High spatial intensity 10 W-CuBr laser with hydrogen additives

High beam quality output has been demonstrated from a single medium-scale (2 cm dia. × 50 cm) CuBr laser with hydrogen additives. The addition of hydrogen leads to beam divergence reduction and laser power lift-up finally yielding 5-fold increase in laser power spatial intensity compared with the case of no hydrogen added. With a simple positive-branch unstable resonator practical (pulse-average) divergence of 80 rad is achieved. The easy-feasible sealed-off version of CuBr laser makes it an attractive source for many high-quality laser beam applications.     

Analysis of the spectral structure of CuBr laser lines

The spectral structure of spontaneous emission of copper atom at 510.6 nm and 578.2 nm was calculated considering the hyperfine structure of energy levels and the isotope shift. The spectral structure of the 510.6 nm and 578.2 nm laser lines was measured in a sealed-off CuBr laser tube with periodic refreshment of the neon buffer gas under different work temperature and excited voltage. The spectral structure of the spontaneous emission of copper atom was found to have similar outline with its laser lines. The spectrum of the 510.6 nm laser line maintains similar outline with three peaks at various discharging parameters while the spectrum of the 578.2 nm laser line is strongly dependent on the reservoir temperature and the discharge voltage.     

复合腔CuBr激光器中的模式研究

本文报道了在两种结构的多镜复合腔中CuBr激光器的纵模淬灭现象,并得到了重复频率几倍于基频纵模间隔的多重锁模脉冲串,最高重复频率可达440MHz.实验结果表明,通过调整复合腔的结构参数,可以在不缩短增益介质长度的条件下,实现CuBr激光多重锁模脉冲高重复频率的大范围调节.     

The effect of an axial external magnetic field on the output power of a small-bore CuBr laser

In this work, the effect of an axial external moveable magnetic field on the output power of a CuBr laser with small-bore tube has been investigated. In all experiments, by applying an EMF along the tube axis, the laser output power has been decreased and by moving the EMF toward the cathode region, more substantial decrease of output power has been observed. The effect is more significant at a magnetic field intensity of 1100 G, Ne gas pressure of 35 Torr, frequency of 19 kHz and voltage of 3.8 kV, such that there was no laser emission when the EMF was placed around the cathode.     

Effect of hydrogen additives on characteristics of the CuBr laser

Kinetic processes in the active medium of the copper bromide vapor laser with hydrogen additives are described in detail. Mechanisms for improvement of its lasing characteristics by hydrogen additives are analyzed.     

Development of a nanosecond oscillator–four pass amplifier XeCl laser system

This paper describes the development of a nanosecond oscillator–four pass amplifier XeCl laser system. A two times increase of the pulse peak power of the free running amplifier module has been experimentally demonstrated. The comparison with numerical results issued from a code describing the amplification of the oscillator beam in the active amplifier medium suggests further experimental improvements to optimize the energy extraction from the amplifier.     

Output parameters and a spectral study of UV Cu Ne–CuBr laser

A comparative study on the temporal behaviour of the average output power of UV Cu⁺ laser with quartz and ceramic inserts is reported. Spectral investigations of the discharge are also made. The use of quartz inserts leads to some impurities introduction into the gas mixture and to an average output power reduction from 500 to 30 mW at multiline output (248.6, 252.9, 260.0 and 270.3 nm) for UV lasing of 25 h. The utilization of ceramic inserts allows to maintain relatively constant gas mixture composition and average output power of about 300 mW for 100-h.     

隔环结构CuBr激光放电管温度场分析

提出了隔环式ＣｕＢｒ激光放电管温度场的简单数学模型,获得了气体温度场径向分布的解析表达式,分析了这种结构对放电稳定性及激不光斑质量的影响。     

Experimental study of a semiconductor laser diode having a Mach–Zehnder interferometer in the cavity

The performance of a semiconductor laser diode that has an asymmetric Mach–Zehnder interferometer all-optical switch in the cavity has been studied experimentally. This novel device was designed to be free from clock pulse insertion, since mode-locked optical pulses are generated internally and change the balance of the interferometer periodically. The device was fabricated using a InGaAsP/InP buried heterostructure and the primary optical properties of the device were investigated. Lasing characteristics that were peculiar to the twin-cavity structure were observed, i.e., continuous-wave lasing power oscillation in relation to the injection current balance between the two arms, and cyclic changes in the single/multiple emission peaks as a function of bias voltage at the saturable absorber. Electrical spectrum analysis indicated 40 GHz modulation of lasing output from the twin-cavity laser.     

