Ce3+:LiCaAlF6 crystal for high-gain orhigh-peak-power amplification of ultraviolet femtosecond pulses and newpotential ultraviolet gain medium: Ce3+:LiSr0.8Ca0.2AlF6

To develop high-peak-power ultrashort pulse laser systems in the ultraviolet region, a large Ce³⁺:LiCaAlF₆ (Ce:LiCAF) crystal, a tunable ultraviolet laser medium with large saturation fluence and broad gain spectrum width, was grown successfully with a diameter of more than 70 mm. To demonstrate high small signal gain, a four-pass confocal amplifier with 60 dB gain and 54 μJ output energy was constructed. Chirped pulse amplification (CPA) in the ultraviolet region was demonstrated using Ce:LiCAF for higher energy extraction. A modified bow-tie-style four-pass amplifier pumped by 100-mJ 266-nm 10-Hz pulses from a Q-switched Nd:YAG laser had 370-times gain and delivered 6-mJ 290-nm pulses. After dispersion compensation, the output pulses can be compressed down to 115 fs. This is the first ultraviolet, all-solid-state high-peak-power CPA laser system using ultraviolet gain media, and this demonstration shows further scalability of the Ce:LiCAF CPA system. Additionally, a new gain medium, Ce³⁺ :LiSr_0.8Ca_0.2AlF₆, with longer fluorescence lifetime and sufficient gain spectrum width over 18 nm was grown to upgrade this system as a candidate for a final power amplifier gain module     

VUV laser in the Lyman band of molecular hydrogen pumped by fstitanium-sapphire laser pulses

We report lasing at 160 nm in the Lyman band of molecular hydrogen. The laser is pumped by 200 mJ/150 fs pulses from the ATLAS titanium-sapphire laser at our institute. The pump pulses are focused at an angle of incidence of 60° onto a 9-cm-long gold target to a line focus, generating traveling-wave excitation. With 80 mbar of hydrogen in the target chamber we measure an average gain of 1.1 cm^-1 and achieve a total gain-length product of 10. The evaluation of the far-field pattern shows that the beam originates from a region with an electron density of 5×10¹⁵ cm^-3. A simple model of the H₂ laser is presented which explains the main part of our observations and supports a pump mechanism of photoelectron pumping     

Hot-carrier reliability comparison for pMOSFETs with ultrathinsilicon-nitride and silicon-oxide gate dielectrics

The degradation of 100-nm effective channel length pMOS transistors with 14 Å equivalent oxide thickness Jet Vapor Deposition (JVD) Si₃N₄ gate dielectric under hot-carrier stress is studied. Interface-state generation is identified as the dominant degradation mechanism. Hot-carrier-induced gate leakage may become a new reliability concern. Hot-carrier reliability of 14 Å Si₃N₄ transistors is compared to reliability of 16 Å SiO₂ transistors     

Miniature erbium:ytterbium fiber Fabry-Perot multiwavelength lasers

We demonstrate stable simultaneous lasing of up to 29 wavelengths in miniature 1- and 2-mm-long Er³⁺:Yb³⁺ fiber Fabry-Perot lasers. The wavelengths are separated by 0.8 (100 GHz) and 0.4 nm (50 GHz), respectively, corresponding to the free spectral range of the laser cavity. The number of lasing wavelengths and the power stability of the individual modes are greatly enhanced by cooling the laser in liquid nitrogen (77 K). The polarization modes and linewidth of each wavelength are measured with high resolution by heterodyning with a local oscillator. The homogeneous linewidth of the Er³⁺:Yb ³⁺ fiber at 77 K is determined to be ~0.5 nm, from spectral-hole-burning measurements, which accounts for the generation of a stable multiwavelength lasing comb with wavelength separations of 0.4 nm     

