Unlike conventional light sources the laser creates coherent, monochromatic and almost non-divergent light.
For the subsurface engraving a laser beam is expanded and then focussed by an objective down to a diameter of approx. 5-10µm.
At the used wavelengths of 1064nm (infrared), 532nm (green) and 355nm (UV), only a negligible part of the radiation is reflected resp. absorbed by most transparent materials such as glass, plastics, sapphire, etc. So these materials are transparent at the given wavelengths.
A non-focussed laser beam would travel through these materials without any interaction with the material.
If the laser beam gets focussed into the material the following happens:
The intensity (I), i.e. the energy per cross-sectional area increases with decreasing beam diameter.
The relating formula is:
The energy (E) of the beam stays constant, the cross-sectional area (A) decreases. Therefore the intensity (I) increases.
From a certain threshold of the intensity Ith, non-linear optical effects lead to an absorption of the laser radiation in the material. The absorption results in a structural change in the material, which appears white in glass due to scattering of the ambient light.
In polycarbonate, the structural changes appear grey due to a carbonisation at the focus.
Thousands of these structural changes („points“) are created in the material to form a two- or three-dimensional image in the material.
The positioning of the laser beam in the x-y-plane is done by a galvanometer-scanner.
In z-direction the distance between focussing lens and transparent material is changed step by step, so that a three-dimensional image is created layer by layer.