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The resonator design is based on a 2.5 m symmetric confocal arrangement with
the spherical end mirrors having a 1.5 meter radius of curvature. The mirrors
have high- power coatings centered at 745 nm with reflectivity greater than 99%.
The cavity utilizes a Medox 700-KDP Pockels Cell and two broad bandwidth thin-film
polarizers (TFP) to achieve switching of the chirped pulse into and out of the
cavity. The gain medium is a 2 cm Brewster-cut Ti:sapphire crystal.
The amplifier is pumped with a 20 ns, 25 mJ (+/- 2 mJ) pulse at 532 nm from frequency
doubled Nd:YAG laser (Continuum NY61-10) and produces an output pulse engery of
2.0 mJ (+/- 0.2 mJ) at 745 nm at a repetition rate of 2 Hz.
Spectral tunability is achieved using the fused prism pair combination with the
spherical mirror at one end of the cavity.
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Fig.3: Ti:Sapphire CPA laser system
Conventional pulse stretchers are comprised of a lens telescope between two antiparallel
gratings. However, for pulse width below 100 femtoseconds, the lenses introduce
such large chromatic aberrations in the beam that it is necessary to use all-reflective
optics.
We use the Ebert-structure design, which allows 70 fs pulses to be stretched
to 300 ps. This design uses only one grating, but due to the high number of reflections
occurring in the stretcher, a high tolerance on the optical components is required.
The oscillator laser beam is incident upon a 1800 lines/mm grating at the Littrow
angle (~42 degrees at 745 nm). The group-velocity dispersion with an effective
grating separation of 36 cm is about 6.2 x 10^6 fs^2.
Fig. 4: Diagram of Ti:Sapphire CPA, pulse compressor and frequency tripler
A traditional compressor uses a grating pair separated by an amount equal to
the stretcher effective grating separation. Our compressor is based on a similar
stretcher design. The final compressed pulse duration is about 100 femtoseconds.
The amplified laser is collimated to a beam size of ~5 mm in diameter, for a
peak intensity of 50 GW/cm^2. The beam is then sent through the two sum-frequency
mixing crystals. Both crystals are KDP. The first is cut with respect to the Type
I frequency mixing at 745 nm, which results in the generation of the second harmonic
of 745 nm at 372.5 nm. The two pulses are then sent into another KDP crystal where
they interact through a Type II sum frequency mixing process. The final output
energy is 25 micro Joules at 248 nm.
After the 248 nm pulse is generated, it propagates through a CaF2 prism pair,
which is used to pre-compensate the GVD in the remainder of the optical train
between the tripler and the final target chamber [4].
References:
1. Compression of optical pulses for chirped pulse amplification, Opt. Comm. 62, 419 (1987).
4. T. Nelson, Ph.D. Thesis, University of Illinois at Chicago 1999.
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