laser systems

www.laser-journal.de ltJ 23 © 2009 WIley-VCH Verlag GmbH & Co. KGaa, Weinheim

understanding materials Processing Lasersa Comprehensive Overview Covering the Capabilities and applicability of the major systems

STuarT WOODSstuart has over 19-years of engineering, market-ing, and management experience in the optical technology industry. Currently at Coherent, Inc., he is the director and general manager of the direct diode and fiber systems group. Previously at sPI lasers (sPI.l), he teamed to take the company from a telecom failure to a aIm publicly floated, fiber laser success. ●●

Dr. stuart WoodsDirector and General managerDirect Diode and Fiber systems

Coherent Inc5100 Patrick Henry Drive

santa Clara, Ca 95054 UsaPhone: +1 (408) 764 4563

e-mail: stuart.woods@coherent.comWebsite: www.coherent.com

THe auTHOra decade ago, flowing gas CO2 lasers dom-inated the market for materials process-ing applications, such as cutting, weld-ing and marking of metals and organics. However, over the past few years, new la-ser technologies have emerged that give the materials processor more options in choosing a laser source. Specifically these developments include the introduction of higher power sealed CO2 lasers, the ad-vent of fiber lasers, and improvements in the brightness of direct diode laser sys-tems. The result is that choosing the right laser for a specific application is now a more complex task than in the past. This article reviews the technology and capa-bilities of all these laser types and pro-vides guidance on identifying the optimal source for a specific need.

market Definitions

the term “materials processing” is used to refer to a very broad range of techniques in a number of different industries. In the context of this article, materials processing specifi-cally means laser-based techniques that cut or melt a material on a macroscopic scale, or cause a macroscopic change in the sur-face properties or appearance of a material. specific techniques that fall under this defi-nition include cutting, welding, soldering, hardening, brazing, cladding and marking of metals, plastics and other organic materi-als (Figure 1a-d). marking specifically means inducing a macroscopic color change in a surface or a change in surface relief (e.g. en-graving).

the materials processing market as just defined has declined from a total of about 1.7 UsD billion in 2008 to approximately 1 billion UsD in 2009 (for the lasers them-selves, not the entire materials processing systems). One thing that will aid a quick recovery for all these market segments is for systems builders to have a clear under-

standing as to the optimum technology for a given application. If there is confusion in the market about applying laser technology, then it’s likely to recover more slowly. that is why it is so important to educate potential customers about the capabilities and appli-cability of the currently available laser types.

Laser Technology Overview

New developments in laser technology have had a substantial impact on the materials processing market over the past decade, and there are now four different laser tech-nologies that predominate. these are high power direct diode lasers (HPDDls), sealed CO2 lasers, fiber lasers, and flowing gas CO2 lasers. It is important to understand the dis-tinguishing output and operating character-istics of each of these sources.

The HPDDL is built from of diode laser bars, which are a single, monolithic semi-conductor substrate on which several emit-

Figure 1: a) (Can)Welding: application for HPDDLaser/CO2 Laser; b) metal Cutting: Typical application for the CO2 Laser; c) Cladding with the HighLight HPDD Laserd) Converting with the DiamOnD e-1000.

laser systems

24 ltJ september 2009 No. 5 © 2009 WIley-VCH Verlag GmbH & Co. KGaa, Weinheim

ters are fabricated. a single bar can have a total output as high as 100 W. these bars can be stacked close together, and mul-tiple stacks can be combined to produce extremely compact assemblies that deliver multiple kW’s of laser power.

Because the output of a HPDDl comes from numerous individual emitters spread over an area several millimeters in size, spe-cialized optics must be employed in order to convert this raw output into a far-field format useful for most applications. this col-lected light can then be focused on to the work piece directly (termed free space deliv-ery), or channeled into a single optical fiber, enabling remote (up to 30 meters) delivery of the laser source from the processing area. a typical output beam from a free space HPDDl system (Figure 2) might be 12 mm x 1 mm at its point of focus, while a fiber delivered system might produce a round spot that ranges from tenth’s to several mil-limeters.

