Temperature-induced laser dynamics of wide-aperture vertical cavity surface emitting semiconductor lasers (VCSELs) is under investigation. We describe the dynamics of VCSELs with circular and square apertures using the full system of two-dimensional Maxwell-Bloch equations. The results of numerical simulations in near and far fields are shown in dependence on frequency detuning, which can be presented as function of temperature. Results of simulations are compared with experimental data and theoretical predictions.
Since the early 1970s ablative laser propulsion (ALP) has promised to revolutionize space travel by reducing the 30:1 propellant/payload ratio needed for near-earth orbit by up to a factor of 50, by leaving the power source on the ground. But the necessary sub-ns high average power lasers were not available. Dramatic recent progress in laser diodes for pumping solid-state lasers is changing that. Recent results from military laser weapons R&D programs, combined with progress on ceramic disk lasers, suddenly promise lasers powerful enough for automobile-size, if not space shuttle-size payloads, not only the 4 - 10 kg "microsatellites" foreseen just a few years ago. For ALP, the 1.6-μm Er:YAG laser resonantly pumped by InP diode lasers is especially promising. Prior coupling experiments have demonstrated adequate coupling coefficients and specific impulses, but were done with too long pulses and too low pulse energies. The properties of ions produced and the ablated surface were generally not measured but are necessary for understanding and modeling propulsion properties. ALP-PALS will realistically measure ALP parameters using the Prague Asterix Laser System (PALS) high power photodissociation iodine laser (λ = 1.315 μm, E<sub>L</sub> ≤1 kJ, τ ~ 400 ps, beam diameter ~29 cm, flat beam profile) whose parameters match those required for application. PALS' 1.3-μm λ is a little short (vs. 1.53-1.72 μm) but is the closest available and PALS' 2ω / 3ω capability allows wavelength dependence to be studied.
Time-resolved force measurements and Intensified Charge-Coupled Device (ICCD) imaging techniques were applied to the study of force generation in the laser ablation of water and ice. A transversely excited atmospheric (TEA) CO<sub>2</sub> laser operated at 10.6 μm, 300 ns pulse width, and up to 20 J pulse energy was used to ablate water and ice held in various containers. Net imparted impulse and coupling coefficient were derived from force sensor data and relevant results will be presented for ice and water. ICCD imaging was used in conjunction with time-resolved force measurements in order to determine the dominating physical mechanism under which the thrust is produced. The effect of shock wave generation and propagation, as well as its contribution to the overall impulse imparted to the targets, was examined from the comparison of the timelines for the pertinent phenomena. The process of mass removal was investigated for each case, and specific impulse and efficiency were calculated from the data. Differences in the force-time curves for ice and water will be presented and discussed. Ballistic experiments were conducted in order to corroborate the force measurements.
A combination of techniques including launch ballistics, force sensing, and time-resolved ICCD imaging was applied to the study of the mechanisms of liquid ablation in the irradiance regimes from 10<sup>6</sup>-10<sup>8</sup> W/cm<sup>2</sup>. A TEA CO<sub>2</sub> laser (λ = 10.6 μm), 300 ns pulse width and 9 J pulse energy, was used for ablation of liquids contained in various quartz glass containers in order to examine dependencies on surface tension, absorption depth, <i>etc</i>. Dominant mechanisms of force generation were analyzed in order to determine their characteristics, and the evolution of the liquid surface was studied in depth. Net imparted impulse and coupling coefficient were derived from the force sensor data and ballistics experiments, and relevant results will be presented for various container designs and liquids used. The key differences between surface and volume absorbing liquids was observed. Various mechanisms including plasma formation, vaporization, bulk liquid flow, <i>etc</i>. will be critically examined and their relevance to force generation and propulsion will be determined.
A new mechanism of ultra-deep drilling and related molten material expulsion during high-power short-pulse laser ablation of metals, semiconductors and dielectrics is proposed. In this mechanism ultra-deep (multi-micron) heat penetration and melting depths in these materials are assumed to result from their bulk absorption of thermal short-wavelength con-tinuous and characteristic radiation emitted by hot near-surface ablative laser plasmas. Multi-microsecond delays for expulsion of subsonic jets of micron-size droplets and for re-radiation of UV bursts from the irradiated targets are ex-plained by subsurface explosive boiling in bulk of the resulting ultra-deep melt pool.
The assessment of energy partition between air and solid propellant has been conducted using a TEA CO<sub>2</sub> laser. The experiments were performed by focusing output pulses of the laser (200 ns pulsewidth at 10.6 μm wavelength and ~10.6 J pulse energy) on aluminum targets mounted on a ballistic pendulum. Coupling coefficients and mass removal rates were determined as functions of air pressure, which varied from 1 atm to 3.5 mTorr. The data from both coupling coefficients and mass removal rates show that there is a sharp transition region ranging between 1.0 and 10 Torr. In this region the momentum imparted to the target via air breakdown appears comparable and, at higher pressures, dominating the momentum due to the breakdown on the target surface.
This work summarizes the combination of experimental and digital image processing technique developed for determination of plasma expansion velocity angular profiles. Such profiles were used further for assessment of specific impulses for ablative laser propulsion. The technique uses time-resolved intensified charge-coupled device (ICCD) camera with 18 ns minimum time delay, 100 μm spatial resolution, and 5 ns gating speed. The plasma was formed in vacuum (~ 3x10<sup>-3</sup> Torr) by focusing output pulses of a laser system (100-ps pulsewidth at 532 nm wavelength and ~35 mJ energy) on surfaces of C (graphite), Al, Si, Fe, Cu, Zn, Sn, and Pb targets. Plasma expansion velocity profiles were derived from plume edge contours. Specific impulse (<i>I<sub>sp</sub></i>) was then deduced from the profiles. New <i>I<sub>sp</sub></i> data appeared in excellent agreement with specific impulses derived from force measurements, conducted earlier. Observed angular profiles of plasma edge velocity and integral intensity are reported and discussed.