“The high energy and high instantaneous peak power can lead to immediate damaging of the optics if the fluence (density of energy) is not well managed all along the optical path”.
MULTITEL is a non-profit organization, private research centre. The activities of MULTITEL include signal processing, wireless networks, railway certification, image processing and applied photonics. The Applied Photonics Department activities cover fiber lasers design and prototyping, laser micro-machining, optical sensors and terahertz imaging and spectroscopy.
The activities related to fiber lasers and micromachining are more particularly focused on the design and application of ultrafast lasers on various materials like for thin films photovoltaics processing, surface texturization, glass refractive index modification, etc.
In the project MULTIPOINT, MULTITEL is in charge of designing the beam handling system that will permit making use of the tremendous amount of energy delivered from the ultrashort kW laser to be developed. This is an exciting challenge to correctly scale the optical system in order to cope with the application needs in terms of the number of beams with specific characteristics and to ensure a sufficient level of robustness and reliability of the whole system. Indeed, this optical module will be a critical link between the laser source and the micromachining unit.
In MULTIPOINT we will use ultrashort pulses with an energy on the range of a few millijoules and average power of more than one thousand Watts. In terms of instantaneous peak power, this corresponds to several GigaWatts! This is more than enough for drilling a “simple” hole in a titanium sheet. The point is that we do not want to make one single hole but many and at high speed. The strategy for this will then be to divide this excess of energy into several beams in order to produce multiple points (holes) at the same time.
The main risks from the optical point of view are related to two aspects of the laser source: its high average power and its high energy. High average power means that any excess of loss on the optical path will convert into heat that can lead to instabilities, drifting of performances and finally degradation of the system. The high energy and high instantaneous peak power can lead to immediate damaging of the optics if the fluence (density of energy) is not well managed all along the optical path. This basically means that the beam diameter at the output of the laser must be enlarged but on the other end of the system, we will encounter some space limitations at the entrance of the focusing heads. Then the optical path must again be carefully dimensioned with various beam expanding and reducing steps in order to avoid premature degradation of its components.
The first experiments carried on at MULTITEL, both with high average power and high energy (the two aspects are considered separately for the moment), are promising and we believe that it will be possible to solve this problem with an appropriate and careful design of the whole optical chain.