Analyzing Small, High Power Beams at Focus in Manufacturing Applications

Author: 
Derrick Peterman, Northern California Sales Manager, Ophir

Lasers are increasingly deployed in manufacturing applications, being pushed to higher and higher output powers and smaller and smaller beam sizes to work faster and with higher precision. Determining the beam profile at the beam focus of these lasers is critical to verify and improve manufacturing product quality. However, there are enormous challenges that must be overcome to capture the beam profiling data.

Consider profiling a 150 micron beam at focus from a 4 kW laser. The power per unit area at focus is a few megawatts (MW) per square centimeter. A CCD sensor saturates at ~1 microWatt per square centimeter. That means profiling a 150 micron from a 4 kW laser requires allowing only a few ten trillionths of the beam power to reach the bare CCD sensor inside the profiler, which is achieved through beam attenuation optics. Of course, the optics must not distort the beam.

Anti-reflection coatings used in standard beam attenuation optics have damage thresholds in the 100 kW per square centimeter range, so they are susceptible to damage in these applications. Finally, in order to determine the beam size at optical focus, the optical path of the beam through the attenuation to the CCD sensor must be known with high precision to ensure the beam profiler measurement plane is at the beam focus.

For these reasons, reliable profiles at the focus of high power lasers are elusive and difficult to obtain. Often, the beam is profiled off-focus where it is larger and easier to attenuate and capture. This provides some useful information about the laser system, but the focus is where the laser beam cutting, drilling, or welding actually takes place and can only be inferred through assumptions and guess work. Until now.

Ophir’s LBS-300s attenuator overcomes the challenges of profiling high power lasers at focus. It uses optical wedges set at right angles to preserve the beam polarity and attenuate the beam four (4) orders of magnitude. Additional absorptive attenuators are used to further reduce the beam power so it can be profiled at focus without saturation of the CCD beam profiler sensor. The optical path the beam travels from the top of the attenuation assembly to the CCD sensor is determined to an accuracy of ±50 microns using an optical non-contact displacement profiler developed in-house by Ophir.

The result is a beam profiler system that easily measures high power beam at focus in a compact package. This system was used to capture a 4 kW fiber laser focused to a spot size of 150 microns (see Figure 1). The system measures beams as small as 37 microns.

The beam profile at focus, where the laser interacts with the material in a critical laser based manufacturing process, no longer needs to be guessed at. Instead, it can now be measured.

Surface Contamination
Figure 1. Beam profile of a 4 kW fiber laser focused to a spot size of 150 microns.

Surface Contamination

Figure 2. Ophir LBP-300s attenuator

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