Marking with near-infrared laser sources

Improvements in laser sources speed marking applications

Industry requirements for marking systems are known and self-evident. Economically speaking, it must be possible to amortize the system over a short time, typically a period of up to two years. Therefore the system must have high processing speed (or short cycle time), maximum possible machine availability with short setup time, and easy handling. Also of concern are ease of integration into existing production lines and low operating costs for raw materials, wear parts, power consumption, and environmental impact. These properties guarantee low marking costs per piece with high throughput.

In general, when an integrated galvanometer system is used, marking costs are estimated at between 0.5% and 3% of the total cost of production. That is, fully automated galvanometer systems in production lines deliver low marking unit costs at high throughput. Flatbed systems produce significantly higher quality (in comparison to galvos) over working areas that are several times larger but at lower throughput.

Furthermore, laser plotters, as well as systems with fixed beams, offer the fundamental advantage of producing a laser beam that is usually better focused with an incident angle that is always perpendicular to the workpiece. A laser-based marking system, especially in a production line, must have the following qualities:

  • Robustness and high MTBF, to guarantee availability, even in harsh industrial environments
  • Low maintenance, to reduce maintenance costs and downtimes
  • High efficiency, especially of the laser source, to minimize energy costs
  • Compactness, to integrate simply. This is especially crucial if the laser system must be integrated into existing lines that do not provide any dedicated space for them
  • Long life, to allow profitable operation for as long as possible after amortization
  • High beam quality, to achieve the same results, at lower laser power, as more powerful lasers do with lower beam quality, and to attain high resolution.

On the application side, the system must be able to ensure applied markings are high quality, durable, and tamper-proof. Quality means good legibility of text, easy-to-decode barcode and Data Matrix code, high resolution of submillimeter markings (for example, for hidden markings), and thermal, mechanical, and chemical resistance of markings for workpieces subjected to high temperatures, mechanical stress, mechanical wear, or aggressive substances in later use.

Commercially available technology

Before a laser source is integrated in devices, it must pass extensive tests. Close cooperation between the machine builder and laser source producer further improves the quality and economy of the laser sources.

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Diode-pumped Nd:YAG lasers and Nd:YVO lasers (vanadate lasers), developed in the 1990s, are near-infrared lasers used to mark metals, plastics, and sometimes ceramic materials. Both lasers have a crystal as their laser-active medium, and the rest of their layout is also largely identical. In comparison to lamp-pumped Nd:YAG lasers used previously, diode-pumped lasers represented a leap forward, offering better beam quality, significantly improved optic-optic efficiency (just a few percent for lamp-pumped systems versus 30%-50% for diode-pumped systems), reduced need for water cooling, more compact construction, and substantially increased service intervals.

Recently available fiber lasers are well-suited to marking tasks. CW fiber lasers can be modulated up to about 25 kHz via pump diodes. This operating mode is also referred to as free-running mode. Pulsed fiber lasers are usually designed according to master- oscillator-power-amplifier (MOPA) systems and have proven themselves in industrial applications with an average power of up to 20 W. Newer pulsed fiber lasers allow modulation frequencies up to 80 kHz and are suitable for medium-speed (2 m/s) flatbed systems. Previous MOPA systems were limited to not even 0.3 m/s in flatbed systems (raster mode).

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Pulsed fiber lasers emit pulses with a defined pulse form. In the case of “laser-on-a-chip,” a laser is integrated on a single chip with the laser-active medium, mirrors, and other optical components integrated. The amplifier consists of an ytterbium-doped glass fiber that is supplied with energy via fiber-coupled pump diodes. The amplification occurs in a single pass (single-pass amplifier).

Technology for today

Near-infrared lasers are an economical laser source for color-change markings on plastics and for marking and engraving on metal. Far-infrared lasers (for example, CO2 lasers) with low power are generally not suitable for metal processing, because the wavelength of 10.6 μm is normally absorbed insufficiently by metals. Lasers in the visible light spectrum or in the ultraviolet spectrum are usually too expensive compared to near-infrared lasers. Therefore, in selecting a suitable laser source it is necessary to consider the application and machine type.

Nd:YAG, Nd:YVO, and fiber lasers are available in large production volumes and from many suppliers. It should be mentioned that the vanadium laser is the better Nd:YAG laser for marking applications at power levels up to 20 W. However VYO compared to Nd:YAG has several advantages: higher repetition rates are possible, up to 200 kHz for YVO versus 100 kHz for YAG, and lower thermal drift in output power, because the absorption bands for the pump light is broader for Vanadium than it is with the YAG.

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Some advantages of lamp-pumped versus diode-pumped methods also apply to a fiber laser versus an Nd:YAG/YVO, although the improvements are, of course, not so severely pronounced:

  • Higher wall-plug efficiency: up to 3% for pulsed Nd:YVO versus 6%-10% for pulsed fiber lasers
  • More compact construction
  • No replacement of pump diodes necessary for fiber (expected life >50,000 hours), life of the pump module is maximum 10,000 hours for YVO lasers
  • All-in-fiber design of the fiber laser means that laser and pump light within the laser is always routed in glass fibers. This makes the system insensitive to dirt and vibration
  • No first pulse problems.

