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The Spitfire Beam Path
Spitfire USF Stretcher and Compressor
The layout of the Spitfire USF is identical to that of the Spitfire F. The only
difference is that the grating ruling density is reduced in the Spitfire USF to
accommodate its shorter pulses (and, therefore, broader spectral bandwidth). The gratings are also set at a different angle to the beam.
Spitfire P Stretcher and Compressor
The narrower spectrum of picosecond pulses (as compared to femtosecond
pulses) requires greater dispersive power from the gratings in order to adequately reduce the peak power of these pulses before they can be safely
amplified. Therefore, the Spitfire P uses gratings with an increased ruling
density, compared to that used on the Spitfire F, and set at a different angle
to the beam.
In addition, the stretcher and compressor incorporate additional folding
mirrors to increase the length of the beam path and give the separated
wavelengths adequate distance to separate and recombine spatially. Figure
7-4 illustrates the stretcher and compressor beam path for picosecond operation.
Note
For clarity, the dispersion of the beam is not shown in the same detail in
Figure 7-4 as in the other stretcher/compressor figures.
Stretcher
Grating
VRR
M2
M3
HRR
Fold
Mirror
VRR
M1
Retro
Mirror
Amplified
Pulse
Compressor
Grating
Figure 7-4: Modifications for the Spitfire P
Spitfire PM Stretcher and Compressor
The picomask version of the Spitfire is identical to the Spitfire F, except
that it uses the same gratings as the picosecond amplifier, and a special
mask aperture is added to the stretcher cavity to reduce the bandwidth of
the femtosecond seed pulses. The Spitfire PM does not require the extra
path length in the stretcher and compressor that is needed by the Spitfire P.
This mask and its position in the stretcher in front of M1 are shown in
Appendix B, “Changing To and From PicoMask Operation,” which provides instructions for converting Spitfire F and Spitfire USF femtosecond
models to and from picomask operation.
7-5