What's the difference between single-mode and multi-mode fiber lasers?

What is single mode? What is Multimode?

The essential difference between single mode and multi-mode lasers is that single mode lasers have only one mode in the output beam pattern, while multi-mode lasers have multiple modes in the output beam pattern;

That is, single mode refers to the laser energy in a two-dimensional plane of a single distribution mode, multi-mode refers to multiple distribution modes superimposed on the formation of the spatial energy distribution pattern. For example, your laser is 1064nm, assuming that you hit out are 1064, but to a target, if at the same time there are a number of points, such as 10 rings, 9 rings, 7 rings, 2 rings, anything, or even a big hole, that is a multi-mode. But if you're shooting out all 10 rings and one dot, that's a single transverse mode [1].

Image from Lao Luo English Training - You can imagine single mode as which bow and arrow in the middle, and multi-mode is this bow and arrow at the bottom.

From the energy distribution:

Industry often said single mode, refers to the transverse mode of the laser, that is, there is only one mode within the cross-section, a Gaussian distribution, the focus is the center to the outer edge, the laser energy density decreases in turn. And multi-mode, is presented on the cross-section of many energy points, the more modes, the more energy is presented as a flat-top distribution, the image of the analogy is a red tasseled gun and a wolfsbane.

Single-mode multi-mode welding applications on the difference is: if you want to go deep welding, it is suitable for single-mode or less mode, single-mode in the splicing of deep welding, stacked welding, fillet welding, etc. has the advantage of high energy density is easier to hit the depth of fusion.

Multi-mode is suitable for shallow welding, good flatness, weld energy uniformity, but also can avoid the base material melting point is too low to bring the center of the weld ablation, perforation and other quality losses. [1]

As shown in the figure above: the left figure for the single fundamental mode energy distribution, over the center of the circle in any direction of the energy distribution are Gaussian curve form (normal distribution); the right figure for the multi-mode energy distribution, the essence of the spatial energy distribution formed by the superposition of a number of single laser modes, multi-mode superimposed on the result of the energy curve is approximately flat-top distribution.

As shown in the figure: assuming that the vertical coordinate of the curve represents the energy density, green class Gaussian energy distribution, blue for the multimode energy distribution, red for the flat-top beam, it can be seen that the single mode in the energy density is more concentrated, the unit energy density is greater.

Generally one can distinguish single mode multimode from the beam quality M2 of the laser:

The M2 factor is calculated by dividing the product of the actual beamwidth and divergence angle by the product of the ideal beamwidth and divergence angle, where the ideal beam is defined by the fundamental mode Gaussian beam and the beamwidth is defined by the second order moments. When a laser beam passes through an aberration-free optical system, its M2 factor is a transmission invariant and M2 ≥ 1; the further M2 deviates from 1, the worse the laser beam quality.

Depending on the M2, lasers can be divided into three types: pure single-mode lasers with M2 < 1.3, quasi-single-mode lasers with M2 between 1.3 and 2.0, and multimode lasers with M2 > 2.0.

Comparison of single mode laser and multimode:

Single mode laser fiber core diameter is small (14um), the energy is Gaussian distribution, the focal spot is small, high energy density (under the same power, the energy density is more than 4-10 times of the multi-mode), and the heat-affected zone is small, especially for the high-reactive alloy (aluminum, copper) can instantly form a molten pool of keyholes (energy density is much greater than the melting threshold of high-reactive alloys), no high-reactive, is not easy to damage the fiber, and to achieve the high reaction of the alloys High-speed processing, but also in the micro-connections have advantages.

Heat input: single-mode energy is more concentrated, small heat-affected zone, small melt pool, small thermal deformation, melt depth, single-mode beam like a sharp knife, multi-mode like a bullet;

Welding process: single-mode keyhole opening is small, multi-mode keyhole opening is large, reflected in the welding stability, single-mode low-speed welding is unstable, easy to have spatter and porosity, need to be matched with the swing head, vibration mirror, or high-speed welding, low-speed welding spatter is larger, thin plate welding, welding is dominated by the welding; reflecting on the metallurgical, that is, single-mode has a greater depth-to-width ratio (the ratio of the depth of metallurgical width); multi-mode can be free to weld in the heat conduction welding and deep melting welding Multi-mode can be freely switched between heat conduction welding and deep fusion welding, suitable for splicing, strong compatibility with gap fluctuations;

Application differences: single mode due to small spot, energy concentration, good penetration, heat input control is more delicate, more suitable for micro-connection processing (3C, medical, etc.) but the power is not high (the current mature commercial up to 3,000W); multi-mode can provide higher power (10,000 watts), suitable for large-area welding, higher compatibility with different material thickness processing, different thicknesses, different gaps, heterogeneous materials can be applied, the cost also has advantages. Multi-mode also has an advantage in cost.

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