A three-dimensional (3-D) image of the Frauenkirche in Dresden, Germany, has been engraved dot by dot into transparent lead glass. The church became known worldwide after its tragic loss in World War II. Its ruins served subsequently as a memorial for the victims of the Dresden bombardment. The Frauenkirche is currently under reconstruction.
The image was inscribed as white dots into the glass with a specially designed Q-switched, modelocked Nd:YLF laser by researchers at the Fraunhofer-Institut für Werkstoff-und Strahltechnik (Dresden, Saxonia). The white dots are actually microcracks caused by microplasma generation at the focus of the laser beam. The surfaces of the cracks are rough so that ambient light is scattered.
At the laser repetition rate of 500 Hz, a new dot can, in principle, be inscribed every 2 ms. The total pulse length is about 500 ns, however, and a pulse consists of many 60-ps spikes that increase and decrease in intensity under the overall pulse envelope. The peak power of the laser is 2 MW. At the focus of the beam, the power density reaches 1012 W/cm2 so that nonlinear effects increase the absorption.
The maximum repetition rate of the laser cannot currently be used because positioning of the glass is achieved by mechanical x-y-z stages. Depending on the complexity of objects, the engraving time varies from a few minutes to a quarter of an hour. By introducing a scanner for the x-y plane, however, processing time will be reduced.
In addition to artistic applications, 3-D engraving can be used for unerasable marking of objects with, for example, company logos.