Speed, simplicity, ruggedness, and cost-effectiveness give fiber lasers an advantage in this "black art"
The laser marking industry has proliferated over the last 10 years, and laser-marking systems are now available from many suppliers worldwide. Almost every industry requires traceability for an increasing number of manufactured products and components, and laser marking has solved many of these requirements due to the inherent flexibility, speed, reliability, and ease-of-use of laser systems when compared to conventional marking techniques. Although many different laser types and several different laser wavelengths have been and are being used, fiber lasers in particular have seen a dramatic increase - almost all marking systems producers have at least one fiber laser-powered model in their range. The benefits of fiber laser technology are well known and well documented, but this article will review some additional less well-known background, and examine the benefits of fiber lasers for the particular case of laser marking.
Low-power continuous wave fiber lasers were in limited use for marking integrated circuits around 1998, and it was sometime after this that the first pulsed nanosecond units, capable of a far wider range of marking applications, were introduced - this was really the start of the fiber laser revolution, which is still gathering pace. Across all market segments global fiber laser revenues rose 48% in 2011, and in the marking and engraving segment, fiber lasers showed a 34% increase in sales as opposed to diode-pumped solid state (DPSS) lasers which grew only 4% last year.1 The rise in the use of fiber lasers for marking has virtually eliminated flash lamp-pumped solid-state lasers at low power levels (<30 W). The last stronghold of other infrared laser types in the marking and engraving field has been at higher power levels (>30 W) for deeper, faster engraving. The development of 50 W pulsed nanosecond fiber lasers, however, means that even this segment is now taking up fiber lasers. Illustrating the growth in use of fiber lasers at all power levels in this sector, more than 10,000 of these units were sold in 2011 by the one dominant supplier.
With the recent 50th anniversary of the laser and the recent loss of the US inventor of the fiber laser, Elias Snitzer, it is perhaps appropriate to discuss why fiber lasers are so radically different from other laser types. Both solid-state lasers and fiber lasers employ one of a number of rare earth elements as the active medium to produce laser beams. The title "rare earths" came about because at the time of their discovery they were indeed thought to be rare. Recent discoveries have shown that the ores of these elements do occur in many places worldwide, although there are some concerns over supply as much of the current reserves occur in Chinese territory in Inner Mongolia. These rare earth elements make up the upper of the two lines at the base of the periodic table of elements. These 15 elements - completely un-pronounceable and probably un-spellable to many outside the laser industry - belong to the group known as the lanthanides because of their chemical similarity to the element lanthanum. The rare earth active element in most widely used fiber lasers is ytterbium, named for the small Swedish village of Ytterby close to where large deposits of this and a number of other rare earth ores were first found.2 The complex electronic structure of ytterbium allows efficient generation of coherent photons when this element is carefully distributed within the core of the laser, the active fiber (FIGURE 1).
|FIGURE 1. Basic construction of the active fiber in a fiber laser.|
The difference between a fiber laser and other free-space solid-state laser technologies are widely misunderstood and sometimes misrepresented. In a fiber laser the beam is actually generated within the fiber. In other technologies, the beam is generated in free space and is then squeezed into a fiber-optic cable to be delivered to the workpiece.
Wavelengths for laser marking
It has been well known for many years that at near-infrared wavelengths, metal reflectivity is significantly lower than at the longer emission wavelengths of carbon dioxide gas lasers at 10.6 μm. A second benefit of using shorter wavelengths is that the divergence of a laser beam is proportional to its wavelength and inversely proportional to the diameter of the beam, summarized in the equation below:
Θ = λ/(πω)
where λ = beam divergence, π = laser wavelength, and ω = beam waist.
So, shorter wavelengths allow smaller focused spots and hence smaller surface features. Despite these focusability limitations, longer-wavelength far-infrared gas lasers still retain a strong position within the marking industry, because many widely marked materials such as paper and thin-film optically transparent polymers simply do not absorb enough of the laser beam. This absorption is required to generate localized features on the surface that are visible to the unaided human eye.
