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Higher Power Plays

Industry experts tackle the question: ‘How much power is enough?’

When talking about laser cutting technology for the steel fabrication industry, the discussion generally involves the highest productivity for the least amount of investment. When evaluating which laser is the best for their operations, manufacturers have specific criteria, one of which is making the highest quality part at the lowest cost per part. There is a balance that they must keep in order to have the capital investment on a new laser match or exceed the productivity and budgetary goals of the company. This should certainly come as no surprise to anyone reading this article now.

Other factors, such as edge quality, must also be taken into account. Of course a fiber laser can cut at astounding speeds, but the need for secondary operations, such as a deburring process, can quickly eliminate any gains made through higher production speeds and lower process times. So, first of all, one must determine if a fiber laser or CO2 laser is the best option based on application. Then, the question of power requirement comes into play.

Historically speaking, in the world of CO2 technology, the development of higher power options has been a gradual process. In more than 35 years, the main power options for CO2 lasers seldom exceeded 6 kW, with the majority of CO2 laser sales leveling in the 4- to 5-kW range. There are various reasons for this, but one of the factors was cost.

Higher wattage CO2 lasers had more optics, more glassware, more of everything. And with this came less tolerance for poorly designed components that could adversely affect the beam quality of the CO2 laser. As technology improved, more high-wattage CO2 lasers entered the market, but the reality of this became more evident in the past 10 years or so, and the capital investment remained high compared to the lower wattage options.

The benefits of the higher wattage CO2 were evident in certain applications, such as clean cutting with nitrogen and thicker overall processing. The additional power allowed for impressive cut speed advantages in nitrogen over the lower wattage 4-kW systems. But often times, the additional cost of operation, electrical consumption and machine price were too much to balance the processing advantages.

Amada’s fiber laser boasts 75 percent maximum light conversion efficiency and consumes only a quarter to a third of the electricity as its CO2 counterpart.

Enter fiber laser cutting

In less than 10 years, the fabrication industry has seen fiber optic wattage levels jump from 2 kW to 6 kW, soon to be 8 kW, soon to be 12 kW, and so on. So why is this happening at such an incredible rate compared to the slow, steady climb of CO2 over the decades? Is it the fact that consumers purchasing these machines are now very familiar with laser technology and simply following the “more is better” philosophy?

Well, sort of. But how much is too much? Where is the line with fiber lasers as was found with CO2 lasers? This answer lies in the very basic structure of the fiber laser itself.

First of all, fiber lasers do not have the optics and spatial cavity that CO2 lasers have. There are no mirrors, glassware, blowers, vacuum pumps, laser gas, and the many other items required for CO2 laser generation.

Fiber lasers are a monolithic structure that use a group of diodes to form a pump package. These pumps are combined to generate a certain amount of combined power to pump light to a special fiber optic cable (active fiber) doped with the proper gain medium, generally ytterbium, to get a specific power and wavelength out the other side. This is known as a module.

A module may have 500 W, 700 W, 1,000 W or more power, depending on the number of laser diodes and the individual output power of these diodes. The ability to build fiber lasers at extraordinary output powers is not hard to do and in fact, nothing new.

The concept is simple. Add more laser diodes to create more power from a module. Combine more modules to create more output power from the engine, and you have lots of power! Perhaps this is simplifying things, but this gives the big picture anyway.

So, armed with this knowledge, how much power do you need? Why don’t fiber laser manufacturers make 100-kW lasers for cutting? This is not as easy to answer. And it begins with a key term, beam parameter product (BPP).

The LCG AJ fiber laser doesn’t require laser gas or mirrors typically associated with CO2 lasers. Users, therefore, see significantly less operating costs and maintenance.

Understanding BPP

The BPP is a measure of the beam quality and size of any laser source. Physically, it is the product of the beam radius and the beam divergence at any point in the system – in the laser, in the process cable or at the workpiece.

Generally speaking, making a small BPP laser is more difficult than making a larger one, but a small BPP allows for a smaller, more collimated beam, smaller kerf and faster cutting. In fact, this is one of the primary reasons fiber lasers cut so much faster than CO2 lasers – the BPP can be as much as 10 times smaller for a fiber laser.

One way to enable small BPP fiber lasers is by using small BPP, or so-called “high-brightness” pump diodes. Let’s take a step back and look at this technology.

