The days of finicky, unstable, high-maintenance laser systems are quickly becoming a thing of the past. Legacy laser systems are being replaced with better sources, components, and total systems, bringing more robust laser processes to production environments. It’s an amazing time to be alive when technology rapidly changes and enhances our lives in so many ways. The digitalization of widgets proves there is no doubt that technology has changed the way we work, think, and play, forever. And industrial laser systems are part of the incredible leaps and bounds technology is making in this modern age.
CO2, Nd:YAG, Fiber Lasers
The CO2 and Nd:YAG lasers are mature technologies that still serve several industrial markets. The technology is well established, trusted, and relatively easy to manage effectively. However, the fiber laser offers some advantages that legacy systems do not, such as higher quality beams, higher wall plug efficiency, relatively lower cost of ownership, and less dependency on frequent routine maintenance schedules. There are still some limitations with fiber laser systems compared to CO2 systems, for instance the thicknesses of metals that the laser can effectively cut. CO2 lasers are preferred for cutting applications where the material is thick, and are lower cost for this type of processing. Nevertheless, there is a possibility that fiber lasers could replace CO2 2D cutting systems in the not too distant future. Fiber lasers are certainly the current best choice for cutting thinner materials.
Application developers have enjoyed the advances in fiber laser technology, especially as power levels continue to climb. There doesn't seem to be an end in sight, nor does there seem to be a limit to the development of even more applications, especially at high power levels. Most recently, automotive manufacturers have begun using higher power fiber lasers in remote welding applications. These applications typically employ relatively larger spot sizes to accommodate the long focal length optics. These higher power lasers have helped supply higher power densities at the workpiece. High power fibers lasers are also being used in laser drilling systems where high-energy peak powers have historically been provided by Nd:YAG lasers.
The Problem of Focus Shift
Naturally, the technology advances have presented some challenges to those developing not only the laser sources, but also the components to be used in the systems. For instance, fused silica transmissive optics have been used for years to deliver and shape 1µm wavelength laser beams. Some limitations exist when high power fiber laser light is applied to transmissive optics. Thermal changes to fused silica can cause a shift in the delivered focal plane of the incident laser beam. Focus shift happens when non-linear heat applied to optic causes that optic to lose its shape. As optical materials heat up, the index of refraction changes causing spot sizes to grow and the focus or minimum spot size to change in location along the propagation axis as well as the lateral X, Y plane. This can cause significant problems in material processing. Reflective optics are now being used by some to combat the problem of focus shift. The use of metal reflective optics has already enjoyed some success in this application. Most reflective optic based processing heads still have a transmissive optic for the protective cover glass. This cover glass can also introduce thermal lensing effects.
While high power fiber laser has solved some problems for industrial laser users, it doesn't produce a completely problem-free process. In addition to the issues already mentioned, the fiber laser also has physical limitations in the type and thickness of material processed and as with all technology is subject to aging and eventual failure. Therefore, these laser processes require regular monitoring and control to ensure a stable laser process over long periods of time and to help prevent catastrophic failure of laser system components.
Laser Measurement Techniques
There are also many ways that laser users have tried to measure the laser system in an attempt to better understand its behavior for the purposes of controlling the process to which the laser is being applied. Measurement products such as a thermopile power puck and the laser mode plate give an indication of how the laser is performing at a given point in time, but they do not provide the laser technician with a complete understanding of the laser's characteristics because the data is incomplete. The power puck provides a single data point describing the intensity, and the laser mode plate provides an image of the cross-section distribution of the laser intensity. Mode plate images are subject to human interpretation.
Water-cooled thermopile and calorimeter based laser power measurement sensors are currently the best way to verify that the power that is being requested to reach the workpiece is indeed doing just that. The introduction of the first commercially available 100kW fiber laser allowed those who develop laser measurement technology the opportunity to introduce the industry's first commercial 100kW power meter based on calorimetric principles, Measurement accuracy of ±5% can be achieved with such a device. By utilizing a system that can measure laser power over long periods of time, the user of the laser can more fully understand how the laser system is performing and adjust process parameters accordingly.
