It is the capability of lasers to vary the rate of energy flux (J/m2s) i.e. intensity, which produces the wide range of material interactions seen during processing. Table 1 compares the wavelengths, frequencies and photon energies available with four types of laser spanning the UV (ultra violet), visible and IR (infra red) parts of the electromagnetic spectrum.
Table 1 Wavelength, frequency and photon energy available from different laser types
|Laser type||Wavelength, λ|
|CO 2 (IR)
||2.8 x 10 13
||2.8 x 10 14
|Cu Vapour (Visible)
||5.2 x 10 14
|XeC1 excimer (UV)
||9.7 x 10 14
Molecular and electronic vibrations generally occur at frequencies of 1012 - 1016 Hz and therefore generate light. Correspondingly, light can interact with electrons and molecules when it is in resonance with their vibrations.
The beam from a material processing laser can carry considerable energy and the low divergence of that beam allows the energy to be concentrated in a small area. If such a beam strikes a surface, some light will be reflected, some will be absorbed and some may be transmitted. In general, in order to provide an efficient process, it is necessary to obtain an adequate intensity on the surface and to couple as high a fraction of incident power into the material as possible. The important beam properties in this respect are:-
- The wavelength, λ, governing in principle the focusability and absorptivity of the beam.
- The power, which together with the achievable spot diameter, determines the intensity in the interaction zone.
- The beam quality, a measure of the available focused spot diameter and beam divergence angle product.
- The polarisation of the beam, which has a considerable influence on the absorptivity for large angles of incidence.
Laser beams are used to process metals, dielectric materials and semiconductors. Figure 1 indicates how the absorptivity of materials at ambient temperature varies as a function of laser wavelength. The wavelength of the UV krypton fluoride excimer laser and the IR Nd:YAG solid-state and CO 2 gas laser, are also shown in this figure.
Due to the properties shown in Table 1, materials processing lasers are capable of both pyrolytic (processes which involve direct heating of the material) and photolytic processes (processes which involve the breaking of chemical bonds). High energy and ultra short pulses of laser light can also produce interactions which come somewhere between the two.
Fig.1 Absorptivity as a function of wavelength for normal (perpendicular) laser beam incidence, smooth surfaces and room temperature. 'Metals 1' are those with full inner electron shells e.g. Au, Ag, Cu and 'metals 2' are transition metals e.g. Fe, Ni, Cr
The pyrolytic processes can be split conveniently into three groups that include heating (e.g. transformation hardening, forming), melting (e.g. cladding, welding, cutting) and vaporisation (welding, cutting and drilling). The photolytic processes include material ablation, cutting by chemical degradation of materials which do not melt, e.g. wood, rubber and thermosetting polymers, and the local setting of resins (e.g. as in rapid prototyping).