Byung Heon Cha

First lasing of the KAERI millimeter-wave free electron laser

Abstract A millimeter-wave free electron laser driven by a recirculating electrostatic accelerator has been developed. The energy and the current of the electron beam are 430 keV and 2 A, respectively. In the first oscillation experiment, the FEL has lased at the wavelength of 10 mm with the pulsewidth of 10–30 μs. The peak power is about 1 kW.

Stability analysis of a diode-pumped, thermal birefringence-compensated two-rod Nd:YAG laser with 770-W output power

Using a ray matrix method, we analyze theoretically how the r and theta polarizations affect the resonator stability condition of two laser heads with or without thermal birefringence compensation. The resonator stability condition is analyzed graphically for a plane-parallel and a concave-concave resonator. The maximum range of stable region is found for both the short and the long cavity. The characteristics of the laser output power are confirmed experimentally in association with the resonator stability condition. The laser output power of 776 W is obtained with the optical-to-optical efficiency of 45% for a plane-parallel resonator with a short crystal separation.

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.

Dependence of the stability and the beam quality on the distance between two rods in a double laser-head resonator

Abstract We investigate the effect of distance between two rods of a double laser-head resonator on stability and beam quality. The characteristics are analyzed in terms of the beam propagation factor ( M 2 ) and TEM 00 mode volume. The results show that, in a thermal-birefringence compensation scheme, low M 2 value can be achieved when each rod is, respectively, placed near each laser mirror. And we find that the distance between two inner principal planes of two rods should be near the sum of the thermal focal lengths to compensate a thermal birefringence as well as to get high power without optical damage.

Output power and polarization characteristics for a diode-side-pumped Nd:YAG rod laser with a diffusive optical pump cavity

We fabricated and analyzed the output power and polarization characteristics of an efficient diode-side-pumped Nd:YAG rod laser with a diffusive optical cavity. The resonator stability conditions are analyzed graphically in the symmetric and asymmetric configurations for a plane-parallel resonator. On the basis of an analysis of the stability condition and mode size for the r and theta polarizations, we clarify how the stable laser operation is possible for various resonator configurations. In particular, we show that the critical stability region of around g1*g2* = 0 provides a stable resonator in the symmetric resonator, even with a slight asymmetry. Experimentally, the output power and polarization characteristics are confirmed in association with the resonator stability condition.

Threshold reduction of stimulated Brillouin scattering by the enhanced Stokes noise initiation

A method of threshold reduction for stimulated Brillouin scattering (SBS) has been demonstrated in a SBS medium combined with a laser dye. It is based on the enhanced Stokes noise initiated by additional field with a Brillouin-shifted frequency component emitted from the laser dye in the SBS medium. Rhodamine 6G dye dissolved in SBS medium (ethanol), which was pumped by a 570 nm wavelength laser, was used to prove the SBS threshold reduction by the additional Stokes fields. The SBS threshold of this solution was reduced to a level about four times lower than that of the pure SBS medium. Also, the experimental results were investigated using the modified formula for the SBS threshold.

The influence of laser gain on stimulated Brillouin scattering in an active medium

The threshold and reflectivity of the stimulated Brillouin scattering (SBS) were investigated in an excited laser gain medium. The SBS in a steady-state was modeled numerically and its results for the threshold and reflectivity were compared with experimental results of SBS in a Rhodamine 6G solution pumped by a frequency-doubled Nd:YAG laser. It is demonstrated that the reflectivity has the maximum value at a certain intensity of an incident laser beam.

Tunable stimulated-Brillouin-scattering resonator started by feedback provided by Bragg scattering from the dynamic grating within an active medium

We report a tunable stimulated-Brillouin-scattering resonator that does not require a starting mirror or a wavelength selector. The resonator is based on the Bragg scattering from the dynamic grating within the active medium itself. The laser dye (Rhodamine 6G dissolved in ethanol) was utilized as the active medium and was pumped by a frequency-doubled Nd:YAG laser. The quality factor (M(2)) of the output beam was 2.2. A single short pulse (<1 ns) with a pumping efficiency of ~4% was observed.

Externally heated copper vapor laser using a carbon heater

An externally heated copper vapor laser using a carbon tubular heater is described. A recrystallized alumina tube of 20‐mm inner diameter is used as the laser tube, which is located inside the tubular heater. When the heating power of the carbon heater is 3 kW, the temperature of the laser tube is 1600 °C. The average laser output power is measured to be 1.2 W at a laser tube temperature of 1500 °C when the charging voltage to the energy storage capacitor is 5 kV, the pulse repetition rate is 5 kHz, and the Ne gas fill pressure is 10 Torr.

Wavefront improvement in an end-pumped high-power Nd:YAG zigzag slab laser

Techniques for wavefront improvement in an end-pumped Nd:YAG zigzag slab laser amplifier were proposed and demonstrated experimentally. First, a study on the contact materials was conducted to improve the heat transfer between the slab and cooling blocks and to increase the cooling uniformity. Among many attempts, only the use of silicon oil showed an improvement in the wavefront. Thus, the appropriate silicone oil was applied to the amplifier as a contact material. In addition, the wavefront compensation method using a glass rod array was also applied to the amplifier. A very low wavefront distortion was obtained through the use of a silicone-oil contact and glass rod array. The variance of the optical path difference for the entire beam height was 3.87 μm at a pump power of 10.6 kW, and that for the 80% section was 1.69 μm. The output power from the oscillator was 3.88 kW, which means the maximum output extracted from the amplifier at a pump power of 10.6 kW.