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> Lowpass Filter For Ultrashort Pulses
migueloreles
Posted: June 08, 2010 03:45 pm
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I'm trying to read laser pulses from a photodetector. the problem is, the pulses are quite short (we use a fast detector, so each pulse is seen as around 2ns width, a few volts in amplitude when using 50ohm input on a scope) but they're at 1kHz rate. I want to read them with a slow scope, so i want each pulse to be broadened to about 1ms. At first i tried to just use an RC filter, problem is that it rises up very fast but decays too slowly, so either it doesn't filter smoothly enough, or i can't see shot-to-shot variations because it's filtering too much. So i tried to make a 6-order butterworth filter using 3 opamps, with a cutoff freq of about 500Hz. It works well if i input pulses with a few microseconds, but not really with the nano-second pulses. Could it be mismatched impedances? Any cleverer idea to solve this problem?
thank you for any help,

miguel
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Dr Wattson
Posted: June 08, 2010 04:42 pm
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How about using a Pulse Stretcher?
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tekwiz
Posted: June 08, 2010 05:17 pm
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Raise your cutoff frequency. It's the pulse length that counts, not the repetition rate.


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migueloreles
Posted: June 08, 2010 06:16 pm
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thanks both of you!
the idea is to make my original pulse become roughly as large as the separation between them. I still want to see the laser intensity varying on a shot-to-shot basis. So the separation between pulses dictates how much i'll have to enlarge each pulse, so in this case i want to make each pulse roughly 1ms.
The pulse stretcher seems a good idea but i'm a bit concerned on how it will work in my case, that is, could it be used to stretch the pulse until the next pulse is detected? And the pulses will be varying a lot in amplitude, so since i will have to set a voltage level for the comparator, i'm afraid it might either skip pulses or trigger on noise. if i manage to get one of those i'll try it anyway!
thanks again,

miguel
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tekwiz
Posted: June 08, 2010 06:33 pm
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A pulse stretcher can be set for whatever output pulse length you want.
However, all output pulses will be of the same amplitude, regardless of the input level.


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For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

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Dr Wattson
Posted: June 08, 2010 10:45 pm
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QUOTE (migueloreles @ June 08, 2010 06:16 pm)
...I still want to see the laser intensity varying on a shot-to-shot basis...

Hmm, as tekwiz mentioned a pulse stretcher has fixed output amplitude, perhaps if you can find (or design) a super fast sample and hold device (circuit) it would be better suited to your application. This would allow amplitude variations to be captured and could also stretch the pulse. The downside would be that you would no longer have a pulse train but a stepped waveform where the amplitude of each step would start off equal to the corresponding pulse.
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CoulombMagician
Posted: June 10, 2010 06:43 pm
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Search ultrafast peak detector. It sounds like you want the peak amplitude but pulse wdith, pulse area, etc. are unimportant.
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