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Spectral behaviors of lasing from semiconductor square ring microcavities with structures for unidirectional laser oscillation were investigated. When a tapered structure was introduced, the lasing envelope shifted to a shorter wavelength region. Statistical estimate of the additional loss caused by the tapered structure was carried out by analyzing spectral data from many sets of cavities with various sizes. When a saw-edged structure was introduced, the unidirectional lasing functioned well but no apparent spectral shift was observed due to negligible additional loss.

Two-phase heat exchanger immersed in a passive containment cooling water storage tank (PCCWST) is a key component of the passive containment cooling system (PCCS) in light water reactors. Condensation and boiling heat transfer phenomena taking place on the inside and outside walls are crucial to the performance of the heat exchanger tube. The performances of such phenomena can be readily affected by the inclination angle of the heat exchanger tube under buoyancy-driven convection conditions in a PCCWST. However, to date no systematic examination of the effects of inclination angle on pool heat exchanger performance has been reported. This paper presents the results of numerical and experimental analyses of how inclination angle affects the performance of a single steam condensation tube immersed in a saturated water pool. To concurrently predict the two-phase heat transfer processes inside and outside the heat exchanger tube, an explicit heat structure coupling of two thermal-hydraulic codes was implemented using open media models. An experimental facility was also constructed to test a single-tube heat exchanger under the same conditions as the simulation. A comparison of simulation data and experimental results obtained for the reference case (tube inclination of 30°) confirmed that the coupled code could predict the heat transfer rate in the pool heat exchanger within the error limits of the experimental measurements. In light of this, additional simulations and experiments were conducted at varying inclination angles, from 3° to 90°. The results of simulations and experimental studies revealed that the heat transfer rate of a heat exchanger tube in a saturated pool is hardly influenced by tube inclination. Although an increase in inclination angle caused the vapor slug to spread around the heat exchanger tube surface, preventing liquid from getting to the surface of the tube, thereby decreasing boiling heat transfer coefficients, this was compensated for by an increase in condensation heat transfer coefficients as the inclination angle increased as a result of accelerated condensate liquid film in the tube. Thus, the overall effect of inclination angle on the thermal performance of a single-tube pool heat exchanger is inconsequential.

When a station blackout accident occurs in a nuclear power plant, active cooling of spent nuclear fuels stored in a water pool using pumps is not available. As a result, the stagnant water in the pool gradually heats up via decay heat from the spent fuels and evaporates, thus exposing the spent fuels to air. This rapidly overheats the spent fuels and produces hydrogen gas due to oxidation of the zircaloy cladding. Accumulation of hydrogen gas can result in harmful detonation in the spent fuel storage building similar to what occurred in the Fukushima accident. In this study, a new concept of a gravity-assisted heat pipe that removes decay heat of spent fuels in a water pool by natural convection of ambient air is proposed. In the design, the condenser at the top of the heat pipe consists of several finned tubes for passive air cooling while the evaporator at the bottom is a single rod submerged in the spent fuel pool, which is called fork-end heat pipe (FEHP). A theoretical analysis model of a FEHP was developed and a laboratory-scale specimen was manufactured and tested for validation. It was found that the developed theoretical analysis model reasonably predicted the heat transfer rate of the FEHP specimen at diverse operating conditions and the operating limit point where the heat transfer rate is maximized due to the counter-current flow limit phenomenon.

Operation properties, such as output power, beam radius, and crystal temperature rise were investigated in a diode-pumped edge-cooled monolithic Yb:YAG laser with ~ 4 W pumping level. The output power showed saturation behavior because of severe temperature rise and poor spatial overlapping between pump beam and lasing mode. Face cooling geometry by using a sapphire plate was also investigated. No apparent saturation behavior in output power was observed due to the reduced temperature rise, which could be attributed to efficient heat removal through the crystal-sapphire interface.

The spatial and temporal evolution of the shock-waves generated by the high energy Nd:Glass laser at KAERI1 were simultaneously measured by Nomarski interferometer and ORVIS. The Nd:Glass laser with pulse energy ~20 J and pulse duration 8 nsec was irradiated onto an Al sample in the air to generate laser-plasma and shock-waves.

