Sungman Lee

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.

Measurement of Absorption Efficiency by a Leakage Power Analysis Method from an Efficient Nd:YAG Rod Laser Pumped by Diode Bars

We proposed a leakage power analysis method to measure the absorption efficiency of a diode side-pumped Nd:YAG rod laser which was fabricated using a 3-fold symmetric gold-coated flow tube. The pump power from the 3 diode bar modules was coupled into the reflector through AR-coated windows. A maximum output power of 150 W with a slope efficiency of 47% was obtained from a Nd:YAG rod of 4 mm diameter at the diode pump power of 434 W, which corresponds to an optical efficiency of 34.6%. We investigated the thermally induced lens and pump light distribution of the 4-mm and 6-mm rods. The high slope efficiency originated from the wing-pumping scheme, uniform pump light distribution and the high absorption efficiency. By fixing the wavelength of the diode at 802.4 nm, we measured the absorption efficiency, using the leakage power analysis method, which agreed well with the results obtained using the Findlay-Clay method. Therefore, the leakage power analysis method was demonstrated to be simpler and more reliable because it gives accurate values without the need to adjust diode wavelength. An absorption efficiency of 94% was measured for the case of a 4-mm rod.

Development and Characterization of a 400-W Slab-type Nd:YAG Gain Module

We have developed a slab-type Nd:YAG gain module based on the techniques of conduction cooling and end pumping. The Nd:YAG slab is end-capped on both ends by undoped pure YAG and is pumped through the end-caps by stacked arrays of laser diode bars. The slabs surfaces of total internal reflection are in contact on both sides with microchannel cooling blocks which are cooled by water circulation. The power oscillator based on the gain module generates more than 400 W at 1-kW pumping with a slope efficiency of 55%. The small-signal gain of the gain module is 10 in a single zig-zag pass, and the amplified beam shows a near diffraction-limited beam quality.

A time-gating scintillation detector for the measurement of laser-induced fast neutrons.

A time-gating scintillation detector, in which a fast high voltage switch is used for gating a channel photomultiplier, was developed for a measurement of laser-induced fast neutrons. The x rays generated from the intense femtosecond laser and the solid target interactions were suppressed selectively and a time-of-flight signal of a laser-generated fast neutron was measured effectively. The detector was used successfully to measure the neutron yield of a femtosecond, deuterated, polystyrene plasma.

Design of a Diode-Pumped Yb:YAG Rod Laser with High Efficiency by Using a Diffusive Optical Cavity.

A diode-pumped Yb:YAG rod laser is designed using a fivefold symmetric diffusive optical cavity to improve the pump absorption efficiency of the pump power. By using a ray-tracing method, the optimum design parameters for the doping density, the radius of the diffusive optical cavity, and the radius of the crystal rod are determined by analyzing the absorbed pump power, the distribution profile of the absorbed power, and the threshold absorbed pump power. Based upon these analyses, we show that a highly efficient Yb:YAG rod laser is feasible with the fivefold symmetric diffusive optical cavity. The pump absorption efficiency of 63.6%, which is calculated for the optimal design parameters, corresponds to a 2.4 times improvement in comparison to that obtained by Sumida et al. [Laser Focus World (June, 1999) 63]. By using the absorbed power distribution, we achieved a more realistic quasi-three-level laser output power calculation. In addition, we developed a realistic code, utilizing the absorbed power distribution, for calculating the output power of a quasi-three-level laser. For an incident pump power of 600 W, the laser output power is calculated to be 126 W with optical-to-optical efficiency of 21% and slope efficiency of 42.9%.

A diode-pumped, eye-safe Er, Yb:phosphate glass laser with 107 mJ output pulse energy at 5 Hz

A compact diode-pumped Er, Yb:phosphate glass laser was developed for applications in eye- safe distance ranging and remote sensing. Under conduction cooling condition of laser diode arrays and laser active medium, the glass laser has the output pulse energy of 107 mJ at 5 Hz, and 80 mJ at 10 Hz, which correspond to optical-to-optical conversion efficiencies of 7.6% and 5.8%, respectively.

A laser-induced repetitive fast neutron source applied for gold activation analysis.

A laser-induced repetitively operated fast neutron source was developed for applications in laser-driven nuclear physics research. The developed neutron source, which has a neutron yield of approximately 4 × 10(5) n/pulse and can be operated up to a pulse repetition rate of 10 Hz, was applied for a gold activation analysis. Relatively strong delayed gamma spectra of the activated gold were measured at 333 keV and 355 keV, and proved the possibility of the neutron source for activation analyses. In addition, the nuclear reactions responsible for the measured gamma spectra of gold were elucidated by the 14 MeV fast neutrons resulting from the D(t,n)He(4) nuclear reaction, for which the required tritium originated from the primary fusion reaction, D(d,p)T(3).