Cryogenic Yb3+-Doped Solid-State Lasers

Cryogenically cooled solid-state lasers promise a revolution in power scalability while maintaining a good beam quality because of significant improvements in efficiency and thermo-optic properties. This is particularly true for Yb lasers because of their relatively low quantum defect and relatively broadband absorption even at cryogenic temperatures. Thermo-optic properties of host materials, including thermal conductivity, thermal expansion, and refractive index at low temperature, are reviewed and data presented for YAG (ceramic and single crystal), GGG, GdVO₄, and Y₂O₃. Spectroscopic properties of Yb:YAG and Yb:LiYF₄ (YLF) including absorption cross sections, emission cross sections, and fluorescence lifetimes at cryogenic temperatures are characterized. Recent experiments have pushed the power from an end-pumped cryogenically cooled Yb:YAG laser to 455-W continuous-wave output power from 640-W incident pump power at an of M² 1.4.     

InGaN-based blue laser diodes

The continuous-wave (CW) operation of InGaN multiquantum-well (MQW) structure laser diodes (LDs) was demonstrated at room temperature (RT) with a lifetime of 100 h. The threshold current and the voltage of the LDs were 50 mA and 5 V, respectively. The threshold current density was 8.8 kA/cm². The carrier lifetime and the threshold carrier density were estimated to be 3.5 ns and 1.8×10²⁰/cm³, respectively. The Stokes shift of the energy difference between the absorption and the emission energy of the InGaN MQW LD's were 140 meV. Both spontaneous and stimulated emission of the LD's originated from this deep localized energy state which is equivalent to a quantum dot-like state. From the measurements of gain spectra and an external differential quantum efficiency dependence on the cavity length, the differential gain coefficient, the transparent carrier density, threshold gain and internal loss were estimated to be 5.8×10^-17 cm², 9.3×10 ¹⁹ cm^-3, 5200 cm^-1, and 43 cm^-1 respectively     

Highly efficient diode-pumped 3-μm Er3+:BaY2F8 laser

A systematic investigation on a series of monoclinic Er³⁺ :BaY₂F₈ crystals with different dopant concentrations (C_Er=5%-30%) and crystal orientations was conducted to optimize the laser performance in this new 3-μm laser medium by laser diode pumping. The highest slope efficiency of 32% near the quantum defect (35%) was obtained with a 10% doped Er³⁺:BaY₂F₈ crystal with the orientation (010) and a length of 3.5 mm. A maximum output power of 160 mW was achieved at an absorbed pump power of 550 mW at a wavelength of 970 nm     

Investigating up-conversion fluorescence of Phloxine B

The steady-state and time-resolved emission of Phloxine B was examined when excited with 90 fs pulses from a mode-locked Ti:sapphire laser. Above the excitation wavelength (775 nm), Phloxine B was found to display two-photon excitation fluorescence with estimated cross-section for excitation of 0.87×10^-49 cm⁴ s/photon, and fluorescence spectrum consistent with one-photon excitation. Over the excitation wavelength range from 590 to 650 nm, Phloxine B was found to display one-photon excitation up-conversion fluorescence. The up-conversion fluorescence of Phloxine B was confirmed by frequency-domain measurements. The intensity decays revealed different double-exponential intensity decays for one-photon excitation at 390 nm and 633 nm. The longer fluorescence lifetimes were observed with 633 nm excitation. Hence, anti-Stokes excitation of Phloxine B causes delay emission at shorter wavelengths. Time-resolved anisotropy decay measurements revealed similar correlation times, but different amplitudes, as has been observed previously for two- versus one-photon excitation     

