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Hi Zdenek,

The refractive index (or vp in the case of a cable) specifies the
group velocity, not the group velocity dispersion (GVD), the latter
being the one that you care about.  You can have a lot of cabling and
amplification in your system so long as you are careful to manage GVD.
For our swept source optical coherence systems, for instance, we
measure femtosecond time of flight differences though tens or even
hundreds of nanoseconds of group delay.  We just very, very carefully
match the group delay between timing and signal arms while minimizing
the GVD.  If you look at some of the really high end transimpedance
amplifiers, they actually spec the group delay vs. frequency for this
reason.

I do something similar for FLIM measurements.

Mike

On Tue, Feb 3, 2015 at 2:01 AM, Zdenek Svindrych <[log in to unmask]> wrote:
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> *****
>
> Hi Peter,
> I tell you 80 MHz is a *high* frequency. What are you aiming at? 1024 x 1024
> pixels at 80 fps? That sounds cool. But how many photons do you think you'll
> get in a pixel? I bet mostly zero. Maybe one, from time to time... So you
> may end up with heavy smoothing anyway. If you get whopping 10^9 photons per
> second in the bright parts of your image, that translates to a SNR of 3.5 :-
> ).
>
> I don't want to put you off, just go ahead and try it (and keep us posted).
> Just remember that every piece of cable needs proper impedance matching
> (termination), also the 'speed of light' is somewhat lower in a cable (20 cm
> / ns), also add some 1 to 5 ns per each comparator or amplifier stage and
> you may end up not knowing which pulse came from which pixel (and of course
> another 3 ns delay due to the lifetime of your fluorescent protein).
>
> Detectors with integrated amplifiers are also available, but it's either low
> speed 'intensity based', with some 6 MHz bandwidth, or fast photon counting
> (and these are currently not able to do 10^9 counts per second). I haven't
> seen anything in between that would be suitable for your project. So
> combining fast detector, fast mplifier and fast digitizer is challenging,
> even if you use commercially available building blocks...
>
> I know I didn't help much, but, good luck!
>
> zdenek
>
>
> --
> Zdenek Svindrych, Ph.D.
> W.M. Keck Center for Cellular Imaging (PLSB 003)
> University of Virginia, Charlottesville, USA
> http://www.kcci.virginia.edu/workshop/index.php
>
>
>
> ---------- Původní zpráva ----------
> Od: Peter Rupprecht <[log in to unmask]>
> Komu: [log in to unmask]
> Datum: 27. 1. 2015 4:41:25
> Předmět: Preamplifier for fast point-scanning
>
> "*****
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> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
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> *****
>
> Dear list,
>
> I'm looking for preamplifiers for use in a resonant scanning 2P microscope,
> using fluorophors like GCaMP or OGB in vivo. Acquisition will be time-locked
> to the 80 MHz laser pulses, so the bandwidth of the PMT preamplifier should
> be at least 80 MHz in order not to smooth adjacent pixels too much.
>
> Right now, I'm using this model, which seems to be used by many others in
> the field (as far as I can see):
>
> http://www.femto.de/en/products/current-amplifiers/variable-gain-up-to-200-
> mhz-dhpca.html
>
> Is there another option which might be better than this standard solution
> for some applications? I'd be glad to hear some opinions on that.
>
> Best regards,
> Peter"