An infrared beam you cannot see, a focal spot around 50 µm wide, and a beam waist that refuses to become a point. Here is why locating the true focal position is difficult, and what accuracy different welding processes actually require.
Of all the parameters that govern a laser welding process, the position of the beam focal point is the hardest to pin down. The task sounds simple — find the point along the beam axis where the diameter is smallest — but several physical constraints work against you at once.
The first difficulty is fundamental: a laser beam is not a physical object, which rules out the simplest measuring tools — rulers, callipers, micrometers. There is nothing to place them against. Compounding this, most industrial lasers are infrared, at roughly 1,060 nm — outside the visible spectrum. The thing you are trying to measure cannot be seen.
For the optical setups used in battery manufacturing — including welding of copper and aluminium — the focal spot diameter falls in the range of 45–60 µm.
Because of wave optics and the beam's Gaussian profile, it cannot be focused to an infinitely small point — as it approaches focus it forms a beam waist. The extent of this region is described by the Rayleigh length: the distance over which the beam's cross-sectional area doubles. Within it, changes in diameter are gradual rather than abrupt, which is precisely what makes the minimum so hard to identify.
An infrared wavelength invisible to the naked eye, a focal spot of roughly 50 µm, and the beam waist phenomenon — together they make locating the point of minimum beam diameter extremely difficult.
Required accuracy should match the specific process — determined by material thickness and the optical system's beam parameters.
Easily weldable materials. For steel at millimetre thicknesses, lower beam-quality lasers with focal spots up to 500–700 µm are common; determining focus to within 0.5–1 mm may be entirely sufficient.
Difficult or impossible-to-weld materials. These require advanced lasers with focal spots as small as 50 µm, on materials rarely exceeding 300 µm thick. Expected focal-point accuracy here falls in the range of 50–100 µm — an order of magnitude difference from the steel case, and the reason a method that works on millimetre-thick steel can be useless on a battery module.
For laser welding of electric battery modules — particularly joining bus bars to cells, where thin, often difficult-to-weld materials such as copper are involved — a method with a precision of at least 100–150 µm should be selected. Correct focal-point positioning is what guarantees repeatable conditions and a stable process. Methods above 500 µm generally prove insufficiently accurate for materials in the 250–300 µm thickness range.
The next article compares the measurement methods available, from a few euros to tens of thousands, and where each one lands on this accuracy scale.
Based on the technical article "RMA FocusFinder — A Reliable Method for Determining the Focal Point of a Single-Mode Laser" by Rajmund Pobereżny, Project Manager, RMA.
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