Beam Quality Enhancement Research Articles

Enhancement of beam quality and transmission using phase bunching in the central region of a cyclotron

Phase bunching in the central region of an azimuthally varying field (AVF) cyclotron contributed significantly to the improvement of the extracted beam quality. The phase bunching effect was demonstrated by measurements of the accelerated and extracted beam properties and by a theoretical analysis using the upgraded geometric trajectory model of the AVF cyclotron at the facility of Takasaki Ion accelerators for Advanced Radiation Application. The phase bunching effect on the beam extraction efficiency and emittance was estimated by a comparison between the 260 MeV 20Ne7+ beam of harmonic number h = 2 in the phase-bunching condition (PBC) and the 107 MeV 4He2+ beam of h = 1 in the non-phase-bunching condition (NBC). The PBC narrowed beam phase width and produced near-single-turn extraction, resulting in high extraction efficiency more than twice as high as that for the NBC. It was proved by beam current measurements. Reduction of the beam phase width was confirmed by the theoretical and experimental analysis of correlation between the radial positions at the phase-defining slits and the phase distribution of the particles passing through the slits. The theoretical analysis also showed that the beam emittance before extraction under the PBC is reduced to less than half of the NBC case. Furthermore, the measured normalized beam emittance in the horizontal direction after the extraction for the PBC was remarkably improved to 0.054π mm mrad nearly one-fourth of the emittance of 0.20π mm mrad for the NBC.

Modulated spectral beam combining of diode laser array stack in an external cavity for the enhancement of beam quality

Modulated spectral beam combining of diode laser array stack in an external cavity for the enhancement of beam quality

Proton beam quality enhancement by spectral phase control of a PW-class laser system

We report on experimental investigations of proton acceleration from solid foils irradiated with PW-class laser-pulses, where highest proton cut-off energies were achieved for temporal pulse parameters that varied significantly from those of an ideally Fourier transform limited (FTL) pulse. Controlled spectral phase modulation of the driver laser by means of an acousto-optic programmable dispersive filter enabled us to manipulate the temporal shape of the last picoseconds around the main pulse and to study the effect on proton acceleration from thin foil targets. The results show that applying positive third order dispersion values to short pulses is favourable for proton acceleration and can lead to maximum energies of 70 MeV in target normal direction at 18 J laser energy for thin plastic foils, significantly enhancing the maximum energy compared to ideally compressed FTL pulses. The paper further proves the robustness and applicability of this enhancement effect for the use of different target materials and thicknesses as well as laser energy and temporal intensity contrast settings. We demonstrate that application relevant proton beam quality was reliably achieved over many months of operation with appropriate control of spectral phase and temporal contrast conditions using a state-of-the-art high-repetition rate PW laser system.

High-efficiency LD-pumped all-fiber Raman laser based on a 100 µm core graded-index fiber

We demonstrate a high-power, high-efficiency Raman laser based on a 100 µm core graded-index (GRIN) fiber directly pumped by 915 nm multimode laser diode modules in all-fiber configuration. Optimization of GRIN fiber length and pumping scheme was performed. As a result, 62 W of CW power has been obtained at a wavelength of 954 nm with a slope efficiency of 85%. The joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of the GRIN fiber core results in significant beam quality enhancement for the generated Stokes beam ( < 3) in comparison with that of the pump radiation ( > 30). In addition, the exact value of the operating wavelength near the Raman gain maximum was varied. To the best of our knowledge, this is the first study of the impact of a Raman gain spectral profile on the power, spectrum and beam quality parameter M2 of multimode Raman laser. It appears that they very much dependent on the spectral region of Raman gain at which we work, when the wavelength is selected by a fiber Bragg grating inscribed in a multimode GRIN fiber.

Dual V-type resonators to enhance beam quality of Yb:YAG thin-disk lasers.

In this paper, the operating characteristics of a Yb:YAG thin-disk laser using double V-type resonators are reported. The laser maintained stable performance by independently activating either the main or the auxiliary V-type resonator depending on the pumping laser power. Experimental investigations revealed that by reducing the dynamic range in which the main resonator was used for stability and instead employing the auxiliary resonator, the laser beam quality factor M2 could be reduced from 40.2 to 32.3, representing a beam quality enhancement of approximately 20%.