This paper reports on the interferometric analysis of laser-induced plasma performed to investigate the spatial and temporal dynamics of a plasma column as a function of the time elapsed after laser-induced ablation on the surface of a copper target. The plasma was driven by focusing Q-switched Nd:YAG laser pulse ( λ = 1064 nm, τ = ~ 5 ns, and E=12. 5 mJ) onto a copper target and the interference patterns were constructed using a Nomarski interferometer. Phase information of the interference patterns was extracted using a fast Fourier-transform method. With the phase information obtained thus the spatial distribution of electron density was inferred from the Abel inversion equation. For the infrared nanosecond pulsed ablation regime, we observed that the resulting plasma expands preferentially toward the incoming laser beam at the early stage of plasma formation. The maximum plasma density was measured to be of the order of ~ 1020 cm-3. As time elapses, the rapid radial expansion of plasma plume and significant reduction of electron density were observed.

A novel and simple microwave-assisted hydrothermal method for synthesizing high-quality TiO2/CdS nanocomposites was developed using TiO2 (P-90), CdCl2, and mercaptopropionic acid (MPA). The optical properties and the physical characteristics of the synthesized TiO2/CdS nanocrystals (NCs) depended on the conditions of microwave irradiation. Samples prepared with the same concentration of precursor were treated at different temperatures from 150 °C to 250 °C by using the microwave power. Consequently, the crystal structure of TiO2/CdS transformed from cubic to hexagonal. Transmission electron microscopy and energy-dispersive X-ray spectroscopy techniques confirmed that the fabricated TiO2/CdS were composed of heterogeneously bonded nanostructures. Analysis of the optical properties showed that the UV absorption spectra had a strong red-shift when the microwave heating temperature increased, which caused an increase in the size of the CdS NCs. The mean radius of the CdS NCs combined with TiO2 tended to increase from 3.7 nm to 13.1 nm. Photoluminescence (PL) measurements were used to investigate the PL peak of the fabricated TiO2/CdS under microwave irradiation at 250 °C, and they revealed a 24 nm Stoke’s shift from the absorption peak.

The 1 kJ Nd:Glass laser at KAERI1 has symmetrically irradiated a SiO2 shell from tetrahedral direction for uniform compression of a spherical target since August 2015. A spherical target chamber with turning mirrors, focusing lenses and monitoring instrument was installed to generate a high density laser-plasma by irradiating four laser beams.

We numerically investigate the generation of wavelength-tunable few-cycle pulses in the visible spectral region through soliton-plasma interactions. We found that in a He-filled single-ring photonic crystal fiber (SR-PCF), soliton-plasma interactions could shift the optical spectra of pulses propagating in the fiber to shorter wavelengths. Through adjusting the single pulse energy launched into the fiber, the central wavelength of these blueshifting pulses could be continuously tuned over hundreds of nanometers, while maintaining a high energy conversion efficiency of >57%. Moreover, we observed that during the nonlinear pulse propagation in the SR-PCF, soliton self-compression effects enhanced the plasma density in the fiber at high pulse energies, which could modulate the phase-matching condition of ultraviolet (UV) dispersive wave (DW) generation. Furthermore, we employed the recently-developed model to study numerically the loss and dispersion of the SR-PCF in its resonant and anti-resonant spectral regions, and demonstrated the remarkable influence of the core-cladding resonance on the process of soliton-plasma interactions. The numerical results demonstrated here pave the way to develop wavelength-tunable, few-cycle light sources in the visible region, which may have considerable application potential in pump-probe spectroscopy and strong-field physics.

A temperature measurement scheme using spectral shift was applied to a diode end-pumped Yb:YAG laser with ∼ 4 W pumping level. The temperature rise was ∼ 90 °C at the input power of 3.3 W.

Laser isotope separation experiments of Gd and B by atomic and molecular methods, respectively, have been performed. Gadolinium-157 was selectively photoionized by means of three linearly polarized dye lasers, the excitation process of which is based on the polarization selection rules. The effect of magnetic field on isotopic selectivity was discussed. 10BCl3 was selectively photodissociated through IR multiphoton dissociation by the irradiation of TEA CO2 laser or free electron laser (FEL). Selectivity was improved by two-color laser irradiation.

A plane wave model has been developed to describe the dynamic behavior of end-pumped CW quasi-three-level lasers. We have derived algebraic equations and analytic expressions from a set of coupled rate equations. The analytic expression predicts that characteristics of a laser are affected not only by the Boltzmann occupation factor of the pump states, but also by that of the laser states. To illustrate the utility of the model, an end-pumped Yb:YAG laser oscillator was investigated and optimized. We have examined the threshold value of the pump beam, pump beam depletion in the gain medium, optimal crystal length, and laser output power for a given pump power. Finally, we have shown the pump and laser intensities and the population of each manifold as a function of crystal length.

