In this Letter we report on the generation of 830 W compressed average power from a femtosecond fiber chirped pulse amplification (CPA) system. In the high-power operation we achieved a compressor throughput of about 90% by using high-efficiency dielectric gratings. The output pulse duration of 640 fs at 78 MHz repetition rate results in a peak power of 12 MW. Additionally, we discuss further a scaling potential toward and beyond the kilowatt level by overcoming the current scaling limitations imposed by the transversal spatial hole burning.
We report on a Yb:YAG Innoslab laser amplifier system for generation of subpicsecond high energy pump pulses for optical parametric chirped pulse amplification (OPCPA) at high repetition rates. Pulse energies of up to 20 mJ (at 12:5 kHz) and repetition rates of up to 100 kHz were attained with pulse durations of 830 fs and average power in excess of 200 W. We further investigate the possibility to use subpicosecond pulses to derive a stable continuum in a YAG crystal for OPCPA seeding. © 2011 Optical Society of America OCIS codes: 140.4480, 190.4410, 190.4970. High repetition rate free electron lasers (FELs) [1], attosecond metrology [2], and coherent control [3] are examples of applied physics fields that require stable laser amplifier systems with very high repetition rates, high pulse energies, and ultrashort pulse durations. Free electron lasers such as FLASH would tremendously benefit when combining extreme UV (XUV) pulses and a laser amplifier with millijoule-level pulse energies, 5-20 fs pulse duration, and an intraburst repetition rate of 0:1-1 MHz to perform pump-probe experiments. Another application is the FEL seeding with similar pulse parameters [4]. Such a state-of-the-art laser system is difficult to develop, most of all because of the additional longterm stability requirements for operation at large scale facilities [5]. Optical parametric chirped pulse amplification (OPCPA) [6,7] is to date the only technique to offer a way to amplify broadband pulses at high pulse energies with several hundreds of watts average power level. An increase of the average output power of an OPCPA system requires novel concepts for the pump amplifier system. Experimental OPCPA pump amplifiers have been successfully used, either to amplify pulses at low repetition rates and high peak powers [8,9] or high repetition rates and lower pulse energies [10,11]. An avenue in the multikilohertz repetition rate regime is the regenerative thin-disk amplifier that can provide millijoule pulse energy in the picosecond regime [12]. The concept has its limitations at high average powers given by difficult cavity outcoupling. Fiber chirped pulse amplification (CPA) laser amplifiers have emerged to be powerful tools for amplification to highest average powers of up to 830 W at femtosecond pulse durations [13,14]. However, combining the fiber laser amplifier with a Yb:YAG Innoslab amplifier [15] is a promising approach to push the average power beyond the kilowatt level with multimilljoule pulse energies. A striking advantage of this amplifier combination is the attainable subpicosecond pulse duration, avoiding complicated and lossy stretcher-compressor schemes for OPCPA [16]. Recent developments have also shown the potential to use a subpicosend pump amplifier driven continuum [17] generated in bulk media to seed optical parametric amplification (OPA) [18]. Additionally, higher peak intensities can be used to drive the OPCPA system due to the inherent scaling of the damage threshold with shorter pulse duration [19]. The current st...
We report on a novel approach of performance scaling of ultra-fast lasers by means of coherent combination. Pulses from a single mode-locked laser are distributed to a number of spatially separated fiber amplifiers and coherently combined after amplification. Splitting and combination is achieved by polarization cubes, i.e. the approach bases on polarization combining. A Hänsch-Couillaud detector measures the polarization state at the output. The error signal (deviation from linear polarization) is used to stabilize the synchronization of different channels. In a proof-of-principle experiment the combination of two femtosecond fiber-based CPA systems is presented. A combining efficiency as high as 97% has been achieved. The technique offers a unique scaling potential and can be applied to all ultrafast amplification schemes independent of the architecture of the gain medium.
A detrimental pulse distortion mechanism inherent to nonlinear chirped-pulse amplification systems is revealed and analyzed. When seeding the nonlinear amplification stage with pulses possessing weak side-pulses, the Kerr-nonlinearity causes a transfer of energy from the main pulse to side pulses. The resulting decrease in pulse contrast is determined by the accumulated nonlinear phase-shift (i.e., the B-integral) and the initial pulse-contrast. The energy transfer can be described by Bessel-functions. Thus, applications relying on a high pulse-contrast demand a low B-integral of the amplification system and a master-oscillator that exhibits an excellent pulse-contrast. In particular, nonlinear fiber CPA-systems operated at B-integrals far beyond pi have to be revised in this context.
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