Tsunami - Beckman Institute Laser Resource Center

I I I I I I B I IL 1 1 i 1111 I L I I a,i I I I UUAI 11 I
Pulse Width Measurement
Example 1: Calculating pulse width measured by a Model 409 -08
without pre-compensation.
Assume an 800 nm pulse at the output coupler surface of a Tsunami laser
is 55 fs long and transform limited. It passes through 1.9 cm of fused silica
before exiting the Tsunami, and 0.25 cm of BK-7 glass and 0.26 cm of
fused silica in the Model 409 -08.
Therefore S = 76O(fs2)/(55 f ~ = ) 0.251 ~
Then, looking at our normalized curve (Figure B-5) S = 0.251, and B =
1.22, toat = 1.22 . tin = 67 f ~.
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 dispersion
equal to zero to eliminate all broadening effects. This allows a direct
calculation of the required prism spacing without finding the actual broadening.
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
system and the length for everything but the prism spacing are known.
The length can be calculated by setting total GVD = 0.
Therefore
L = 25.3 cm ( = 10 in.).
Note: the spacing L is the distance between the two tips of a prism in a
double-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.