CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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2. 8. 2. 2. Amplifier
As mentioned earlier femtosecond pulse energies from Ti-S oscillator are too low to
perform transient absorption measurements. Therefore these pulses have to be amplified
several orders of magnitude for experiments. This can be achieved by use of amplifiers.
Because pulse durations are in fs time domain and during amplification peak power of the
pulse increases by orders of magnitude, it can lead to irreparable damage to the optics.
Therefore the fs pulses are chirped and broadened temporally to ~100ps before amplification.
Therefore this technique is called as chirped pulse amplification [2.22]. After amplification
we again compress the pulse to fs time domain. The typical amplifiers generally achieve an
enhancement of five orders of magnitude. A typical layout of the process is given in the
diagram. Stretching the pulse involves use of dispersive elements like prism or gratings to
create group velocity dispersion. A typical schematic of stretcher is given in the figure 2.7.
In the present set up used, the pulse width is ~50nm. On incidence of the femtosecond
pulse on the stretcher red and blue wavelengths are diffracted at different wavelength.
Therefore the path difference between the different wavelengths is used to stretch the pulse.
This creates an effect of stretch in terms of time. The schematic also describes the stretcher
used in present study. The present set up uses a type of stretcher called an Offner stretcher.
The stretcher consists of a Grating and a combination of silver coated concave and convex
mirrors. The focal length of the convex mirror is half of the focal length of concave mirror
and the mirrors are separated by ~30cm. The stretched pulse on hitting the grating is
elongated therefore after stretching the pulse needs to be reconstructed without losing the
chirp introduced by stretcher. The exact arrangement of mirrors allows us to do just that. The
stretch introduces in time is ~300ps after the stretching stage.