Laserati

The remote sensing of gases in complex mixtures at atmospheric pressure is a challenging problem and much attention has been paid to it. The most fundamental difference between this application and highly successful astrophysical and upper atmospheric remote sensing is the line width associated with atmospheric pressure broadening, ${sim}$ 5 GHz in all spectral regions. In this paper, we discuss quantitatively a new approach that would use a short pulse infrared laser to modulate the submillimeter/terahertz (SMM/THz) spectral absorptions on the time scale of atmospheric relaxation. We show that such a scheme has three important attributes. 1) The time resolved pump makes it possible and efficient to separate signal from atmospheric and system clutter, thereby gaining as much as a factor of 10 $^{6}$ in sensitivity. 2) The 3-D information matrix (infrared pump laser frequency, SMM/THz probe frequency, and time resolved SMM/THz relaxation) can provide orders of magnitude greater specificity than a sensor that uses only one of these three dimensions.

High-power fiber lasers can be incoherently combined to form the basis of a directed high-energy laser system which is highly efficient, compact, robust, low-maintenance and has a long operating lifetime. This approach has a number of advantages over other beam combining methods. We present results of the first field demonstration of incoherent beam combining using kilowatt-class, single-mode fiber lasers. The experiment combined four fiber lasers using a beam director consisting of individually controlled steering mirrors. Propagation efficiencies of ${sim}$ 90%, at a range of 1.2 km, with transmitted continious-wave power levels of 3 kW were demonstrated in moderate atmospheric turbulence. We analyze the propagation of combined single-mode and multimode beams in atmospheric turbulence and find good agreement between theory, simulations and experiments.

Electroluminescence (EL) measurements have been performed on a set of In(Ga)As–GaAs quantum-dot (QD) structures with varying spacer layer growth temperature. At room temperature and low injection current, a superlinear dependence of the integrated EL intensity (IEL) on the injection current is observed. This superlinearity decreases as the spacer layer growth temperature increases and is attributed to a reduction in the amount of nonradiative recombination. Temperature-dependent IEL measurements show a reduction of the IEL with increasing temperature. Two thermally activated quenching processes, with activation energies of ${sim}, $ 157 meV and ${sim}, $ 320 meV, are deduced and these are attributed to the loss of electrons and holes from the QD ground state to the GaAs barriers. Our results demonstrate that growing the GaAs barriers at higher temperatures improves their quality, thereby increasing the radiative efficiency of the QD emission.

The beam properties of 980-nm tapered lasers with separate current drives for the ridge waveguide and tapered sections are analyzed by means of a comparison between simulations and experimental results. The simulations are performed with a new model for this type of tapered lasers, providing a good qualitative agreement with experiments. The observed improvement in the beam quality by a stronger pumping of the ridge waveguide section with respect to the tapered section is attributed to the reduction of the backward field intensity. The simulations show that this improvement, far from being a general rule, depends on the details of the device geometry.

WDM systems cascaded by ${hbox {Er}}^{3+}-{hbox {Tm}}^{3+}$ -codoped tellurite fiber amplifiers pumped by 800- and 980-nm lasers are analyzed for the first time in this paper. We use an amplifier model based on propagation and population-rate equations that are solved numerically using the Runge-Kutta algorithm and Newton's iterative method. In addition to analyzing the performance of the 800-nm pump, we also study the performance improvement after adding a new pump of 980 nm. With this new 980-nm pump, channels in the 1420–1620-nm wavelength region can reach a bit error rate (BER) less than $1times 10^{-9}$ , with a transmission distance of 400 km.

In this work, we present an analytical model describing the density of states and spectral behavior of the ordered nanopore array diode laser. The nanopore structure consists of an ordinary quantum well perturbed by a periodic lattice of energy barriers. It is shown that such a perturbation leads to the formation of energy subbands in both the conduction and valence bands. The theoretically predicted gain spectrum shows excellent agreement with experimental results. Finally, the unique effects of in-plane quantization and periodicity on the intersubband selection rules are described in detail.

Strain-compensated InGaN–AlGaN quantum wells (QW) are investigated as improved active regions for lasers and light emitting diodes. The strain-compensated QW structure consists of thin tensile-strained AlGaN barriers surrounding the InGaN QW. The band structure was calculated by using a self-consistent 6-band $kcdot p$ formalism, taking into account valence band mixing, strain effect, spontaneous and piezoelectric polarizations, as well as the carrier screening effect. The spontaneous emission and gain properties were analyzed for strain-compensated InGaN–AlGaN QW structures with indium contents of 28%, 22%, and 15% for lasers (light-emitting diodes) emitting at 480 (500), 440 (450), and 405 nm (415 nm) spectral regimes, respectively. The spontaneous emission spectra show significant improvement of the radiative emission for strain-compensated QW for all three structures compared to the corresponding conventional InGaN QW, which indicates the enhanced radiative efficiency for light emitting diodes.

A self-consistent model comprising rate equations and thermal conduction equation is used to analyze the influence of self-heating on the carrier occupation, quantum efficiency, and output power of 1.3- $mu{hbox {m}}$ InAs–GaAs quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs). The simulation results show that the poor hole confinement in QDs is due to the thin wetting layer, and increase in QD density and layer number can significantly improve the self-heating effect and quantum efficiency of the device. The output power of the QD VCSEL is mainly determined by the quantum efficiency. High output power can be achieved by the high number of QD layers and QD density. However, there exists an optimized number of QD layers ( $sim$ 15) to achieve the highest output power.

This work reports the properties of stimulated Rayleigh–Bragg scattering (SRBS) from a two-photon absorbing CdSe–Cds–ZnS quantum-rods (QRs) solution in chloroform, excited by 1064-nm and $sim$ 13-ns laser pulses. The two-photon absorbing capability of the scattering medium, as well as the pump threshold, spectral structure, and pulse waveforms of the backward stimulated scattering were measured. Comparing to a pure solvent or an organic dye-solution, the semiconductor QR system has many advantages such as the lower pump threshold, higher energy transfer efficiency, and better photo-physical and photo-chemical stability. The measured output/input characteristic curve shows that the backward SRBS can enhance the optical power limiting performance that is based on two-photon absorption, backward stimulated scattering, and other nonlinear absorption mechanisms. In addition, the backward SRBS beam from our sample medium exhibits a fairly good optical phase-conjugation capability, so that the distortion influence from an inserted aberrator can be automatically removed.

A theoretical model is presented describing continuous-wave operation of diode-end-pumped ${hbox {Er}}^{3+}$ :YAG lasers. Implemented as a numerical computer model it takes into account the full spectral information of the pump and laser transitions as well as all important ionic levels, their Stark splitting, decay schemes, up-conversion processes, excited-state absorption and especially the thermal dependencies of these important parameters. The model is compared to experimental results with good agreement and predicts highly efficient operation of Er:YAG lasers even at high temperatures.