How can I measure the power of a pulsed laser with a low repetition rate?
Friday, August 11, 2023
In fact, this is a problem that some of you may not have even imagined existed. Without knowing it, you might just have a laser/detector combination for which it’s actually not a problem at all.
It really depends on three things: how slow is a “low repetition rate”, how long is the pulse, and what technology your power meter relies on to make accurate measurements.
On the market, you’ll come across various technology options. These tend to be linked to a certain power range so you’ll realize that some applications might be more susceptible to face measurement problems related to low repetition rates than others.
Energy meter or power meter?
This can be confusing since one is just the average of the other one over a second. But there are actually two types of detectors, some specifically designed to measure the energy of your laser, pulse-to-pulse, and others to average it out to power.
Naturally, talking about energy detectors won’t make any sense when continuous wave lasers are involved, but for pulsed ones, both types of devices are useful. If you’re working with very energetic pulses and very low repetition rates, you might not have the choice to go with energy meters, but when possible, go with power meters, they’re usually easier to use and less expensive.
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Rise time and pulse width
Each technology usually displays a typical range of rise time. This is intrinsically intertwined with the responsiveness of a device. For those not familiar with it, rise time is the delay a detector takes to get its response from baseline to peak (actual measurement).
The fall time is usually the same but the other way around. It’s the delay needed for its response to come back to baseline from the peak. We published an article in the past that also cover this, so check it here if you want more details on that.
Imagine throwing a single instantaneous pulse at a power meter. What you’ll then see is a power reading that goes from zero to a specific value and then back to zero. This whole process would take, temporally, the sum of the rise and the fall time.
Now, let’s say you throw multiple pulses at it during that delay, if you throw enough of them, the response of the detector would look as if the signal was continuous and cancel the rise and fall effect you would otherwise see from one pulse to the next. “10” is the magic number here. 10 pulses thrown at our power meter inside the rise time would generate a stable enough response to give accurate power measurements. On top of that, to maximize the stability of your readings, we recommend users turn off “anticipation” and add a moving average.
We know that a pulse is not instantaneous, but as long as you can fit 10 of them while the detector response is still on the rise, your readings will remain accurate.
This leads us to the subject at hand, how low a repetition rate can be to measure laser power.
Photodetectors – Low power
Photodetectors tend to have a really low rise time. That’s something you might actually appreciate because it means the device is very responsive and will give you accurate power readings in a matter of milliseconds. When you’re trying to measure power from a low repetition rate laser though…not so much! In that case then, if your laser output pulses at a few hertz, you would be better off using an energy meter and doing the math yourself to convert it to power.
Thermal detectors – Low to mid-power range
This is a totally different playing field. Even with fast thermal detectors, when anticipation is turned off, you can expect a rise time that is counted in seconds. The same rule of thumb applies here, 10 pulses need to fit in the rise time. This means repetition rates can be quite low and pulses can be long too.
Water calorimeter – High power
Water calorimeters are in a different category than thermal detectors since they measure the increase of water temperature to give power measurements. They are generally slow, meaning it takes multiple seconds to get a reading. Repetition rates can get as low as one pulse every couple of seconds and still give accurate and stable measurements.
That being said, if you’re firing one pulse every couple of seconds or maybe even minutes, it might be a better idea to look at the energy in each pulse instead of averaging them out to power.
How about an energy meter?
When using a laser energy meter instead of a power meter, there are still some rules that apply regarding rise time. No matter how slow your rep rates are, the pulse width needs to fit within the rise time. Meaning that the total energy of a pulse must be transferred to your detector while its response is still on the rise.
Final note
Keep in mind that, although your detector may have a specific rise time, you cannot eliminate the effect of the electronics it’s connected to. It’s usually the slowest of both that will set the adequate rise time to consider. It’s counterintuitive but, when using very low repetition rates, the less responsive your detector is, the better. In case of doubt or if you simply forget, don’t worry, look at your 2 hands and count, 10 is the number!
