Analysis of basic processes inside the keyhole during deep penetration ND-YAG CW laser welding

During laser welding, the keyhole is generated by the recoil pressure induced by the evaporation processes occurring mainly on the front keyhole wall (KW). In order to characterize the evaporation process, we have measured this recoil pressure by using a plume deflection technique, where the plume generated for static conditions (i. e. with no sample displacement) is deflected by a transverse side gas jet. From the measurement of the plume deflection angle, the recoil pressure can be determined as a function of incident intensity and sample material. From these data one can estimate the pressure generated on the front KW, during laser welding. Therefore, the corresponding dynamic pressure exerted by the vapor plume expansion on the rear KW, in contact with the melt pool, can be also estimated. These pressures appear to be in close agreement with those generated by an additional side jet that has been used in previous experiments, for stabilizing the observed melt pool oscillations or fluctuations.

Laser-vapour interaction in high-power cw Nd:YAG laser welding

During high-power cw Nd:YAG laser welding a vapour plume is formed containing vaporised material ejected from the keyhole. Spectroscopic studies of the vapour emission have demonstrated that the vapour can be considered as thermally excited gas with a stable temperature (less than 3000K), not as partially ionised plasma. In this paper, a review of temperatures in the vapour plume is presented. The difficulties in the analysis of the plume spectroscopic results are reviewed and explained. It is shown that particles present in the vapour interact with the laser beam, attenuating it. The attenuation can be calculated with Mie scattering theory, however, vaporisation and particle formation also both play a major role in this process. The laser beam is also defocused due to the scattering part of the attenuation mechanism, changing the energy density in the laser beam. Methods for mitigating the effects of the laser beam-vapour interaction, using control gases, are presented together with their advantages and disadvantages. This 'plume control' has two complementary roles: firstly, the gas must divert the vapour plume from out of the laser beam path, preventing the attenuation. Secondly, the gas has to stabilise the front wall of the keyhole, to prevent porosity formation.

A Diode-Side-Pumped Dynamic Fundamental Mode Nd:YAG Laser

A fundamental mode Nd YAG laser is experimentally demonstrated with a stagger pumped laser module and a special resonator. The rod is pumped symmetrically by staggered bar modules. A dynamic fundamental mode is achieved with the special resonator under different pump levels. A maximal continuous wave output of 61 W (M-2 = 1.4) is achieved with a single rod. An average output of 47 W, pulse width of 54 ns, pulse energy of 4.7 mJ and peak power of 87 kW are obtained under the Q-switched operation of 10 kHz.

Antiphase state in passively Q-switched Yb : YAG microchip multimode lasers with a saturable absorber GaAs

We report on recent experimental results of the spontaneous antiphase dynamics that occurs in a laser-diode-pumped multimode passively Q-switched microchip Yb:YAG (where YAG is yttrium aluminum garnet) lasers with a saturable absorber GaAs. We observe that the pulse sequence of the first mode characterized by one, two, and three pulses as a group and all the modes display an antiphase state as the pumping ratio rises. We modify the multimode rate equations to account for nonlinear absorption due to GaAs in the presence of spatial hole burning. We perform numerical simulations based on the proposed rate equations and reproduce the observed antiphase state of two and three active modes.

Observation of antiphase states in a passively Q-switched multimode microchip Yb : YAG laser with a GaAs saturable absorber

Antiphase dynamics has been observed experimentally for the laser modes operation in a laser-diode-pumped Q-switched microchip Yb:YAG laser with GaAs as a saturable absorber in the presence of spatial hole-burning. The Q-switched pulses sequences of two modes at different pump power have been obtained. The experimental results have shown that the pulses sequences displayed classic antiphase dynamics. (C) 2003 Elsevier B.V. All rights reserved.

