Fiber Laser Micro Welding: Single Mode or Multi Mode?

Fiber laser micro welders come in two flavors: single mode and multi mode. What are the differences and which should you choose for your fiber laser micro welding application?

Single mode fiber lasers are typically delivered via fiber with a core diameter of around 9 microns producing a narrow, high intensity beam which can be focused down to a spot size as small as 10 microns. This high intensity, small spot is ideally suited for laser cutting applications, but generally not great for welding, as weld widths are too narrow to accommodate most production fit-up tolerances.

Multi-mode fiber lasers, by contrast, utilize fibers with core diameters between 50 – 300 microns resulting in lower intensity, more uniform, “flat top” beams which promote an enlarged melt zone more in line with laser micro welding requirements.

This figure shows a schematic of the laser exiting the fiber and the cross section of power intensity through the beam diameter for the two modes:

Consider also laser alignment to the weld joint –stack-up positional tolerances and gaps between weld edges mean that aligning the beam to the joint line generally requires a spot size of at least 200 microns. To attain that large a spot with a single mode fiber laser, you’d need a focal length of 2-3 feet! Not practical in any welding system… Furthermore, the single mode’s high central intensity which tapers rapidly to the edges, concentrates all of its power in a small volume of material. If there is any gap in the joint, the weld will be undercut or underfilled, and, if the intensity is too high, the laser will remove material.

The multimode fiber laser beam more equally distributes its intensity across the weld, resulting in more stable welding conditions. It is less sensitive to gaps between welding surfaces, and its larger flat top intensity profile melts more base material volume, effectively bridging gaps as needed.

The figure below shows cross sections of bead on plate welds for single mode and multimode lasers in 0.06” thick stainless steel utilizing (a) Single Mode Fiber Laser at 500W, 300ipm with a 30 micron spot size (b) Multi Mode Fiber Laser at 700W, 100ipm with a 150 micron spot size (c) Multi Mode Fiber Laser at 1kW, 80 ipm with a 250 micron spot size:

There are some cases where single mode fiber lasers can be implemented effectively in laser micro welding applications; high speed lap welding, for example, or very close fitting joints that can be welded with significantly lower laser power, but still achieve a certain penetration over multi mode lasers.