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SHARP Report Summary

Project ID: HPRI-CT-2001-50037
Country: France

XeF direct femtosecond amplification

A compact photolytical XeF(C-A) amplifier for direct amplification of high-power visible femtosecond laser pulses has been developed.

This device uses a low density gaseous amplifier medium, attractive for ultrafast laser amplification due to its low non-linear index of refraction making possible direct amplification, without pulse stretching, of high-power ultrashort pulses, its high breakdown threshold and scalability to very large volumes. Among all gas laser media, application of the photolytical XeF(C-A) laser for high energy amplification is attractive for the development of ultra-high power laser systems up to the petawatt power level due to the XeF(C-A) broad amplification bandwidth (80nm FWHM centered near 475nm) and a rather high saturation fluence (0.05{-2}), as well as a very low level of Amplified Spontaneous Emission.

In that context, a XeF(C-A) amplifier cavity of 50cm long with a clear aperture of 20 x 4.5cm{2} has been designed and put into operation. The XeF(C-A) active medium, consisting of XeF2/1:N2/37:Ar/730 gas mixture under 1 bar total pressure, is photolytically pumped by two VUV optical planar pumping sources located in parallel oppositely to each other and having the following parameters: a large radiating area (hundreds of cm2) and a high brightness temperature (> 20 000 K) of the discharge plasma, a rapid discharge circuit with submicrosecond light-pulse duration and a low jitter of initiation (<50ns). Measurements of optical characteristics (pump power, gain, ASE, etc.) of the amplifier have been performed.

A pilot experiment to test the capability of XeF(C-A) medium for femtosecond amplification has been realized. A commercial femtosecond laser system delivered linearly-polarized 150fs seed pulses with energy of several microjoules at 480nm to the amplifier cavity. The seed pulse entered the amplifier through a CaF2 window and made a dozen roundtrips inside, reflecting from the mirrors.

The medium was pumped by a microsecond pulse of VUV radiation from a multi-channel sliding discharge consisting of 50 parallel discharge channels simultaneously initiated on the area of 40 x 18 cm{2} along one side of the rectangular amplifier cavity. This pumping configuration corresponds to a one-side pumping scheme (two sides are today operational).

Amplification of the seed pulse energy by a factor of 5 was registered corresponding to a small-signal gain of 0.0016 per cm. The amplified pulse spectrum exhibits narrow-band absorption features but much weaker than for a e-beam pumped medium. This is of prime importance for amplification of femtosecond pulses as any distortion in the amplified spectrum leads to an increase of the final pulse duration.

These weak narrow-band absorption lines, corresponding to the Rydberg series of transitions from the Xe (6s3P0) excited state, saturate at several times smaller energy density than XeF(C-A) one, thus allowing to avoid pulse width distortions during the amplification process. This very particular gas medium also shows a low level of ASE due to its low small signal gain and a relatively long radiative lifetime (100ns).

Moreover, it is important to underline that the pumping used (photodissociation by a VUV radiation) is the most suitable for preserving the spatial and temporal phase of the amplifying beam.

As a conclusion, this makes this photolytically XeF(C-A) amplifier device very promising for obtaining high contrast high power femtosecond pulses. The high contrast is obtained through frequency conversion of Ti:Sa ultrafast laser sources ensuring laser pulse cleaning at low energy and direct amplification in a gaseous medium having a very low non-linear index of refraction and generating a very small ASE pedestal (< 1{-2}).

A further advantage of the laser chain resides in the suppression of the vacuum grating compressor at the end of the laser chain thus eliminating a source of pre-pulses and significant energy loss and greatly reducing complexity and cost of the laser system. This should allow the development of multiterawatt femtosecond laser with > 10^10 temporal contrast. This particular amplifier might then replace or be an alternative to conventional solid-state power amplifiers in multiterawatt and even petawatt ultrafast laboratory facilities.

Due to its emission bandwidth in the visible, this power amplifier shall also provide very high energy ultrafast laser pulses in the blue-green region outclassing the best performances of conventional infra-red frequency-doubled ultrafast lasers in this spectral region.

Applications of XeF(C-A) amplifier should concern upgrade of high energy ultrafast laboratory facilities and related high-field physics experiments concerning time-resolved dense plasma diagnostics, generation of laser plasma and X-ray sources, ignition of photonuclear reactions, particle acceleration or to explore the relativistic regime of the interaction of radiation with matter.

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Laboratory LP3 - Case 917, 163, Avenue de Luminy - 13288 Marseille cedex 9
13288 Marseille
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