Free electron laser based on the Smith–Purcell radiation

A Smith–Purcell (S–P) free electron laser (FEL) composed of a metallic diffraction flat grating, an open cylindrical mirror cavity and a relativistic sheet electron beam with moderate energy, is presented. The characteristics of this device are studied by theoretical analysis, experimental measurements and particle-in-cell (PIC) simulation method. Results indicate that coherent radiation with output peak power up to 50 MW at millimeter wavelengths can be generated by using relativistic electron beam of moderate energy.     

The equilibrium and kinetics of intermolecular hydrogen bonding in nitroarene anion radical–alcohol system: 1,3‐dinitro‐benzene anion radical – R–OH (R═CH3–, C2H5–, (CH3)2 CH–)

To explore the possibility of hydrogen bonding of a stable anion radical with DNA – component sugar, hormones, steroid, and so on (through hydroxyl group), as a first step, the possibility of hydrogen bonding of 1,3‐dinitrobenzene anion radical (1,3‐DNB^??) with aliphatic alcohols was studied. It was found that 1,3‐DNB^?? anion radical undergoes hydrogen bonding with alcohols: methanol, ethanol, and 2‐proponal. The hydrogen‐bonding equilibrium constant K_eq and the (hydrogen‐bonding) rate constants k₂ were evaluated through the use of linear scan and cyclic voltammetry theory and techniques. The K_eq was found to be in the range of 1.4–6.0 m ^?1, whereas the rate constants k₂ were found to be in the range of 1.5–3.6 m ^?1 s^?1, depending upon the hydrogen‐bonding agent and the equation used for the calculation of the rate constants. The hydrogen‐bonding number n was found to be around 0.5 or 1.0. The implication of this study in, for example, the replication of DNA, the prevention of the formation of super oxide, and so on is discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Theoretical treatment of RF discharges in coaxial He–Ne laser

Based on the ambi-polar diffusion, a model to simulate the coaxial RF-excited He–Ne laser plasmas is set up, and the microprocesses in laser plasmas are studied. Some parameters such as the spatial distribution of electron density, RF electric field, RF power density and excitation efficiency for upper laser level are calculated and discussed, which are suitable to describe the electrical properties of the discharges. The theoretical results can explain some experimental phenomena and guide the experimental study.     

Theoretical and experimental study of V–I characteristics of UV pre-ionized TEA CO2 laser for variety of laser gas mixtures

Experimental and theoretical study of V–I characteristics of UV pre-ionized TEA CO₂ laser has been carried out for a variety of gas mixtures emitting different optical pulse shapes suitable for various applications. Coupled differential equations have been solved to model the pulse excitation circuit using the numerically calculated values of ionization coefficient (α), attachment coefficient (β) and drift velocity (U_d) as functions of E/N (i.e. electric field to neutral particle density ratio) for chosen gas mixture. Calculated and experimental V–I characteristics for gas mixtures (CO₂:N₂:He::1:2:3, 1:1:4, 1:1:5 and 1:0:4.7) show a good agreement. It has been shown that gas mixture has a dominant effect on the delay between pre-ionization and main discharge; thus, determining the long-term stability of discharge. The excitation pulse duration increases with increase in molecular content of gas mixture (i.e. amount of CO₂ and N₂ in gas mixture).     

Reaction laser sintering of Ni–Al powder alloys

Laser reactive sintering, i.e., laser-induced self-propagating reaction sintering synthesis was carried out on Ni–Al powder alloys. The exothermic behaviors for the alloys with different Al content were characterized by sintering temperature curves produced from reactive heat. The phases transformed from the sintered alloys were identified by X-ray diffraction. Properties and microstructure of the sintered alloys were studied.     

Consolidation of hydrogenation–disproportionation–desorption–recombination processed Nd–Fe–B magnets by spark plasma sintering

We have successfully consolidated hydrogenation–disproportionation–desorption–recombination (HDDR) processed Nd–Fe–Co–Zr–B–Ga powder by spark plasma sintering (SPS). The field compacted samples were sintered at different temperatures (T_S) from 550 to 600 °C with compressive pressure of 80 MPa for 20 min. Microstructural investigations by transmission electron microscopy indicated that the sintered specimen exhibits Nd₂Fe₁₄B grains of ~300 nm with Nd-rich grain boundary phase. The optimum magnetic properties of B_r: 1.22 T, H_c: 928 kA/m, _BH_c: 600 kA/m, (BH)_max: 210 kJ/m³ were obtained in the sample sintered at 550 °C. The strategy for further improving the coercivity and remanence is discussed based on the microstructure-property relationships.     

