Suppression of Surface Waviness Error of Fresnel Micro-Structured Mold by Using Non-Integer Rotation Speed Ratio in Parallel Grinding Process

Fresnel micro-structured lenses are widely used in the field of modern optoelectronic technology. High-precision Fresnel micro-structured mold is the key technology to achieve its large-scale replication production. Focusing on the surface waviness error of Fresnel micro-structured mold machined by parallel grinding process, this paper conducted theoretical modeling and experiment research. Based on the grinding kinematics theory, the simulation models of the surface waviness topography and the circular waviness profiles of the ground Fresnel micro-structured mold were developed, considering the combined influence of the non-integer rotation speed ratio and other grinding parameters. A series of grinding experiments were carried out to verify the proposed simulation models. The influence of a non-integer rotation speed ratio and a wave-shift value upon the surface waviness error of the ground Fresnel micro-structured molds were analyzed. Both the simulation and experimental results proved that choosing the non-integer rotation speed ratio and a proper wave-shift value could greatly reduce the surface waviness error and improve the surface quality and uniformity.

Brief communication: Characteristic properties of extreme wave events observed in the northern Baltic Proper, Baltic Sea

A significant wave height of 7 m has been measured five times by the northern Baltic Proper wave buoy in the Baltic Sea, exceeding 8 m twice (2004 and 2017). We classified these storms into two groups by duration and wave steepness. Interestingly, the two highest events exhibited opposite properties, with the 2017 event being the longest storm on record. This storm is also the first where the harshest wave conditions were modelled to occur in the western part of the Baltic Proper. The metrics quantifying the storm's duration and steepness might aid in issuing warnings for extreme wave conditions.

Brief communication: Characteristic properties of extreme wave events in the Baltic Sea

A significant wave height of 7 m has been measured five times by the northern Baltic Proper wave buoy in the Baltic Sea, exceeding 8 m twice (2004 & 2017). We classified these storms into two groups by duration and wave steepness. Interestingly, the two highest events exhibited opposite properties, with the 2017 event being the longest storm on record. This storm is also the first where the harshest wave conditions were modelled to occur in the western part of the Baltic Proper. The metrics quantifying the storm's duration and steepness might aid in issuing warnings for extreme wave conditions.