Lasers producing near-infrared wavelengths, such as fiber lasers, are used for marking a very wide range of materials, both metals and non-metals. In these cases, marks that must be visible by the un-aided eye are created either by ablating material or by creating oxide layers on the surface, or by a combination of both of these. An ablative mark might appear very precise to the un-aided eye, but examined under higher magnification one can usually see evidence of the small-scale but very dynamic and energetic heating and vaporization processes that are occurring. Although most of these features cannot be resolved by the un-aided eye and are so shallow as to not affect the functionality of the component in most circumstances, the slightly roughened edges are probably responsible for light scattering and hence making the mark visible (FIGURE 2).
|FIGURE 2. Laser-marked 304 stainless steel, 20 kHz, 0.5 mJ, 2 m/s. Melt spots are 70 μm dia.|
Many polymers can also be laser-marked by inducing a range of surface effects such as foaming, carbonization, and ablation. For marking lighter-colored polymers, thin polymer films, or semiconductor materials such as silicon when small features are required, even better absorption may be necessary - although the reasons for this are beyond the scope of this article, in some cases shorter wavelength lasers in the visible spectrum are used.
What can laser marking systems do?
Laser marking system manufacturers all use very sophisticated commercially available laser marking software to control the galvanometer scanners that produce relative motion between the laser spot and the workpiece. It is the laser and the optics at the heart of the machine, however, that control the marking mechanisms the system can produce. These marks can be almost infinite combinations of characters and graphics, logos, unique serialized alphanumerics, or one of a number of different barcode designs. There are many justifications for this: traceability, anti-counterfeiting, material, batch or manufacturer identification. The primary function of all marks is they must be readable, either by machine or by the unaided eye. Other secondary requirements may be:
• The functionality of the part must not be compromised throughout its lifetime in any way - for example, it should not mechanically weaken or cause corrosion of the part;
• The mark must endure for the lifetime of the part; and
• The mark must be aesthetically pleasing.
The sophistication of laser marking systems make the marking process look very simple, but laser marking software allows many different approaches to producing an optimized mark on a particular surface. A range of scan speeds, scan line overlaps, scan patterns, and laser delays are available, and different operators may use very different approaches to achieve a similar mark. This tells us that laser marking is still something of a "black art" - although some general rules can be applied, a great deal of laser marking is still experientially based.
Technical benefits of fiber lasers
One major benefit of ytterbium-doped fiber lasers is that the near-infrared 1070 nm wavelength emitted is close enough to the 1064 nm wavelength of neodymium-doped Yttrium aluminum garnet (Nd:YAG) lasers as to make no difference during the actual process of laser marking. This made for a relatively easy replacement of continuous wave Nd:YAG lasers by fiber lasers for most marking applications. This early success exposed the marking industry to fiber lasers and their many additional benefits became better understood. This led, in turn, to more advanced applications where fiber lasers were able to challenge the still relatively new diode-pumped solid-state laser technology.
Another often unappreciated aspect of fiber lasers is that the whole optical path of the laser is fully maintained and hermetically sealed within zero-loss fully coated optical fibers - it must be made this way when the optical fibers are produced. The continuous optical path is achieved by combining all of the fiber-based optical components using advanced optical fiber splicing techniques. This approach has enormous benefits and is unlike any other laser technology, in that no optical misalignment is possible until the laser beam exits into the focusing optics. Another related aspect is that in principle it is very simple to generate higher average power; one simply uses longer active fibers or additional fiber amplifier stages with more pump diodes. Of course, this scaling simply cannot be achieved without an in-depth understanding of the science and technology of fiber lasers, which in turn leads to an understanding of precisely where damaging optical effects such as Raman scattering will occur and can be avoided.
