What are the limitations of my laser power meter's damage threshold, and how can I avoid exceeding it?
Tuesday, July 25, 2023
Even though some laser power meters on the market can withstand tremendous amounts of power, like our HP400A-100kW (that can sustain 100kW of continuous power), you can still damage one if you’re not careful.
To do so, yes you could simply throw at it 150 kW of power, which would exceed the 100 kW it was built to sustain and it would eventually reach breaking point. Assuming that this obvious scenario was not the initial motivation that led you to read this article, we’ll explain the physics behind it and go for less obvious ways to damage your meter.
Average power
The first way to damage a power meter is to throw too much average power on it for a prolonged period (like in the previous example). The key factor linking it to damage threshold here relies mostly on the ability of your device to get rid of all the heat that is building up in your device as a result of you firing a laser at it.
So, when you see a high-average power detector, that means this meter is pretty efficient at absorbing and dissipating heat. You could damage it by either throwing too much average power on it or by shooting at it for too long.
Besides, by limiting the duration of each shot, you could reach way higher average power measurements with a well-built detector. That’s exactly what we did with our PRONTO-250-PLUS, a handheld laser power meter that can give very accurate measurements up to 250W in 5 seconds single shot mode.
When used in continuous measurement mode though, that damage threshold drops to 8W. Great product, but drastically different damage thresholds depending on how you intend to use it, so keep that in mind.
Power Density
If you want to know more about the depths of power density and how to measure it, go check out this previous article we made that explains how to properly calculate it.
But for a quick answer, power density refers to the average power divided by the beam size, so, in simple terms, how concentrated is that average power, or, the other way around, how big is the surface on which your laser power will be spread. Ultimately, how much power is there in each cm2 where your beam hits the detector surface.
If it’s too high, despite our relentless efforts to make indestructible power meters, your laser will start to cut through the detector, just like it would cut a thick piece of metal in some applications. Considering you want to measure the exact power of your laser, turning the power down is not an option. The choice is quite simple then, to avoid damage, just make the beam bigger.
How to do it you may ask? Because non-diverging laser beams are not of this world (yet!), you can easily make the beam bigger on the detector surface by taking your power measurements further away from the focal plane. Therefore, diluting the power density of your beam.
If you don’t want to simply guess what the distance from the laser source should be to have an adequate beam size, our spot size & beam waist calculator will do the trick! For scenarios that don’t involve lenses or for very long distances, use our beam divergence & diameter calculator instead.
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Energy density
This way of damaging a power meter is only related to pulsed lasers. The damage threshold can be busted here because, despite having an acceptable average power, a pulsed laser delivers that power not in a continuous manner but in a series of pulses, so in a limited time.
When you consider pulses width ranging in the femtoseconds, for example, during these short pulses, the power seen by the detector is way higher than the laser's average power. The local heat built-up can literally vaporize matter on the detector surface, blasting off parts of its coating at each pulse.
Pulse width
When using ultra-short pulses like those mentioned above, you are more likely to reach what we call an electrical breakdown, so the amplitude of the incoming electric field of your laser pulse is so high that electrons from your detector surface will jump into the conduction band, making it behave like a conductive metal.
It could get even further and you could witness multiphoton ionization. That being said, for those of you that didn’t understand a single word I just said, let’s keep our cool here and, simply remember that by shortening pulse width you’ll reach the damage threshold way quicker.
To avoid damage, you could try to stretch the pulse or lower its energy, but in both cases, you would most likely affect the power output of your laser, so it’s not an attractive option considering you’re trying to measure just that, its power! The best solution remains the same as when you have too much power density… simply make the beam bigger.
Wavelength Absorptivity
Wavelength relates to fundamental interactions between each photon emitted by your laser and the absorber material. Each different material absorption spectrum varies, so some may be more sensitive to visible light and others to infrared, etc. So, if a material is a really good absorbent for your laser main wavelength, the heat built-up will be quicker than if it’s not.
At Gentec-EO, our devices are calibrated to account for that, therefore keeping accurate measurements for different wavelengths. We also do try to make this absorption shift as linear and flat as possible. Still, this does mean that at a specific wavelength, it could be safe to measure your laser continuously, but at a more absorbent one, it might not be.
Final note
It might be obvious for some of you, but remember that, as you would with any other type of optics, the absorber surface should not be touched directly. This might put some contaminant on it and therefore interact with the incoming laser beam. This could affect either accuracy of measurement or damage threshold. But life being what it is, if you happen to touch it with your “not so clean fingers” or simply drop something on it, here is how you can properly clean it!
