Suppression of gain narrowing with a combination of negative and positive CPA

ABSTRACT A laser scheme implementing a combination of negatively- and positively-chirped pulse amplification (NPCPA) is demonstrated. This method of amplification suppresses spectral narrowing typically appearing in chirped pulse amplification (CPA) lasers thus supports pulse spectrum s ubstantially broader than a conventional CPA. With a NPCPA Ti:sapphire laser we have achieved laser pulses of 50 nm spectral width and 150 mJ energy without any additional spectral correction. The scheme appears as an easy and relia ble solution to preserve spectral bandwidth in Ti:Sapphire lasers, especially at Petawatt and higher power levels. Keywords: Chirped pulse amplification, Ti:sapphire lasers. 1. INTRODUCTION At present Ti:sapphire can be considered as the most advanced medium for ultrashort and high peak power lasers. The progress of Ti:sapphire technology achieved during the few la st years in increase of the out put energy allows at present commercial lasers with the output power up to 200 TW. Desp ite of this progress, the current and standard Ti:sapphire technology based on the principle of chirped pulse amplification (CPA) supports laser pulses of ~ 30 fs duration only. This value is nearly one order of magnitude larger, than that supported by the bandwidth of Ti:sapphire medium itself. The high peak power ultrashort laser systems are mostly used for laser-matter interaction experiments at high intensity. The main directions of this research and development are to increase the pulse intensity on the target, and to sufficiently suppress the level of pre-pulses and amplified spontaneous emission (ASE). This requires to increase permanently the peak power of ultrashort lasers. At present several Petawatt laser systems are under construction all over the world [1-4], the Exawatt laser project, that will suppor t ultra relativistic laser-matter interactio ns, is already planned [5]. The increase of the peak power with the growth of the output energy requires substantial investments. Preferable is the obvious solution to shorten duration of the amplified pulses, that is, broaden their bandwidth. As well known, however, the broad emission spectrum of the amplifier medium (Ti:sapphire) is substantially narrowed upon high gain amplification [6]. This gain narrowing effect depends strongly on the total gain of the laser system. Hence, the higher is the energy of the amplified pulses the longer is their duration [7-9]. During the recent decade much efforts have been made to develop techniques for overcoming the gain narrowing effect. An obvious approach is to feed the laser syst em with more energetic seed pul ses, so that the total gain can be reduced. As it was shown recently this would also enhance the temporal contrast [10,11]. Unfortunately, the construction of an appropriate laser oscillator is by far not evident. Shaping the spectrum of the seed pulse prior to the amplifier chain was a reliable solution to increase the temporal contrast. However, these techniques based on acousto-optical crystals [12-14] or spatial light modulators [6] reduce substantially the energy of the seed pulse. Consequently, to achieve the same level of multi-TW amplification, the Ti:sapphire amplifiers need to be driven harder, which would in turn increase the ASE level and hence decrease the temporal contrast. A further attempt of inserting etalons into the regenerative amplifier turned out to produce side peaks and, hence, also reduced the temporal contrast [15-16]. Shaping via deformable mirrors [17] does not give high enough spectral modulation for compensation of gain narrowing at high gain systems. Suppression of gain narrowing with special dielectric coated mirrors is another method, which was successfully used for broadband amplification in Ti:Sa [18]. Unfortunately this method requires special dielectric coatin gs in addition, the temporal co ntrast achievable with this method has to be testified. Another promising solution was the invention of optical parametric chirped pulse amplification (OPCPA) [20-21]. Since it offers a bandwidth over 200 nm even at high gain (10