|Ablation rates using 50 W nanosecond fiber laser for percussion drilling 0.6 mm thick 304 stainless steel.|
Fixed or variable pulse length nanosecond fiber lasers
Both fixed and variable pulse length nanosecond lasers have been used extensively for laser marking, and the simplicity, ruggedness, and cost-effectiveness of fixed pulse length fiber lasers has, as we have seen, allowed significant market penetration. There are, however, a limited number of circumstances where the added flexibility of a shorter laser pulse can provide benefits. One good example of this in the field of laser marking is marking clear polycarbonate components. The mechanism is rather different from most other materials in that small micron-sized bubbles are generated beneath the surface of the material and these bubbles appear black to the unaided eye. Reducing the pulse length to 30 ns along with careful control of other marking parameters such as speed, pulse energy, and the distance between fill lines allows these bubbles to be generated beneath the surface without agglomerating into features that disrupt the surface of the component (FIGURE 3).
|FIGURE 3. Subsurface mark in polycarbonate. Letters are 2 mm tall.|
This approach is highly desirable for marking medical devices as unwanted debris entrapment can be eliminated. There are benefits for some highly specialized marking processes in using even shorter pulses, as low as 1.5 ns. Once again, fiber lasers have a significant advantage because these short pulses and high pulse repetition rates can be achieved without compromising average power to any great extent. For example, one particular model available from the leading supplier delivers 18 W average power at 300 kHz with 1.5 ns pulses (60 μJ), an M2 of 1.3, and a peak power >40 kW. Although pulse duration is an important laser processing variable, it is only one of a number of factors that contribute to producing a particular feature size. This parameter combination allows off-the-shelf infrared fiber lasers to produce feature sizes with conventional optics that have only previously been obtainable using more complex and costly shorter wavelength diode-pumped solid-state lasers.
What does higher average power do for laser marking?
Because of the complex nature of the phenomena involved in laser marking, it is difficult to predict whether marking speed or marking depth will double by doubling the power of the laser. In most cases, however, a higher average power laser will allow users to mark either faster or deeper, or a combination of both. For applications where a significant depth to the mark is required, 30 W or even 50 W lasers have been developed without any increase in footprint and without any compromise in the focusability (or brightness) of the laser. The results shown in the table above were gathered during percussion drilling experiments with a 50 W fiber laser. Although this is the best possible case for material removal (the mechanism that is observed when deep engraving metals), removal rates as high as 5 mm3/s have been measured. It should be noted that using a higher average power laser translates directly into a higher heat input to the part, and distortion on thinner components may well limit the average power that can be used. When mark depth is >100 μm, these marks retain their readability even after serious abuse of metal surfaces; this may be deemed "tamper evident" in that a significant amount of adjacent material needs to be removed to render the mark illegible.
Mid infrared-wavelength fiber lasers
There are a number of other rare earth elements from the same Lanthanides group in which ytterbium resides that have been used as active media in solid-state lasers to generate alternative wavelengths. Holmium (Ho,) erbium (Er,) and thulium (Tm) are all adjacent to each other in the periodic table, and all of these have been used for some years in fiber lasers for various non-industrial laser applications such as laser surgery, largely due to the high absorption of this wavelength by H2O. Thulium fiber lasers emitting longer wavelength beams (in the spectral region of 1900-2010 nm) have now been developed to very high power levels (>100 W) for melting processes such as polymer welding due to their higher volumetric absorption in unfilled polymers. These lasers are not yet available at power levels in the pulsed nanosecond regime to be of interest to the laser marking industry, but they will be available in the not-too-distant future.
It is quite a technological leap from using fiber optic cables to simply guide near-infrared laser beams, to both generating and guiding the beam using all fiber optics - but the benefits of combining these functions are now obvious for all to see. As one well-known manufacturer of laser marking systems told me recently: "The reason we love these lasers is simple; we take them out of the box, we plug them in, we test the system, we ship it straight out of the door, and we never see them again." What more can be said!
1. David A Belforte, "2011 Annual Economic Review and Forecast", Industrial Laser Solutions, January 2012.
2. Sam Kean, " The Disappearing Spoon", Back Bay Books, ISBN 978-0-316-05164-7.
Dr. Tony Hoult (email@example.com) is the general manager of West Coast operations for IPG Photonics, Santa Clara, CA. He is also an Editorial Advisor to Industrial Laser Solutions and a frequent and valued contributor to this publication.