Thomas Graf

Thermooptical compensation methods for high-power lasers

Thermally induced optical effects can be exploited to generate adaptive optical devices such as self-adjusting lenses. An adaptive lens in a resonator can be used to compensate for the thermal lens in a high-power solid-state laser rod (LR) and herewith significantly improve the beam quality and increase the output-power range of solid-state lasers. With suitable materials and an appropriate design of the compensating device, resonators with self-balancing thermal lenses can be developed. In this paper, we review the material requirements for a self-adaptive compensating element and discuss a selection of suitable materials (glasses, liquids and curing gels) and schemes to compensate for the thermal lens of a Nd:YAG LR. Finally, we present a very simple and promising design of a thermooptically self-compensated laser amplifier.

Generation of Super-Gaussian modes in Nd:YAG lasers with a graded-phase mirror

A dielectric graded-phase mirror was developed to change the intensity distribution of the fundamental mode in an Nd:YAG laser resonator to a top-hat profile. A super-Gaussian mode of the sixth order was generated by means of a graded-phase mirror with a simple ring-shaped phase step on a spherical reflector. The depth of the ring on the mirror was 90 nm. The graded-phase mirror was manufactured with an ordinary vapor deposition technique using an appropriate mask. Laser experiments with CW and repetitively pulsed diode-laser pumping were performed and compared. Slope efficiencies up to 19% were achieved. The experimental results are in excellent agreement with the theory.

Proton and gamma radiation tests on nonlinear crystals

We report on the results of proton and gamma irradiation tests performed on nonlinear crystals for second- (SHG) and third-harmonic generation. Beta-barium borate (BBO), lithium triborate (LBO), and KTP crystals were exposed to three different energies of proton radiation (8, 70, and 300 MeV) and incremental doses of gamma radiation (up to 139 krad) in order to investigate the change in SHG performance and transmission spectra. BBO and LBO crystals turned out to be a suitable choice for SHG under radiative conditions.

Vector coupled-mode theory of dielectric waveguides

A consistent derivation of a system of vector coupled-mode (VCM) equations for parallel dielectric waveguides is presented and compared with earlier versions of the improved coupled-mode theory (ICMT). As a validity test, it is shown that the effectively scalar transverse electric and transverse magnetic (TM) coupled-mode (CM) equations are direct limits of our full VCM formulation. In particular, our formulation does not lead to the fundamental error found with earlier coupled-mode theories (CMTs) in a case of TM fields. Functional equations of our VCMT are consistent with Maxwells equations and lead to higher precision. They can be applied to complicated arrays of strongly coupled parallel dielectric waveguides with true vectorial behavior.

Moving humps at the capillary front in laser welding

Welding is a widespread application in laser materials processing and the current development of laser systems with high beam quality helps to enhance the number of applications furthermore. However, the weld-pool dynamics is still a limiting factor leading to weld defects and it seems to be more important for lasers with high focusability and for radiation around 1 µm compared to the 10 µm radiation.In the time past a lot of investigations were performed to analyse the reasons for those instabilities: high speed imaging, X-ray analysis and also modelling. In addition to experiments with real workpieces (iron and aluminium alloys) a set of experiments with transparent materials such as water and ice were performed at IFSW in the last years.As a result, flow components of the melt parallel to the laser beam were identified to be a major reason of the unwanted behaviour of the melt, driven by humps at the capillary front. Although such humps were discovered in X-ray analyses already in the 1980s and described in some models, they were discussed relatively seldom.For this contribution, welding with tracer material in the specimen was used to identify typical flow patterns, whereas the “welding” in transparent material shows directly typical flow structures. Finally, calculations of the beam propagation within the capillary clarify interesting differences for different wavelengths and beam quality. In this context, also the role of the polarisation will be discussed.Welding is a widespread application in laser materials processing and the current development of laser systems with high beam quality helps to enhance the number of applications furthermore. However, the weld-pool dynamics is still a limiting factor leading to weld defects and it seems to be more important for lasers with high focusability and for radiation around 1 µm compared to the 10 µm radiation.In the time past a lot of investigations were performed to analyse the reasons for those instabilities: high speed imaging, X-ray analysis and also modelling. In addition to experiments with real workpieces (iron and aluminium alloys) a set of experiments with transparent materials such as water and ice were performed at IFSW in the last years.As a result, flow components of the melt parallel to the laser beam were identified to be a major reason of the unwanted behaviour of the melt, driven by humps at the capillary front. Although such humps were discovered in X-ray analyses already in the 1980s and describ...

