RSS 2.0

For a lot of people following this market, it feels like high-power fiber lasers are becoming a commodity, with prices getting so low that they can now be used in industries where the cost of the machines would have been prohibitive only a few years ago. However, technological innovation is still very much happening; it is not all about cost reduction (although it is certainly a central consideration).

New applications each have specific requirements, and to keep costs low enough for the application to be profitable, a single laser type would not only be extremely difficult to make but would also be too expensive. To tackle these specific needs, lasers must be designed to have precise features to provide an edge to the manufacturer. This implies that laser engineers must be creative and flexible, and come up with new ways of achieving targeted specifications. This also requires having access to versatile components to work with.

Of the most recent emerging applications of high-power fiber lasers, the following three are generating the most interest (and money): multi-tens of kilowatt systems, laser welding, and additive manufacturing. The following section details the specific challenges of each application and unveils the most common techniques used to tackle them.

Multi-tens of Kilowatts Systems

Industrial laser systems of 20 to 60 kW of output power have become commercially interesting in recent years because advances in fiber laser technology have made it possible to create such systems at a reasonable price. The interest in these systems is not only the increase in cutting speeds but mostly to address the heavy industries like naval manufacturing, railway industry, or mining equipment. These industries require not only thicker but also larger pieces. To make ultrahigh-power laser cutting systems relevant, the laser head needs to travel far to manufacture those giant metal gizmos. Long delivery fibers must be used for the light blade to have enough reach.

While technical innovations were the foundations for this new application, it is the innovations in the cost structure that truly allowed it to take off. Such high-power systems are made by combining laser modules of lower power. While it is not seen as glamorous as technological advancements by many, advances in operations and cost structures make the difference between a cool result and a technological advancement. The most recent model that laser manufacturers are embracing is the modular design, which allows a production line of standardized fiber lasers to minimize costs while having the flexibility to offer systems that are just right for the client. Enough power to reach the thickness and cutting speed targets, but not too much to avoid the high price of unnecessary power. This model enables more applications to embrace fiber laser technology, making the business case more appealing for applications that would not otherwise be profitable if different power levels were not available. This model creates pressure on the fiber laser design because, to make modular designs manufacturable, the output fiber must be longer compared to a system designed for a specific power level.

Thus, the ability to use long passive fibers is key for these ultrahigh-power systems. While the fiber itself is not very expensive, having longer fibers can be detrimental to the laser quality because of Stimulated Raman Scattering (SRS). SRS is a non-linear effect that will not only limit the output power but will also lower the output power stability and destabilize the beam shape, which will make the cutting uneven and unreliable. Since SRS is a non-linear effect that will be amplified during propagation in the fiber, reducing its level at the start when it is low (i.e., at the output of the oscillator or amplifier) will have a great effect on the final system performance.

Welding

Green initiatives and regulations are propelling consumers toward sustainable, low-carbon transportation by accelerating the shift to electrified vehicle drivetrains, making electric vehicles (EV) the technology of the future for the automotive industry – which is estimated at about 3.56 trillion $ worldwide in 2023. EVs have been invented more than 100 years ago, but they have not been relevant until recent advances in battery technology.

What many do not know is that current battery technology would not be possible without laser processes. The ability to weld increasingly smaller features while maintaining electrical contact and extreme reliability is not something that was possible a few years ago. To achieve the high-speed, no-failure, and multi-material welds needed to democratize EVs, an astonishing level of control of the laser must be used. Nowadays, high-speed process monitoring is used to adjust the laser parameters at the millisecond timescale during the weld. Often made by optical coherence tomography (OCT) techniques or high-speed cameras, this monitoring will account for the chaotic movements of the melt pool to change either the laser beam shape, the deposited power, or both. High-speed process monitoring is also used to evaluate the quality of the weld without having to make an electrical test, streamlining quality control considerably.

Of course, to ensure that these laser adjustments are relevant, the laser has to itself be of the utmost stability, both in output power and beam quality. Most laser manufacturers have developed a technology that allows them to change the shape of the output beam, but all those methods have precise requirements of beam quality to function properly. This means that even low levels of SRS that are usually tolerable are not acceptable for this application.

Additive Manufacturing

Additive manufacturing (AM) simply means to machine pieces by adding material, instead of removing it like it has been done since the dawn of time. It should be safe to assume that the ancient Greeks would have made even more impressive statues if they had the option to sculpt by adding material instead of carving stone. While many AM techniques exist, this section focuses on powder bed fusion, which is the act of placing a layer of metal powder and using a laser to melt a "slice" of the part, then adding another powder layer and so on.

With this method, engineers can unleash their creativity to imagine highly complex, lightweight, and efficient parts that can finally be made. The laser used will determine the precision of the features that can be made. In order to have regular segments, laser output power requires the highest levels of stability. Moreover, the smaller the features, the more freedom is given to designers. The size of the features is limited in part by the beam quality, which has much more stringent requirements in this application.

The promise of AM is great in so many industries, but it is not yet being adopted very much. The reason is simple: this process is slow. It takes much longer to build a piece plane by plane than it takes to use a tool to drill a hole, for instance, making AM pieces quite expensive compared to their traditional counterparts. To bridge this gap, a lot of focus is being given to increasing the output power of the lasers, which is technically very challenging to do while keeping the high beam quality requirements of this application. Small core size, highly single-mode fibers must be used, which intrinsically generates a lot of non-linear effects (like SRS) that degrade the beam quality and stability. Nonetheless, the payoff is great since an increase in power for the same beam quality can translate into an increase in processing speed and reduced cost per part.

How to Tackle Those Common Challenges

About 5 years ago, it seemed that managing SRS was only a way to scale the power of either a laser oscillator or a master-oscillator-power-amplifier (MOPA) while minimizing cost. Today, the interest in suppressing SRS is much wider than this.

One very efficient way to achieve lower SRS is to use the RSS - Raman Scattering Suppressor. Now available in both single-mode and multimode fibers (from 5 to 35 µm core diameter), this in-line transmissive filter suppresses SRS while its power is still low. Whether to use smaller core size for optimum beam quality or to greatly increase the delivery fiber length, the RSS will prevent the buildup of SRS that happens in a non-linear fashion during propagation, allowing otherwise impossible laser specifications.

The recently improved RSS can handle up to 5 kW of signal power while maintaining a ≤0.15 dB insertion loss. With many thousands of units now deployed on the field, the RSS is one of the many innovations required to make such advances in material processing.

Advantages

• Low temperature dependence
• Available for single-mode oscillators and multimode power amplifiers
• Low insertion loss
• Custom versions available