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Femtosecond Lasers For Unmatched Micromachining

Femtosecond Lasers for Unmatched Micromachining

Klaus Kleine and Michael LaHa of Coherent Inc review a new type of cutting laser, the ultra-short pulse (USP) industrial laser, which can cut feature sizes of tens of microns, with virtually no heat affected zone.

There is an increasing need to produce high precision, miniaturised components across several industries such as medical devices, automotive manufacturing and microelectronics. In many cases, traditional cutting methods cannot deliver the required combination of feature resolution and cut quality for these applications. Even lasers, which have historically offered the highest level of cutting precision, sometimes produce an unacceptably large heat affected zone (HAZ), that is, a region where melting or microcracking in surrounding material degrades part quality and/or performance. This article explores a new type of cutting laser, the ultra-short pulse (USP) industrial laser, which can cut feature sizes of tens of microns, with virtually no HAZ, and presents some specific examples of the use of this technology in medical device manufacturing.

Figure 1: Laser micromachining; in drilling, cutting and texturing applications, the use of lasers with shorter pulse widths avoids some of the limitations associated with longer pulse widths, including thermal effects and recast debris and surface micro-cracking.

USP Laser Advantages

Most precision micromachining lasers have pulse durations of 40 to 60 nanoseconds (10-9 seconds).  But, when cut quality is of primary importance, such as for producing very smooth HAZ-free edges, or when processing thin or delicate substrates, these lasers are not always optimum.

In these cases, even shorter pulse durations, in the picosecond (10-12 seconds) realm, can offer superior results.  This is because with this very short pulse duration, this is an athermal process which doesn’t cause unwanted heat to spread into surrounding material and cause a HAZ. And, the fact that the ejected material consists of very small particles (eg: as atoms) means that picosecond laser pulses do not produce recast, therefore leaving clean, smooth surfaces.

USP lasers are typically characterized by much lower pulse energies than nanosecond lasers, but with very high pulse repetition rates – usually in the 1 to 50 MHz range. So, each pulse removes a minute amount of material with minimal thermal damage, enabling unmatched depth control. Yet, the high pulse repetition rate delivers sufficient overall material removal speeds for many tasks.

Figure 2: A nitnol stent, cut with the Coherent Monaco femtosecond laser, shown at two different magnifications.  Notice the precision, edge quality and clean smooth surfaces in each case.

Femtosecond Lasers

Recently, interest has grown in using lasers with even shorter pulsewidths, specifically in the femtosecond (10-15 seconds) domain. In medical device manufacturing in particular, three factors have driven this trend. The first is the growing need for increased miniaturization, superior edge quality and surface smoothness. The extremely short pulse duration of femtosecond lasers further increases the advantages of athermal processing described previously. This is particularly valuable when processing thin films and delicate materials where no HAZ can be tolerated.

A second reason is the increasing use of mixed and layered materials, eg: bioabsorbable plastics on metal, or polyimide on glass. Femtosecond lasers produce very high peak pulse powers, which, in turn, drive non-linear (multiphoton) absorption in the material. Unlike traditional (linear) absorption, this is less dependent upon wavelength; the femtosecond laser can process virtually any material, even if it is transparent, such as glass. This allows coated and laminated substrates to be processed in a single step, enabling streamlined and lower cost fabrication in many cases.

Finally, femtosecond lasers are becoming increasingly attractive to industrial users because of recent improvements in their performance, lifetime, reliability and cost of ownership characteristics. Originally, femtosecond lasers were used exclusively for scientific applications. But, in the last few years, femtosecond laser manufacturers like Coherent have implemented a new laser material, ytterbium-doped fibre, which is scalable to much higher power. And because the laser material is in the form of a fibre, this new generation of industrial femtosecond lasers has simpler internal design and construction, leading to lower costs and significantly increased reliability.

For example, the Monaco series from Coherent provides up to 60W of processing power in a compact (667 mm x 360 mm x 181 mm) sealed package, which, given its lower capital cost and increased reliability, makes femtosecond laser processing economically competitive for a host of enabling applications in a variety of industries.  Moreover, these capabilities these lasers bring are available at several levels of integration. Options include standalone lasers, laser sub-systems (light “engines”) with scanning/focusing optics, complete machines with integrated part handling, and even complete solutions with custom software pre-optimized for a specified set of results for particular applications.