Diode-end-pumped passively mode-locked ceramic Nd : YAG Laser with a semiconductor saturable mirror

We report on a diode- pumped CW passively mode locked ceramic Nd: YAG laser with SESAM ( semiconductor saturable absorber mirror), wavelength 1064nm. At a pump power of 7.6w, the pulse width was estimated to be similar to 8.3ps with repetition rate similar to 130MHz and the average output power was 1.59w. To our knowledge, this was the first demonstration that ceramic Nd: YAG was used for diode pumped CW passively mode locking. (C) 2005 Optical Society of America.

Structure and magnetic properties of ScFe6Ga6-type RCo5Ga7 (R = Y, Tb, Dy, Ho and Er)

The structure and magnetic properties of the RCo5Ga7 (R = Y, Tb, Dy, Ho and Er) compounds with the ScFe6Ga6-type structure have been studied. The stability of RCo5Ga7 is closely related with the ratio of the metal radii R-RE/R-(Co,R-Ga). With R-RE/R-(Co,R-Ga) less than or equal to 1.36, the compounds can be stabilized in the ScFe6Ga6-type structure. The lattice of RCo5Ga7 shrinks as the atomic order of R increases, and it is consistent with the lanthanide contraction. The structure analysis based on X-ray diffraction patterns reveals that in the orthorhombic RCo5Ga7 (Immm), R occupies the 2a site, and Co enters into the 8k and the 4h sites, and Ga is at the 4e, 4f, 4g, 4h and 8k sites. The interatomic distances and the coordination numbers of RCo5Ga7 are provided from the refinement results. The short interatomic distance (less than 2.480 Angstrom) between the Co ions results in the negative magnetic interaction, which does not favor ferromagnetic ordering. The magnetic moment of YCo5Ga7 is absent, and RCo5Ga7 (R = Tb, Dy, Ho and Er) may have long-range magnetic ordering with the paramagnetic Curie temperature lower than 5 K. (C) 2004 Elsevier Inc. All rights reserved.

Q-switched and mode-locked diode-pumped Nd : YAG laser with an LT-GaAs

Simultaneous Q-switching and mode-locking (QML) is accomplished in a diode-pumped Nd:YAG laser using low-temperature GaAs (LT-GaAs) as the saturable absorber, which also acts as an output coupler at the same time. The repetition rate of the Q-switched envelope increased from 25 to 40 kHz as the pump power increased from 2.2 to 6.9 W. The mode-locked pulses inside the Q-switched pulse envelope had a repetition rate of 714 MHz. A maximum average output power of 770 mW was obtained. (c) 2005 Elsevier GmbH. All rights reserved.

A passively mode-locked diode-end-pumped Nd : YAG laser with a semiconductor saturable absorber mirror grown by metal organic chemical vapour deposition

We report the experimental results of a mode-locked diode-end-pumped Nd:YAG laser with a semiconductor saturable absorber mirror (SESAM) from which we achieved a 10 ps pulse duration at 150 MHz repetition rate. The SESAM was grown by metal organic chemical vapour deposition at low temperature. The recovery time was measured to be 0.5 ps, indicating the potential pulse compression to sub-picoseconds.

Diode-pumped passively Q-switched Yb : YAG microchip laser with a GaAs as saturable absorber

A passively Q-switched Yb: YAG microchip laser has been constructed by using a doped GaAs as the saturable absorber as well as the output coupler. At 13.5 W of pump power the device produces high-quality 3.4 muJ 52 ns pulses at 1030nm with a pulse repetition rate of 7.8kHz in a TEM00-mode.

LD side-pumped high beam quality passive Q-switched and mode-locked Nd:YAG laser based on SESAM

We report a LD side-pumped fundamental-mode (Mx(2) = 1.35 and My(2) = 1.27) passive Q-switched and mode-locked Nd:YAG laser based on a semiconductor saturable absorber mirror (SESAM). At a pump current of 12.5 A, the average output power of 5.68 W with 80 kHz repetition rate and 2 mu s pulse width of the Q-switched envelope was generated. The repetition rate of the mode-locked pulse within the Q-switched envelope of 88 MHz was achieved.