Sven Breitkopf

Coherently-combined two channel femtosecond fiber CPA system producing 3 mJ pulse energy

We present a fiber CPA system consisting of two coherently combined fiber amplifiers, which have been arranged in an actively stabilized Mach-Zehnder interferometer. Pulse durations as short as 470 fs and pulse energies of 3 mJ, corresponding to 5.4 GW of peak power, have been achieved at an average power of 30 W.

88 W 0.5 mJ femtosecond laser pulses from two coherently combined fiber amplifiers.

The generation of 0.5 mJ femtosecond laser pulses by coherent combining of two high power high energy fiber chirped-pulse amplifiers is reported. The system is running at a repetition frequency of 175 kHz producing 88 W of average power after the compressor unit. Polarizing beam splitters have been used to realize an amplifying Mach-Zehnder interferometer, which has been stabilized with a Hänsch-Couillaud measurement system. The stabilized system possesses a measured residual rms phase difference fluctuation between the two branches as low as λ/70 rad at the maximum power level. The experiment proves that coherent addition of femtosecond fiber lasers can be efficiently and reliably performed at high B-integral and considerable thermal load in the individual amplifiers.

Coherently combined fiber laser system delivering 120 μJ femtosecond pulses

We report on the coherent combination of two chirped pulsed fiber lasers. The beams coming from two 100 μm core diameter ytterbium-doped rod-type fibers were coupled in a Mach-Zehnder-type interferometer by means of a polarization beam splitter. Active stabilization of the interferometer was achieved by using a piezo-mounted mirror driven by a Hänsch-Couillaud polarization detection system. Pulses with 120 μJ energy and a compressed duration of 800 fs were obtained. This, compared with the 66 μJ coming from each single amplifier, results in a combining efficiency as high as 91%.

Performance Scaling of Ultrafast Laser Systems by Coherent Addition of Femtosecond Pulses

In the last decades, ultrafast lasers and amplifiers have achieved an extraordinary power increase and have enabled a plethora of scientific, medical or industrial applications. However, especially in recent years, it has become more and more challenging to keep up with this pace since intrinsic physical limitations are becoming difficult to avoid. A promising way to get around this problem is the technique of spatially and/or temporally separated amplification and subsequent coherent addition of ultrashort pulses. It turns out that fiber amplifiers are perfect candidates for this concept due to their outstanding average-power capability and their simple single-pass setups, which can be easily parallelized. Herein we provide an overview of the most important experimental implementations of this concept and recent results. We discuss the ability of these approaches to generate laser parameters that, only a few years ago, seemed impossible to achieve.

Optimizing high-power Yb-doped fiber amplifier systems in the presence of transverse mode instabilities

The average output power of Yb-doped fiber amplifier systems is currently limited by the onset of transverse mode instabilities. Besides, it has been recently shown that the transverse mode instability threshold can be significantly reduced by the presence of photodarkening in the fiber. Therefore, reducing the photodarkening level of the core material composition is the most straightforward way to increase the output average power of fiber amplifier systems but, unfortunately, this is not always easy or possible. In this paper we present guidelines to optimize the output average power of fiber amplifiers affected by transverse mode instabilities and photodarkening. The guidelines derived from the simulations do not involve changes in the composition of the active material (except for its doping concentration), but can still lead to a significant increase of the transverse mode instability threshold. The dependence of this parameter on the active ion concentration and the core conformation, among others, will be studied and discussed.

Non-steady-state enhancement cavities using pulse-dumping as power scaling concept of femtosecond lasers

We present a promising concept to reach femtosecond pulses with over 1 J energy at repetition rates of 15 kHz using a combination of fiber-amplifiers and ultra-long, non-steady-state enhancement-cavities. We show first preliminary results for a 10 MHz cavity.

Approaching TW-peak powers at >10kHz repetition rate by multi-dimensional coherent combining of femtosecond fiber lasers

Novel laser applications such as laser-wake-field acceleration of particles require extreme parameters from ultra-short-pulse systems. We propose a concept capable to realize simultaneously multi-TW peak powers and multi-kW average powers by employing spatially and temporally separated amplification of chirped laser pulses delivered by fiber-amplifiers. As a combining element for the temporally 100 ns separated pulses (10 MHz repetition rate) we suggest a non-steady-state enhancement cavity using a fast switching-element to dump out the enhanced pulses at 15 kHz.

Velocity- and pointing-error measurements of a 300 000-r/min self-bearing permanent-magnet motor for optical applications

Compact, ultra-high-speed self-bearing permanent-magnet motors enable a wide scope of applications including an increasing number of optical ones. For implementation in an optical setup, the rotors have to satisfy high demands regarding their velocity and pointing errors. Only a restricted number of measurements of these parameters exist and only at relatively low velocities. This manuscript presents the measurement of the velocity and pointing errors at rotation frequencies up to 5 kHz. The acquired data allow us to identify the rotor drive as the main source of velocity variations with fast fluctuations of up to 3.4 ns (RMS) and slow drifts of 23 ns (RMS) over ∼120 revolutions at 5 kHz in vacuum. At the same rotation frequency, the pointing fluctuated by 12 μrad (RMS) and 33 μrad (peak-to-peak) over ∼10 000 round trips. To our best knowledge, this states the first measurement of velocity and pointing errors at multi-kHz rotation frequencies and will allow potential adopters to evaluate the feasibility of such rotor drives for their application.

Optimizing the mode instability threshold of high-power fiber laser systems

In this work we present guidelines to increase the transverse mode instability threshold of high power fiber amplifiers and also, for the first time to the best of our knowledge, of fiber oscillators. These guidelines do not involve changes in the composition of the active material (except for its doping concentration), but they can still lead to a significant increase of the transverse mode instability threshold. The dependence of this parameter on the active ion concentration, the core conformation, the pump configuration and the mirror reflectivities in a fiber oscillator will be studied and discussed.

Investigation of a 10 MHz, non-steady state cavity for pulse energy enhancement of ultrafast fiber lasers

Here, we present a passive 30-m long enhancement cavity that supports a steady-state enhancement of 198, which is the highest enhancement that has ever been reached in such a long cavity. Furthermore, we demonstrate the extraction of a short burst with a total energy of 53.6 μJ employing an acousto-optic modulator (AOM) as a switching device. The cavity was seeded with pulses of 1.49 μJ energy at 10 MHz repetition rate. The individual output coupled pulses showed an energy enhancement of up to 8.5 while the whole burst contained the entire energy of 36 input pulses. In the last section theoretical considerations for the single pulse extraction are presented and briefly discussed.