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     

Electrical breakdown of SF6 at small values of theproduct pd

The breakdown of SF₆ when static and impulse voltages are applied at small values (from 10^-4 bar-mm to 1 bar-mm) of the product pd was investigated. It was found that the prevailing breakdown mechanism up to 8×10^-3 bar-mm was the Townsend mechanism. Between 8×10^-3 and 5.5 bar-mm the Townsend and streamers mechanisms combined, whereas from 5.5 bar-mm on the streamers mechanism took over. The Paschen law was found to hold for the points lying to the right of the minimum when static voltage was applied, whereas edge-type breakdown occurred on the points lying to the left of the minimum. When impulse voltage was applied, the Paschen law was valid only for the points to the right of the 1 bar-mm point. The electrode material affected the static and impulse breakdowns through the values of its work function     

Infrared microscopy studies on high-power InGaAs-GaAs-InGaP laserswith Ga2O3 facet coatings

InGaAs-GaAs separate confinement, heterostructure single quantum-well (SCH-SQW) lasers (λ=0.98 μm) with lattice-matched InGaP cladding layers, using a new Ga₂O₃ low reflectivity (LR) front-facet coating, are reported. The CW peak power density (17 MW/cm²) of 6 μm×750 μm ridge-waveguide lasers is limited by thermal rollover, and repeated cycling beyond thermal rollover produced no change in operating characteristics. The high-power temperature distribution along the active stripe has been measured by high-resolution infrared (3-5 μm) imaging microscopy. The temperature profile acquired for a very high optical power density P_D=11 MW/cm³ was found to be uniform along the inner active laser stripe, and revealed a local temperature increase at the LR front facet ΔT_f of only 9 K above the average stripe temperature ΔT_s=24 K. An excellent front-facet interface recombination velocity <10⁵ cm/s has been inferred from the measured low local temperature rise in the front facet     

Diode-pumped neodymium lasers repetitively Q-switched by Cr4+:YAG solid-state saturable absorbers

We report on the experimental results of a continuously diode-laser pumped Nd:YAG laser, operating at 1064 nm and repetitively Q-switched by a Cr⁴⁺:YAG solid-state saturable absorber. End-pumping the Nd:YAG with a 10-W fiber-coupled diode-laser we could either optimize the energy or the average output power, depending on the choice of the saturable absorber and the output coupler. The maximum energy was ≈200 μJ in single TEM₀₀, 17 ns pulses at 6 kHz, whereas a maximum average power of ≈2 W with 32-ns pulses at 20 kHz was obtained. We also present preliminary results of a repetitively Q-switched Nd:YVO₄ laser at 1064 nm. The repetitive Q-switching operation is described by an improved model, which accounts for the behavior of both the active medium and the solid-state saturable absorber. The results of the model agree fairly well with the experimental data. Experimental results of second harmonic conversion are also reported and interpreted using a depleted pump model     

Polarization behavior of ferroelectricBi4Ti3O12 ceramics

The effect of oxidation and reduction atmospheres on the ferroelectric behavior of bismuth titanate was investigated. The effective surface density σ_eff of free charges in samples polarized by applying an electric field E_p=1 MV m^-1 at a temperature of 100°C for 90 min was determined by compensation voltage measurements. The thermally treated samples in oxidation atmospheres exhibit a significant decrease of σ_eff while those treated in a reducing atmosphere exhibit an increase accompanied by an improvement in the stability of the electret charge. Such observations indicate that the formation as well as the transformation of the electret state of Bi₄Ti₃O ₁₂ ceramics may be related to the enhancement and diminishing of oxygen vacancies. In all cases homocharge with a time independent sign was obtained     

High-power and high-efficiency operation of a CW-diode-side-pumpedNd:YAG rod laser

We have demonstrated high-power and high efficiency performance of a continuous-wave (CW) Nd:YAG laser with a simple and scalable side-pumping configuration. The maximum output power of 147 W was obtained in low brightness operation of M²=45. The corresponding electric efficiency is 14.8%. To our knowledge, this is the highest value reported for diode side-pumped Nd:YAG lasers. By using a rate and photon transport calculation, we have estimated the pumping efficiency of 72%. High brightness operation was also carried out by applying bifocusing compensation of the Nd:YAG rod. The brightness of 272 MW/cm² sr with beam quality of M²=5.9 and output power of 107 W was obtained at the electric efficiency of 11.6%. The brightness and the electric efficiency are comparable with those of industrial high-power CO₂ lasers that have been the first option for industrial applications     

