No doubt fiber lasers are the most common choice in the industry today due to their monolithic design, close to diffraction limited beam quality, low operational cost and large average power capabilities. In the engraving sector, fiber lasers can achieve depth of the mark with ease while delivering maintenance-free operation for tens of thousands of hours.
However, the seemingly at-a-disadvantage diode-pumped solid-state (DPSS) laser veteran can still teach these new fiber lasers something about performance. Let’s examine how this is even possible.
DPSS vs. fiber
To achieve marking, lasers are often pulsed – short bursts of light are delivered to the target where they are concentrated into a tiny area to achieve a change in the structure of the material. This could be ablation (removal) or some other form, such as annealing (formation of oxide layer on some metals). A common form to achieve pulsing mode is Q-switching. Inside the laser resonator, losses are modulated and the internal energy rapidly rises and falls below the lasing threshold. Each time the laser is above threshold, it emits a pulse.
So how do the two competitors – DPSS and fiber – perform against each other? In Q-switched mode, the round trip time of a fiber laser is on the order of 10 to 20 times longer than its DPSS laser counterpart, which leads to same ratio longer pulses at the output. Longer pulse durations tend to overheat the substrate hit by the laser beam, as atoms and molecules have more time to respond and absorb the energy without being ripped apart by the electromagnetic wave. The shorter the pulse duration, the less likely heating is to occur.
The A-10 laser system is capable of annealing on curved surfaces due to its deep focal depth, which allows it to mark on surfaces that change as much as 10 mm in height. This curved piece of steel shows an example of a dark annealing mark.
Shorter pulse durations are still achievable in fiber laser master oscillator power amplifier (MOPA) configurations, but the price point of these devices can be more than two times higher than DPSS systems, and effects like self-phase modulation still limit the peak power. This is due to the fiber laser’s comparatively much longer gain medium length, which is on the order of meters.
When using the same pulse generating techniques, DPSS lasers are more economical to assemble than fiber lasers as they don’t need special equipment like arc splicers. They can also be serviced rather than replaced.
The A-10 system produces a dark annealing mark without breaking the surface. Surface marking metals like steel, titanium and nickel is useful for corrosive environments and medical applications where there must not be cavities for bacteria to grow in.
To a customer, the real advantage of a DPSS laser comes when application-focused performance is needed. Because of their short pulse duration (about 10 ns vs. 100 ns for fiber), DPSS lasers not only achieve higher peak intensities, but seemingly experience longer focal depth during marking. This means that for a fiber laser, while the beam geometrically is intense over a certain region around the focal plane, the useful region is limited because the peak intensity across the beam waist reaches levels above ablation energy for only ±1 mm around the focus.
For DPSS systems, even a beam that’s not as tightly focused still has enough energy to remove the material from the targeted surface. This enables marking on a curved surface without the need of refocusing, which can be expensive and time consuming.
Refocusing on the fly usually requires complicated electronically actuated devices, which adds to the cost of the laser system as well as introduces larger opportunities for failure and increases downtime of the system. Various companies provide a compact monolithic variable focal length available in the 1,064-nm and 532-nm wavelength spectrum, but it is not available for UV laser systems due to polymer transparency issues.
In many circumstances, such accessories add size and complexity to the marking system and also significantly increase the cost due to hardware and software development. For fiber systems, such an increase might be warranted because it is impossible to mark consistently on variable height surfaces. DPSS systems, on the other hand, are able to produce such marks over a 10-mm range without the need to refocus.
But achieving a high peak power is not the only parameter required for a quality mark. As mentioned, a significant part of the laser’s bill of materials is allocated to the beam delivery system, such as galvoscanners and focusing lenses. In a laser and scanhead duo, the source can cost sometimes even less than delivery optics and shapers. A lot of companies in the market choose to integrate various fiber lasers from outside manufacturers and focus only on software, packaging and integration. The end result is a laser that performs equivalently with every other company’s flavor of integration.
At RMI Laser, a company with deep roots in DPSS laser development as well as fiber laser integration, the focus is on providing the best solution for the application at hand. A recent project within the company that has seen positive responses from customers is RMI’s A-10 laser system. Built on the company’s U-10 series, the laser was originally conceived as a dark annealing laser. However, it has proven to be extremely versatile and has spawned a series of product redesigns.
Chemion plastic is highly sensitive to the 1,064-nm laser energy and foams badly. The A-10 system produces high contrast marks without melting the plastic, leaving the surface perfectly smooth.
Achieving the beam
As discussed, there are several ways to achieve custom beam delivery in a system. A highly Gaussian single mode laser beam is the design goal of every laser engineer. From there, a beam shaper can be incorporated to change the beam profile of the system. However, when designing a laser from scratch, there is a much more cost-efficient, robust approach. This entails achieving the beam shaping directly out of the laser itself instead of having the beam shaper component, meaning it then would be delivered via simple optics, such as mirrors, to the marking plane. When annealing a metal like steel or marking on a thin layer of painted plastic, the flatter the intensity across the spot size, the more uniform the result will look. To elaborate, it’s helpful to look at these marking types separately.
Dark annealing (as well as color): Dark annealing is a process that redistributes the surface atoms of a metal like steel via heating while preserving the surface integrity and forming an oxide layer that can be easily distinguished by the viewer. Such application is highly desired in the medical field because surgical tools and other equipment can remain sterile after the marking process. To achieve uniform color change without breaking the surface integrity, the intensity of the beam needs to be uniform across the spot size while high enough to create the color change. A fiber laser is able to anneal out of focus, but the results are inconsistent and operator dependent. The A-10 laser system is capable of producing dark and color marks on steel with ease while preserving the commonly used techniques of finding the laser focus and having great repeatability.
Uniform beam intensity: This type of marking is useful on plastics because most of the time a fine control over the depth of the mark is required. When a beam profile with a Gaussian shape hits the soft plastics, the peak of the Gaussian beam penetrates much deeper than the sides. This can result in foaming, poor color change and an overall unclear mark. When the flat top beam of an A-10 laser hits the surface, it varies little and thus can be dialed precisely to the right peak intensity and repetition rate that induces the cleanest possible mark while reducing foaming and marking time.
Additionally, the A-10 system can do deep marking comparable in time to a 20-W fiber module, but on a curved surface without the need of a rotary chuck. These characteristics make this seemingly tired technology lucrative for a wide range of applications.
Overall, the resonator beam modification approach has restarted the redesign of RMI Laser’s 20-W DPSS MOPA counterpart (U-20) where it has led to an increased pump efficiency, cleaner spatial beam and better marking performance. No doubt, the DPSS technology can still outperform the fiber laser.
When precision and finely tuned performance is needed, the RMI laser delivers with specifically tuned systems like the A-10. While fiber laser has dominated the engraving market and has spawned a great deal of integrator businesses, the future of niche laser systems that RMI Laser specializes in is still bright.