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.
WORKING WITH A CAMERA LENS
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 the Gentec-EO 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
Regarding the camera view, you must select from two models available: 25mm focal length and 50mm focal length. The differences are field of view (FOV) and minimum working distance.
MEASUREMENT PROCEDURE
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.
- Setup the camera on a translation stage and put the camera at minimum working distance and as close as possible to the propagation axis (almost 0° angle) in order to minimize distortions.
- The translation stage is used to move the camera perpendicular to the beam propagation axis, which is used for the pixel multiplication factor in the calibration procedure.
- Connect the camera to the computer and start PC-Beamage.
- The first adjustment wheel on the camera lens (closest to the camera) controls the iris (aperture), which changes the amount of light entering the lens. Start with the iris as small as possible (moved toward ‘’C’’ on the camera lens).
- The second adjustment is the focus with the second wheel. To do so, first put a business card or something with text on it directly on the power meter’s absorber, exactly where the image of the beam would be if the laser was ON. Then turn the wheel until you clearly see the text in the 2D display of PC-Beamage.
- REMOVE THE BUSINESS CARD.
- In the PC-Beamage software, select the “Flip Horizontally” option in the Setup tab to compensate the camera lens’ inversion.
- Perform a background subtraction to only view the laser. This is important because the camera lens will image the laser, but also the entire scene.
- Turn the laser ON.
- Follow the pixel multiplication factor calibration procedure in the Camera Lens tab, which can be found in the Advanced tab. The X axis is perpendicular to the beam propagation axis.
- It’s very important to be meticulous which each adjustment step because they have an impact on accuracy.
- Turn on the Despeckle filter.
- You can now make your measurements.
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.
From the Gentec-EO blog.
