Laser Beam Focus Analyser

The quantitative and qualitative description of laser beam characteristics is important for process implementation and optimisation. In particular, a need for quantitative characterisation of beam diameter was identified when using fibre lasers for micro manufacturing. Here the beam diameter limits the obtainable features in direct laser machining as well as heat affected zones in welding processes.

This paper describes the development of a measuring unit capable of analysing beam shape and diameter of lasers to be used in manufacturing processes. The analyser is based on the principle of a rotating mechanical wire being swept through the laser beam at varying Z-heights. The reflected signal is analysed and the resulting beam profile determined.

The development comprised the design of a flexible fixture capable of providing both rotation and Z-axis movement, control software including data capture and finally data analysis based on the ISO approach. The device was calibrated and tested on commercially available laser systems. It showed good reproducibility.

It was the target to be able to measure CW lasers with a power up to 200 W, focused down to spot diameters in the range of 10µm. In order to eliminate any undesired machining of the rotating wire by the laser beam, a careful balance between the speed of the wire and the chosen wire material had to be chosen.

The photodiode detector used had a rise time of 5 ns limiting the speed of the wire. This rise time limit of the detector also limits the resolution of the device. Therefore the circular speed of the wire through the laser beam, the rise time of the detector and the oscilloscope have to be comparable.

Another limit on the resolution is the position and size of the detector; the angle to the rotating wire, the distance to the wire, and the aperture of the detector. In order to make the device small and flexible, a multi mode fibre was used to collect the signal. This gives the option to change the fibre, and thereby the aperture.