How to Profile a Laser Beam

From slicing solar panels to measuring the distance to the moon --- lasers are asked to do a lot.

Lasers are used for welding, telecommunications, 3-D mapping, semiconductor processing --- the list could go on for pages. Lasers have advantages over alternative methods for doing all these jobs, but they lose part of that advantage if they're misshapen. Each time a laser beam is put through a lens or bounces off a mirror, there's a potential to distort the profile of the beam. The distortions can't be corrected if they can't be measured. Semi-automated beam-profilers are available, but not absolutely necessary --- if you're patient enough to make some careful measurements.

Things You'll Need

• Photodiode or other single-element detector responsive to the laser wavelength
• Small aperture --- a pinhole or a slit
• Movable stage for aperture
• CCD or other multi-element detector responsive to the laser wavelength

Instructions

Scanning Aperture Method

1. 

   Turning the laser on is the easy part. Ensuring beam quality is just a bit harder.

   Place the photodiode in the laser beam. Measure the output power.

2. Place the aperture at the edge of the beam. Record the aperture position and the measured power.

3. Move the aperture horizontally into the edge of the beam. Select the distance based on the width of the beam and the desired resolution. For example, if the beam is 15 mm wide and the profile is to be measured at ten points, move the aperture 1.5 mm per step.

4. Record the aperture position and the measured power. Repeat until the aperture is past the opposite edge of the beam.

5. Using a slit aperture, turn the slit horizontal and perform repeated measurements, moving the slit vertically.

6. Using a pinhole aperture, move the pinhole up one step and perform repeated measurements until the pinhole has been moved in a grid covering the entire beam.

7. Divide each measurement by the total power and plot the measurements. The result is the beam profile.

Multi-Element Detector

1. 

   Multi-element detectors can measure beam profile --- if they're calibrated.

   Fill the detector with a uniformly-illuminated target. Record the output from each detector element or pixel.

2. Increase the illumination on the target and record the output from each element.

3. Calculate the slope of the response of each detector element to different levels of illumination. Record the calibration data.

4. Direct the laser beam to the detector. Adjust the peak intensity to fall within the range of the calibration.

5. Record the image of the laser beam. Correct the measured values using the calibration. Divide each pixel's value by the value of the maximum pixel reading. The result is the laser beam profile.

Tips & Warnings

• If you have the need to profile a laser beam frequently --- or even more than just once --- it will almost certainly be cost-effective to use a purpose-built laser beam profiler.

• The coherent light in a laser beam can easily cause temporary or permanent eye damage. Take care to avoid exposure to the direct or reflected beam.

References

• University of California, San Diego: Current Technology of Laser Beam Profile Measurements
• Ophir-Spiricon: Beam Profiling---A Primer