What is the minimum pulse width of a pulsed laser when used with an energy detector?
Monday, March 29, 2021
A question that often comes up at Gentec-EO is about the minimum pulse width that a pulsed laser should have to be properly measured by one of our energy detectors. Our specs sheets almost always show a maximum pulse width, but what about the minimum pulse width, eh?
Well, the answer is actually quite simple:
There is no minimum pulse width with any Gentec-EO energy detector. There is no exception to that statement.
Now, we could end the discussion right here, but that would be a little bit lame. Keep on reading to find out why exactly there’s no minimum pulse width with our detectors.
The reason why there is no minimum pulse width
To better answer this question, it would be best to first understand where the assumption that there is a minimum pulse width with our detectors comes from. This is however difficult to know for sure, but if we were to bet, we believe that it comes from an intuition that if the pulse is too short, the detector won’t ‘’have the time to react’’, if you will, and therefore, that it wouldn’t generate a proper signal (i.e. measurement).
If that is the case, then that would be an incorrect way to see how our detectors work. You should instead see the pulse and the detector as a classical thermodynamic system, where no energy is destroyed or created. Take, for example, a scenario where you first fire a pulse of 1 fs, which is pretty short (10E-15), and then later fire another one of 1 µs (10E-6). Imagine the energy in both pulses is exactly the same, which you achieve thanks to…I don’t know, your deep knowledge in photonic wizardry, for example.
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Now, it doesn’t matter whether the energy that is absorbed occurs in a femtosecond (10E-15) or a microsecond (10E-6), because in both cases, the same amount of energy has been absorbed by the detector, and therefore, in both cases, the same voltage response from the detector will be generated. The ‘’shape’’ of the response signal would be exactly the same. This is coming from the fact that the response time of the detector is much longer than the pulse width itself, and therefore, the pulse will be fully absorbed before the detector response ends.
Both lasers have the same energy per pulse (y axis), yet because the pulse width (x axis) is shorter than the response time of the detector in both cases, the same response signal is generated.
But then, you might have another question after reading this: what happens if the pulse width of the laser is actually longer than the response time of the detector? We won’t go as deep into the details here, but basically, the detector would not generate a proper response signal, and therefore the measurement would be erroneous. This is because, in simple terms, the pulse is still "ongoing" or "being measured", while the detector response is trying to go down.
This is why there is actually a maximum pulse width, and no minimum pulse width, with Gentec-EO energy detectors!
Note that the above reasoning also applies to power detectors used in single-shot energy mode.
The laser here has a pulse width that is too long compared with the response time of the detector. As the detector reaches its maximum expected signal output, the pulse is still occurring and being measured. The end result of all this is a measurement that will appear as lower than expected.
Take-home message about the question of minimum pulse width
The bottom line is that if you have a pulsed laser, you should check whether its pulse width is shorter than the maximum pulse width that appears in our specs sheets. If it is, you’re in the clear. If it isn’t, you may need a special energy detector tuned to handle longer pulses.
To wrap this up, here’s a general rule of thumb: if you have a laser with nanosecond pulses or shorter, you shouldn’t worry at all about minimum or maximum pulse width, because first, there is no minimum, like we just established, and then the maximum pulse width is generally always 1 µs or longer on most of our energy detectors.
