Effect of sensitizers on flashlamp-pumping efficiency and heat deposition in Er glasses

This report is a study of the atomic iodine laser pumped with nuclear- excited XeBr fluorescence. Preliminary experiments, conducted in the TRIGA reactor investigated the fluorescence of the excimer XeBr under nuclear pumping with {sup 10}B and {sup 3}He, for use as a flashlamp gas to stimulate the laser. These measurements included a determination of the fluorescence efficiency (light emitted in the wavelength region of interest, divided by energy deposited in the gas) of XeBr under nuclear pumping, with varying excimer mixtures. Maximum fluorescence efficiencies were approximately 1%. In order to better understand XeBr under nuclear excitation, a kinetics model of the system was prepared. The model generated the time-dependant concentrations of 20 reaction species for three pulse sizes, a TRIGA pulse, a fast burst reactor pulse, and an e-beam pulse. The modeling results predicted fluorescence efficiencies significantly higher (peak efficiencies of approximately 10%) than recorded in the fluorescence experiments. The cause of this discrepancy was not fully determined. A ray tracing computer model was also prepared to evaluate the efficiency with which nuclear-induced fluorescence generated in one cavity of a laser could be coupled into another cavity containing an iodine lasant. Finally, an experimental laser cell was constructed to verify that nuclear-induced XeBr fluorescence could be used to stimulate a laser. Lasing was achieved at 1.31 micron in the TRIGA using C{sub 3}F{sub 7}I, a common iodine lasant. Peak laser powers were approximately 20 mW. Measured flashlamp pump powers at threshold agreed well with literature values, as did lasant pressure dependency on laser operation.

A dependence of lasing threshold on laser rod diameter is investigated for Cr:Yb:Er phosphate glass under flashlamp pumping. The dependence is linear within limit of diameter variation from 1.6 mm to 2.5 mm and it can be described within the framework of Yb:Er:glass model. Minimum obtained electrical threshold is 2.2 J. A method of the output energy passive stabilization, based on stored energy saturation, is demonstrated. It is shown that the laser output energy can be maintained constant in spite of laser rod heating. Minimum output pulse duration measured is below 17 ns (FWHM) at relatively slow FTIR Q-switching. It is concluded that the output pulse duration can be reduced down to few ns at 10-mJ level of output energy.

Nuclear heating analyses have been performed with the MCNP6.2 and McDeLicious-17 codes for the Test Cell of the accelerator-based IFMIF - DONES (International Fusion Materials Irradiation Facility – Demo Oriented Neutron Source) irradiation facility. Heat density for the IFMIF/EVEDA beam profile with 10x5 cm2 footprint studied with D+ beam energy deposition in the Li jet at the TA d-Li footprint area. •A more focused deuteron beam with a 10x5 cm2 Li footprint caused a factor of 2 increase in heat deposition in Li (peak of 211 kW/cc vs. 110 kW/cc). Considering that the initial temperature of Li entering the target is 300 C, for a 20x5 cm2 footprint the temperature increase to 428.8 C is 128.8 C, while for 10x5 cm2 it rises up to 552.2 C by 252.2 C. That means a more concentrated beam of 10x5 cm2 resulted in a similar ratio of temperature heating up as the heat deposition, delta temperature increased due to the halved beam by the same factor of 2. Operation of two accelerators with double D+ current of 250 mA with IFMIF parameters caused double-times heating of the TC materials. Integral heating in the TC components: one beam IFMIF-DONES (I = 125 mA) vs. two beams IFMIF (I=250 mA). A beam profile with a 20x5 cm2 footprint has been used in both cases.

Optimization of the excitation characteristics of a flashlamp-pumped titanium sapphire laser

A mathematical model of a passively Q-switched solid-state laser based on ytterbium and erbium codoped active media with transverse pumping by linear laser diode arrays has been developed. The gain in the laser is calculated using rate equations taking into account the space-time dependence of the pump radiation intensity in the laser element. The output laser characteristics, the pulse energy, the peak pulse power and the pulse duration, are calculated using analytical equations obtained for a passively Q-switched solid-state laser in the approximation of a "slow" saturable absorber. The model allows one to find the range of parameters of the active element, passive modulator, resonator, and pumping system at which the generation threshold is reached and the laser generates pulses with the required energy and duration. Modeling results were used for the experimental development of lasers with an active element based on phosphate glass doped by ytterbium and erbium ions. The energy and duration of the output light pulses were ≈ 1 mJ, ≈ 40 ns, and ≈ 2 mJ and ≈ 20 ns, depending on the content of ytterbium and erbium ions in the active element, as well as on the initial transmission of the passive modulator and the resonator parameters.

Science and technology of laser glass

Output characteristics of a traveling-wave monomode laser with an intracavity three-level absorber are investigated. The laser frequency is assumed to be symmetrically detuned from the two ground-state sublevels for a $\ensuremath{\Lambda}$-type transition scheme. It is shown that bistability and hysteresis in the laser output, characteristics of a first-order phase transition, may arise much before the absorber saturates. For this reason, bistability may be observed even if the gain medium saturates faster than the absorber, in contrast with the case of a two-level saturable absorber. The physical mechanism for bistable behavior is coherent population trapping which accumulates all absorbing atoms in the ground-state sublevels with a sharp decrease in absorption.