Figure 3: Three examples of stainless steel surface texturing produced with the Coherent Monaco femtosecond laser.

Cutting Precision Medical Devices

Many medical devices are formed from tubular blanks; common tasks are to make cylindrical cuts as well as produce intricate patterns for cardiovascular and peripheral stents. Testing has shown that femtosecond laser cutting delivers superior feature consistency and residual strength. For this application, the laser is typically integrated in a workstation in which the blank is mounted to a moving stage with 4-axes of motion, (three translation and one rotation). The use of a femtosecond laser allows for kerf cutting of tube stock or flat stock material with micrometre-scale precision and tolerances. Processing is sometimes accompanied by high-pressure co-axial assist gas to help remove vaporised ablation debris when cutting thicker-walled materials.

For surface texturing of contoured materials, such as catheter balloons, or surface ablation of flat stock materials such as stainless steel, another approach is used. Here, a 2D scanner workstation is usually the optimum solution, employing a high-speed 2-axis galvanometer scanner to cover a 20cm radius. The use of a femtosecond laser enables highly precise results with sub-micrometre depth control.

Yet another approach has been optimized to perform tasks such as precision hole drilling in irrigated ablation tip catheters with controlled wall taper, precision placement of slots and grooves, or to create complex shapes in tubes or flat stock material. In this case, the workstation contains a 5-axis trepanning scan head with co-axial assist gas along with a 5-axis motion control system. Again the femtosecond laser provides sub-micrometre dimensional precision and clean surfaces with typically no need for post-processing.


Many industries face a challenge to produce ever smaller and more precise components, while simultaneously reducing cost. Ultra-short pulse laser micromachining supports this trend in several ways, since it naturally delivers small features without damaging, heating, cracking or otherwise affecting the bulk material, while its minimisation of debris and recast material almost completely eliminates the cost of post-processing cleaning.


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Fiber Laser Welding Cuts Costs And Improves Results

Fiber Laser Welding Cuts Costs And Improves Results

Lasers have been employed in a variety of welding applications for many years. And, as laser technology further develops and diversifies, its uses in welding continue to expand. This article by Coherent provides an overview of high power lasers in keyhole welding.

Most traditional (non-laser) welding techniques currently in use are variations of arc welding. In these methods, two pieces of metal are first brought into contact or close physical proximity. The edges of the pieces may have been shaped to facilitate their joining. A high voltage is established between an electrode and the contact region, creating an arc which melts the material (or, in some cases an additional filler material or the electrode itself). The melted material fills any gap between the workpieces, or overlays them, and then solidifies to join the parts.

The primary advantage of most arc welding methods is their relatively low cost, particularly in terms of the capital equipment expenditure. Furthermore, arc welding techniques are well understood and widely employed, and standards for producing and testing them are well established, so there’s not much of a learning curve in bringing these processes on line.

The major disadvantages of arc welding mostly derive from subjecting parts to high heat. This can result in microstructures in the melted material that yield poor strength in the weld joint, and a relatively large heat affected zone in the material adjacent to the weld. Additionally, the parameters of the arc are influenced by the local electric field, and can therefore not be set independently.

Laser Keyhole Welding

Most laser welding techniques can be classified into two basic categories, “keyhole” and “conduction mode” welding. Both of these welding modes are capable of being performed autogenously, that is, without filler metal, as well as with filler, if so desired.

Keyhole, or deep penetration welding, is commonly encountered when welding thicker materials at high laser powers. In keyhole welding, the laser is focused so as to achieve a very high power density at the work piece. At the focus of the laser beam, the metal actually vaporises, opening up a blind hole (the keyhole) within the molten metal pool. Vapour pressure holds back the surrounding molten metal and keeps this hole open during the process. The laser power is mainly absorbed at the vapour melt boundary and the keyhole walls. The focused laser beam and the keyhole continuously move along the welding path. At the front of the keyhole, new material is molten, and at the back, it resolidifies to become the welded joint.

