Hybrid Ytterbium–Neodimium Chirped Pulse Amplifier, with dual output for High Repetitive and High Energy Experiments


  Yariv Shamir [1]  ,  Zaharit Refaeli [1,2]  ,  Marcelo Wyszkin [1]  ,  Almantas Galvanauskas [3]  
[1] Soreq NRC, Yavne 8180000, Yavner, Israel
[2] Department of Applied Physics, The Hebrew University of Jerusalem, Jerusalem, Israel
[3] Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan, 48109, USA

Hybrid Ytterbium–Neodimium Chirped Pulse Amplifier, with dual output for High Repetitive and High Energy Experiments

Among the core topics involving high energy ultrafast laser technology is the design of a Front End (FE) subsystem, such that it enables appropriate pulse properties, e.g.: spectrum, dispersion, polarization, nonlinear phase control, beam quality, and is compatible with the main amplifiers and compressor. Nowadays, multi Joules to some kilojoule ultrashort chirped-pulse amplifier (CPA) laser facilities are dominated by large size Neodymium-Glass (Nd:Gl) disk amplifiers [1,2], with recent beyond 100 Joule Titanium doped Sapphire worth mentioning [3]. For all CPA [4] or in OPCPA schemes [5], the FE must be implemented highly stable and reliable, with special emphasis in the case of ~Hz to kHz repetitive use, e.g., particle acceleration or X-ray generation,applications that shall be used in, e.g. borders security or hadron therapy [6]. These properties emphasises the attractiveness of fiber based FE. Nd:Gl amplifiers support broad spectrum (>24 nm) [7,8] around 1053 nm, that, under high gain / multi-pass amplification tends to narrow the seed spectrum and challenges repetitive use for its thermal limitations. An interesting candidate to seed Nd:Gl, that is significantly less complicated compared to, e.g., regenerative amplifier, can be Ytterbium (Yb) doped fiber, with gain spanning the 1000 - 1100 nm and further advantages of integration simplicity, aligning-free, compactness, excellent beam quality, stability and reliability. On the downside are the typical long interaction of narrow beam waist, posing strong nonlinearities that limit the extractable energy, and gain shaping favoring 1030 nm. To our knowledge, previous reports discussed Yb-fiber preceding Nd amplifiers are very few [9,10], mostly of narrowband nanosecond [9-11] or tunable [12] pulses. Few reported on Yb-fiber CPA directly interacting with Nd:Gl amplifiers, but either showed partial use of its linewidth or as a preceding OPCPA rather, not directly seeding Nd:Gl [13].

(a): Schematic. (b): Temporal and (c) spectral shapes at the output. Yb amplification linewidth is 13 nm (red) whereas after Nd amplifiers it narrowed down to ~ 6 nm (blue). Dotted blue: Estimated output spectrum.

In this report, we present a hybrid Yb-fiber - Nd:Gl CPA that operate successively to generate pump beam for OPCPA. The scheme starts with modelocked laser that is also the timing synchro-clock for the subsystem. After stretching to <0.5 ns, pulses are diluted to 1 MHz, and amplified with small and large core single mode Yb fiber amplifiers. The system was tweaked so as to limit the short wave gain shaping effect. It was then succeeded by a novel ultra-large core fiber (85 µm), that practically supports nearly the fundamental mode by applying angular momentum perturbation in the form of helical satellite cores, named Chiralli Core Coupling (CCC) [14,15]. This mechanism leaves nearly only large Gaussian-like beam propagation. For relaxed nonlinear phase, we currently limited its output energy to few µJ, yielding an estimated B-integral of ~8. A secondary picker selects ~Hz rate pulses into two sequential Nd:Gl amplifiers, while simultaneously, the non-diluted pulses were directed to a separate compression for aperiodic frequency conversion study, reported separately by Z. Refaeli et-al. The low rate pulses go through double and four-pass Nd:Gl pump chambers, yielding ~ 50 µJ and ~ 4 mJ correspondingly. The output spectrum showed narrowing to ~ 5 nm, that is expected to be compressed to sub 500 fs. Preliminary compression tests show ~ 800 fs pulse duration yet without the dedicated designed compressor. Final verification shall be reported separately. In parallel, a work shall take place to enhance glass amplifiers to handle repetitive operation. Such scheme can contribute to stable FE that can operate in conjunction with Nd:Gl amplifiers prior to seeding high energy ultrashort laser systems.

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