Interaction of intense laser pulses with hydrogen atomic clusters

The interaction between intense femtosecond laser pulses and hydrogen atomic clusters is studied by a simplified Coulomb explosion model. The dependences of average proton kinetic energy on cluster size, pulse duration, laser intensity and wavelength are studied respectively. The calculated results indicate that the irradiation of a femtosecond laser of longer wavelength on hydrogen atomic clusters may be a simple, economical way to produce highly kinetic hydrogen ions. The phenomenon suggests that the irradiation of femtosecond laser of longer wavelength on deuterium atomic clusters may be easier than that of shorter wavelength to drive nuclear fusion reactions. The product of the laser intensity and the squared laser wavelength needed to make proton energy saturated as a function of the squared cluster radius is also investigated. The proton energy distribution calculated is also shown and compared with the experimental data. Our results are in agreement with the experimental results fairly well.     

Wavelength-spacing continuously tunable multi-wavelength SOA-fiber ring laser based on Mach–Zehnder interferometer

A Mach–Zehnder interferometer (MZI) which is used as a wavelength-spacing tunable comb filter in a fiber ring laser is built by employing an optical variable delay line (OVDL). Stable multi-wavelength semiconductor optical amplifier (SOA)-fiber ring laser based on an SOA and the MZI comb filter is achieved. Wavelength spacing can be continuously tuned by adjusting the OVDL and, as an example, multi-wavelength lasing with the wavelength spacing of 0.4, 0.8, or 1.6 nm is demonstrated. The output of the proposed multi-wavelength SOA-fiber ring laser is quite stable at room temperature and the output spectrum can be adjusted by controlling the bias current of the SOA.     

Bose–Einstein condensation in atomic hydrogen

The addition of atomic hydrogen to the set of gases in which Bose–Einstein condensation can be observed expands the range of parameters over which this remarkable phenomenon can be studied. Hydrogen, with the lowest atomic mass, has the highest transition temperature, 50 μK in our experiments. The very weak interaction between the atoms results in a high ratio of the condensate to normal gas densities, even at modest condensate fractions. Using cryogenic rather than laser precooling generates large condensates. Finally, two-photon spectroscopy is introduced as a versatile probe of the phase transition: condensation in real space is manifested by the appearance of a high-density component in the gas, condensation in momentum space is readily apparent in the momentum distribution, and the phase transition line can be delineated by following the evolution of the density of the normal component.     

Dihydrogen bond and X–H…σ interaction as sub‐classes of hydrogen bond

Dihydrogen bond (DHB) and X–H…σ interaction are discussed and compared here. Both interactions possess numerous characteristics of the hydrogen bond (HB). The Natural Bond Orbitals method results show that σ → σ* is the most important interaction connected with the electron charge transfer from the Lewis base to the Lewis acid for the DHB as well as for the X–H…σ HB. However, there are distinct differences between these interactions, and this is evident from the analysis based on the Quantum Theory of Atoms in Molecules as well as from the decomposition of the energy of interaction. The X–H…π interaction is also discussed here since it possesses few characteristics typical for the X–H…σ interaction and not for the DHB. Copyright © 2013 John Wiley & Sons, Ltd.     

A study of balancing the competing effects of ultrashort laser induced plasma for optimal laser machining

We report that plasma generated during processing of materials with ultrashort pulse lasers and the associated high intensity optical beam have both favourable and unfavourable impact on the machined surface quality. Intensity of the optical beam propagating through ambient air medium enhanced further by self-focusing is sufficiently high to cause gas breakdown forming air plasma. The generated plasma reduces the effect of self-focusing but also distorts the beam profile. Duration of the pulse being too short for thermal equilibrium to establish, ablation occurs largely by direct removal of the material forming another plasma plume. Normally, the scattering effect of plasma results in distortions of the fabricated features. However, for certain parameter ranges, the competing self-focusing and gas plasma plumes supplemented with the material plasma can combine to cause filamentation, eliminating the distortions. Filament of hot plasma also acts as a well-shaped energy source. In the present study, brass is taken as an example for the investigation. Experiments were conducted to capture the spectrum of the light scattered by plasma using a spectrometer. Analysis was done to estimate the material plasma. Theoretical calculation on the intensity distribution in an optical beam propagating through air was then followed for a range of parameter values taking the self-focusing effect of the medium and the impact of the plasma generated by its breakdown. Approximate values of the machining parameters for clean fabrication are deduced from the calculations, which were used to conduct a laser machining test on brass.