Any steered beam fiber laser marking system requires a means of directing the light from the fiber laser into the steering mirrors of the scanhead. The output device of a fiber laser is called an “isolator” or “collimator”. When the fiber laser beam exits the “isolator” it is about 8.5mm in diameter. The fiber laser beam must ultimately go into the scan head, reflect off of the X and Y galvo mirrors, pass through the F theta flat field lens and then be focused on the work piece to be marked or engraved.
The easiest way to get the fiber laser beam from the isolator into the scanhead is to just build a mount that connects the 2 pieces together. The problem with this design is that the resulting focused spot at the work piece is going to be determined by the output properties of the laser beam and the characteristics of the lens. Regardless of what type of material you are marking, the laser spot size at the focal point of the lens is the same. Laser marking is not a “one focused spot size fits all” process. Different materials and different marking appearances require different spot sizes.
Marking generally falls into three categories: Ablation, Engraving and Stain marking.
Ablation marking involves removing a film or layer of material on the part. Ablation marking is not particularly sensitive to laser spot size.
Engraving is just as the name implies. Generally speaking, smaller spot sizes yield better results when an engraving technique is used.
Stain marking is one of the most useful marking techniques for fiber lasers, but it is also one of the most misunderstood techniques.
Stain marking can provide a permanent, dark mark on stainless steel, titanium and hard chrome. The most commonly used technique to increase the size of the focused laser spot size is to simply move the lens away from the work piece so that it is no longer in focus.
This is a poor technique for the following reasons:
1. Marking at a distance other than the focal point of the lens can be subjective and hard to repeat.
2. When the lens on a steered beam marker is moved away from the focal point, the field size calibration is no longer correct and marking will not be to size.
3. When marking on a curved surface, the focused spot size will change as the laser beam is moved over the curvature of the part. How far one can mark around that curvature is mainly determined by the “depth of focus” of the optical system. If the lens is already been placed out of focus in order to achieve the correct spot size for a stain mark then there is no depth of focus available. Marking around a curved surface with an out of focus laser beam makes stain marking difficult, if not impossible. (For more information on focused laser beams check out http://en.wikipedia.org/wiki/Gaussian_beam, if you are interested). The diagram below illustrates how the laser beam acts at the focal point and on either side of the focal point. There is no depth of focus with an out of focus lens.
For the reasons described above, the Jimani Langolier Optical Housing does not attach the laser isolator directly to the scanhead. The beam is passed through a reversible beam expander before it enters the scanhead. The beam expander makes the lens capable of achieving 3 distinctly different spot sizes at the work piece. With the beam expander in the forward position, the lens is able to focus to a smaller spot than can be achieved by taking the beam directly from the isolator into the scanhead. This smaller spot increases the laser power density at the work piece by a factor of 2. It allows the system to provide deeper engraving at a higher speed than could be accomplished without it.
If the beam expander is removed from its housing, the laser beam from the isolator passes directly into the scanhead mirrors and the focused spot size is larger than with the beam expander in the forward position. That size spot is perfect for most ablation marking applications.
If the beam expander is placed in the reverse position, the focused spot size is even larger at the focal point. In this position stain marking can be achieved with the lens at its focal point and the entire depth of focus of the lens is available for marking around curved surfaces.
All optical components on the Langolier Optical Housing are dowel pinned into position. The entire housing can be disassembled and then reassembled without any alignment whatsoever. Components only fit into the housing one way and everything is self aligning. The reversible beam expander is secured in its cradle by two thumb screws and its position can easily be changed without affecting alignment at all. The fiber laser isolator fits into a similar cradle and also self aligns. The Langolier Optical Housing also contains a failsafe laser safety shutter. Opening and closing of the safety shutter is controlled by a front panel switch and the safety shutter goes to the closed position if the Optical Housing cover is removed for any reason. The scanhead is dowel pinned to the front plate of the Optical Housing and can also be removed or reinstalled without alignment. All components are mounted on a rigid flat base plate.
Contact Jimani today to see how we can help you with you laser marking system needs.