Laser beam mode
A laser beam’s mode refers to the distribution of energy through its cross section (Fig. 4, also see the Laser Basics brochure). The mode affects the cutting process, because:
- It affects the size of the focused spot and the intensity of the focused beam
- It affects intensity distribution in the beam and focus and, as a result, cut quality
A good mode is therefore essential in cutting. The best mode is Gaussian intensity distribution, which is required for optimum focusing properties. The Gaussian mode is often termed TEM00 (Transverse Electromagnetic Mode of the zero order).
Wavelength of the laser beam
The absorption in the material being cut is dependant upon the wavelength of the beam (Fig. 5). The absorption rate of CO2 radiation in cold steel, for example, is approx.11%, to the effect that 89% of the radiation is reflect. The absorption rate of Nd:YAG radiation, on the other hand, is approximately 30%. Nd:YAG and CO2 can overcome the high initial reflectivity of many metals, provided the intensity of the focused beam is sufficiently high. The absorption rate increases remarkably as the temperature of the material rises. To give another example, laser light of visible or near infrared (Nd:YAG) wavelength can not cut glass. Because it is transmitted through the glass without any energy absorption.
While some highly reflective materials (e.g. aluminium or copper) might absorb some wavelengths better than others, a specific type of laser is generally more suitable for a specific application than others. Suitability usually depends on other parameters, such as peak power, pulse length and focus ability rather than wavelength characteristics. Therefore, if for example, Nd:YAG are claimed to cut with better quality or more precision than CO2, this is true only when comparing pulsed Nd:YAG with cw CO2.