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Hi Mike, there's also the issue of pulse-to-pulse stability. The variations
in peak energy tend to average out when you have many pulses. My 1 MHz rep
rate system performs more evenly at 2 pulses per pixel than 1 pulse, but
the entire source is optimized for pulse-to-pulse stability. A regular
Ti:Saph laser may not have this stabilization, and at 20 or more pulses per
pixel would not need it as the effect averages out very nicely.

Craig


On Sun, Jan 11, 2015 at 2:49 PM, Michael Giacomelli <[log in to unmask]> wrote:

> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> Post images on http://www.imgur.com and include the link in your posting.
> *****
>
> >did you synchronize the pixel clock with the laser repetition rate? I
> guess
> you did, but if not, then some pixels receive the dose of two pulses and
> others just single pulse. This will create some (non-random) pattern that
> degrades your image.
>
> Yes, I sampled at 240 MHz phase-locked to the Ti:Sapphire using a 3x
> multiplier.  However, the comparison wasn't really scientific, and
> were complicated by the fact that I was using a resonance scanner and
> then dewarping the images.   I haven't had a chance to really look
> into it, so it is quite possible that there was some other problem.
>
> Mike
>
> On Sun, Jan 11, 2015 at 4:24 PM, Zdenek Svindrych <[log in to unmask]>
> wrote:
> > *****
> > To join, leave or search the confocal microscopy listserv, go to:
> > http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> > Post images on http://www.imgur.com and include the link in your
> posting.
> > *****
> >
> > Hi Mike,
> >>images taken at low numbers of pulses per pixel (1-2) tend to be fairly
> > grainy
> >
> > did you synchronize the pixel clock with the laser repetition rate? I
> guess
> > you did, but if not, then some pixels receive the dose of two pulses and
> > others just single pulse. This will create some (non-random) pattern that
> > degrades your image.
> >
> > It's generally not an issue when the number of pulses per pixels is
> high, e.
> > g. 80 MHz Ti:S and 1 us pixel dwell time gives some 80 +/- 1 pulses per
> > pixel (when not synchronized), limiting the dynamic range to some 40 dB.
> > Given the SNR of typical two-photon image is much lower than that, the
> > beating between the laser clock and the pixel clock can't be observed.
> > 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: Michael Giacomelli <[log in to unmask]>
> > Komu: [log in to unmask]
> > Datum: 11. 1. 2015 15:28:03
> > Předmět: Re: 2p dead time
> >
> > "*****
> > To join, leave or search the confocal microscopy listserv, go to:
> > http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> > Post images on http://www.imgur.com and include the link in your
> posting.
> > *****
> >
> > Hi Guy,
> >
> > This is something I've been meaning to test by rep rate
> > doubling/quadrupling our Chameleon and doing SNR measurements. Its
> > hard to say from a theoretical point of view which would be better.
> > Certainly higher rep rate gives you lower multiphoton absorption
> > probability per unit power, but you don't make as effective use of the
> > detector dynamic range. Another thing on my mind is that my
> > (admittedly unscientific) impression is that images taken at low
> > numbers of pulses per pixel (1-2) tend to be fairly grainy no matter
> > how high the illumination power, although I've never been entirely
> > clear what that mechanism would be or if its somehow specific to my
> > samples.
> >
> > Mike
> >
> > "
> >
>