Laser diodes, are in fact, simple diodes in the electrical sense. Current flows in one direction, but not in the other when a voltage is applied. However, laser diodes are special in that they generate light when current flows and actually convert almost each electron of current into a photon of light at very high efficiency.

This process requires that the diode is made with very high quality. Diodes are made from semiconductors, and laser diodes for fiber laser pumping are generally made from the semiconductor, gallium arsenide (GaAs). GaAs is a crystal that is “grown” onto wafer substrates and processed in a wafer fab, similar to how computer memory and processor chips are produced. Crystalline defects must be exceedingly low – below one part per trillion for reliable operation.

While you may not realize it, laser diodes are used by most people every day. They are the light sources that read or write information on CD, DVD and Blu-ray disks, and they send the digital 0s and 1s of the internet through optical fiber around the world.

They must be exceedingly reliable. No one wants their movie interrupted by a failed DVD player, or their overseas long-distance call disconnected by a laser diode failure in an undersea fiber optic cable. The high reliability nature of these devices makes them ideal for industrial applications where up time is a must, and operation over many years without service is desirable.

As an example, the laser diodes in the Amada/JDSU fiber laser cutting tools use the same technology as is used for laser diodes powering fiber optic communications in 24/7 operation beneath the world’s oceans and across continents. Indeed a service call is a very expensive proposition in that application, so with an expected service time of 25 years, these diodes are built to last. When this technology is used in the fiber laser, the diodes have a characteristic lifetime of 630,000 hours, and this ensures a worry-free pump source.

The state-of-the-art in laser diodes is driven by making higher reliable optical power in a smaller spot size, or BPP. Higher power within a smaller BPP from the pump diodes allows for higher power fiber lasers with an overall smaller BPP. With the latest technology development, this means pump powers now exceed 140 W from a single pump package and have a market-leading BPP and brightness.

These pumps are then combined together and attached to the input end of the active fiber laser. Having low-BPP, high-brightness pumps is critical to enable a simple and reliable fiber laser design, while also ensuring the low BPP needed from the fiber laser for efficient, fast cutting.

Currently, a single fiber laser module has an output power of 2 kW, with typical BPP of 0.9 mm-mrad. Although perhaps modest in output power, the low BPP is key for cutting speed and quality, as well as to serve as a great building block to scale to higher power levels. Two of such modules may be combined together to generate 4 kW, or three for 6 kW.

Although scaling to higher powers does increase the overall BPP, with the individual fiber module BPP so low, the resulting BPP from these combined modules is also relatively low. This enables differentiated cutting speeds and quality from these higher power fiber laser engines.

The LCG AJ fiber laser system is designed for high-speed operations and features a high-speed cutting head with increased sensing speed for faster cutting.

Achieving reliability, robustness

This story is nothing new with regard to power scaling in fiber lasers. As stated earlier, the ability to create high-kilowatt systems has been possible for many years. However, the pump source technology was not capable of producing high enough output power with the low enough beam quality needed to maintain a usable beam for quality cutting.

The goal is to have more power with fewer components or modules to ultimately provide the best overall BPP. To the end user, this simply means it’s possible to have a 4-kW fiber laser cut at the same or better speed than a 6-kW fiber. Or even have a 6-kW cut at the same or better speed than an 8-kW with better edge quality.

There are, of course, other factors, which can influence the cutting result. These include lenses, nozzles and operator experience, but overall beam quality combined with power output has a major influence. So, when evaluating your fiber laser choices, don’t automatically assume more is better.

Capital investment, cost per part, process time, edge quality and secondary operations are all important when looking at the big picture. Additionally, the number of components and reliability of the source technology also plays a major factor.

The ST laser diode pump is powered by a next-generation diode laser chip made possible thanks to JDSU’s innovative designs, which minimize maintenance and recalibration needs.

Solid-state technology is evolving fast. It is estimated that around 60 to 65 percent of all flat cutting lasers sold in 2015 will be fiber. But don’t feel pulled into the power wave. Just like CO2, fiber is not a one-size-fits-all technology.

There is a right wattage and right price for everyone. With that said, it is a good idea to at least jump in the water. Fiber is here to stay.

Amada America
JDSU