What is single mode? What is multimode?
The essential difference between a single-mode laser and a multimode laser is that a single-mode laser has only one mode in the output beam pattern, while a multimode laser has multiple modes in the output beam pattern;
That is, single-mode refers to a single distribution mode of laser energy in a two-dimensional plane, while multimode refers to a spatial energy distribution mode formed by superimposing multiple distribution modes together. For example, your laser is 1064nm, suppose you hit out all 1064, but to a target, if there are several points at the same time, such as 10 rings 9 rings 7 rings 2 rings, anything, even a big hole, that is a multi-transverse mode. But if you shoot out All 10 rings one point, that's a single horizontal die.
In terms of energy distribution:
Industry often speaks of single mode, referring to the transverse mode of the laser, i.e., there is only one mode within the cross-section, which is Gaussian distributed, with the focal point being the center to the outer edge, and the laser energy density decreasing in order. Multi-mode, on the other hand, presents many energy points in the cross-section, and the more modes there are, the more the energy is distributed in a flat-top manner, figuratively compared to a red tassel and a wolf's tooth stick.
The difference between single-mode and multi-mode in welding applications is that: if you want to go deep fusion welding, it is suitable for single mode or less mode, single mode has advantages in splicing deep fusion welding, stack welding, fillet welding, etc. High energy density is easier to hit the fusion depth. Multi-mode is suitable for shallow welding, good flatness, uniform weld energy, but also to avoid quality losses such as ablation and perforation in the center of the weld brought about by the low melting point of the base material.
Assuming that the vertical coordinate of the curve represents the energy density, the green class Gaussian energy distribution, blue is the multimode energy distribution, and red is the flat top beam, it can be seen that the single mode is more concentrated in energy density, and the unit energy density is greater.
In general, single-mode multimode can be distinguished from laser beam quality M²:
The M² factor is calculated by dividing the product of the actual beam width and the divergence angle by the product of the ideal beam width and the divergence angle, where the ideal beam is defined by the fundamental mode Gaussian beam and the beam width is defined by the second order moment. When the laser beam passes through the aberration-free optical system, its M² factor is the transmission invariant and M² ≥ 1; the farther M² deviates from 1, the worse the laser beam quality is.
Depending on the M², lasers can be classified 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.
Single-mode laser versus multimode:
Single-mode laser fiber core diameter is small (14um), the energy is Gaussian distribution, the focal spot is small, high energy density (the same power, the energy density is 4-10 times more than multimode), and the heat-affected zone is small, especially for high anti-alloy (aluminum, copper) can instantly form a molten pool keyhole (energy density is much greater than the high anti-alloy melting threshold), no high reversal, not easy to damage the fiber, and can achieve high anti-alloy 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, large melting depth, single-mode beam like a sharp knife, multi-mode like a bullet tip.
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 not stable, easy to have spatter and porosity, need to match the oscillating head, vibration mirror, or high-speed welding, low-speed welding spatter is larger, thin plate stacking, sputtering welding; reflected in the metallographic, single-mode has a greater depth to width ratio (the ratio of metallographic depth to width); multi-mode can be freely in the thermal conduction welding and deep fusion welding switching, suitable for splicing, and highly compatible with gap fluctuations.
Application differences: single mode due to the small spot, energy concentration, good penetration, finer control of heat input, more suitable for micro connection processing (3C, medical, etc.) but the power is not high (the current maximum 3000W mature commercial); multi-mode can provide higher power (10,000 watts), suitable for large area welding, higher compatibility with different material thickness processing, for different thicknesses, different gaps, different materials can be applied. The cost of multi-mode also has advantages.