One key advantage of HPDDls is their wall plug (electrical conversion) efficiency, which is many times higher than for any other laser type. this translates directly into lower operating cost for the system, since less electricity is required to produce a given amount of output power.

HPDDls are also very physically compact and lightweight compared with most other industrial lasers, therefore making their inte-gration cost very low. In addition, a closed loop cooling system can be connected to the diode stack affording a typical operating life-time of tens of thousands of hours.

the end result is that HPDDls offer sub-stantially lower cost of ownership than other laser technologies. In addition, the initial capital cost is usually lower for a HPDDl than

for another laser type of equivalent output power.

The sealed CO2 laser is a compact source where the gas fill is provided at the factory, and the resonator is hard sealed, so no ex-ternal gas supply is used. the use of a slab discharge resonator offers the highest power per size ratio available for sealed lasers, and can also deliver up to about 1 kW of output power. In this configuration, the resonator is sandwiched between two flat electrodes that initiate the discharge and effectively cool the gas mixture.

their small size makes these lasers easy to integrate into robotic applications or even in desktop equipment. For example, Coherent recently introduced the DIamOND e-1000, the world’s smallest CO2 laser having 1 kW of output to date, which measures less than <1500 mm x 500 mm x 400 mm (the head which includes the power supply).

When coupled with the right beam con-ditioning optics, the slab discharge configu-ration can produce a high quality Gaussian beam with a low M2 (<1.2). this translates directly into the ability to achieve a small fo-

cused spot, with nearly all the laser power in the central beam. the result is more precise cuts or higher cut speeds, a smaller heat af-fected zone (HaZ), and the most efficient use of laser power.

another important characteristic of the slab discharge design is that it naturally produces nearly square wave shaped pulses with very rapid rise and fall times. this is im-portant, because pulses with faster rise and fall times deliver more of the total pulse en-ergy above the material processing thresh-old. laser power delivered below this level merely heats or melts the material, increas-ing the size of the HaZ and lowering overall efficiency.

the high cutting efficiency of the slab discharge laser also minimizes the electrical consumption for a given process speed. la-ser efficiency is in the area of 17%. the lack of an external gas supply eliminates all the costs associated with the handling and stor-age of gas tanks, as well as the downtime as-sociated with changing them. these factors, together with the high reliability of the de-sign, give the slab discharge laser the most attractive cost of ownership characteristics of any CO2 laser type in this power range.

Fiber lasers use semiconductor diodes to pump light into a doped optical fiber, which then emits laser light at about 1.1 µm (de-pending upon the dopant). Output power is directly dependent on the quality of the fiber (efficiency) and the available pump power. Currently available fiber lasers fall into two categories, single mode and multi-mode.

Figure 2: Coherent’s HighLight Laser: High Power Direct Diode Laser for indus-trial manufacturing.

Coherent inc.

Coherent designs and manufactures a broad selection of lasers and supplies electro-optic instruments for laser test and measurement. the company‘s pro-ducts include laser diodes and laser diode systems, carbon dioxide (CO2) lasers, excimer lasers, ion lasers, CW and Q-switched DPss lasers and systems, ultra-fast lasers and amplifiers. the company provides worldwide service and applica-tions support.

Contact:Petra Wallenta, Pr manager europe e-mail: Petra.Wallenta@coherent.comWebsite: www.coherent.com, www.coherent.de

THe COmPany

TabLe 1: Summary of materials processing lasers characteristics.

