Zoltán Várallyay

Energetic sub-2-cycle laser with 216 W average power.

Few-cycle lasers are essential for many research areas such as attosecond physics that promise to address fundamental questions in science and technology. Therefore, further advancements are connected to significant progress in the underlying laser technology. Here, two-stage nonlinear compression of a 660 W femtosecond fiber laser system is utilized to achieve unprecedented average power levels of energetic ultrashort or even few-cycle laser pulses. In a first compression step, 408 W, 320 μJ, 30 fs pulses are achieved, which can be further compressed to 216 W, 170 μJ, 6.3 fs pulses in a second compression stage. To the best of our knowledge, this is the highest average power few-cycle laser system presented so far. It is expected to significantly advance the fields of high harmonic generation and attosecond science.

Random access three-dimensional two-photon microscopy

We propose a two-photon microscope scheme capable of real-time, three-dimensional investigation of the electric activity pattern of neural networks or signal summation rules of individual neurons in a 0.6 mm x 0.6 mm x 0.2 mm volume of the sample. The points of measurement are chosen according to a conventional scanning two-photon image, and they are addressed by separately adjustable optical fibers. This allows scanning at kilohertz repetition rates of as many as 100 data points. Submicrometer spatial resolution is maintained during the measurement similarly to conventional two-photon microscopy.

2.1 kW single mode continuous wave monolithic fiber laser

A robust, alignment-free monolithic 2.1 kW single-mode continuous wave fiber laser, operating at 1083 nm is demonstrated. The laser is pumped with commercial fiber pigtailed multimode diodes through all-fiber pump-signal power combiners in a MOPA architecture. The oscillator was formed with high reflector and output coupler fiber Bragg gratings written in 11/200 μm (mode field/cladding diameter) single-mode fiber. The gain medium was a 19m OFS commercial 11/200 μm double clad Yb-doped fiber (DCY). Pump light was coupled to the oscillator using two 11/200 μm pump-signal power combiners (PSC). A total of 20 commercially available 58W pump diodes at 915 nm were used to generate 800W of signal, as measured before the amplifier. The Raman power after the oscillator was more than 60 dB below the signal power. The amplifier was built using 13 m of 14/200 µm DCY and two (18+1)x1 PSC combiners with more than 95% pump and signal light transmission. A total of 2 kW of power was used to bi-directionally pump the amplifier. The output was measured after 3 m 14/200 μm fiber, and 10 m 100/360 μm delivery cable. Total signal output power was 2.1 kW, corresponding to an amplifier slope efficiency of 77%. The Raman power is more than 30 dB below the signal power. At maximum power, no modal instabilities, thermal effects, nor power rollover were observed. With higher power pumps, it is predicted that a power level of 2.6 kW can be achieved with the Raman level below 20 dB.

Design of high-bandwidth one-and two-dimensional photonic bandgap dielectric structures at grazing incidence of light

We propose one-dimensional photonic bandgap (PB) dielectric structures to be used at grazing incidence in order to obtain an extended bandgap exhibiting considerably reduced reflection loss and dispersion compared to similar structures used at a normal incidence of light. The well-known quarter-wave condition is applied for the design in this specific case, resulting in resonance-free reflection bands without drops in reflection versus wavelength function and a monotonous variation of the group delay dispersion versus wavelength function, which are important issues in femtosecond pulse laser applications. Based on these results we extend our studies to two-dimensional PB structures and provide guidelines to the design of leaking mode-free hollow-core Bragg PB fibers providing anomalous dispersion over most of the bandgap.

Concept for CEP-stable few-cycle pulses at 100 W average power

We present the system design and first experimental results of the ELI-ALPS HR1 laser system. At the output CEP-stable few-cycle pulses with 1 mJ energy, >0.1 TW peak power and 100 W of average power will be generated.

200 W Average Power Energetic Few-cycle Fiber Laser

A state-of-the-art 8 channel fiber-chirped-pulse-amplifier system delivers 680 W of average power. Two-stage nonlinear compression in gas-filled capillaries yields 400 W, 30fs, >300µJ pulses and 220W, sub-7fs, 170 µJ pulses, respectively.

Higher order mode photonic bandgap fibers for dispersion control

We investigate the dispersion properties of solid core and hollow core photonic bandgap fibers and propose a novel, partial reflector layer around the core for dispersion control of femtosecond optical pulses.

Cubic phase distortion of single attosecond pulses being reflected on narrowband Mo/Si filtering mirrors

We show that cubic phase distortion caused by narrowband Mo/Si multilayer filtering X-ray mirrors may considerably increase the time duration of single attosecond pulses.

The extreme light infrastructure—attosecond light pulse source (ELI-ALPS) project

Globally, large international research infrastructures have over many decades promoted excellence in science and technology. Aligned with the international practice, the Europe Strategy Forum for Research Infrastructures (ESFRI) has developed and keeps updating a roadmap for research infrastructures. The Extreme Light Infrastructure (ELI) is one of the two large scale Laser Research Infrastructures (RI) proposed in the ESFRI Roadmap published in 2006. ELI aims to provide access to some of the most intense world-wide lasers for the international scientific user community, as well as secondary radiation and particle sources driven by them, offering to the users new interdisciplinary research opportunities. ELI is currently implemented as a distributed infrastructure in three pillars: ELI-Beamlines (ELI-BL) in Dolni Břežany, Czech Republic, ELI-Attosecond Light Pulse Source (ELI-ALPS) in Szeged, Hungary and ELI-Nuclear Physics (ELI-NP) in Magurele, Romania. This chapter is devoted to introduce the Hungarian pillar, ELI-ALPS, which will be operational in Szeged in 2018, with the primary mission to provide to the users the highest laboratory spatiotemporal resolution and a secondary mission to contribute to the technological development towards 200 petawatt (PW) lasers for high-field science, which is the ultimate goal of the ELI project. The chapter includes descriptions of the primary and secondary sources, while emphasis is given to selected examples of the scientific case of ELI-ALPS, presenting unique access offered by the technologies to be hosted in the infrastructure.

115 W, 10 GHz, femtosecond pulses from a very-large-mode-area Er-doped fiber amplifier

We demonstrate high average power, high peak power amplification of 10 GHz, picosecond and femtosecond pulses in a Very-Large-Mode Area (VLMA), Er-doped fiber with an effective area of ~1050 μm2. A high power, singlemode Raman fiber laser with up to 183 W of power at 1480 nm served as a pump source. 130 femtosecond pulses with an average power of 115 W, peak power of 88 kW, and M2 of 1.18 were achieved. Simulations that take into account pair-induced quenching give excellent agreement with measurements.