The small size of the keyhole region results in a precise, narrow fusion zone, with a high aspect ratio (depth to width) as compared to arc welding methods. Furthermore, the highly localised application of heat means that bulk of the work piece acts as an effective heat sink so the weld region heats up and cools down rapidly. This minimises the size of the heat affected zone, and reduces grain growth. Thus, the laser can generally produce stronger joints than arc welding, which is one of its primary benefits.

Laser welding also offers greater flexibility than arc welding, since it is compatible with an extremely broad range of materials, including carbon steel, high strength steel, stainless steel, titanium, aluminium, and precious metals. It can also be used to join dissimilar materials, as differences in material melting temperatures and heat conduction are of minor importance in the process.

In addition, laser welding delivers significant cost advantages over traditional methods, when all the process steps are considered. In particular, the precise application of heat minimises distortion in the weld and overall part, thus eliminating the need for post processing in many cases. Plus, the ability to project the laser beam over relatively long distances with essentially no power loss makes it easy to integrate laser welding with other production processes, and lends itself well to integration with manufacturing robotics. Last, but not least, new product configurations with reduced flange sizes can be realized, which is critical for light weight vehicles in the automotive industry.

Fibre Lasers for Welding

Modern CO2 and fibre lasers easily deliver the beam parameters and power requirements for keyhole welding. Since almost all metals become increasingly absorptive at shorter wavelengths, process efficiency is enhanced at the shorter fibre laser wavelength of ~1 μm, as compared to CO2 laser wavelength of 10.6 μm.

Fibre lasers, in particular, match the requirements of keyhole welding extremely well. They typically offer output powers in the range of 500 W to 10 kW, and can readily achieve focused spot sizes in the necessary range between 40 μm and 800 μm, even at relatively large working distances. From a practical standpoint, the use of beam delivery fibre expands integration options and facilitates the use of the laser in the manufacturing environment. Finally, the high reliability, excellent uptime and favourable cost of ownership characteristics of fiber lasers make them an economically viable and attractive choice for production welding applications.

There are currently several manufacturers of high power fibre lasers for welding and other materials processing applications. Coherent | Rofin fibre lasers, for example, can deliver a combination of performance, reliability, ease of integration and cost characteristics that is optimum for welding and other materials processing applications. To understand how this is achieved, it’s useful to examine some of the design and construction details of these lasers.

The drawing shows the main elements of the fibre laser oscillator employed by Coherent | Rofin. The laser resonator is formed by a large mode area (LMA), Yb-doped, double clad optical fibre and fibre Bragg gratings for resonator mirrors. This is pumped from each end by a series of diode laser pump modules, whose outputs are fibre coupled into the gain fibre.

Coherent | Rofin fibre laser oscillator schematic, including 12 diode laser pump modules, and 6×1 fibre coupling modules which inject pump light into the gain fibre, and allow efficient extraction of the laser output.

Based on this design, one set of pumps and gain fibre can produce output powers of up to 3kW. The output from up to four of these single mode fibre laser units can then be combined into one multimode fibre to achieve powers of up to 10kW. Alternately, the “standard” cabinet supports splitting the output from a single fibre laser into four separate fibres through the use of the integrated fibre-to-fibre switches.

Thus, this modular construction approach allows Coherent | Rofin to offer several options in terms of output power, delivery fibre diameter, and beam parameter product. The benefit is the ability to readily adapt the laser beam characteristics to precisely match the exact requirements of a specific process.

Some users have experienced fibre laser damage or process inconsistencies caused by back reflections when processing highly reflective metals, such as copper and brass. Coherent | Rofin lasers utilise an optimised power generation and delivery technology, as well as sensors at different positions within the system, to protect laser components from such damage. These safeguards eliminate the problem of back reflections, and allow reliable welding of brass, aluminium and copper without any concern for damaging the laser.

Of course, the fibre laser is just one part of the entire welding system, which also includes a beam focusing welding head, as well as control electronics. In addition to fibre lasers, Coherent | Rofin also offers beam delivery components which mount into customers’ machines. These can be fixed optics or complete, integrated scanning solutions, which include control of all relevant laser parameters, to fully optimize the welding process. Moreover, these integrated solutions often feature fast and flexible beam scanning technology which allows rapid beam movement from one welding contour to the next. This increases the productivity of a laser processing system enormously.


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