Picosecond Signal Timing

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

  1. fsonnichsen

    Thread Starter New Member

    Jun 6, 2013
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    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. WBahn

    Moderator

    Mar 31, 2012
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    Light travels about 300μm in 1ps. And that is in a vacuum. In electronic circuits you are generally talking about something more like 100μm.

    Some form of ECL (emitter-coupled logic) has always been the fastest logic and I don't know whether they can even achieve internal inverter gate delays of 1ps. You'd be talking about devices that can operate at frequencies in excess of 500 GHz.
     
  3. fsonnichsen

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    Jun 6, 2013
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    Duly noted. I am clearly foraying into the dark realm of high speed pulse timing and I am sure there are some coveted trade secrets here. Phasing is sometimes used for this purpose but I expect there are other methods. I have on my bench, a pulse timer that works down to about 1ns for example--a commercial device by Berkley Electronics. Hopefully I can find a few papers or text that discusses this topic. I am expecting something using delay cables based upon length for example may be used here.

    thanks!
    Fritz
     
  4. Stuntman

    Active Member

    Mar 28, 2011
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    Interesting post. I can tell you that I have encountered a somewhat similar task in dealing with timing issues for laser microscopy.

    From what I've found, ps resolution is really past the point of even modern descrete IC's. At this point, the gate capacitance of FETs and general propagation delays can trump what you are trying to measure.

    It seems to me, as I found in my situation, is you really need some kind of PLL, or chirped signal to be able to time with this kind of resolution.

    My point, you may be looking at purchasing a (pricey) complete system exactly for this purpose, not an IC you can slap on a PCB to do what you want.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Could you use an optical interference effect by combining the returned light with some reference?
     
  6. fsonnichsen

    Thread Starter New Member

    Jun 6, 2013
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    Alec-yes--
    I think as Stunman indicates some type of chirped signal in in order here. I believe this is how low cost distance measuring lasers work-a type of lidar basically.
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Pico second is easy - you just need the right equipment. Commercially available and probably sitting on the back shelf of Ahmed Zewail's lab at Cal Tech. He won the Nobel Prize in chemistry in 1999 for FEMTO second spectroscopy of fast chemical transformations. Note, this means his major work was at least 3 years before that.

    Read his old papers. There is plenty of equipment out there, new and used. Also, the CCD detectors and amplifiers cost $300k to $1M for a reason (those were 1990 prices for pico-second parts. They are not just made of a single fast op amps you solder to a pcb. As pulses become that short, the energy per pulse goes way down and amplification techniques become the second major problem. Then there is noise, focusing mirrors, need for higher laser power per pulse, ...

    http://en.wikipedia.org/wiki/Ahmed_Zewail
     
  8. fsonnichsen

    Thread Starter New Member

    Jun 6, 2013
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    I'll pan a few papers by Zewail but the papers I have looked at so far aren't helpful. I find that most journals do not get into the electronic details regarding methodology. Engineers still communicate knowledge much as the Greek Scholars did 2400 years ago-word of mouth while strolling the streets.

    I don't know that pico sec is really easy-at least with conventional electronic methods that I use. I have some high speed ICCDs here but I am using a PMT and filter technique for my work-pretty conventional in fluorescence -spec and easy to work with. Focusing is not important here-we have plenty of signal-and I am investigating supplanting my lasers with LEDs. So-the burden will revolve around difficulties of signal timing. What I have seen of time-domain spec is leading me to the conclusion that it is not simply a matter of "clocking" pulses. Physics must be invoked and it is not trivial to implement as you imply. The need for methods like TCSPC bear this out. Most of the TD work that I have seen is partially user implemented-e.g. still in the research mode. HORIBA makes a turn key unit I think and they have one for $30K, but we always need to rebuild or scratch build our devices to accommodate spec and power constraints.

    Thanks for the reply
    fritz
     
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Look up some stuff from Gerald Babcock, Nick Turro at Columbia and Harry Gray from cal tech from the early to mid-1990s. It is not all listed in every paper but they surely reference their own paper where a complete setup is described. If I remember correctly, they all had commercial pieces that could be referenced by brand and model number that were connected in an easily- described layout. The only thing custom was the interfaces for data collection but that is trivial now days.

    As per "easy", I meant that boxes can be purchased and bolted together. The "physics" portion of the electronics is done in the black boxes for you. I don't know of any chemists that are making board-level designs of pico-second equipment.
     
  10. fsonnichsen

    Thread Starter New Member

    Jun 6, 2013
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    Yes-your points well made. Thanks for the tips on the authors. I'll look them up.
    fs
     
  11. GopherT

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    Nov 23, 2012
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    I just looked you up and see your location on the cape. Call Dan Nocera at Harvard. He has all of this equipment set up and one of his grad students will likely run some samples for you. He likes collaborations. He used to collaborate with Babcock when he was at Michigan State. Really nice guy who answers his own phone. Big into sensors, spectroscopy and ...
     
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