Multi-beam UV-picosecond laser micromachining of temperature sensitive piezoelectric ceramic materialsWednesday (24.06.2020) 16:50 - 17:10 Room 2
Modern high-power picosecond laser sources have been continuously developed over the years. These reach nowadays average powers beyond 100W in the MHz regime in industrial environments. The on-going development of picosecond lasers sources has demonstrated up to kW average power, at repetition rates of several hundreds kHz and pulse energies in the mJ level. However, efficient use of such high available average power for common laser micromachining tasks remains a challenge. The optimal peak fluence in terms of volumetric removal rates, needed for high-quality processing of a given substrate, is e2 times the ablation fluence threshold. In practice, this implies rarely applying average powers above 10W when processing most materials. This is even more pronounced for substrates which are intrinsically temperature sensitive, such as polymers, or that may lose their key desired properties when heated up. Piezoelectric ceramics such as lead zirconate titanates (PZT) constitute an example of the latter. These materials start losing their piezoelectric properties if heated to temperatures close to and beyond their Curie temperature. This loss of performance starts at 200°C for some types of PZT.
It has been shown that PZT partially loses its piezoelectric properties when processed by ultra-short laser pulses of about 1ps. This can be attributed to the limited -yet existing- heat flux to surrounding volume. A very careful heat input management is required then to limit performance loses, and to manufacture functional PZT-based devices without postprocessing steps. This severely limits the speed of the laser micromachining process, and therefore the question on how to take advantage of the high available laser power remains.
In this work, a strategy consisting of first optimizing processing conditions for single-beam UV-picosecond laser micromachining of PZT electrode grids and then up-scaling the process by beam splitting and parallel processing is adopted for manufacturing high resolution ultrasound endoscopes. Single-beam PZT processing is shown here to require low average powers around tens of mW, to avoid losing piezoelectric performance. Fixed structured glass diffractive optical elements (DOEs) are then employed for beam splitting and parallel processing, owing to their high damage threshold that allows withstanding high average powers. Several multi-beam micromachining strategies are investigated here in order to determine the process up-scalability.