FASTLITE DazScope all, Operating Manual

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DazScope
—
manual Chapter 4 : Optimizing the pulse duration using ChirpScan
Pulse bandwidth @800nm Pulse duration Order 2 Order 3 Order 4
10nm 94fs 5 500fs
2
1 000 000fs
3
250 000 000fs
4
20nm 47fs 1 400fs
2
100 000fs
3
17 000 000fs
4
30nm 31fs 600fs
2
36 000fs
3
2 500 000fs
4
40nm 24fs 350fs
2
15 000fs
3
900 000fs
4
50nm 19fs 220fs
2
8 000fs
3
300 000fs
4
60nm 16fs 150fs
2
4 900fs
3
190 000fs
4
Table 4.1: Phase coefficients values that causes a doubling of the (TF limited) pulse duration, as
a function of bandwidth at 800nm for a gaussian spectrum.
Pulse bandwidth @800nm Order 2 (gaussian) Order 2 (flat-top)
10nm 1 000fs
2
7 000fs
2
20nm 250fs
2
1 700fs
2
30nm 100fs
2
750fs
2
40nm 60fs
2
450fs
2
50nm 40fs
2
275fs
2
60nm 30fs
2
185fs
2
Table 4.2: Order of magnitude of negligible phase coefficients values as a function of bandwidth
at 800nm and as a function of the spectral shape.
four phase coefficients as a function of the bandwidth at 800nm. These values correspond to a
pulse lengthening of about a factor of two with respect to the shortest achievable pulse duration
for a gaussian pulse
1
. All pulse durations and bandwidths are FWHM values. For non-gaussian
pulses, the phase coefficient values must be multiplied by a factor depending on the exact spectral
shape. For a supergaussian of order 6, this factor is about 3-4.
Another point of interest is the value of second order for which the change in time domain is
negligible. The second orders corresponding to a broadening of 5% in the FWHM intensity pulse
duration can be found in table
4.2 for both gaussian and supergaussian (flat top) spectra.
When the measured second order derivative of the phase displayed at the end of a chirp scan
lies within a small vertical extent of added second order (this extent strongly depends on the
bandwidth and exact shape of the spectrum as shown in table
4.2 ), the optimization is done and
the pulse is compressed as best as possible.
1
For orders 3 and 4, these values have been minored to take into account the extension of the pulse in the feet.
FASTLITE-9 september 2008 Table of contents 24/ 42