GaInNAs: a novel material for long-wavelength semiconductor lasers

GaInNAs was proposed and created in 1995 by the authors. It can be grown pseudomorphically on a GaAs substrate and is a light-emitting material having a bandgap energy suitable for long-wavelength laser diodes (1.3-1.55 μm and longer wavelengths). By combining GaInNAs with GaAs or other wide-gap materials that can be grown on a GaAs substrate, a type-I band lineup is achieved and, thus, very deep quantum wells can be fabricated, especially in the conduction band. Since the electron overflow from the wells to the barrier layers at high temperatures can he suppressed, the novel material of GaInNAs is very attractive to overcome the poor temperature characteristics of conventional long-wavelength laser diodes used for optical fiber communication systems. GaInNAs with excellent crystallinity was grown by gas-source molecular beam epitaxy in which a nitrogen radical was used as the nitrogen source. GaInNAs was applied in both edge-emitting and vertical-cavity surface-emitting lasers (VCSELs) in the long-wavelength range. In edge-emitting laser diodes, operation under room temperature continuous-wave (CW) conditions with record high temperature performance (T₀=126 K) was achieved. The optical and physical parameters, such as quantum efficiency and gain constant, are also systematically investigated to confirm the applicability of GaInNAs to laser diodes for optical fiber communications. In a VCSEL, successful lasing action was obtained under room-temperature (RT) CW conditions by photopumping with a low threshold pump intensity and a lasing wavelength of 1.22 μm     

InGaAs-GaAs quantum-dot lasers

Quantum-dot (QD) lasers provide superior lasing characteristics compared to quantum-well (QW) and QW wire lasers due to their delta like density of states. Record threshold current densities of 40 A·cm ^-2 at 77 K and of 62 A·cm^-2 at 300 K are obtained while a characteristic temperature of 385 K is maintained up to 300 K. The internal quantum efficiency approaches values of ~80 %. Currently, operating QD lasers show broad-gain spectra with full-width at half-maximum (FWHM) up to ~50 meV, ultrahigh material gain of ~10⁵ cm^-1, differential gain of ~10^-13 cm² and strong nonlinear gain effects with a gain compression coefficient of ~10^-16 cm³. The modulation bandwidth is limited by nonlinear gain effects but can be increased by careful choice of the energy difference between QD and barrier states. The linewidth enhancement factor is ~0.5. The InGaAs-GaAs QD emission can be tuned between 0.95 μm and 1.37 μm at 300 K     

Loss and recovery of hydrophobicity, surface energies, diffusioncoefficients and activation energy of nylon

The loss of hydrophobicity of nylon 6/6 caused by immersion in saline water for up to 336 h at different conductivities (0.005 to 100 mS/cm) and different temperatures (0 to 98°C) and its subsequent recovery in air (during 4500 h) have been investigated. The hydrophobicity is determined by measuring the static contact angle &thetas; between the tangent to a droplet of distilled water and the horizontal surface. The changes in the surface roughness and in the weight of the specimens were determined and correlated with the changes in the contact angle. It has been found that &thetas; decreased with increasing conductivity and increasing temperature of the saline solution. After removal from the solution, the higher the conductivity and temperature, the longer it took for &thetas; to recover in air. &thetas; decreased from 70° to 54° after nylon was subjected for 521 h to a uniform field of 15 kV_dc/cm in air. The surface free energy of nylon was determined as a function of time of immersion, the conductivity and temperature of the solution and during the recovery in air. The surface energies calculated for the virgin specimen are in good agreement with the literature. The diffusion coefficient of water into nylon increased from 0.23×10^-12 m²/s at 23°C to 7.4×10^-12 m²/s at 75°C. The activation energy was determined to be 59.4±2.2 kJ/mol. For unaged nylon the surface energies were determined at 23°C to be γ_S=44.7 mJ/m², γ_SD=29.3 mJ/m², γ_SH=15.4 mJ/m², W_SL =97.7 mJ/m² and γ_SL=19.8 mJ/m²      

