Procedure for the identification of the focal plane position in high power fiber laser systems

The paper presents a procedure for the identification of the focal plane position when a high power fiber laser source is used. The proposed procedure is based on the analysis of the phenomena that occur during the fusion of a metallic surface. The identification of the focal plane position is based on the monitoring of laser radiation back reflected by the melting pool directly acquired by a photodiode placed inside the optical combiner of the fiber laser source. This monitoring architecture, namely Through Optical Combiner Monitoring (TOCM), can be applied in all the fiber laser sources that are equipped with an optical combiner. Due to the favourable position of the monitoring architecture the suggested procedure does not require equipment directly installed on the laser head and can be used for instance during close or remote welding. In the paper the proposed procedure is presented and its performances are determined and compared with the techniques commonly used for the identification of the focal plane position. The standard procedures based on the direct measurement of the laser beam caustic (according to ISO 11146-1 standard) and the indirect measurement of a fusion welding track (according to UNI 10300 standard) are in fact the standard methods that constitute the alternative reference for the new one.