Active beam controlling of high power Q-switched Nd:YAG lasers for stable fiber coupling with small numerical aperture for material processing

Abstract

For mobile surface treatment it is convenient to work with flexible fiber coupled lasers. To keep the focusing optic sufficiently small it is necessary to have small fiber core diameters and a small outgoing numerical aperture. The knowledge of the resonator beam parameters is essential for long term stable fiber coupling conditions. We demonstrate that the out-coupling beam diameter is suitable as a control process variable for this purpose. For high power side pumped Nd:YAG lasers it is necessary to have an effective rod cooling to compensate thermal lensing. By adapting the rod cooling to the thermal lensing it is possible to keep the beam diameter constant. In this case the water flow of the rod cooling circuit has to be independent from pump diode cooling circuit and has to be controlled by a beam profile measurement inside the resonator. The half angle of the laser beam outside the resonator is dependent on the resonator length and the pump power and therefore the thermal lensing. If the beam diameter on the mirror is constant, the numerical aperture of the laser is also constant. If the beam diameter changes due to diode degradation or other effects inside the resonator, the diameter as a process control variable should control the flow of the rod cooling. Thus, the thermal lensing could be adapted, to control the beam diameter and as a result the numerical aperture for the fiber.