User`s manual

5
2. PRINCIPLES OF OPERATION
Femtosecond Pulse Stretcher
Temporal pulse stretching is required in avoiding the effects of
peak power damage in high energy ultrafast amplifiers. This
peak power damage is due to the tendency of bright beams to
self focusing (a result of non-linearity in the index of refraction),
which makes it necessary to limit the intensity present in
amplifiers. The technique of chirped pulse amplification (CPA)
gives a possibility to avoid this obstacle. The idea of CPA is to
stretch femtosecond pulse duration reducing peak power before
amplification and to compress pulse duration back to
femtoseconds after the amplification. A principal scheme of
femtosecond pulse stretcher is shown in the Figure 2. This is
purely optical device containing diffraction grating, spherical
mirror and plane mirrors. Femtosecond pulse going into pulse
stretcher has a broad bandwidth. For a 100 fs Gaussian pulse the
corresponding bandwidth is about 9 nm. A diffraction grating
sends different frequencies in different directions at different
angles of diffraction shown in the Figure 2 for long wavelength
(shown as red) and short wavelength (shown as blue) spectral
components of femtosecond pulse. After double pass, bluer and
redder components exit the stretcher as shown in the Figure 2.
One can see from the figure that bluer frequency components
have to travel further through the stretcher than the redder
frequency components. The result is that the redder frequency
components exit the stretcher first, the pulse has been stretched.
In the Wedge 50 pulse stretcher the input pulse is dispersed in
the horizontal plane. The stretched pulse is directed back to the
stretcher with help of vertical retroreflector, and four passes
through the stretcher are achieved. Four-pass configuration is
necessary to ensure that the stretched beam is spatially
reconstructed. Femtosecond pulses with pulse duration 100 fs
are stretched to more than ten picoseconds pulses before
amplification. High reflective gold coated holographic grating
gives stretcher efficiency higher than 50% for specific
wavelength regimes.