On the other hand, an YVO laser can typically be operated between 10 and 200 kHz, high repetition rates that are indispensable on flatbed systems with high resolution. And an YVO laser has better pulsability or shorter rise and decay times. That is why a fast raster engraving tool with a flatbed laser at high resolution cannot use a fiber laser. Moreover, the pulse peak power of YVO lasers is substantially greater than that of fiber lasers. This can be advantageous in color-change markings on polymers. Experiments show that this advantage is decisive in 10-15% of plastic applications. Finally, when using fiber lasers, back reflections must be avoided.

Typical for fiber lasers-in contrast to the YAG/YVO lasers-is that the average output power is not a function of the repetition rate. In YAG/YVO lasers, at a repetition rate of 10 kHz only about 50% of the maximum average power is output. With increasing repetition rate, the output power rises: starting at about 40 kHz greater than 90% of the maximum power is output.

This means that the fiber laser supplies significantly higher pulse energies between 20 and 40 kHz. Despite the longer pulse duration of the fiber laser and its significantly lower pulse peak power, it is not possible to produce a metal engraving in the same amount of time and at the same quality using a YAG/YVO laser at 20 kHz. Fiber lasers are usually the first choice for galvanometer-based laser systems, because pulse-repetition rates greater than 100 kHz are not necessary, and it is easy to compensate for the slower power rise- and falltimes at beginning or end vectors.

On fast flatbed systems with higher resolution, high pulse-repetition rates are absolutely necessary (resolution x travel speed = minimum repetition rate). Moreover, the high dynamics in power rise and fall guarantees that individual pixels are really circular in shape and not elliptically distorted, for example.

Which source and when

Fast flatbed systems (laser plotters) with high resolution usually have a short focal distance lens (81.3 mm) and a focal diameter of approx. 25 μm. This enables a resolution of 1000 dots per inch (dpi): 1000 consecutive foci that contact one another (no overlap, no gap) yield 1 in. (25.4 mm). If a flying lens now travels at 100 in./s (2.54 m/s), a repetition rate of at least 100 kHz is necessary to create a continuous line. This travel speed is the theoretical upper limit for a laser source that is pulsable at max. 100 kHz and has a focal diameter of 25 μm.


FIGURE 1. Schematic plot of power vs. repetition rate for fiber lasers vs. vanadate lasers
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In reality, the laser rise- and falltimes are exhausted at speeds up to 1 m/s. These types of systems must therefore be combined with an YVO laser. High feed rates (rates up to 3.5 m/s have already been achieved) can be used primarily on plastics due to the high intensity of focus (small focal spot due to short focal distance and high pulse peak power).

Current development has led to a pulsed fiber laser for which rise- and falltimes are significantly optimized. Furthermore, the optical response, that is, the time between rising slope of the gate signal and optical emission, is a constant value with many fewer fluctuations compared to older systems. This laser can be modulated up to 80 kHz.

In addition, a small flatbed system that allows marking speeds of max. 2 m/s with 1000 dpi is now commercially available. This system has about 50% slower speed compared to very fast YVOs but is four to six times faster and has much higher quality compared to “pure” CW systems. Furthermore, the pulsed system has a peak power around 5 kW where the CW system has up to 20W power.

Slower flatbed systems (or flatbed systems with lower resolution), which should result in low investment costs, are inevitable for CW fiber lasers. These are relatively cost-effective, can be modulated up to about 25 kHz, and are well-suited to vector marking. This assumes that the laser plotter also permits this operating mode (and not just raster markings). This method enables anneal markings on metals and-despite the lack of pulse overshoot-markings on some plastics.

Galvanometer systems with standard optics (focal length of 160 mm) have a laser focal spot of about 40 μm. At a repetition rate of 100 kHz, feed rates of up to 4m/s are possible. At this speed, the averaged laser power is the limiting factor in greater than 90% of applications and not the upper limit of the laser’s repetition rate. Because galvanometer systems are often integrated in production lines, the attributes of the fiber laser are decisive arguments here: maintenance-free, highly efficient (resulting in lower energy costs), intrinsic protection against dirt, and insensitivity to vibration.

Prospects

Nd:YAG lasers integrated on a silicon chip are pushing into power ranges that are well-suited for marking applications. The compactness and robustness of these systems signify a distinct improvement compared to existing Nd:YAG laser sources. In the area of pulsed fiber lasers, systems have recently become available that can be pulsed up to 500 kHz. Pulse peak powers and pulse durations of these systems are higher or shorter than those described. But, as already mentioned, these new systems must be able to deliver this performance over their entire life, before they will be integrated into machines. Moreover, total life cycle costs must be evaluated compared to existing systems.

To continuously succeed in offering economic solutions, each technological innovation must be examined for its suitability in specific machines. In its fiber laser systems, Trotec, for example, is utilizing a new, alternative laser source that has been available in industrial quality for a relatively short time. This source has its strengths, and it will partially replace “classic” YAG/YVO lasers.

The first evolution of pulsed fiber lasers to better modulation properties allowed the combination with a medium-speed (2 m/s) flatbed system. However, as is often the case with innovations, what is new is generally not a cure-all. Well-known technologies that have proven themselves over time, such as vanadium lasers, will continue to be irreplaceable for fast, high-resolution laser plotters. However, in the case of cost-effective, slower plotters, the CW fiber laser may replace some “classic” YAG/YVO galvo workstations. In galvanometer systems operated on production lines in harsh industrial environments, a fiber laser is significantly more economical than a YAG/YVO laser.


Gernot Schrems is with Trotec (www.trotec.net) in Wels, Austria. In the USA, contact Trotec Laser (sales@troteclaser.com).

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