Numerical simulation of 30-kW class liquid-cooled Nd:YAG multi-slab resonator.

A numerical modeling is developed for 30-kW class liquid-convection-cooled elastically-mounted Nd:YAG multi-slab laser resonator configuration. The modeling exhibits the thermal effects and resultant wavefront aberration of the gain module under flow cooling and CW pumping at 100-kW level, the self-reproducing oscillating mode within the large-aperture cavity, as well as the beam quality enhancement by adaptive optics. The simulation results predict a CW output power of 31 kW with the optical-optical efficiency of 26.1% obtained from a modified resonator configuration with dual gain modules that have opposite flow directions, while the beam quality can be improved to β<2 after the correction of a deformable mirror.

Beam Quality Management in Multi-Stage Side-Pumped Nd:YAG MOPA Laser Systems

A method of beam quality management in multi-stage Master-oscillator power-amplifier (MOPA) laser systems has been presented. Beam quality always gets worse when a laser beam passes though an amplifier due to the thermally induced aberrations in the intensively pumped gain medium. Numerical calculations show that the spherical aberration (SA) coefficient of the aberrated beam wavefront from the amplifier can change the sign in free space propagation. The wavefront can thus be compensated by another amplifier, which has nearly identical pumping parameters with the first amplifier. A single-pass two-stage side-pumped Nd:YAG amplifier is developed in experiments. The seed beam from the oscillator with 42 W output power and beam quality factor of M 2 = 1.21 is scaled up to 74 W by the first stage of the amplifier, while the beam quality is degraded to M 2 = 1.80. The beam quality is improved to M 2 = 1.30 after the second stage by properly wavefront compensation and the output power is 101 W. Experiments are also carried out with a double-pass one-stage amplifier. The output beam with 84 W power and M 2 = 1.29 is achieved. To our best knowledge, it is the first time that beam quality enhancement in a side-pumped MOPA is demonstrated by SA compensation.

侧面泵浦主振荡功率放大器中的光束质量提高

侧面泵浦主振荡功率放大器中的光束质量提高

The combined guiding effect in MOPA lasers

The combination of the thermal induced refractive index guiding and gain guiding in an end-pumped Nd:YVO4 MOPA laser is reported. A nonlinear Schrödinger-type wave equation including the combined guiding effect is derived for the amplification process. In experiment, a novel experimental phenomenon, enhancement of beam quality during amplification, is observed in the end-pumped MOPA laser. After amplification by two amplifiers, the 33 W seed laser is scaled up to 107 W, and the beam quality is also enhanced from M2 ≈ 2.4 to M2 ≈ 1.2. The theoretical results agree reasonably well with the experiment, which indicates that the evolution of the transverse mode is dominated by the combined guiding effect in the end-pumped Nd:YVO4 MOPA laser. A new mechanism is proposed by which the combined guiding effect can be applied to control the beam quality during the amplification in the MOPA laser.

Explicit exact three-dimensional analytical temperature distribution in passively and actively cooled high-power fibre lasers

Efficiency and beam quality enhancement of high-power lasers require effective thermal management. In order to determine the temperature distribution in double-cladding high-power continuous wave fibre lasers, the heat transfer equation is analytically solved. Using the exact derived temperature distributions, the ultimate pump power before thermal damage, which is one of the key subjects of consideration in high-power laser design, is achieved.

Beam quality enhancement for a radio-frequency excited annular CO2 laser

We report the preliminary experimental results obtained from an annular radio-frequency excited CO2 laser with a Talbot cavity and a phase correcting mirror external to the cavity. The Talbot cavity is adopted in order to reduce the number of oscillating azimuthal modes. The use of the external profile-modulated mirror is aimed at reducing the beam phase modulation and thus increasing the amount of fundamental annular mode energy content. The beam quality results to be sensibly increased.

Status and prospects of the ELETTRA control system

The ELETTRA third-generation synchrotron light source facility consists of a linac injector, a transfer line and a storage ring with electron beam energies between 1.5 and 2 GeV. The ELETTRA control system, which is still under development, is being operated for the commissioning of the whole plant. After an overall description of the control system, the first operational results are reported. The present activities and future developments foreseen for beam quality enhancement are reported.