A broadly tunable erbium-doped fiber ring laser: experimentationand modeling

Broad-band tunability of erbium-doped silica fiber ring lasers in the 1.48-1.62 μm wavelength band is demonstrated through modeling and experiment. Tunability over the erbium-doped fiber amplifier (EDFA) C- and L-bands is achieved with a simple laser design using a single gain medium working in deep saturation. A comprehensive numerical model based on an iterative solution of propagation rate equations and spectrally resolved Giles parameters was used to analyze the impact of various laser variables. The dependence of laser output power on total cavity loss, erbium-doped fiber length, pump power, and lasing wavelength has been investigated. The calculated laser characteristics have been found in good quantitative agreement with the experimentally obtained data. Experimental results concerning wavelength tunability, output power, and lasing wavelength repeatability/stability and spectral purity are also presented     

Transient collisional excitation X-ray lasers with 1-ps tabletopdrivers

Recent results are presented for 1-ps driven X-ray laser amplification in Ne-like and Ni-like transient collisional excitation research at the Lawrence Livermore National Laboratory. Plasma formation, ionization and collisional excitation are optimized using two laser pulses of 600- and 1-ps duration at tabletop energies of typically 5 J or less in each beam. Gain of 35 cm^-1 and higher has been measured on the 147 Å 4d→4pJ=0→1 transition of Ni-like Pd and is a direct consequence of the nonstationary population in, version produced by the high intensity picosecond pulse. We report the characterization of the X-ray laser properties including the transient gain lifetime and beam divergence for different Ne-like and Ni-like X-ray lasers     

Continuous-Wave (CW) operation of GaInP-AlGaInP visiblecompressively strained multiple quantum-wire (CS-WQWR) lasers

GaInP-AlGaInP compressively strained multiple quantum-wire layers were fabricated by the in situ strain induced lateral layer ordering process, during gas source molecular beam epitaxial (GS-MBE) growth. The effect of compositional modulation was described in terms of PL spectra, and TEM images for GaInP-AlGaInP MQWR lasers with 18 period (GaP)_1.5-(InP)_1.5 SPBS active layers. Based on transmission electron microscopy (TEM) images, the size of quantum-wire width was estimated, and the size fluctuation of quantum wires were discussed. Quantum-wire effect was discussed in terms of anisotropic lasing characteristics and EL polarization, which were reflected by an anisotropic oscillation strength in quantum wires and the comparison with GaInP-AlGaInP compressively strained quantum-film lasers was examined in terms of threshold current density. The condition under which quantum wires were formed by strained induced lateral layer ordering process was discussed in terms of anisotropic behaviors of lasing characteristics, such as threshold current density and lasing wavelength for GaInP-AlGaInP MQWR lasers with (GaP)_m/(InP) _mSPBS active layers. The lowest obtained J_th value was 278 A/cm² under the room temperature (r.t.) pulsed condition. The first CW operation of GaInP-AlGaInp quantum-wire laser was described. Threshold current was 294 A/cm² and CW operation up to 70°C was obtained     

1.047-μm Yb:Sr5(PO4)3F energystorage optical amplifier

The pumping and gain properties of Yb³⁺-doped Sr₅ (PO₄)₃F (Yb:S-FAP) are reported. Using a tunable, free running 900-nm Cr:LiSAF oscillator as a pump source for a Yb:S-FAP rod, the saturation fluence for pumping was measured to be 2.2 J/cm² based on either the spatial, temporal, or energy transmission properties of the Yb:S-FAP rod. The emission peak of Yb:S-FAP (1047.5 nm in air) is shown to overlap with that of Nd:YLiF₄ (Nd:YLF) to within 0.1 nm, rendering Yb:S-FAP suitable as an effective power amplifier for Nd:YLF oscillators. The small signal gain, under varying pumping conditions, was measured with a cw Nd:YLF probe laser. These measurements implied emission cross sections of 6.0×10^-20 and 1.5×10^-20 cm ² for π and σ polarized light. Respectively, which fall within the error limits of the previously reported values of 7.3×10^-20 and 1.4×10^-20 cm² for π and σ polarized light, obtained from purely spectroscopic techniques. The effects of radiation trapping on the emission lifetime have been quantified and have been shown to lead to emission lifetimes as long as 1.7 ms, for large optically dense crystals. This is substantially larger than the measured intrinsic lifetime of 1.10 ms. Yb:S-FAP crystal boules up to 25×25×175 mm in size, which were grown for the above experiments and were found to have acceptable loss characteristics (<~1%/cm) and adequately large laser damage thresholds at 1064 nm (~20 J/cm² at 3 ns). Overall, diode-pumped Yb:S-FAP amplifiers are anticipated to offer a viable means of amplifying 1.047-μm light, and may be particularly well suited to applications sensitive to overall laser efficiencies, such as inertial confinement fusion energy applications     

