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How should I choose the correct focusing optic for CO2 or Nd:YAG laser keyhole welding?


Frequently Asked Questions

Optics for laser welding focus the raw laser beam that is typically tens of millimetres in diameter down to a spot size significantly less than a millimetre in diameter. This dramatically increases the power density (power divided by area) in the laser beam, in the same way as a magnifying glass can focus sunlight to burn paper. Optics are, therefore, a crucial element of any laser system for materials processing, particularly for laser keyhole welding, which requires a very high power density in the laser spot (> 10 6 W/cm 2 for steels) to enable rapid melting and vaporisation of metals. These power densities are usually achieved with focused spot sizes between 0.2 and 0.6mm diameter.

The focusing optics used are one of two types: reflective (mirrors), or transmissive (lenses). CO 2 lasers generally use gold-plated copper or coated silicon mirrors for beam guidance, with a parabolic-form mirror at the end of the beam-path to focus the laser beam to a small spot size. For CO 2 laser powers up to ~3-4kW, much cheaper and easier to align anti-reflection ZnSe or KCl lenses can also be used. However, at higher laser powers those materials tend to suffer from thermal distortion, as they absorb part of the laser light at the CO 2 wavelength, reducing their effectiveness and possibly causing their failure. For Nd:YAG lasers, transmissive focusing optics made of anti-reflection coated special glasses are used almost exclusively.

As the laser beam is focused, it converges to reach its minimum diameter, after which it diverges again. The location where the beam is at its minimum diameter is generally referred to as the focal point, with the beam diameter at this location referred to as the focused spot size. Since the laser beam converges into - and then diverges out of - the focal point, it also has what is known as a depth of focus. A convenient definition of this is the distance over which the beam diameter does not vary by more than 5% from the minimum, i.e. the focused spot size. For a particular optic, the spot size and depth of focus that can be achieved are dependent on a number of factors, including the wavelength of the laser light, the raw beam diameter and the focal length of the optic. The latter is defined as the distance from the focal point to the centre of the optic.

Ideally, as small a spot size as possible would be used, to give the highest power density, and the depth of focus would be as large as possible, to have as large as possible a tolerance to variations in focus position. Unfortunately, these two requirements are in conflict. A very important parameter characterising focusing optics is, therefore, the so-called 'f-number'. The f-number is defined as the quotient of the focal length and the raw beam diameter. The smaller the f-number, the smaller the spot size will be but, consequently, also the smaller the depth of focus will be. A good starting point for choosing optics is an f-number of 7.5 for CO 2 lasers, and 4 for Nd:YAG. If the user then establishes the raw beam diameter of the laser beam, and multiplies this with the suggested values for the f number, this will then give a suggested focal length giving a good compromise between spot size and depth of focus.

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