A temperature measurement scheme was proposed in a diode end-pumped thin monolithic Yb:YAG laser by analyzing the red-shifting behaviors of each lasing peak. The amount of peak shift was measured on the basis of the threshold lasing spectrum by using a chopped pump beam. In order to determine the effective scale factor, the ratio between the peak shift and the temperature rise, the dynamics of the spectral shift, the output beam profile, and the output power were investigated. The effective scale factor was determined to be about 0.0114 nm/°C in the case of the crystal sandwiched by copper bocks with a hole, wherein the plane stress approximation is valid. On the other hand, the effective scale factor significantly decreased in the case of the crystal sandwiched by sapphire plates.

The spectrum of scattered light by an ionized two-level atom is investigated. The ionization is considered as a damping out of the ordinary two-level system and its effect on the spectrum is extracted. Because of the ionization damping, the central peak in the spectrum changes significantly according to the parameters involved. For a large ionization damping, a central dip is observed; for a small ionization damping, an additional sharp peak appears on top of the central peak.

The spatial and temporal evolution of plasma generated via laser-induced breakdown in air was investigated. The plasma was produced in air using a focused Nd:YAG Q-switched laser (λ = 1064 nm; pulse duration ~5 ns; pulse energy ~60 mJ). The interference patterns of the resulting plasma were measured as a function of time using a Nomarski interferometer. The elapsed times were in the range 59–232 ns. A frequency-doubled Q-switched Nd:YAG laser (λ = 532 nm; pulse duration ~10 ns; pulse energy ~10.5 mJ) was coupled with the Nomarski interferometer to form an interferometric probe beam. The electron density was inferred from Abel inversion and fast Fourier transformation analysis of the recorded interference patterns. The measured electron densities were on the order of ~1018 cm−3. Using the Saha equation, assuming that the plasma is in local thermodynamic equilibrium, and that the ionization reaction is (N → N+ + e), the electron temperatures were estimated to be in the range 17600–15500 K.

In order to study autoionizing states of zirconium, we performed three-photon polarization spectroscopy. Three dye lasers were used as atomic excitation sources. The first two dye lasers charged with rhodamine 570 were pumped by the second harmonics of a Nd:YAG laser. Atoms in the high-lying even parity levels were excited to autoionizing states and ionized by scanning the third dye laser, which was pumped by the third harmonics of the Nd:YAG laser.

An interferometric analysis was performed to investigate the influence of argon (Ar) buffer gas on the characteristics of laser-induced aluminum (Al) plasma at atmospheric pressure. The plasma was produced by focusing a Q-switched Nd:YAG laser pulse (λ=1064 nm, pulse duration ∼5 ns, E=6.0 mJ) onto an Al target. The interference patterns were constructed using a Nomarski interferometer incorporated with a frequency-doubled, Q-switched Nd:YAG laser (λ=532 nm, pulse duration ∼10 ns) that generates an interferometric probe beam. The interferometric measurements were carried out as a function of the elapsed time after the onset of breakdown under the conditions of open air and an Ar gas jet flow (5 l/min). With the injection of an Ar buffer gas jet in the ablation process, an increase in electron density and a preferential axial plasma expansion of the plasma plume were observed during the early stages of plasma formation as a consequence of increased inverse-Bremsstrahlung (IB) absorption efficiency.

Spectral enhancement of a mode-locked fiber oscillator has been proposed by using gain spectral filtering technique. A wide spectral width of 83 nm at 1030 nm, with 5.2 nJ pulse energy was achieved with positive net cavity dispersion. Fiber oscillator output spectrum was analyzed numerically, showing larger gain spectral filtering effect in longer gain medium systems. We show that the gain spectral filtering plays the same role as spectral filter element inside the resonator, hence could replace it in dispersion managed lasers improving the output spectrum and simplifying fiber oscillator designs.

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.

High-resolution laser-induced fluorescence spectroscopy has been applied to investigate the hyperfine structure constants and isotope shifts of high-lying even-parity levels of zirconium in the energy range 34 000–37 000 cm-1. By analysis of the measured spectra, the hyperfine-structure constants of seven levels and isotope shifts of eight transitions are determined.