A coupled-mode model for the Fabry-Perot semiconductor laser kink instability

This paper derives a coupled-mode Fabry-Perot semiconductor laser model from the longitudinal-mode laser rate equations with parabolic gain dispersion. Truncating the full coupled-mode model permits stability analysis of the Fabry-Perot laser kink instability, which is a mode hop with increasing current. Two mechanisms contribute to the mode hop: the Silberberg-Bar-Joseph instability transports energy from higher frequencies to lower frequencies and gain dispersion stops the energy transport. Numerical simulations have shown that higher order carrier modes are excited during intervals of energy transport among the optical modes. These numerical results suggest that modes greatly detuned from the peak gain affect the laser dynamics.     

Densely arrayed eight-wavelength semiconductor lasers fabricated bymicroarray selective epitaxy

This paper describes a novel epitaxial growth technique, called microarray selective epitaxy (MASE), for fabricating extremely small integrated photonic devices. The MASE technique makes it possible to form densely arrayed (pitch <10 μm) multiple-quantum-well (MQW) waveguides without semiconductor etching as well as to control the bandgap energy of each waveguide. The technique is demonstrated for fabricating an eight-channel 10-μm-spacing microarray MQW structure, and the bandgap wavelength of each channel is successfully controlled by changing the SiO₂ mask pattern over a range of 90 nm. The technique is also applied to the fabrication of densely arrayed, eight-wavelength, Fabry-Perot laser diodes. The laser section is only 70 pm wide and 400 μm long. Eight different lasing wavelengths (each over 80 nm), a uniform threshold current of less than 9 mA, and an output power of over 10 mW are obtained     

Sixty Thousand Hour Light Output Reliability of AlGaInP Light Emitting Diodes

Both fixed current density and variable current density stress conditions are used to study light output degradation of (Al_xGa_1-x)_0.5In_0.5P light-emitting diodes (LEDs) as functions of LED stress current and LED stress time. Quantification of the resulting data indicates that (Al_xGa_1-x)_0.5In_0.5P LED degradation, D, is a linear function of current density, J, and a logarithmic function of stress time, t, for stress times as long as 60 000 hours in duration. For stress times long enough and current densities high enough to saturate any short-term effects (Al_xGa_1-x)_0.5In_0.5P LED degradation is therefore quantified by the empirical equation D=D₁ +D₂J+(D₃+D₄J)ln(t), where D₁ , D₂, D₃, and D₄ are independent of LED stress current and LED stress time. Within the limits of the data presented here, this equation is shown to accurately describe light output degradation of individual LED lamps, the average degradation behavior of individual LED wafers, and the average degradation behavior of a distribution of multiple LED wafers. The resulting expression may thus provide helpful guidance in quantifying the tradeoff between LED flux and LED degradation, both of which depend linearly on current density     

Room temperature operation of InAs quantum-dot laser utilizing GaAs photonic-crystal-slab-based line-defect waveguide with optical pump

We have successfully fabricated InAs quantum dots (QDs) embedded in a line-defect waveguide in an air-bridge type GaAs photonic-crystal slab (PCS) and observed laser action from optical-pumping. This lasing is found to occur without any optical cavity, such as a set of Fabry-Perot mirrors. Comparison of the observed transmittance spectrum with the calculated band dispersion of the triple-lines defect mode enables us to specify the lasing wavelength as that at the band edge. From this fact, it follows that the distributed feedback mechanism at the band edge with an infinitely small group velocity is responsible for the present lasing.     

