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Picosecond Pulse Timing

Discussion in 'General Electronics Discussion' started by fsonnichsen, Jan 10, 2014.

  1. fsonnichsen

    fsonnichsen

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    Oct 23, 2013
    I am working with fluorescence and have interest in measuring the time between a returned light pulse referenced to the firing of a laser. Typical return times are in the pico-second range.

    This is well beyond the reach of the board level electronics that I work in (I top out well above 10ns). I have investigated streak cameras for this purpose but I expect there are some modern circuits, ICs out there that may work here. Anyone have any suggestions?

    Thanks
    Fritz
     
  2. richardb

    richardb

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    Dec 21, 2013
    Measuring timing difference is really hard at anything below a ns because light travels 300mm per ns so @1ps it travels 3mm so all your trace length and cable length light paths need to be known very accurately.

    realistically what resolution are you expecting to get to?

    can't you use a scope?
    what's the rise-time on your detector ? if its slow any amplitude jitter will affect timing.


    what laser are you using?

    Rich

    p.s. even if I can't help (and I probably can't) maybe someone else can with more info.
     
  3. jpanhalt

    jpanhalt

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    Nov 12, 2013
    I assume you are talking about measuring fluorescence half-lives. Many years ago, we used phase to do that. Here is a recent thread where I mention the same thing:
    https://www.electronicspoint.com/dissolved-o2-switch-t266738.html

    Unfortunately the link to Hach is broken, but it was OK when I posted. I will check a little later, and if still broken, fix it. Otherwise, just search on optical dissolved oxygen sensors.

    Picosecond timing can be tough, but it is clearly doable at low cost. Witness the large number of TOF laser distance measuring devices with mm accuracy. I suspect they may also phase or interference based, but am not sure.

    Edit: Let's see if these links work

    http://www.hach.com/powerldo

    http://www.hach.com/asset-get.download.jsa?id=7639984815

    John
     
    Last edited: Jan 10, 2014
  4. fsonnichsen

    fsonnichsen

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    Oct 23, 2013
    Thanks Rich.
    I am using 405nm CW lasers with modulation ports (Laserglow and powertech). I also have used a 1064 NdYag pulsed laser, the latter putting out a 9ns pulse that is pretty clearly cut off.

    Regarding travel time of light and signals thru the system this should not be an issue since this can be calibrated out of the system- the laser sends a pulse at t0, a PMT is opened at t1. The latter can be varied and a fluorescence curve results. Usually there are some "known" peaks in the curve which serve as timing posts for calibration.

    There are a few philosophies that can work here. Try to time your laser and PMT firing pulses. Us a streak cam to continuously monitor progress of the system. Fire random pulses, measure them after the fact and bin the results. One method (TCSPC) relies on counting rarefied photons after the fact and binning them according to time of arrival. This relies on accurate timing of course.

    My 500Ghz scope falls off in the sub nano-second range. Also this device will need to be compacted so a scope wouldn't work past bench top testing. As you mention jitter is one of the big issues that needs to be addressed in this type of thing!

    Thanks for the post
    Fritz
     
  5. fsonnichsen

    fsonnichsen

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    Oct 23, 2013
    John
    I am indeed looking at fluro-half lives. I actually integrated an O2 optode type sensor a few years ago. The electronics was "canned" so I don't know how it was built but I believe phasing was employed. I think this relies on a low background noise and more important-a sharply defined return peak. I will read your paper later today to see if this complies with my understanding. We are looking at various organics that give very broad peaks both in the wavelength domain and the time domain. Also a lot of return noise.

    Your links downloaded fine. Thanks!

    Your premise on the low cost of TOF is a very valid point. I having been casting about the web trying to learn more on how these devices work. As you mention they may rely on phasing.
    Thanks for the valuable comments and the papers-off to the lab for me
    Cheers
    Fritz
     
  6. woodchips

    woodchips

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    Feb 8, 2013
    Interesting. As far as I can find the cheap laser distance measurers just use TOF. Not yet bought one to see how. With an accuracy quoted at 0.5mm then just putting a coin infront of the target should be measurable. If so then pretty amazing.

    Don't forget to consider a sampling scope for sub nanosecond measurements. Unlike a normalnscope they don't suffer the very high gain rolloff over their bandwidth. I have had a nominal 4GHz sampler happily displaying 12GHz, reduced amplitude, but displaying.
     
  7. jpanhalt

    jpanhalt

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    Nov 12, 2013
    I talked directly with Stanley technical and was told it was TOF. It very clearly is not triangulation.

    For a fun experiment, mount the device on a stand vertically above an empty bucket. Measure the distance to the bottom of the bucket. Then fill the bucket with clean water and measure the distance again.

    Did you move the bucket? Which direction?

    John
     
  8. fsonnichsen

    fsonnichsen

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    Oct 23, 2013
    I think the "TOF" phrase includes the chirped phase delay method in its meaning. In that case the absolute return pulse time would not be measured.

    Anyone have any experience with devices like these? For my original intended purpose, tens of picoseconds might work.

    http://www.acam.de/products/time-to-digital-converters
     
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