Georg Bostanjoglo

Processing of Ni-based aero engine components with repetitively Q-switched Nd:YAG lasers

Aircraft engine industry uses free running high power Nd:YAG lasers for drilling cooling holes into nickel base alloy turbine components. A cw-pumped, Q-switched, high beam quality laser system with 400W laser power is presented. The laser is used to trepan drilling of 1.6mm. Hastelloy X sheets and ceramic coated combustion chamber tubes of the same metal. Cylindrical shape, uniformity, and reproducibility are achieved with a trepan-like drilling setup. The heat load of the workpiece as well as the process time is considerably decreased by employing high-repetition Q-switched lasers.

6-kW Nd:YAG laser system for welding applications

High average laser output power was realized by combining the beams of three Nd:YAG lasers of the 2 kW class, each being coupled into an individual 600 micrometers fiber. The end faces are imaged onto a 1 mm fiber, which then transmits 6 kW cw power to the processing station. With this laser system, butt and edge joints with up to 5 mm thick sheets of AlMgSil and AlMgMn4,5 were performed. Velocities of about 3 m/min in combination with filling wire and helium shielding gas yielded satisfying results. The sheets of 1.5 mm thickness were best welded with 8 m/min and a O 1 mm filler wire delivery speed of 5 - 7 m/min. One of the drawbacks of the system is the deterioration of beam quality due to the tilt angle between the incoming beams, so that a 1 mm fiber has to be used. With a modular oscillator-amplifier system with fiber link it should be possible to use fibers with smaller core diameter.

Frequency doubled Nd:YAG laser with high average power in processing of ceramics, metals and composites

This paper describes the use of 532 nm green light generated via frequency doubling or Second Harmonic Generation (SHG) of a continuously pumped and acusto-optically, AO- Q-switched, and a passively Q-switched Nd:YAG laser using KTP and LBO crystals. The resulting pulse energies were lower than at the fundamental wavelength but parameters such as pulse duration and pulse repetition frequency (PRF) remained the same. The better focusability of the green light compensates for the losses during frequency conversion when used for drilling.The SHG laser beam was successfully used for drilling and cutting hard to machine ceramics (e.g. cubic boron nitride) as well as metals and fibre reinforced composites using both percussive and trepanning techniques. The use of SHG Nd:YAG-laser for deep hole drilling may therefore be a possible alternative to Cu vapour lasers. A comparison of hole drilling at the fundamental and second harmonic wavelengths is presented.This paper describes the use of 532 nm green light generated via frequency doubling or Second Harmonic Generation (SHG) of a continuously pumped and acusto-optically, AO- Q-switched, and a passively Q-switched Nd:YAG laser using KTP and LBO crystals. The resulting pulse energies were lower than at the fundamental wavelength but parameters such as pulse duration and pulse repetition frequency (PRF) remained the same. The better focusability of the green light compensates for the losses during frequency conversion when used for drilling.The SHG laser beam was successfully used for drilling and cutting hard to machine ceramics (e.g. cubic boron nitride) as well as metals and fibre reinforced composites using both percussive and trepanning techniques. The use of SHG Nd:YAG-laser for deep hole drilling may therefore be a possible alternative to Cu vapour lasers. A comparison of hole drilling at the fundamental and second harmonic wavelengths is presented.

Stable resonators with variable reflectivity mirrors for multi-kW Nd:YAG lasers

A new method for obtaining high-power laser beams with drastically improved beam quality for fiber coupling is presented. The central idea is to use variable reflectivity mirrors (VRM) as outcoupling mirrors for high-power solid- state lasers with stable resonators, operated near the maximum power. The reflectivity profile of the VRM, properly chosen, acts similar to a mode aperture. Without any further modifications of the laser, the beam parameter products were substantially reduced: the maximum values at intermediate pumping powers were reduced to the beam parameter product occurring at the highest input power. The laser power, however, was not reduced within the principal working interval. Different Nd:YAG laser systems with 350 W, 1.9 kW, and 3 kW maximum output power were equipped with VRM. The 1.9 kW system was coupled without any loss of power into a 400 micrometer fiber instead of the 600 micrometer fiber normally used. The influence of the VRM spot size on laser power and beam parameter product (BPP) was investigated. Furthermore, it was shown that better beam quality is achieved by adding further laser cavities inside the resonator instead of using them as external amplifiers.

1.06-µm and 1.44-µm Nd:YAG lasers with apodized reflectors

One of the present tasks in high-power solid-state laser development is to increase the beam quality. For 1.06 micrometer Nd:YAG lasers with unstable resonators, superGaussian outcoupling mirror can be used to obtain a high central intensity peak. Still, it contains less than half the power. Part of the rest should be focusable if bifocusing is avoided by birefringence compensation. If used with stable resonators, graded reflectivity mirrors were able to reduce the maximum beam parameter product from 16 to 10 mm*mrad. The maximum output power of 360 W remained unchanged. For 1.44 micrometer Nd:YAG lasers, which are used in laser medicine, apodized gratings have to be used instead of gradient mirrors. Preliminary investigations and first results are presented