When you want to profile your laser, you’ll notice that CCD cameras are generally way too sensitive to use as is.
I mean, try directing your laser at an unprotected CCD and see what happens.
Umm, but when your boss asks you what you were thinking, please don’t blame me. Tell him… you were performing an important experiment to test the camera’s damage threshold.
Ok. So now we know NOT to profile a laser beam without proper attenuation. There are generally 3 parts to the attenuation process, although you won’t necessarily need them all:
- ND filters (almost always necessary, barring very low power lasers)
- Beam splitters (for really powerful lasers)
- Beam expanders and reducers (for small or large beams, respectively)
Make sure you place these items in the proper order.
Oh, you’re wondering what that is?
I’ll tell you.
The first thing the laser should touch are the beam splitters since they allow most of the beam to pass through and reflect only a small percentage of the beam into the beam profiler for analysis. This means the laser’s effect on the beam profiling system is a much smaller power density than the original laser.
Next, use your expander or reducer (if applicable), which usually has a damage threshold below that of the beam splitter, but higher than an ND filter.
The last line of defense before your CCD camera is your ND filters. These filters are usually spec’d to 50 W/cm2. However, we don’t recommend using them beyond 5 W/cm2 since thermal lensing can be an issue and I want you to get the most accurate beam measurements possible.
Also, not all filters are created equal. Use the weakest filters first (since they absorb over a larger volume) and the strongest last, or closest to the beam profiler.
Summing it Up
The order of attenuation should always look like this. (If you aren’t using something, just remove it from the list; the rest of the attenuators will keep the same order.)
In the order that the laser hits them:
- Beam Splitters (one or more)
- Beam Expander/Reducer (please don’t use both, that’s just silly)
- Low ND filters
- High ND filters
- CCD Beam Profiler