Practical Aspects of Surface Generated Third Harmonic for Highly Precise Beam-workpiece Alignment

摘要

Precise beam-workpiece alignme nt is highly important in la ser processing. Tho ugh many meth- ods exist for this task few allo w high-precision measurements using the pro cessing laser its elf. One such method detects the laser 's backreflectio n from the workpiece's surface using a co nfocal setup and yie ld s ideally an accuracy of about 10% of t he Rayle ig h length of the focused beam. Ne verthe- less, under certain co nditions the accuracy of this method may su ffer from a mismatch o f beam di- vergence s between laser and co nfocal detect ion o ptics. This becomes important when transparent ma teria ls are pro cessed in three dimensions using optics with very high numerical apertures or if expensive materials or samples have to be processed where any misalig nment would be cost ly. A possible solution t o this is surface based third harmo nic generatio n that allo ws for absolute surface detection while having the same accuracy as the confocal method. However, this method requires a comp lex setup while exhib iting low conversion efficiency. Both rea sons make a proper a lignment o f this measurement sche me difficu lt. Herein we pre sent results sho wing a cle ar conve rsio n effic ie ncy dependence on the reflect ion coefficient of the generating interface for a nu mber of material combi- natio ns. This makes the selection of suitable mat erials more straightforward and reduces the a lign- me nt effort. The highest conversio n efficiency w as achieved fo r air/silicon which exhib its the hig h- est reflect ion coefficient of the investigated materials.