HPDDl sealed CO2 Fiber Flowing Gas CO2

Wavelength (µm) 0.98 10.6 1.07 10.6

Power range10’s of watts to 10 kW

10’s of watts to 1 kW

100’s of watts to 10’s of kW

100’s of watts to 10’s of kW

size Very small small medium large

electrical efficiency 40% 17% 25% 10%

maintenance Interval

2 years >2 year 2 years 6 months

Initial Capital Cost low low High medium

Cost of Ownership* low low low High

laser systems

www.laser-journal.de ltJ 25 © 2009 WIley-VCH Verlag GmbH & Co. KGaa, Weinheim

Commercial single mode fiber lasers de-liver output powers of up to 1 kW with ex-cellent beam quality (M2 < 1.2). this makes them particularly useful for so called “re-mote welding and cutting,” that is, welding and cutting where the focusing optics are located a significant distance from the work surface. this configuration is particularly use-ful when the focusing optics are mounted on a robotic arm.

By increasing the mode volume, com-mercial multi-mode fiber lasers can provide powers of up to 20 kW. even with multi-mode operation, their spot size is still well matched to the needs of many applications – making the 1 or 2 kW multi-mode fiber la-sers the most commonly used in today’s ma-terial processing applications. multi-mode fiber lasers are less complex, and hence, less costly than single mode devices. as a result, they are typically the first choice for any ap-plication that doesn’t specifically require re-mote focusing.

Both fiber laser types offer excellent re-liability characteristics and feature minimal maintenance requirements. their electrical

conversion efficiency is second only to HP-PDls. as a result, they have a relatively low cost of ownership.

Flowing gas CO2 lasers have been the workhorses of many industrial processes for decades, and are still widely used in ap-plications that require higher power. they are available with multi-kilowatt output in a multi-mode beam.

Flowing gas CO2 lasers are a mature technology, enabling them to typically of-fer the lowest purchase price per watt at a given power (at least at the high end of their power range). Furthermore, their perfor-mance and maintenance requirements are well understood and characterized. In addi-tion, relatively inexpensive refurbished lasers are readily available.

However, there are drawbacks to this technology, which is why the other la-ser types have been gaining market share against it over the past few years. For exam-ple, flowing gas CO2 lasers are electrically inefficient and require external supplies of gas and cooling water. this translates into a high cost of ownership and the need to

devote a large amount of space to the laser on the production floor. the large size of the laser head also means that it can’t usually be mounted on a gantry or robotic arm and brought close to the work surface, thus often necessitating the use of a complex beam de-livery system (and this wavelength can’t be fiber delivered)

applications

the output characteristics of most materials processing lasers are typically highly multi-mode, or in the case of the HPDDl, an ex-tended beam. as a result, the mode quality parameter M2, which is typically used to de-scribe the ability to focus a laser beam, is not particularly useful for these sources. since the ability to focus an extended source is limited by the size of the emitting region and the beam divergence, a more useful quantity is the beam parameter product (BPP), which is the product of the source size and beam divergence, and is given in units of mm x mrad. to visualize the performance range of a specific materials processing laser, its BPP

1000

100

1010 100 100001000

Laser Power (W)

Beam

Par

amet

er P

rodu

ct (

mm

× m

rad)

Figure 3: bPP versus power graph. Several important mate-rials processing applications are shown , divided into low brightness and high brightness regions.

Figure 4: First completely sealed 1 kW CO2 Laser from Coherent

laser systems

26 ltJ september 2009 No. 5 © 2009 WIley-VCH Verlag GmbH & Co. KGaa, Weinheim

is typically plotted against output power in a double log plot.

the required operating regime for vari-ous materials processing applications can also be plotted on the BPP versus power graph. this provides a quick visual way to assess the source brightness requirements of a given application. a BPP graph of several important materials processing applications is shown in Figure 3. For the purposes of dis-cussion, it is roughly divided into low bright-ness and high brightness regions.

While any of the lasers discussed here could at least potentially service the low brightness applications shown on the graph, the HPPDl makes the most sense in practical terms. this is because HPDDls offer the low-est capital cost of any of industrial laser type, as well as the lowest operating costs, due to their high electrical efficiency.

the output characteristics of the HPDDl are also well matched to the needs of some of these applications. For example, the line beam shape of a free space delivered HPDDl is actually advantageous in many heat treat-ing and cladding processes, because these require that heat be applied over a relatively large area. Fiber delivery is well suited for many welding applications; it enables re-mote placement of the laser for space sensi-tive manufacturing uses, and also supports high throughput fabrication because the focusing head can be moved rapidly.

