Laser ablation as a process has many advantages over mechanical machining, certainly in the area of micromachining and engraving.
Apart from being a non-contact technique, it allows a wide range of materials to be machined and very small dimensions and high resolution and accuracy to be achieved. Open tool paths are easily achieved by simply switching the laser off while moving to the starting point of the next tool path rather than lifting the tool out of the workpiece. And it is not necessary to drill pilot holes in order to move the tool into position in the first place.
Also there is no tool radius, so corners can be infinitely sharp.
On the other hand, laser ablation is a much more complex process than for example milling. Figure 1 illustrates in a schematic and simplified way the differences between milling and laser ablation.

Figure 1: Schematic comparison of milling (top sequence) and laser ablation (bottom sequence).

Whereas with milling, the result of a machining operation is completely determined by the geometry and the position of the tool, this is not so with laser ablation. One fundamental issue is the depth control: Given the length of a milling tool you can easily position it so that it will remove material exactly down to a predefined absolute z-level. A laser beam, however, machines down to a certain depth relative to the surface – wherever the surface is. In particular, overlap of tool paths results in extra depth.
Similarly, with milling the profile of the cut is entirely determined by the shape of the milling tool. With laser ablation, on the other hand, there are many factors that influence the profile of the cut: The beam profile and shape, laser parameters such as the pulse repetition rate and the shot overlap, physical properties of the material (e.g. optical penetration depth, thermal diffusivity), the curvature of the tool path (the cut will be deeper at the inside of a curve due to higher shot overlap) and the shape of the surface of the substrate.
Thankfully this is not a problem when laser ablation is used for engraving trophies, plaques, lables and the like.

In many cases, the most efficient way to machine a slot is not cutting along its edges, but raster scanning the whole slot. This technique is the analogue to the spiraling technique for hole drilling and has the same effect: Material is removed from the whole area of the slot in order to avoid a narrow cut. Again, it is not the technique itself that is innovative, but the ease of use and convenience provided by our software. As an example of an application for slot cutting we can mention the machining of ink feeding slots for ink jet printer heads.

2D area clearing is mainly used for single shot processes where no depth control is required, e.g. for patterning of layers on glass, metals or flexible substrates as required for flat panel displays. Tool paths are generated that allow all the material in the areas defined by the borderlines in the CAD drawing to be removed. In the case of nested borderlines, we build up a tree structure reflecting which borderline is inside which. This allows us to identify the different regions, each of which is delimited by an outer border and possibly contains islands. The rastering prossess is shown in Figure 8.

For more information on laser engraving For more information on image resolution/DPI

ACKNOWLEDGEMENTS : Information in this article are extracts from a paper "CAD/CAM software for an industrial laser manufacturing" written by Exitech Ltd, Oxford Industrial Park, Yarnton, Oxford, UK; and Imperial College of Science, Technology and Medicine, Dept. of Electrical and Electronic Engineering, London, UK.