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Beijing Laserworld International Imp&Exp Co.,Ltd 
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Laser Marking Process Parameters
(1) Effects of average, peak power and pulse energy
The best marking results are obtained only when there is a proper combination of pulse energy, pulse duration, average, and pulse repetition rate. The pulse duration (ms) is defined as the period during which the laser power intensity exceeds 50% of the maximum power intensity. The peak power is determined by the following equation:
For a laser with high pulse repetition rate, the peak power is usually expressed in terms of average power:
where P is the average power, and PRR is the pulse repetition rate.
A high peak power is often preferred in marking process for fast vaporisation. As described in Equation (1), the peak power is determined by the pulse energy and the pulse duration. Shorter pulses have higher peak power. The thermal interaction time is also shorter, which lead to smaller heat-affected-zones, and thus better hole quality. However, it should be noted that the pulse energy is usually high for high order beam modes, which produce large divergence angles. In the case of very fine marking, this situation is undesirable except for mask projection marking such as excimer laser marking.
(2) Effects of beam focal position
A beam of finite diameter is focused by a lens to obtain a smaller beam spot, as shown in Figure 6. If the diameter of the focused spot, d0 , is defined as the diameter which contains 86% of the focused energy, the focus spot size is determined by
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where, f is the focal length of the focus lens, D is the entrance beam diameter, and l is the wavelength.
If the total beam divergence angle q is known, the diameter of the focus spot size is given by
As the Gaussian beam focuses from a lens down to a waist and then expands, there is a need to define a depth of focus. Normally, it is defined as the distance between the d0 spot size points or 2 times Rayleigh range. It can be written as
or
where F is the f-number of a focusing lens, which is defined a
It is concluded from Eqs. (3) to (7) that a lens with a longer focal length gives a greater depth of focus and a larger focus spot size than a lens with a shorter focal length. Thus the focal length of the focus lens should be selected properly according to the marking requirements.
(3) Effects of beam mode and spot size
Because the order of the beam mode has great effect on both the focused spot size and the depth of focus, the beam mode structure plays an important role in laser materials processing. A laser beam with a higher-order mode structure diverges more rapidly, focuses to a larger spot and has a shorter depth of focus than a TEM00 Gaussian beam.
Because in laser marking, it is generally desirable to achieve highest possible speed and therefore the highest possible power density, the lowest order mode is desirable (TEM00 or Gaussian mode for stable resonators). However, a low-order mode structure often means a lower conversion efficiency and thus less laser output power. Therefore the process must be optimised for good processing quality, proper processing speed, and laser output power.
(4) Laser wavelength
Generally, shorter wavelengths are much better absorbed by materials. The wavelength also determines the theoretical minimum focused spot size. For a TEM00 laser with diffraction-limited optics, the focused spot size, s, is given by
where l is the laser wavelength, f is the lens focal length, and d is the diameter of the beam (entering the lens). It is obvious that the focused spot is proportional to the laser wavelength. When the laser wavelength is halved, the spot size is reduced by a factor of two.
The wavelength also determines the interaction mechanism - thermal or photochemical. The reflectivity of a material is a function of the wavelength, as shown in Figure 7.
Figure 7: Absorption vs. wavelength
(5) Material properties
For any material, absorptivity, reflectivity, and transmissivity will satisfy
absorptivity + reflectivity + transmissivity =1
In general, metals absorb the Nd:YAG laser beam energy well, while paper and most transparent materials (e.g. polymers and glass) absorb the CO2 laser energy well. Almost all materials absorb well the short wavelengths of excimer laser beams.
Surface finish or coating affects the absorptivity. Bare metal surface will be difficult to mark by CO2 lasers, but can be easily marked by Nd:YAG or excimer lasers. Glass and transparent plastics are not suitable for Nd:YAG laser marking. Nearly all materials can be marked by excimer lasers with a shallow engraving.
(6) System requirements
In order to obtain the minimum linewidth and highest power density, laser beam shall focus on the work piece surface. Overlap is another important factor affecting mark depth, width, and continuity. The PRR and the marking speed together determine the percentage overlap in the laser spots. A good deal of overlap can ensure that the engraving lines are continuous and that splattering will be kept small. If the percentage overlap (in %) is defined as m = x/s, then
where s is the spot size, x is overlap length, and l is the centre-to-centre spacing between the pulsed spots, which is given by
where PRR is the pulse repetition rate in pps, and v is the engraving speed in m/sec.
Therefore, the required spot size is related to marking speed and pulse repetition rate by
General speaking, the spot overlap is required around 70% to 90% to ensure a good marking.
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