For the higher brightness applications,

there are several factors which must be con-sidered in choosing a laser source. the first of these is the material absorption characteris-tics. In particular, most organic materials ab-sorb very strongly at or near the CO2 output wavelength of 10.6 µm. that makes process-ing these materials with a CO2 laser inher-ently more efficient. the CO2 laser also deliv-ers better results when processing organics, because it is less likely to burn or char the material than shorter wavelengths, which are more poorly absorbed. On the other hand, the absorption of metals is higher at the near infrared output wavelengths of fiber lasers and HPDDls.

For thin metals (<2 mm thickness) high material absorption translates directly into increased cutting speeds. For this reason, fi-ber lasers have come to dominate thin metal cutting applications over the past few years. In applications for which speed is the primary factor, this situation is unlikely to change any time soon.

From the author’s experience and review-ing published data, for metals in the 2 mm to 4 mm thickness range, the nature of the la-ser/material interaction is such that material absorption is no longer the primary factor in cutting speed, and fiber lasers and sealed CO2 at the same power offer essentially the same processing speeds in this domain. However, recent advances in sealed CO2 la-ser technology have made this technology much more attractive from a practical stand-

point. For example, since they are compact and don’t need a gas supply, sealed CO2 lasers can be directly mounted on a robotic arm, eliminating the need for fiber delivery. they also offer reliability equal to that of a fiber laser, yet their capital cost and cost of ownership are substantially lower. Further-more, some manufacturers have recently been able to dramatically improve bright-ness; for example, in the DIamOND e series (Figure 4) from Coherent, a 1 kW sealed CO2 laser offers the same brightness and pro-cessing power as a 2 kW flowing gas laser, but with all the advantages of lower cost, greater compactness and higher reliability. the result of all this is that sealed CO2 lasers are an increasingly attractive alternative to fiber lasers for more cost sensitive uses, or especially for situations where both metal and organics must be processed. a typical example of this is cutting the carbon fiber reinforced composite materials used in au-tomotive interiors.

For processing metals of 4 mm to 6 mm in thickness, the main contenders are fiber lasers and flowing gas CO2 lasers. Both of-fer similar cut quality and processing speeds. For situations in which fiber delivery offers a significant practical advantage, the edge goes to fiber lasers. When purchase cost is the main consideration, or where there is the need to cut both metals and nonmet-als, then the flowing gas CO2 laser has an advantage.

For cutting thick metals (above 6 mm thickness), the need to remove molten metal from the cut typically makes it advantageous to use a larger beam. the relatively narrow beam of the fiber laser isn’t a good match for this requirement, making the flowing gas CO2 laser the source of choice.

Conclusion

the materials processing market now has available a range of laser sources, each of which offers its own particular advantages and processing characteristics. By guiding customers to the optimum technology for their particular needs, laser manufacturers can ensure a quick recovery of this market segment, as well as healthy growth for some time to come.

material thickness Optimum laser Comments

≤ 2 mm

Fiber laserDelivers high cutting speed and good cut qual-ity for metals

sealed CO2Best choice when both metals and nonmetals must be processed

2 mm – 4 mm

Fiber laserProcessing details depend upon material and thickness, but generally delivers high cutting speed and good cut quality for metals

sealed CO2

more economical alternative, and especially at-tractive when both metals and nonmetals must be processed

>4 mm – 6 mm

Fiber laserHigher purchase price, but lower operating cost and less maintenance downtime

Flowing gas CO2Offers lowest cost per watt but requires more complex beam delivery optics

> 6 mm Flowing gas CO2Offers the necessary raw power and beam characteristics

TabLe 2: Laser Cutting Comparison Chart