Answers to webinar questions: How to measure energy in any laser system?
Tuesday, November 24, 2020
On November 11, our International Sales Director, Charles Dumas, presented a webinar titled From photons to nuclear fusion: How to measure energy in any laser system?
We want to say a special thank you to everyone who attended and also everyone who watched the replay available here. Many good questions were asked during the webinar and we wanted to answer all of them.
Terahertz detectors are based on pyroelectric sensor technology, just like the energy detectors we discussed in this presentation, and as such, it will only correctly measure either a pulsed or chopped CW signal. This could explain the OVR message that you are seeing. If the issue persists, please do not hesitate to contact our support team.
An energy detector will measure energy per pulse directly but may also calculate average power indirectly. Average power appears in the ''Statistics'' display of the meter to which the energy detector is connected (if the meter supports this).
This average power measurement is not as accurate as the one you would get from an actual power detector because it is indirectly calculated using average energy per pulse and repetition rate. Still, this is a great way to get both measurements at the same time.
It would be possible to measure the ASE energy level if the off time between 2 pulses is longer than the fastest pyroelectric detector rise time. It’s also possible to set a trigger on the energy meter so the pyroelectric detector will only measure the ASE pulse that’s matched with the trigger. To confirm this is possible, we need to know the full laser specs with the expected off time between the real pulse and the ASE.
There should be no difference between the measurement at the focal point and before/after the focal point: only the energy density is changing because the beam is smaller at the focal point than elsewhere. Speaking of which, you should never make a measurement of a beam at its focal point because its power/energy density can be so high that it will damage the detector. On top of that, the measurement will be the same whether you are at the focal point or not.
To help visualize this, imagine the situation where you try to burn an ant using a magnifying lens. When the light crosses the lens, the actual amount of light is the same whether you are at the focal plane or right next to the lens. However, the light is more ''dense'' at the focal plane because the beam is smaller. This illustrates how measuring at the focal plane can cause damage (or, burn the ant), but the measurement will be the same at another plane than the focal plane.
One final thing to note is that your lens, in this example, would absorb part of the incoming light, so it would make sense to characterize this loss before making the measurement. But again, there should not be any measurement difference between being at the focal plane and not being at the focal plane.
The best way to increase the beam size is by simply measuring it at another plane than the focal plane. In other words, place the detector at another position along the beam path where the beam is larger, like before the converging lens. For example, some of our customers using laser welders will remove the integrated lens when doing measurements, or they keep it but instead lower the working surface at a much lower (or higher) position than normal, so that the beam is larger there.
Another good way is by using a convex lens along the beam path, which will increase beam size. There are also beam expanders out there, which are specifically made to expand beams, quite simply.
Again, we recommend to characterize the power/energy loss coming from such a lens before making your measurements. You can determine what will be the correction factor to apply to your measurements at a low energy level and you measure it with and without the lenses. From the difference measured, you’ll be able to input a correction factor in the display/interface.
Feel free to contact us and share about your needs, we’re happy to help you with your lasers!
Tell us as much as you can about your laser specs and we will make sure you use the right measurement device for safe and calibrated laser power, energy, profile measurements in your operations.
Charles Dumas is Gentec-EO's International Sales Director. With a strong technical background in engineering physics, he has worked in the photonics industry for the past 15 years.
He’s been in different positions, but mostly sitting in a plane while either pre-sales or post-sales, from technical support to direct sales presentations covering Americas, Europe, Asia, Africa, Middle-East and everywhere in between.
Now with the slowdown in flying, he has reunited with the "in front of webcam" position.