High-energy chirped- and divided-pulse Sagnac femtosecond fiber amplifier

Abstract

We report on the generation of 1.1-mJ, 300-fs pulses at 50 kHz by implementing an amplifier architecture whereby four stretched pulse replicas are created in the temporal and spatial domains, allowing pulse energy scaling by the same factor. The whole spatiotemporal coherent combining geometry is passive, avoiding the need for active electronic stabilization loop systems. The combining efficiency remains above 90% at all power levels.High-power femtosecond fiber lasers are now wellestablished as compact and reliable sources of ultrafast pulses. The guiding properties of the amplifying medium guarantees excellent optical-to-optical efficiency and beam quality, but limits the achievable pulse energy because the mode area cannot be scaled arbitrarily. Despite this limitation, femtosecond pulses carrying energy of several microjoules have been generated from such systems [1]. This has been made possible by using very large mode area rod-type fibers and the chirped pulse amplification (CPA) architecture with significant stretching ratios of several nanoseconds. The pulse energy is thereby spread in time and space, limiting the peak intensity that causes detrimental optical nonlinear effects. However, these large stretching ratios are obtained by using very large and expensive diffraction gratings that cannot be reasonably further scaled.To reach higher energies or reduce the footprint of high energy ultrafast fiber sources, recent experimental efforts have focused on coherent combining of several independently amplified pulses, either in the space or time domains, to further decrease the peak intensity in the amplifying medium. Active coherent beam combining [2], passive coherent beam combining [3], and divided pulse amplification (DPA) [4,5] have all been recently demonstrated to allow energy scaling. Active parallel beam combination can in principle be scaled arbitrarily, and record performances have been recently obtained using four rod-type fiber amplifiers [6].In contrast, it is more difficult to scale the number of pulses that can be coherently combined passively, i.e., without the need for active feedback on the optical phase of the pulses. High-energy passive DPA systems [7] are limited by the onset of gain saturation that breaks the symmetry between successive pulse replicas in the amplifiers. This effect limits the number of replicas that can be combined efficiently at high energy to only two [8]. The same is true for passive coherent beam combining [9], because the Sagnac interferometer (SI) only produces two counter-propagating beams. Multiple beams same-path interferometers can in principle be devised, but these architectures are not well suited to high-gain fiber amplifiers due to parasitic laser effect. Scaling the performances of passive coherent combining systems therefore requires the use of spatial and temporal strategies simultaneously. In particular, DPA can be used in conjunction with CPA [8], or with passive coherent combining [9], to allow even further increase of performances.In this report, w...

PERLE. Powerful energy recovery linac for experiments. Conceptual design report

“…Using a LBO crystal, the laser beam frequency can finally be doubled with more than 50% efficiency before entering the optical cavity to provide a high average power beam at a wavelength close to 515nm. Eventually one can also parallelise two fibre amplifiers to compensate for the second harmonic generation limited efficiency [107][108][109].…”

Coherent combination of ultrafast fiber amplifiers

“…Since both approaches are limited in current practical setups to two replicas, it is a simple way to conceive an entirely passive coherent combining system that is compatible with highenergy fiber femtosecond amplifiers and uses 4 replicas. A recent demonstration (Guichard et al 2015) implements this idea in a CPA system, using freespace delay lines to generate the temporal replicas, with a Sagnac interferometer including two 0.8 m-long rodtype fiber amplifiers. The setup allows the generation of 1.1 mJ 300 fs pulses with a combining efficiency of 90%, illustrating the relevance of the passive architecture: the energy is scaled by a factor of four compared to performances achieved without any combining strategies, without the need for any active opto-electronic feedback loop system.…”

Single-Shot Phase and Amplitude Fluctuations of Narrow-Line Pulse Bursts in Divided-Pulse Amplifier

“…Also known as coherent pulse stacking amplification (CPSA), it is typically used for femtosecond pulses [4]. Combined with chirped-pulse amplification (CPA), the pulses can be stretched to ~1 ns and then further effective stretching by DPA avoids excessive nonlinearities and damage even at high energy extraction [5], with burst energy several times the saturation energy. CPSA works with other types of coherent pulses, too.…”