Single-prism shaping of laser beams: Mathematical background and application
In the last few decades, diode lasers have established in a number of fields of application in research and industry. Such light sources feature a high degree of efficiency as well as a long duration of life. Thanks to steady further developments, modern diode lasers exhibit an output power in the range of some watts and have become a cost-efficient alternative solution for other types of lasers. In addition, diodes are suitable sources for pumping solid state lasers. However, due to the functional principle and the setup, diode lasers exhibit an elliptical beam profile, resulting in a limited focusability. In addition, both the short and the long axis of diode lasers propagate at different angles of divergence. As a consequence, astigmatism can occur. In order to overcome these disadvantages, several techniques for shaping diode laser beams are in hand. For example, anamorphic lenses, consisting of an assembly of cylindrical lenses or gradient-index elements, can be used for shaping a circular beam profile. For this purpose, anamorphic micro-lenses can be applied moreover. Further, anamorphic prisms or prism pairs allow converting circular laser beams from elliptical diode laser irradiation. Such a shaping is also achieved by combinations of aspheric, cylindric and toric micro-lenses and macroscopic lenses. Other techniques for generating circular beam profiles are based on micro-optical elements such as crossed micro step-mirrors in order to shape the short and the long axis successively. This method can be used for fibre-coupling applications. In addition, micro-mirrors are used to assemble beam-shaping devices where the functional principle is comparable to imaging micro-lens beam homogenisers. The incoming beam is divided into a number of partial beams by a series of tilted plane micro-mirrors. Subsequently, the partial beams are recomposed by a micro-mirror recombiner. A similar technique is based on a stepped mirror. Here, only the short axis is divided in subunits, leading to a rectangular beam profile after further reflection. This rectangular beam profile is then shaped by macroscopic lenses. Instead of micro-mirrors, diffractive optical elements or arrays of right-angled prisms can be used furthermore for shaping diode laser beams. Beam shaping can also be achieved using two offset plane mirrors. Due to its simple design, this technique presents a costefficient solution.