End-pumped Nd:YAG laser with self-adaptive compensation of the thermal lens

Thermally induced lenses are the most critical problem in the development of high-power solid-state lasers. To compensate for thermal lenses, we have been investigating self-adaptive compensation methods based on thermal effects themselves. Recently, we demonstrated a novel compensation scheme for transversally pumped lasers. This scheme has now been adapted to an end-pumped laser system. The reduction of the thermal lens has been simulated and measured experimentally. The experiments were carried out with a diode-pumped Nd:YAG laser with a maximum output power of 15.6 W.

Self-Adaptive Compensation for the Thermal Lens in High-Power Lasers

An adaptive negative thermal lens that compensates for the power-dependent positive thermal lens in a transversally diode-pumped Nd:YAG laser rod is presented. We demonstrate that the proposed technique leads to a reduction of the total thermal lens in the resonator by more than an order of magnitude.

High-quality laser welding of copper using appropriate power modulation

Due to its very good electric and thermal conductivity, copper is an important material with a wide range of applications, which require an efficient and reliable welding process, preferably with laser beams. The same material properties make laser welding of copper a challenging task. To achieve welds with penetration depths of several millimeters in copper using commercially available laser sources relatively low feed rates below 10 m/min are needed. Welds in copper at such low feed rates with 1 µm wavelength lasers often suffer from numerous weld seam defects such as melt ejections and pores.In this paper welds in copper using different laser wavelengths will be presented. Due to a higher absorption green laser light (515 nm wavelength) is obviously far better suited for welding copper. Hence the potential of green laser light on welding copper is discussed. Furthermore the influence of power modulation during copper welding on weld defects is shown. Presuming the correct parameters, modulation of the power shows very distinct advantages. On the one hand a strong reduction in number of weld defects was measured. On the other hand a significantly more regular and homogeneous solidified weld seam surface for the modulated welds was achieved. Welding results for different copper alloys using laser power modulation with different laser wavelengths are presented. Furthermore the influence of the modulation frequency is discussed.Due to its very good electric and thermal conductivity, copper is an important material with a wide range of applications, which require an efficient and reliable welding process, preferably with laser beams. The same material properties make laser welding of copper a challenging task. To achieve welds with penetration depths of several millimeters in copper using commercially available laser sources relatively low feed rates below 10 m/min are needed. Welds in copper at such low feed rates with 1 µm wavelength lasers often suffer from numerous weld seam defects such as melt ejections and pores.In this paper welds in copper using different laser wavelengths will be presented. Due to a higher absorption green laser light (515 nm wavelength) is obviously far better suited for welding copper. Hence the potential of green laser light on welding copper is discussed. Furthermore the influence of power modulation during copper welding on weld defects is shown. Presuming the correct parameters, modulation of the ...

Adaptive thermal optics in high-power laser resonators

In order to compensate for the thermally induced lenses in high-power laser rods we investigate self-adaptive techniques based on thermo-optical processes. Recently we have demonstrated that the influence of the thermal lense in high-power lasers can be reduced significantly by means of a thin liquid layer located within the resonator. Here we report on the investigations of different liquids and gels for the generation of the adaptive lens and discus an improved implementation of the technique, with the compensating layer placed directly in contact with the laser rod.

High-efficiency laser processing of CFRP

Industrial laser processing of carbon fibres is very promising for large-volume production of CFRP lightweight parts. Yet reduced quality and the large process energy needed for processing carbon requiring high average powers actually limits the use of laser technology.However, choosing appropriate laser parameters and processing strategies the thermal damage caused by the laser radiation can be utilized to realize a very efficient processing of CFRP: In the first step only small kerfs are created by sublimating the carbon material. By producing two close kerfs fibre fragments are created which can be removed in a second step by just sublimating the matrix material. The detached fiber fragments are either removed by the ablating material pressure or by an additional process gas jet.The present paper compares the required absorbed energy densities for the different processing strategies of CFRP. The theoretical considerations are compared with experimental results achieved with ns-drilling of blind holes.Industrial laser processing of carbon fibres is very promising for large-volume production of CFRP lightweight parts. Yet reduced quality and the large process energy needed for processing carbon requiring high average powers actually limits the use of laser technology.However, choosing appropriate laser parameters and processing strategies the thermal damage caused by the laser radiation can be utilized to realize a very efficient processing of CFRP: In the first step only small kerfs are created by sublimating the carbon material. By producing two close kerfs fibre fragments are created which can be removed in a second step by just sublimating the matrix material. The detached fiber fragments are either removed by the ablating material pressure or by an additional process gas jet.The present paper compares the required absorbed energy densities for the different processing strategies of CFRP. The theoretical considerations are compared with experimental results achieved with ns-drilling of blind holes.