How to measure the beam profile of a very large laser beam
Thursday, September 02, 2021
Laser beams tend to be very small when you want to look at the beam profile at their focal point or when they are collimated. It’s a challenge that we won’t cover here. What we will cover here is what to do when the beam doesn’t fit in your beam profiler sensor.
Typically, a beam profiling camera sensor is about 1 cm width or smaller. Some sensors can even go up to 20mm X 20mm, but beware that the price of the camera increases with its sensor dimension.
What you can do if you want to measure a beam profile that is bigger than the beam profiler's sensor is to work with a camera lens. The sensor on the beam profiler is similar in size and behavior to one you’ll find in a reflex camera. Therefore, if you shoot a laser directly at it you will see the energy distribution of your laser.
If you install a camera lens in front of the sensor, such as our CL-25 camera lens, you will be able to aim the laser at a non-reflective surface and image its profile on the beam profiler. We must avoid a surface that will have a specular reflection such as a mirror. You need a surface that will have a diffuse Lambertian reflection.
If you want a good suggestion for a very resistant Lambertian surface to look at to get your reflected beam profile is a power meter absorber surface. As a bonus you will be able to measure the beam profile and the power in real time on the same computer.
Figure 1: 12 kW fiber laser beam profile measurement setup
Remove the neutral density (ND) filter from the camera and screw the appropriate spacer and camera lens onto the camera’s aperture. Be careful not to let any dust enter the camera. This could damage the CMOS sensor.
See below the beam profile you can achieve in reflection.
As a final point, one of the most typical questions we get from customers is: "Will the angle between the camera and the incident angle from the laser affect the measurement uncertainty?" The answer is: if the angle is small (less than 10%), its impact is very limited and it remains inside the uncertainty coming from the pixels size and the ISO calculation method, so the beam size can be considered as accurate.