Measurement of the maximum specific removal rate: unexpected influence of the experimental method and the spot size

Ultra short laser pulses often are the tool of choice when high requirements concerning machining quality are demanded. But for industrial use the process has also to be efficient, meaning that the removal rate (ablated volume per time and average power) should be as high as possible. Many publications deal with the threshold fluence and the removal rate for various materials but often use different methods and beam parameters to determine these values. To demonstrate the influence of the different methods, the removal rate for steel and copper was determined for different pulse durations and different spot sizes using the following three different methods: With the first method the removal rate is calculated from the threshold fluence and the energy penetration depth deduced by machining craters at low repetition rates, measuring its depths and using the logarithmic ablation law. With the second method the removal rates were directly determined by measuring the volume of these craters and with the third method they were determined by measuring the volume of squares machined with a pulse overlap and higher repetition rates. This systematic study shows differences between the investigated methods themselves. Additionally it reveals for all three methods an unexpected influence of the spot size which is much more pronounced in the case of steel.