With respect to beam quality measurement, even modern technology paints a vague picture of how the laser performs with respect to the problem of focus shift because these systems typically measure a single pane. Electronic methods of laser measurement which intrude on the laser beam, especially at the high power densities that the modern fiber lasers produce, are susceptible to damage by the laser or by operator error. These issues can result in inaccurate measurements, damage, or failure of the equipment. With the increasing powers of the fiber laser, current laser measurement techniques simply cannot supply the laser user with the data that is needed for control of the laser application. But how can a laser be measured without coming into some kind of contact with the beam?
Non-Contact Laser Beam Measurement
There is a phenomenon that happens with light called Rayleigh scattering. Rayleigh scattering happens when light reflects off of molecules, atoms, or any particle smaller than the wavelength of the light. This optical phenomenon most commonly occurs when light reflects off of gas molecules in the air. The reason the sky is blue is because of the Rayleigh scattering off of air molecules and because light at the blue end of the visible light spectrum is more likely to be reflected due to its shorter wavelength.
In state-of-the-art laser measurement products, this phenomenon is used for beam measurements. Rayleigh scattering from focused lasers produces a signal strong enough, even at around 1µm wavelengths, to be detected with a silicon-based camera. When the camera is coupled with a telocentric lens that views the laser beam waist from the side, the image produced can be directly correlated to the behavior of the laser. Measurements can be made that correlate the spot size through the laser caustic, thus providing immediate information about beam propagation and focusing. Because this information is available as soon as the laser is turned on, it allows for real-time measurement of these laser characteristics, including the focused spot location over time. If focus shift is occurring, the amount of shift and the rate of shift can also be measured. Because the system does not come in contact with the laser, these properties are not changed in any way by the measurement device. In addition, there is no specified upper limit at which the product can measure these laser properties.
The development of this technology has resulted in beam measurement data that many who have worked with lasers for several years are seeing for the first time. And it is allowing high power fiber laser users develop new applications and understand commonly used applications with greater insight when issues arise. It is quickly being realized as a tool that could help solve many of the problems that higher power fiber users are experiencing.
Handling Focus Shift
For instance, EWI in Columbus, Ohio has used Ophir’s BeamWatch™ laser measurement system for testing on their patented, high power, reflective focusing optics and received never-before captured results related to focal shift of their products and other products in the market.
Automotive laser users who employ remote welding techniques are taking notice of this new technology. They realize that being able to view and measure the focused spot location in real-time allows them to more effectively and accurately develop laser applications. The actual power densities can be assured throughout the process. This technology can be used to qualify the components being integrated into the laser and work cells. It permits certification of the cell during system runoff, before installation. The technology is also valuable for periodic checks, including laser power checks, to ensure a stable, consistent laser process over long periods of time. Periodic checks can guarantee a stable process over the life of the work cell.
Aviation and aerospace drilling laser users are also considering this new data collection method. They realizing that it will help measure focused spot location over time more accurately than traditional methods. The location of the focused spot with respect to the part being processed is critical with drilling applications. Often, the parts being processed will be in excess of several thousands, and could contain tens or even hundreds of thousands, of holes. These holes need to be as identical in shape and depth as possible and the power density applied to the material is critical for this to happen. Frequent verification of laser power is already standard practice at the facilities performing such drilling. But until recently there has not been a way to verify the real-time location, intensity, and size of the focused spot. A noncontact, in-line measurement tool that does not affect the laser beam is the best possible solution.
There is no doubt that the fiber laser has changed the game for industrial laser applications. By helping to provide more reliable, efficient, robust processes, fiber lasers bring much to assure a continued home for industrial laser processes in modern manufacturing. For many, the concepts behind the control and improvement of the process, regardless of the type of laser employed, remain the same. Process improvement must start with control, and control must start with measurement. It is good to know that as the powers of fiber lasers increase, new, more complete, and non-destructive methods of measuring these lasers are available.