1,4-Diazabicyclo[2.2.2]octane (DABCO) was tested as a flashlamp-pumped laser lifetime-extender with a variety of coumarin dyes and solvents under both air and argon. In a few cases, DABCO could be considered to have extended the lifetime as much as threefold; however, the selection of the appropriate cover gas and/or solvent could often yield a much greater improvement in the absence of DABCO. In general, DABCO was not found to be very valuable as a lifetime-extender. Furthermore, under some conditions DABCO was found to markedly reduce the laser lifetime and output.

The output characteristics of linear cavity Yb-doped double-clad fiber laser have been studied theoretically and experimentally. Analytical expressions are evaluated for threshold pump power, output power and slope efficiency of the laser under steady state conditions. The expression of minimum of fiber length for designing a linear cavity Yb-doped double-cladding fiber laser has been given. Then, the Yb-doped double-clad fiber laser has been investigated experimentally. By exploiting various output coupler, output power, threshold pump power and the slop efficiency have been measured. Comparison between the experimental results and theoretical data has been performed. They are in good agreement with each other.

Output characteristics of a transient Ni-like Ag X-ray laser pumped by a single picosecond laser pulse are demonstrated. The small-signal gain coefficient, beam divergence, output pulse profile and beam dimensions in the near-field were measured. The influence of the pump pulse structure on the lasing process and the output signal is analysed theoretically.

An approximate method of analysis of nonlinear operation of Fabry-Perot lasers including longitudinal electric field dependence and the spatial hole burning effect is presented. The method is based on a semiclassical approach and provides an approximate expression relating the normalized small signal gain to the normalized output power and laser parameters. The influence of the spatial hole burning effect on the laser characteristics is investigated. It is shown that this effect determines the value of the optimal coupling strength, as well as causing changes in the laser characteristics.

Lasing characteristics of Rhodamine 6G dye-doped microdroplets containing highly scattering fat emulsion Intralipid-10% are studied experimentally. Noteworthy findings are that well-defined lasing threshold can be observed and one order or more magnitude enhancement of emission intensity with suitable (optimum) conditions of the Intralipid mixing ratio, in comparison with original neat-dye lasing microdroplets. We present and discuss the measured results of input-output intensities for different dye concentrations and dye-Intralipid mixing ratios in this high-gain laser dye-soft scatterer system and microscope images of spatial distribution of light emission from both the microdroplets containing neat-dye and dye-Intralipid mixture. It was found that almost no-lasing neat-dye microdroplets, which have either much higher or much lower dye concentration, can achieve lasing by substituting suitably certain amounts of the Intralipid, causing multiple light scattering. Spectral measurements of lasing outputs from the Rhodamine 6G dye-Intralipid microdroplets show the tendency of the disappearance of the well-known mode structures, owing to the morphology-dependent resonances of this microspherical cavity inherent to the neat-dye microdroplets. It is our belief that the present results make this novel method of dye-Intralipid microsystem very attractive for a variety of future applications, including diagnostic tools for highly sensitive detection and identification of small quantity objects and species embedded or hidden in highly scattering media, such as biological tissues and cellular structures.

An experimental investigation was made of the main output and electrical parameters of cw diode lasers with an output of 1 W in the spectral range 808 nm at temperatures in the range 10–45 °C. The width of the stripe contact was 100 mm. A numerical simulation yielded the values of the threshold current density, of the external differential quantum efficiency, and of the total efficiency of diode lasers generating multimode radiation. The results obtained made it possible to optimise the design parameters of the cavity.

Output and threshold characteristics of small-bore pulsed xenon ion lasers are presented in detail as a function of current and gas pressure for ranges of these parameters that are consistent with high optical power output in the green-blue spectral region. It has been found that six wavelengths characteristic of xenon exhibit peak output powers greater than 100 watts, from a 5-foot laser tube over a limited (8-24 mtorr) range of xenon tube pressure. Laser action has also been obtained at high peak powers for longer current pulse ( 5-50 \mu s) operation of the tube. In addition, observation of three new laser wavelengths 5340, 5501, and 5590 A is reported. These lines are only observed at very low tube pressures and very high peak currents.

The intracavity photon densities and the initial population inversion density were assumed to be Gaussian distributions in the rate equations of the laser diode end-pumped actively Q-switched intracavity Raman laser. These space-dependent rate equations were solved numerically. In the experiment, an efficient self-Raman laser was realized based on the multifunctional Nd:YVO4 laser crystal with the acoustooptic Q-switch. The output, temporal, and spectral characteristics of the self-Raman laser were investigated experimentally in detail. The performance of the self-Raman laser was studied numerically, and the theoretical results showed the main trends of the intracavity Raman laser, which are in agreement with the experimental ones