The following article reviews the basic technology behind Carbon Dioxide and Fiber laser marking systems – two of the most popular laser engravers used for making pad printing plates in the USA. The primary differences between these laser systems are the maintenance required, the spot size, and ability to ablate (etch) various materials.
Maintenance. The diode-pumped YAG laser has a diode bar that heats up and can warp after about 10,000 hours of use, thereby requiring costly replacement. The CO2 laser also does not require a significant amount of maintenance. A type of YAG – Ytterbium fiber laser has no diode bar and therefore, has no maintenance for up to 70,000 hours of operation.
Spot Size. The wavelength of a YAG laser (1.064 microns) is exactly ten times smaller than the CO2 wavelength (10.64 microns) and therefore, has a resulting spot size that is 10 times smaller than a CO2 (in the same set-up). YAG lasers are able to provide more detailed graphics than CO2 when both laser types are put into the same machine set-up.
Materials. YAG laser engravers are ideally suited for metals, but their wavelengths are not easily absorbed by many other materials (wood, acrylic, plastics, fabrics, etc.) A CO2 laser beam has much more latitude and can be absorbed easily by many organic materials such as wood, paper, plastics, glass, textiles, and rubber, but is not easily absorbed by metal.
The article below further discusses such specifications of Carbone Dioxide and Fiber laser marking system as applications, optical characteristics, wavelengths of both lasers; also components, Q-switching, power, marking field of YAG laser system.
Fiber and Carbon Dioxide (CO2) lasers emit radiation at different wavelengths. Fundamentally, the 1.064-micron wavelength of the Fiber is an order of magnitude shorter than the 10.6-micron radiation from a Co2 laser. In materials processing, the shorter wavelength of the Fiber couples better to metal while the longer CO2 wavelength is more suitable for cutting plastics, ceramics, and other organic materials. The electromagnetic spectrum below shows where the Fiber and CO2 laser emissions are located relative to other lasers and radiation sources.
The fundamental wavelength of the Fiber laser is 1.064 microns while the CO2 laser emits at 10.6 microns. Most industrial laser marking systems use either the fundamental output of these lasers. The Fiber laser wavelength can also be shifted into the green at 0.355 microns or the UV at 0.266 microns using non-linear optics. These shorter wavelengths are now being adopted for very small (micro-marking) applications in a broad range of materials. The shorter wavelength of the Fiber laser couples to metal better than the CO2 laser. However, for many marking applications on plastics or painted objects, the CO2 laser is equally as practical as the Fiber laser and may be somewhat less costly. Fiber laser beams also have the advantage of focusing to a smaller spot diameter, with higher power density. As a result, they produce a smaller heat-affected zone and less thermal distortion. Fiber laser energy may also be delivered via a fiber optic cable which allows for greater flexibility in the factory environment.
In general, Fiber lasers at the fundamental 1.064 wavelengths are best suited for marking metals while the CO2 laser is more suited for plastics, painted or organic marking. This is the Global Standard.
Laser markers are used to produce alpha-numeric characters, barcodes, serial numbers, logo’s, artwork and other graphic images using a non-contact thermal process. Beam characteristics of Fiber laser markers are classified according to their mode structure. For applications requiring higher average power, the multi-mode output is most desirable. Multimode output spot diameters are typically 30 to 60 microns. TEMoo or End Pumped by Diode Array (DPSS) Systems uses much substantially lower powers to get comparative Outputs as compared to Lamp Pumped Systems due to superior Beam Characteristics. Fixed or fiber optic beam delivery may be employed from the resonator to the scanning assembly.
Note: This is the system followed by all topmost global companies. The flash lamp used is a Krypton Lamp.
When an RF signal is applied to the transducer an acoustic wave is projected through the quartz which momentarily compresses the material and changes its index of refraction. Some of the light passing through the Q-switch is diffracted to a small angle and misses the rear mirror momentarily, causing the lasing action to cease. During this phase, the atoms in the laser rod gain energy which is stored until the RF signal is removed. The resulting burst of laser energy may be several kilowatts of peak power, significantly more peak power than the laser would emit without the Q-switch. Q-switching is the most common means of producing very short pulse widths and very high peak power from a relatively low power Fiber laser.
Q-switches globally typically operate at frequencies from 1 to 50 kHz.
Average Power and Power Density: A continuous wave (CW) Fiber laser marker rated at 50 watts produces the same average power as a 50 watt light bulb. The difference is the light bulb projects white light of all wavelengths in all directions. The laser emission is coherent (traveling in one direction), monochromatic (of one wavelength) and in phase. The power density, expressed in watts/cm2 tells us how concentrated the laser energy is at the point of focus. A laser producing 50 watts of power in a 0.005″ diameter spot will produce considerably higher power density than the same laser with a 0.500″ spot diameter. The effective marking power of the laser is the net power density produced at the point of focus or where the best laser mark is achieved
Galvanometer or XY Scanner: The output of the laser marker is typically directed by moving mirrors operating in both the X and Y planes. The XY mirror sets are each independently connected to highly accurate, repeatable and extremely fast galvanometer drivers (motors). Precision beam steering, and thus the laser mark, in both the X and Y axis, is controlled by the software and control electronics.
Flat Field Lens: The most common method to focus the laser beam in a laser marker is after it is positioned by the XY galvanometer mirrors by using a flat field lens. Typical flat field lens sets provide marking areas from 60 mm to over 300 mm in diameter. These flat field lenses provide a relatively large marking area and permit fast, high-quality laser marks and distortion free pictures to be made on a broad range of metals and other materials.
**Excerpts taken from “Rita Pad Inc.” http://www.ritapad.com/lasertechnologies.htm June 20, 2005