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Types of Lasers


The TherMark process utilizes long wavelength (IR and near-IR) lasers to permanently bond the laser marking material to the surface. There are a number of laser systems designed for laser marking technology, classified according to their emission source, mode of operation, beam steering method etc. which will work with TherMark laser marking materials. The following paragraphs briefly explain the main characteristics of laser marking systems and explain their differences.


Laser source: There are two major classes of lasers which are being used in laser marking and cutting technology:
gas phase CO2 and solid state lasers. The major difference between these two types of lasers is the wavelength of emitted radiation. While most solid state lasers operate in near IR region (~1060 nm) the CO2 lasers operate at IR (~10 µm). This difference in wavelengths renders them suitable for a specific application. The absorbers in laser marking materials have characteristic absorption spectrum and absorb light more efficiently at specific wavelengths. This peak in absorption spectrum makes different lasers particularly suitable for certain applications. Due to differences in wavelengths (e.g. CO2 vs. solid state lasers) required intensity of marking on the same sample can be quite different for different lasers. Moreover, intensity of the laser beam at the focal plane depends on the focal length of the focusing lens and is inversely proportional to the focal length and hence, the focal spot size.

 

Laser beam steering: There are two main methods for beam delivery to the substrate. In so-called XY-flatbed systems the beam is manipulated in the horizontal plane through mirrors mounted on linear stages. As the stages move in x- and y-directions the beam is steered according to the programmable moving pattern of these mirrors. In galvo-based laser scanning systems, the laser beam is bounced off of two mirrors which are mounted on rotating motors. The beam steering occurs through angular motion of mirrors which deflect the beam through a focusing lens to the substrate. The focusing lens in these systems serves a dual purpose: focusing and steering. The idea behind it is that small angular movements of the deflecting motors in galvo systems translate to large distances in the projected focal plane.

 

Speed setting: In both cases the velocity of beam steering movement on the substrate can be controlled externally, buy adjusting the speed setting. This setting is measured either in in/sec (IPS: inches per second) or cm/sec (for some European models). Most laser control software allows the user to choose between metric or English settings. The speed settings are crucially important for the successful marking and should not be overlooked.

 

Vectoring vs. rastering: In computer graphics, a raster graphic image, digital image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a computer monitor, paper, or other display medium. In the laser marking industry rastering refers to a pixilated method of image processing and marking as opposed to vectoring, in which the graphic represents a structure of lines and the laser follows these lines to reproduce the desired mark. In raster images each pixel has an individually defined color, which corresponds to particular laser settings and can be programmed. Some laser engraving systems allow the user to choose between these two modes of operation. When rastering is chosen any imported graphic is converted to a bitmap (pixilated) image, and appropriate resolution should be chosen during importing process. This setting is characterized by DPI (dots per inch) settings and usually ranges between 100-1000 DPI. Generally for lower melting point materials lower values are chosen.

 

Modes of operation: There are two different modes of operation: CW (continuous wave), and QS (Q-switched, or pulsed). Some lasers may operate in one or the other mode; others may be able to operate in both modes. These two modes of operation are significantly different and may require different settings for power and speed. In CW mode the intensity of laser marking stays constant during the marking process unless programmed by the user to gradually increase or decrease. In QS or pulsed mode, as the name suggests, the laser supplies a train of pulses: the intensity is on and off repeatedly with regular intervals. The time delay between the pulses is characterized by its repetition rate measured in Hz (1Hz, means 1 pulse per second, 1000Hz, means 1000 pulses per second, etc.).

 

Due to this intermittent mode of operation the energy of the laser beam is concentrated in relatively short time intervals (duration of the pulse). Each laser pulse may deliver very high power and will have the capability to cause more heating or damage than the same average power in a CW operation. Consequently, the settings in QS mode differ from CW settings significantly.

 

In several laser engraving systems, instead of controlling the repetition rate (measured Hz’s), it is possible to introduce a slightly different parameter: PPI (pulses per inch). In this case, instead of controlling the timing between the pulses, the physical distance in the plane between each consecutive laser firing can be controlled such that as the laser moves along a line there is a finite number of laser firings occurring as the laser moves 1 inch distance. This option can be particularly useful for marking various materials with low melting points.