Pulse Width Measurement
B-9
Example 2: Calculating the prism spacing necessary for pre-compensating
the Model 409-08.
Since dispersion is additive, it is only necessary to make the total disper-
sion equal to zero to eliminate all broadening effects. This allows a direct
calculation of the required prism spacing without finding the actual broad-
ening.
Again, start with a 55 fs transform-limited, 800 nm pulse going through
2.16 cm of fused silica and 0.25 cm of BK-7. Also assume the use of an SF-
10
prism-pair pre-compensator where the beam passes through a total of
2 mm of prism tip per pass, or 8 mm total. The
GVD for all parts of the sys-
tem and the length for everything but the prism spacing are known. The
length can be calculated by setting total GVD = 0.
[5]
= 300 ·2.16 + 450 ·0.25 + 0.8 ·1590 +
L ·(-80.2) = 0
Therefore
L = 25.3 cm (10 in.).
Note: the spacing L is the distance between the two tips of a prism in a dou-
ble-pass configuration, or the distance between the two tips in one leg of a
four-prism sequence.
The calculated L is shorter than recommended above, but since the material
dispersion value of SF-10 prisms is so high, sliding just a bit more glass in
will add a large amount of positive GVD, thereby balancing out the prism
spacing.
CW Breakthrough
Under certain circumstances, it is possible for a mode-locked Tsunami
laser to exhibit a small continuous wave (CW) component along with the
pulsed output. This phenomenon is generally restricted to systems operat-
ing in the fs regime where intracavity prisms are used for dispersion com-
pensation. It usually occurs when either the input pump power is too high
or the intracavity dispersion compensation is incorrect. It can easily be
removed by reducing the ion laser pump power or by adjusting the prism
dispersion compensation control on the Tsunami laser. There appears to be
no analogous dual mode operation in the ps regime (where a GTI is used
for dispersion compensation). Too high an input power in a ps system usu-
ally results in satellite temporal pulses.
Because of the dynamic range limitation of photodetectors, a direct mea-
surement of the CW background for a fs mode-locked Tsunami is impossi-
ble. For a mode-locked repetition rate of 100 MHz (10
5
duty cycle), with
1% of the energy in a CW component, this amounts to a base line signal 10
7
times smaller than the peak pulse height. In order to detect this CW signal
level directly, a detector with a resolution of 100 fs and dynamic range of
>10
7
is required. A successful measurement can be made if we take into
account the fact that the CW component has a very different bandwidth
than that of a sub 100 fs pulse. It is possible to deter mine a CW component
D
ω tot()
L
ω tot()
D
ω 1()
L
ω 1()
D
ω 2 )(
L
ω 2()
D
ω 3()
L
ω 3()
D
ω 4()
L
ω 4()
0=+++=
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