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Multi-GHz femtosecond pulses from linear andnon-linear propagation of a phase-modulated laser


We propose and demonstrate a non-mode-locking approach to generating multi-GHz repetition rate, femtosecond pulses in burst mode by shaping a continuouswave (CW) seed laser in an all-fiber configuration. The seed laser at 1030 nm is first phase modulated and dechirped to low-contrast, ∼2 ps pulses at 17.5 GHz repetition rate, then carved to bursts at 60 kHz repetition rate, and finally shaped to sub-2 ps clean pulses by a Mamyshev regenerator. This prepared high-quality picosecond source is further used to seed a Yb-doped fiber
amplifier operating in the highly nonlinear regime, delivering output pulses at 23 nJ/pulse and 20 μJ/burst, compressible to ∼100 fs level. The system eliminates
the need of mode-locked cavities and simplifies conventional ultrafast electro-optic combs to using only one phase modulator, while providing femtosecond pulses at multiple GHz repetition rate, enhanced pulse energy in burst mode and the potential of further power/energy scaling.

Experimental analysis on CPA-free thin-disk multipass amplifiers operated in a helium-rich atmosphere


We present an experimental investigation on the benefits of helium as an atmospheric gas in CPA-free thin-disk multipass amplifiers (TDMPAs) for the amplification to average powers exceeding 1 kW and pulse peak powers reaching 5 GW. Both the performance of the amplifier and the properties of the amplified sub-400 fs laser pulses centred at a wavelength of 1030 nm are compared for different helium concentrations in air, outlining and quantifying the benefits of
a helium-rich atmosphere. The amplification of 100 μJ pulses in an atmosphere with 60% helium instead of air led to a maximum increase in efficiency from 24% to 29%. This translated into an increase of average output power and pulse energy of 34 W (i.e +19%) and 0.34 mJ (i.e. +19%) respectively. At the same time an improvement of the beam quality from M2 = 1.18 to M2 = 1.14 was achieved. For the amplification of 10 μJ pulses to over 1 kW of average power an atmosphere with 33% helium led to an improved beam pointing stability by a factor of 2. Moreover, the beam propagation factor M2 improved by 0.1, and the power stability improved by approximately 10%.

X-ray emissions during laser machining of cylindrical 

Roswitha Giedl-Wagner and Thomas Lang

Ultrashort pulse lasers are the perfect tool for micromachining of components for a wide range of applications.  Even hard and brittle materials may be processed with high precision in state-of-the-art machines like the from GFH. With pulse lengths in the pico- and femtosecond range, an irradiation > 1012 W/cm2 is incident on the work piece with the potential to generate ionizing radiation. This article deals with the x-ray emissions recorded during 
laser turning operations and the corresponding safety measures. 

Thin-disk multipass amplifier delivering sub-400 fs pulses with excellent beam quality at an average power of 1 kW

André Loescher, Florian Bienert, Christoph Röcker, Thomas Graf, Martin Gorjan, Jürg Aus der Au, and Marwan Abdou Ahmed

We present an improved multipass amplifier design, enabling the amplification of ultrashort pulses with excellent beam quality to more than 1 kW of average output power. 260 fs short pulses at an average power of 105 W and a repetition rate of 1 MHz were directly amplified up to an average power of 1033 W. The pulse duration at this power level was measured to be 388 fs assuming a Gaussian temporal shape. This corresponds to a peak power of 2.5 GW. The power stability was measured to be 0.16% RMS over a duration of more than two hours at a sampling rate of 2 Hz. High beam quality is proven with measured values of M2x = 1.16 in the horizontal and M2y = 1.19 in the vertical plane according to ISO Standard 11146.

Electro-optic comb pumped optical parametric oscillator
with flexible repetition rate at GHz level

Hanyu Ye, Valerian Freysz, Ramatou Bello-Doua, Lilia Pontagnier, Giorgio Santarelli, Eric Cormier, Eric Freysz

We present a gigahertz (GHz)-repetition-rate optical parametric oscillator (OPO) pumped by an electro-optic (EO) comb at 1.03 mm, delivering sub-picosecond signal pulses across 1.5-1.7 mm from a MgO-doped periodically poled LiNbO3 crystal. Using a pump power of 5 W at 14.2 GHz repetition rate, 378 mW of signal power is obtained at 1.52 mm from a subharmonic cavity, corresponding to a signal extraction efficiency of 7.6%. By cascading a Mach-Zehnder modulator, the pump pulse repetition rate can be divided by any integer number from 1 to 14, allowing the OPO to operate with flexible repetition rate from 1 to 14.2 GHz. Besides, a strategy leading to quasi-continuous repetition rate tunability of the OPO is also discussed.

Multi-GHz repetition rate,femtosecond deep ultraviolet source in burst mode derived from an electro-optic comb


We present a femtosecond, 11.48 GHz intra-burst repetition rate deep UV source at 258 nm based on forth-harmonic generation (FHG) of an electro-optic (EO) comb operating in burst mode. Second-harmonic generation (SHG) of the burst-mode EO comb in LiB3O5 (LBO) leads to 3.7Waverage power and 242 fs root-mean-square pulse duration. A second stage of SHG is further performed using two separate β-BaB2O4 (BBO) crystals, delivering deep UV pulses at 523mW and 294mW, with estimated pulse durations of half-ps and sub-300 fs, respectively. At divided pulse repetition rates of 5.7 GHz and 2.9 GHz, FHG is also demonstrated, highlighting the potential of flexible repetition rate operation at the GHz level.

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