What is the maximum beam size my laser power meter can accommodate?
Monday, June 12, 2023
That question is kind of tricky, mainly because there are many assumptions here. So, let’s try to reframe that by making some precisions about your laser beam itself and the conditions in which measurements are performed.
Measurements conditions
First, we assume here that your beam is reaching the detector surface at normal incidence. So, the cross-section of the beam and the detector surface are both in one single plane. Second, we consider that your beam is perfectly centered in the aperture of the power meter. Third, the power distribution of your beam can vary largely according to its shape and profile. So, let’s assume here that your beam has one of these profiles: Gaussian or flat-top, which are the most common types.
For the flat top beam, the power is theoretically evenly distributed along its cross-section and outside of this section, the power drops to zero. The diameter of the beam is clearly defined. On the other hand, the Gaussian beam mimics the shape of a three-dimensional Gaussian function which has a gradual decrease from high intensity in the center to gradually lower intensity. Therefore, to define a Gaussian beam’s diameter we must rely on some conventions since a Gaussian function does not have a clear edge from which you can measure the beam width. Many definitions exist, but the diameter is commonly defined as the width of the beam at the point where the intensity drops to 1/e² (about 13.5%) of its maximum value. This point is known as the beam waist radius. Accordingly, the diameter is twice that.
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Gaussian profile
If your beam is perfectly centered on the detector and its diameter at 1/e² is the same as the power meter aperture, 13.5% of the power will reside outside the absorber surface, meaning it will not be measured by your meter. When your diameter represents 65% of the aperture diameter, more than 99% of your beam power will be absorbed by your detector. Then, if you drop down to 50% of the aperture size, you will obtain more than 99.9% of your beam power, which is way better than the instrument’s uncertainty.
For a quick and precise calculation of that value tailored to your laser, go check our aperture transmission calculator. Just enter your power meter’s aperture size, your beam diameter, and average power and you’ll get an instant result.
That being said, any beam diameter south of 50% of the aperture size can be considered as fully absorbed by your laser power meter. Be careful though that, by making your beam smaller, you might reach the damage thresholds of your detector by having too much power density there. Always aim to make your beam as large as possible in order to guarantee the safe and reliable use of your meters.
Flat-top profile
Also, Gaussian beams aside, if you take a top-hat-shaped beam, you can see it as a kind of very long cylinder. So, the diameter is quite obvious and it contains roughly 100% of its power. In that scenario, the beam diameter can then be almost as big as the detector aperture. It really depends on the quality of your beam here and how much it behaves like the theoretical model or not. It’s up to you, but for accurate measurements, we recommend that you keep your beam diameter below 90-95% of your aperture size.
