection algorithm for the Shack-Hartmann wavefront sensor

A new center position detecting algorithm of the spot image for the Shack-Hartmann wavefront sensor was developed. The new algorithm is a modified center of weight algorithm, which uses some power of the grey level intensity of the spot images instead of the grey level intensity itself of the spot images. From experiments, the repeatability and accuracy of the center position detection of the spot images of the Shack-Hartmann wavefront sensor which used the new algorithm were improved compared with the conventional center position detection algorithm using the center of weight. Applications of the algorithm to measurement of the displacement of the spot images and the Shack-Hartmann wavefront sensor for measuring wavefront distortion and the experimental results of closed-loop wavefront correction are described in this paper.

Diode Laser‐Excited Optogalvanic and Absorption Measurements of Uranium in a Hollow Cathode Discharge

Optogalvanic spectra for fifty two transition lines of uranium in the wavelength ranges of 662–683, 774–792, and 834–862 nm were measured by using external‐cavity diode lasers. Among these transitions, 860.795 nm and 682.691 nm were chosen for a detailed investigation of the detection limit for uranium by wavelength modulation spectroscopy due to its stronger signal magnitudes. A detection limit of about 2 × 10− 5 absorbance achieved at 860.795 nm is more sensitive than that obtained at 682.691 nm, but the absorption spectrum at 682.691 nm is preferable to determine the isotope ratio due to the narrower hyperfine structure as well as the larger isotope shift. A preliminary result for an isotope ratio determination in a depleted sample is presented.

Stable Isotope Production of 168Yb and 176Yb for Industrial and Medical Applications

We have developed a laser isotope separation technology for the production of the 168Yb and 176Yb isotopes. 168Yb is very useful for the generation of a non destructive testing source, 169Yb. 176Yb can be used to produce 177Lu which is known to be a promising radioisotope for a medical application. For these applications, the abundances of 168Yb and 176Yb isotopes should be enriched to more than 15% and 97%, respectively. Our developed system consists of three dye lasers pumped by a diode-pumped solid-state laser, a Yb evaporator, and a photo-ion extractor. Up to now, we could enrich 168Yb to more than 31% with a productivity of 0.5 mg/h. Also, we succeeded in enriching 176Yb to more than 97% with a productivity of 27 mg/h.

BHP jet stabilization of COIL

A COIL (Chemical Oxygen-Iodine Laser) of 2.2 kW output power has been developed in Korea. The effects of stability and uniformity of the BHP(Basic Hydrogen Peroxide) jet columns on the operation of the COIL are reported. In this paper, the uniform jet speed distribution ofjet SOG (JSOG) in a supersonic COIL is achieved by using the centrifugal pump. And the vane plate in front of BHP jet plate is installed to suppress the turbulent BHP flow generated by the centrifugal pump. The stability and uniformity of BHP jet columns are increased by installing the vane plates. Substantially, the chemical efficiency is increased up to 21 .7 %.