I find that manufacturing engineers tend to devote a lot of energy to thinking about the laser, motion, tooling and process, while overlooking both the laser beam focus and delivery to the workpiece. So, I thought I’d take some time to to review some best working practices for the implementation, standardization and maintenance of optical delivery components, which I firmly believe are key to the manufacturing equation. In a later post, I plan to give you my thoughts on applying this to maintaining high production yields and troubleshooting methods.
- Launching the laser into the fiber
For an Nd:YAG laser, a laser coupling optic is used to launch the laser into the fiber, with one side accepting the laser and the other containing the fiber connection. There is usually a one-time fiber launch alignment setup, though I recommend you verify alignment by delivered power or pulse energy and if possible beam mode if you use a smaller fiber diameter.
If you are using a fiber laser, the fiber laser and delivery fiber are spliced within the laser by the manufacturer optic. The disadvantage of an internal splice is that replacement of a damaged delivery fiber may require returning the laser to the supplier for repair. There is the option of an external beam coupler whereby the delivery fiber can be replaced in the field. This is an additional cost item to be balanced with the specific budget, application and production needs/volume.
Routing the fiber to the workstation
Make sure the fiber does not exceed the minimum bend radius (usually around 6 inches, or 150mm) because bending a fiber beyond this limit may overstress the fiber, leading to potential power losses or failure. Best working practice is to route the fiber above ground, not touching the floor between the laser and workstation to avoid the possibility of crushing the fiber, then coil and hang excess fiber length at the workstation. If the focus head will be moving, give sufficient slack, and again make sure the fiber’s connection to the focus head avoids exceeding the minimum bend radius.
Focus Head & Optics
Select the right focus head and optics for space, working distance, part access and tooling accommodation. With each focus head style, the laser diverges from the fiber cable and is “collimated” by a first optic, transforming the diverging light to light that is propagating parallel to the travel direction. The second lens, known as the "focus lens," focuses the laser to a spot where processing will occur.
Here are some simple rules to follow for selecting the fiber diameter and focal lengths for the collimator and focus lens: The focal length of the focus lens should be maximized to provide sufficient working distance, the magnification ratio (ratio of focus lens/collimating lens) should be 1:1 (x1) and no less than 1:2 (x0.5), the largest core diameter should be selected to ensure the first two rules. The following table summarizes these selection considerations.
Be sure to check, clean, and occasionally replace, the lens cover slide, because contamination may lead to power transmission loss. For non-critical welds a loss of 5 percent is not significant, but heavy dirt on the cover slide can lead to a 10 percent power loss, and this is a problem.
The focus optic and the collimation optic may need replacing over time, and make sure you ensure the correct lens orientation when returning cleaned or new lenses. The lenses have a curved shape on one and a flat on the other, and the efficiency of lens and the resulting optical focus spot size are affected by the lens orientation. If replacing with news lenses the lens to lens variation will mean the focus point, z distance of focus head from workpiece, will need to be re confirmed. Maintaining the focus point is a key setup parameter and should be regularly verified as part of the manufacturing process and whenever any focus head optics are replaced or removed to be cleaned.
Do you have other tips to share for optimizing beam delivery and focusing optics? Feel free to comment below!