High power GHz femtosecond laser for ablation efficiency increase

Abstract

Nowadays the relevance and the robustness of ultrafast lasers are well established for many industrial applications. The main limitation to the penetration of femtosecond processing in the industrial manufacturing market is the insufficient productivity compared to the needs of the application markets and alternative technologies. Increasing the power, pulse energy or repetition rate, of femtosecond lasers has been a general trend in in order to comply to the needs of the market for higher throughput. One of the strategies of increasing the power has resulted in the development of a GHz femtosecond laser. When increasing the repetition rate during the process beyond several hundred kHz, the delay between the laser pulses is less than the thermal relaxation time and there is thermal accumulation in the target material. The resulting rise in temperature leads to an ablation threshold lowering. The authors report on a simple and easy-to-use GHz amplified femtosecond laser source. The laser source is based on a passively mode-locked oscillator with a near GHz repetition rate. GHz pulses are then selected, and the obtained bursts of pulses are further amplified in a high-power amplifier chain. The presented GHz femtosecond laser source is used with a galvanometric scanner to perform ablation experiments on copper, aluminum, and stainless steel. Specific ablation rates of 0.7, 2.3, and 1.4 (mm3/min)/W are reached. The role of the important experimental parameters, such as the number of sub-pulses in the burst, is highlighted. Thanks to a specific ablation scheme in the GHz mode, the ablation efficiency is then comparable to the case of single nanosecond pulses, but with the usual quality of femtosecond processing.