As the technology of laser marking has advanced, new markets have evolved to take advantage of increasingly faster marking speeds as well as greater marking precision and imaging capabilities. Continuing developments in laser-cavity design, beam-steering and focusing optics, and computer hardware and software are expanding the role of the systems.
Steering the beam
Of the available marking technologies, beam-steered laser marking systems provide users with the greatest amount of image flexibility in a fast, permanent, noncontact marking process. As manufacturing processes become more automated and after-sale tracking more prevalent, laser markers are frequently the only method available to produce individually unique, permanent images at high speed.
Beam-steered laser marking systems usually incorporate either a CO2 or Nd:YAG laser. The CO2 laser emits a continuous-wave output in the far-infrared (10.6-um wavelength) while the Nd:YAG laser emits in the near-infrared (1.06 um) in either a CW or pulsed mode (1 to 50 kHz). The Nd:YAG laser is also unique in its ability to produce very short, high-peak-power pulses when operated in the pulsed mode. For example, a typical 60-W-average-power Nd:YAG laser can produce peak powers on the order of 90 kW at 1-kHz pulse rate.
The delivery optics consist of either a simple focusing lens assembly or a combination fixed upcollimator and flat-field lens assembly. In either instance, the laser beam is directed across the work surface by mirrors mounted on two high-speed, computer-controlled galvanometers.
The simple focusing assembly offers the advantages of low cost and fewer optical components and is routinely used with CO2 lasers. The flat field lens design, though more expensive, maintains the focal point of the marking beam on a flat plane for more consistent image characteristics throughout the marking field. The flat-field lens also produces higher power density on the work surface than the simple focusing assembly due to the shorter effective focal length. The flat-field lens design is always preferred for high-accuracy and high-image-quality applications and is usually incorporated with Nd:YAG lasers.
Both designs provide the user with a selection of lenses that establish both the diameter of the marking field and the marking-line width. Longer-focal-length lenses provide larger working areas, but the line width is also enlarged, thus reducing the power density on the work surface. The user must compensate by either increasing the laser output power and/or decreasing the marking speed which usually consists of two lenses and may be placed anywhere in the beam path before the focusing lens. A beam expander often is used instead of extending the beam path approximately 10 more feet, in which the beam expands through its inherent tendency to diverge as it exits the resonator cavity. A spatial filter inserted within the beam expander produces the best mode quality in close-coupled systems, by passing the beam through a small aperture.
The last optical element that a laser beam encounters is the focusing lens. With CO2 lasers, this lens is usually made from one of several materials: Zinc selenide (ZnSe), gallium arsenide (GaAs) or germanium (Ge). ZnSe, a dense, yellow material that is transparent to visible wavelengths, is by far the most common of these materials, and it allows a low-power, HeNe laser beam through for alignment purposes. This is a great advantage over GaAs or Ge which are opaque to light from the visible portion of the spectrum.
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