By John McCauley, Product Specialist, Ophir Photonics Group
|Most anyone working with lasers today will tell you that it is an exciting field. Applications of coherent light at different wavelengths are ever increasing and show no signs of plateauing. The skills required to understand, troubleshoot, and correct problems with lasers, and the systems they are a part of, are in high demand. It's all part of an evolution whereby yesterday's laser technician has turned into today's laser technology specialist.||
Being a laser expert often requires a strong working knowledge of science, specifically physics. Being able to understand how light is behaving (and how to make it behave when it is not) is paramount to success in the field. But even when laser experts have this general understanding of laser light, they haven't always had the tools to provide them with the information needed about how the laser is behaving.
Historical Measurements of Laser Behavior
Over the years, there have been several simple tools that have provided valuable, but limited, information about laser performance. Using a basic thermopile element, known as a "meat thermometer" or "power puck," and measuring the rise in temperature that the laser adds to the device gives the laser technician a rough indication of laser power. That data point is then tracked over time to see how much and how quickly laser power is drifting, mostly due to degradation of one or more components within the laser system.
Another valuable piece of information about a laser's health is its beam profile. Historically, this has been determined using burn paper, wall projections, mode plates, and acrylic blocks. The beam profile shows how round and balanced the beam is, which can tell the technician many things about the laser's stability and alignment, and the state of components internal to the system.
|Today, many of these simple tools are still in use. Technicians that have been working with lasers for several years may still rely on them for laser maintenance because they have used them for so long and feel confident in the information they provide. "Why fix what isn't broken," right?||
However, when we're talking about what is undoubtedly one of a company's largest investments (lasers), the protection of that investment shouldn't rely on some of the simplest and rudimentary analysis tools. Just as laser technology continues to advance, so has the technology associated with measuring the performance of these lasers.
As valuable as the information these simple tools provide is, they often tell an incomplete story of how the laser is performing. Most of these products provide single data points over a very short period of time that the laser is operating, usually from a few short milliseconds to just a few seconds after beam-on time. Those working with lasers will universally tell you that several characteristics about the laser could change from the time the laser is turned on to several seconds after because of the thermal effects that the laser has on its internal components.
Three Cases for Modern Laser Measurement Techniques
It is only through modern-day electronic methods of collecting laser performance data that the full story of the laser can be determined. Let's discuss three laser characteristics where electronic laser measurement can better provide this data and better help the laser expert.
Figure 1 shows beam profiles from a CO2 cutting laser from the time the laser was turned on to several seconds after. In this example, it was determined that a damaged output coupler was causing instability in the laser, resulting in its poor performance. One of the more common methods of obtaining a beam profile is using an acrylic block. This is where the laser is fired into the block for about a second and a half and a beam profile is obtained. However, the use of an acrylic block would not have given the needed information to get to the real story about the damage to this laser.
Figure 1. Changes in a beam profile from beam-on time to several seconds after the laser starts.
In another example, Figure 2 shows a graph of laser power over a period of about 60 seconds. This laser was set for a power output of 5500 watts and in a normal situation would achieve that power level within just a few seconds after the laser was turned on. Initially, the laser reached a power level of about 3600 watts and stabilized. However, as seen in the graph, the power then became unstable for several seconds and eventually stabilized about 45-50 seconds after the beam-on time (which is an eternity in a production environment). The dated but simple approach to analyzing the laser would be to use the "meat thermometer," previously discussed. And although the power reading would have probably been low, it would never have given the detailed amount of information seen with the electronic power measurement. Therefore, the stabilization issue would not have been difficult to diagnose. It turns out, the cause of the power fluctuation was a gas leak in the laser's resonator.
Figure 2. Laser power over a 60 second time period.
As laser powers continue to climb, thermal effects on the laser system become more of an issue. The more light that is pushed down the pipe, the more the possibility that components integrated into the system will be susceptible to physical changes in shape. The end result will likely be a change in location of the laser's focused spot with respect to the process. This phenomenon is most commonly known as "focus shift."
Figure 3 shows the measurement of spot size (or "beam waist") location over time on a 100KW laser using reflective optics to focus the beam. Even though reflective optics are usually less susceptible to focus shift, the laser power was high enough to cause an approximately 8mm shift over the course of about 4 seconds; these were due to thermal effects on the laser system. When this happens, the power density being applied to the process can change, potentially to the point where the laser is no longer processing correctly. This results in parts rework or scrap
Figure 3. A 100KW laser system with reflective optics exhibiting an 8mm shift in the focused spot.
While most may not realize it, being a laser technician has become a fast-paced, continually evolving job. It can be seen as either exciting or stressful, as a large amount of pressure is placed on the laser specialist to ensure a highly efficient laser system and a quality process. No doubt there is plenty of stress that comes with the job, but having an incomplete story of the state of the quality and efficiency of the laser system will undoubtedly add to the stress.
As laser sources and systems have evolved, so has the laser technician. Today, they are laser technology specialists that are moving past historical laser measurement techniques in order to paint a more complete picture of their laser's health. As laser technologies develop, the measurement tools of the past will become extinct, allowing the laser technology specialist to evolve into a more efficient, better educated, and more valuable and respected member of the laser community.