Image quality analysis for dual energy subtraction imaging with afemtosecond laser-based hard X-ray source

We present a study of image quality for dual energy subtraction imaging using an iodinated contrast agent and a femtosecond laser-based hard X-ray source. The INRS CPA laser (400 fs pulse focused on solid targets in a 3 μm spot at 4 × 10¹⁸ W cm^-2) was used to create a bright hard X-ray source (conversion efficiency of 10^-5 in the characteristic K_∝ line emission, 12 μm X-ray source diameter). A model of image quality has been developed and been benchmarked with specific experiments using specially made angiography phantoms     

Dielectric recovery of a residual SF6 plasma between twoparallel plane electrodes

The present work is a theoretical investigation into the dielectric strength of a thin column of hot SF₆ gas left between two contacts after the thermal quenching of an arc at current zero in a modern HV gas-blast circuit breaker. Because of time rapid decay of arc temperature during current zero, this hot gas is still in a plasma state (referred to as the residual plasma) with sufficiently high charge density (both electrons and ions) although electrical conductivity is low. The dielectric recovery of such a residual plasma differs from that of an infinitely large, uniform plasma in two aspects. Firstly, the presence of space charge can distort severely the local electric field and can influence strongly the generation and loss of charged particles by ionization and chemical reactions. Secondly, due to the finite size of the plasma column, electrons diffuse in an ambipolar manner, thus enhancing the loss of charged particles in the plasma. In the present investigation, the gap length between the contacts (electrodes) is 1.0×10^-2 m and the diameter of the plasma column varies between 50×10^-6 and 500×10 ^-6 m. Distribution of the number density of charged species on the axis is obtained by solving their continuity equations using the method of MacCormack with Shuman filter. It is found that formation of space charge creates strong ionization layers close to the electrodes and that the whole column could deviate from electrical neutrality due to the drift of charged particles. The breakdown field of such a bounded plasma at atmospheric pressure or above is approximately equal to the critical field for an infinite plasma within a numerical uncertainty of 5%     

Simulation of corona discharge. Positive corona in SF6

Positive coronas in SF₆ under three voltage levels, 3.0, 3.5 and 4.0 kV, are studied for a 0.5 cm gap between a hyperboloid needle and a plane electrode at a gas number density N=2.12×10¹⁸ cm³. The current in the form of a burst of pulses is found to increase with increasing voltage. The initiation and development of successive avalanches are traced as function of time after voltage increases. The development of electron avalanches and streamers are due to ionization and photoionization and quenching of avalanches is due to space charge. The simulation provides a detailed structure of avalanches from which essential differences between positive and negative corona with regard to total field distribution, propagation of successive avalanches and ion distribution can be discerned     

Electrical conduction in double-layer P-SiO2/polymerdielectric structures

The dc electrical conduction in MI₁M and MI₁I₂M composite specimens has been studied, where I₁ is polyimide (PI) or polyester (PET), and I₂ is a thin plasma deposited silicon compound film (oxide, P-SiO₂, or nitride, P-SiN). The current-electric field (j(E,T)) characteristics have been measured over a wide range of E values (O⩽E⩽2×10 ⁵ V cm^-1) and temperatures (20⩽T⩽170°C). The observed transport characteristics are in broad agreement with data reported in the literature, except for PI based MI₁I₂M specimens, for which polarity dependent transport behavior occurs for T>60°C and E>2×10³ V cm^-1. The observed behavior of all specimen types can be explained in terms of protonic hopping conduction; the charge carriers are created by thermal dissociation and ionization of carboxylic acid (-COOH) groups, present in PI as several percent of unimidized polyamic acid, and in PET at chain ends. When the polarity is +MI₁I₂M-, the relatively abundant free protons in PI flow to the I₁/I₂ interface, where they can accumulate and give rise to a space charge-induced potential barrier of ~0.2 eV. By modifying Lewis' molecular dipole model so as to include this potential barrier, we derive expressions for j(E,T) which describe the experimentally observed polarity dependence in PI rather well. The effect is not observed in PET based MI₁I₂M specimens on account of the much lower concentration of free protons in this polymer     

