Abraham Lincoln once said, “You can fool all of the people some of the time and some of the people all of the time, but you cannot fool all the people all the time.”¹ When approaching the question of monitoring the performance of a laser integrated into a system, the same is true. You can monitor all of the system some of the time, you can monitor some of the system all of the time, but you cannot monitor all of the system all of the time. In the age of Industry 4.0, a.k.a. smart manufacturing, it’s important to understand the difference.

Industry 4.0 is changing manufacturing across all industries. Technology is helping manufacturers produce parts more efficiently, more quickly, and, in general, more intelligently. To properly apply smart machines, there is a need for data. Eventually, this data must be interpreted and filtered to improve the process. Too little data hinders process improvement—but at the same time, too much data can be counterproductive.

Systems that involve a laser process have their own set of operational characteristics and associated problems. Too little data about how the laser is performing does not help the laser operator fully understand how to manage changes within the laser system. Too much data about how the laser is performing is overwhelming, confusing, and ultimately counterproductive.

When to measure laser performance 

There are four approaches to measuring a laser’s performance characteristics. The first is what most laser system operators gravitate toward: Periodic maintenance. In this approach, the laser’s performance is measured based on scheduled downtimes of the laser, usually quarterly, semiannually, or annually. During this time, measurements of the laser’s performance characteristics are taken and compared to previous measurements to analyze laser behavior trends.

The second approach is to measure the laser during some kind of process failure in which the laser is involved. For example, a welding laser will show a degradation of the welds, or a cutting laser will lose its cut or stop cutting altogether. Laser performance characteristics are measured to bring the laser system back into its designed operational parameters.

The third and fourth approaches are what this article will discuss: In-process monitoring and at-process monitoring. Both approaches have pros and cons that need to be understood by laser operators when trying to determine how best to understand the way their lasers are processing. Laser operators must also understand which measurements of the laser system are important at the production stage of the laser’s life.

How the laser processes material

At a high level, a laser operator must understand how the laser processes the material, regardless of which process the laser is being applied to—for instance, how a type of laser welds vs. how that laser welds a doorframe on an automobile. The simplest way to understand this is through laser power density.

Power density is defined as the amount of laser light that is applied to the material per unit area. Power density is often expressed through the formula: W/cm², where the “W” stands for “watts,” the unit of laser continuous-wave (CW) power or average power (for pulsed lasers), and the “cm²” represents the area of the laser’s spot size at the working plane. For example, a laser operating at 100 W of CW or average power focused down to a spot size of 100 µm would yield a power density of 2.6 × 10³ kW/cm²).

The laser’s power density can be affected by changes in either the amount of laser light or size of the beam being applied to the material. It’s important that the laser operator measures, analyzes, and understands both variables to maintain a successful laser process.

Vital laser behavior measurements

Measurement of the amount of laser light is achieved through what is commonly known as a power meter. A power meter is a sensor that collects laser light, converts that laser light to an electrical signal which is then extrapolated into the amount of power or energy that the light is producing, and it ultimately provides a reading to an attached meter or local PC for analysis. This process usually only takes a few seconds, but can vary depending on the technologies used. These measurements are important to collect and analyze, especially during the production phase of the laser’s life, because it tells the laser user how laser performance is changing and how these changes affect the laser’s application to the process.

Overall beam diameter must also be measured. There are different methods that can be used: D4σ, 13.5% of peak, and 10/90 knife-edge each calculate beam diameter, but can produce very different results. All are used by people in different industries, with different backgrounds and experiences.

The roundness of the beam, or ellipticity value, must be considered when calculating beam diameter. Beam shape, and how the energy is distributed across the beam profile, must be understood. Is the beam a Gaussian beam? Is it a flat-top beam? All these measurements, and more, can and should be made via an industry-standard beam profiling system when attempting to understand how the laser is applied to the process.

Beam quality must also be considered, whether you''re selecting a laser, developing the application with a laser, or integrating or commissioning a laser source into a system. In most cases, beam quality analysis is rarely considered once the laser is deployed into production, so it is important to consider this analysis before this last phase of the laser’s life cycle.

Beam quality can be expressed as an M² value, with the value of 1.0 being a perfect laser. Beam parameter product (BPP) and K are also expressions of laser quality. Laser sources have improved with respect to beam quality and the efficiency with which the laser light is produced, and different laser sources have their own strengths when it comes to the different processes where a laser is used.

It is important for the laser user to understand how the laser interacts with the process. Measuring the amount of laser light, the beam size at the process, and how and why these two important measurements change over time is vital to fully understanding system performance and ensuring more consistent long-term performance.

In-process vs. at-process monitoring

Today, there is a need for as close to real-time data input as possible. This calls for a technique commonly referred to as “in-process monitoring,” which involves monitoring laser performance measurements while the laser is processing. In the additive manufacturing world, this technique is known as “in situ monitoring.”

Another area that is important to understand is “at-process monitoring,” which is performed in between parts as they are processed and measures the laser performance at the plane where the laser is processing. There are important pros and cons that come with each technique (see Fig. 1).

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