360°风作用下线条风荷载分配系数特性

线条风荷载的准确计算是输电杆塔设计的关键一步,角度风荷载分配系数的选取是否合理将直接影响到设计指标的合理性。对常规线条风荷载（0.5Φ₁=0.5Φ₂=0）的计算原理进行梳理和分析,得到了0~90°范围内的角度风荷载分配系数。同时,通过对风向角θ和线路转角Φ的剖析,推导了线路前后侧360°风吹时的线条角度风荷载分配系数计算公式,并给出了前后侧挡距不同分配比例（0.5Φ₁=0.5Φ₂,0.5L₁∶0.5L₂=5∶5、4∶6、3∶7）时的线条角度风荷载分配系数,分析了分配系数的特点,并进行了对比研究。研究结果揭示了线条角度风荷载分配系数的特点,可作为输电杆塔抗风设计的一种参考。     

Design and characteristics of high-power (>0.5-W CW)diode-pumped vertical-external-cavity surface-emitting semiconductorlasers with circular TEM00 beams

We describe the design, fabrication, and measured characteristics of the high-power optically pumped-semiconductor (OPS) vertical-external-cavity surface-emitting lasers (VCSELs). Using diode laser pumping, we have recently demonstrated operation of such lasers, which for the first time generate high (watt-level) power and a circular Gaussian beam directly from a semiconductor laser. These OPS-VECSELs have a strain-compensated multi-quantum-well InGaAs-GaAsP-GaAs structure and operate CW near λ~1004 nm with output power of 0.69 W in TEM ₁₁ mode, 0.52 W in TEM₀₀ mode and 0.37 W coupled to a single-mode fiber. With multiple pump and gain elements, OPS-VCSEL technology is scalable to the multiwatt power levels. Such lasers will prove useful in a variety of applications requiring compact and efficient sources with high-power output in a single-mode fiber or with diffraction-limited beam quality     

Picosecond pulse amplification in tapered-waveguide laser-diodeamplifiers

The amplification characteristics of picosecond Gaussian pulses in conventional nontapered and both linear and exponential tapered-waveguide (TW) laser-diode amplifier (LDA) structures have been studied. The analysis is based on numerical simulation of the rate equation which also takes into account the effect of lateral carrier density distribution. The amount of pulse distortion experienced within the amplifier for input pulses having energies E_in=0.1 E_sat(in)=0.475 pJ (where E_sat(in) is the input saturation energy of the amplifier) and E_in=E_sat(in) =4.75 pJ have been analyzed for each structure which has a length of 900 μm and an input width of 1 μm. It has been found that the TW-LDA provides higher gain saturation and hence imposes less distortion on the amplified pulse as compared with a conventional nontapered LDA. The amplified 10-ps pulse used in this study experiences almost no broadening in the TW-LDA, whereas it suffers from broadening in the conventional nontapered LDA. The carrier density distribution and the dependence of the amplifier gain on the input pulse energy have also been studied for both nontapered and tapered amplifier structures. For example, in a TW-LDA with an output width of 20 μm and a length of 900 μm, the exponential structure provides 9-dB improvement in saturation energy as compared with the conventional amplifier. This improvement is about 10.5 dB in linear TW-LDAs     

A Rigorous,Coupled-Wave Transfer Matrix Method for Two-Dimensional Threshold Analysis of Distributed Feedback Semiconductor Lasers

A rigorous, truly two-dimensional method for threshold analysis of distributed feedback (DFB) lasers based on a coupled wave transfer matrix formalism is presented. The method makes it possible to systematically study the effect of the various structural and material properties parameters of the laser on the threshold gain and lasing frequency. Since the optical fields inside and outside the laser are very accurately represented in our analysis, the small differences in gain of pairs of longitudinal laser modes symmetrically located on both sides of the gap can be more accurately calculated than in any previous work. The analysis is applicable to gratings of any shape for both the TE and TM modes, but numerical results are given only for first-order gratings with rectangular and triangular tooth-shape. Reflectivities of the laser end facets have been calculated from first principles in some typical cases rather than treated as given parameters.