Characterization device for measuring beam parameter product and beam quality of collimated and uncollimated diode lasers

摘要

Diode laser systems have been established for material processing and pumping solid state lasers in the recent years, due to flexibility, efficiency and lifetime. In the meantime, diode laser bars with an output power of more than 120 W and a beam parameter product less than 70 mm mrad are available (see fig. 1). Depending on the optical system an energy density in focus of more then 10~6 Wcm~(-2) can be achieved. But for several applications like hardening metal surfaces or welding thin blanks/plates the output power is insufficient. To increase optical output power several diode laser bars are arranged vertically and/or horizontally. With these so called stacks an optical output power of more than 4 kW can be achieved. Due to the incoherent beam coupling the beam parameter product is increased at the same rate. But the energy density or intensity in focus is rather less than constant. Other applications, e. g. welding or marking, require higher intensities, which can not be achieved with diode lasers. For these applications diode pumped solid state laser are mostly applied. All systems and applications based on diode lasers have in common, that a special intensity profile of the diode laser source is required (for e. g. coupling into a fiber or crystal or for beam homogenization). In order to achieve the required intensity profile an optical system has to be designed. Therefore the optical properties of the diode laser system have to be well known. Especially the knowledge of the divergence angle and the beam parameter product is indispensable for the design of an optical system. Several methods for measuring these properties are well known, also their advantages and disadvantages. In focus of ISO-standard 11146 three different methods for measuring beam parameter product and the resultant beam quality are presented. Their advantages are discussed and measuring errors are estimated. Reflecting this discussion a new approach for a fast measurement of beam parameter product and the resultant beam quality is developed. Filially a complete automated characterization device based on this approach is presented.