Dynamics of prebreakdown phenomena in a uniform field in water

For a breakdown time lag of 10^-7 to 10^-3, both slow subsonic and thin supersonic streamers were found to emanate from a point electrode in deionized water (σ=5×10^-5 S/m). The slow streamer-formed dendrite probably consists of gaseous microbubbles. Relaxation of the slow streamers was studied. The development of fast streamers resulted from kinetic phase transition in the liquid, with subsequent ionization of thin discharge channels. It was shown that heating of water and electrohydrodynamic flow do not affect the prebreakdown phenomena under these conditions. A model has been proposed of an anodic supersonic streamer which is generated by the plasma in the condensed medium     

Studies on collisional pumping of soft X-ray lasers at ILE

X-ray laser research performed at the Institute of Laser Engineering, Osaka University, are reported. The performance of X-ray lasing pumped by a train of 100-ps pulses at the wavelength range between 14 and 4 nm was investigated. The irradiance on the target was up to ~3×10¹⁴ W·cm. The double target amplification was successfully demonstrated with two beam irradiation for Ag, Nd, Yb, Hf, and Ta lasing. Moreover double-pass amplification at 7.9 nm has been demonstrated. The higher atomic number X-ray lasing strongly depends upon the transverse distance of the two targets     

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     

Diode arrays, crystals, and thermal management for solid-statelasers

We summarize our efforts in the development of solid-state lasers, including the laser diode arrays, pump light delivery, approaches to thermal management, and novel gain media. Our interests are in developing unique solid-state lasers, including those operating at higher powers, offering less common wavelengths, and having other specialized features. In this paper, we discuss high-power Tm:YAG and Yb:YAG lasers. The gas cooled slab laser concept using Yb:S-FAP, and side-pumped Er:YAG and Cr:ZnSe lasers. We address the optical and thermal physics of these systems and also mention several additional gain media that have the potential of offering unique performance characteristics: Ce:LiSAF, APG-2 laser glass, Dy:LaCl₃, and Yb:BCBF     

Optical and Photoluminescence Properties of Erbium-Doped Chalcogenide Glasses (GeGaS:Er)

We have examined the optical and photoluminescence (PL) properties of Er³⁺-doped GeGaS glasses of near-stoichiometric composition Ge₂₈Ga_6.2S_65.3:Er_0.5. We have also used powdered samples of various mean sizes (L) to examine the dependence of the 1.54 -mum PL emission spectrum and the PL decay time on the average sample size. Optical absorption spectra of Er³⁺ ions arising from transitions between different energy manifolds, such as 4 I_{15 /2} -⁴ I_13/2,⁴ I_{15 /2} -⁴ I_{11 /2} , etc., have been used to extract Omega₂, Omega₄, and Omega₆ values using the Judd-Ofelt analysis and a Judd-Ofelt radiative lifetime T_JO = 2.6 ms for the ⁴ I_{13 / 2} -⁴ I_{15 / 2} transition. The PL emission spectra and the decay time have been found to depend on the mean sample size. The spectra are broader and the decay times are longer for larger sample sizes, due to photon trapping occurring in the sample. The extrapolated decay time to zero particle size yields a decay time that matches the Judd-Ofelt radiative lifetime almost perfectly, and confirms the argument that the true PL lifetime needs to be measured in fine powders to avoid reabsorption effects. We have estimated the maximum emission cross section as 15.5 X 10^-21 cm².