Henning Carstens

Fiber chirped-pulse amplification system emitting 3.8 GW peak power

We report on the experimental demonstration of a fiber chirped- pulse amplification system capable of generating nearly transform-limited sub 500 fs pulses with 2.2 mJ pulse energy at 11 W average power. The resulting record peak power of 3.8 GW could be achieved by combining active phase shaping with an efficient reduction of the acquired nonlinear phase. Therefore, we used an Ytterbium-doped large-pitch fiber with a mode field diameter of 105 µm as the main amplifier.

Megawatt-scale average-power ultrashort pulses in an enhancement cavity

We investigate power scaling of ultrashort-pulse enhancement cavities. We propose a model for the sensitivity of a cavity design to thermal deformations of the mirrors due to the high circulating powers. Using this model and optimized cavity mirrors, we demonstrate 400 kW of average power with 250 fs pulses and 670 kW with 10 ps pulses at a central wavelength of 1040 nm and a repetition rate of 250 MHz. These results represent an average power improvement of one order of magnitude compared to state-of-the-art systems with similar pulse durations and will thus benefit numerous applications such as the further scaling of tabletop sources of hard x rays (via Thomson scattering of relativistic electrons) and of soft x rays (via high harmonic generation).

Generation of µW level plateau harmonics at high repetition rate

The process of high harmonic generation allows for coherent transfer of infrared laser light to the extreme ultraviolet spectral range opening a variety of applications. The low conversion efficiency of this process calls for optimization or higher repetition rate intense ultrashort pulse lasers. Here we present state-of-the-art fiber laser systems for the generation of high harmonics up to 1 MHz repetition rate. We perform measurements of the average power with a calibrated spectrometer and achieved µW harmonics between 45 nm and 61 nm (H23-H17) at a repetition rate of 50 kHz. Additionally, we show the potential for few-cycle pulses at high average power and repetition rate that may enable water-window harmonics at unprecedented repetition rate.

1 MHz repetition rate hollow fiber pulse compression to sub-100-fs duration at 100 W average power

We report on nonlinear pulse compression at very high average power. A high-power fiber chirped pulse amplification system based on a novel large pitch photonic crystal fiber delivers 700 fs pulses with 200 μJ pulse energy at a 1 MHz repetition rate, resulting in 200 W of average power. Subsequent spectral broadening in a xenon-filled hollow-core fiber and pulse compression with chirped mirrors is employed for pulse shortening and peak power enhancement. For the first time, to our knowledge, more than 100 W of average power are transmitted through a noble-gas-filled hollow fiber. After pulse compression of 81 fs, 93 μJ pulses are obtained at a 1 MHz repetition rate.

Large-mode enhancement cavities

In passive enhancement cavities the achievable power level is limited by mirror damage. Here, we address the design of robust optical resonators with large spot sizes on all mirrors, a measure that promises to mitigate this limitation by decreasing both the intensity and the thermal gradient on the mirror surfaces. We introduce a misalignment sensitivity metric to evaluate the robustness of resonator designs. We identify the standard bow-tie resonator operated close to the inner stability edge as the most robust large-mode cavity and implement this cavity with two spherical mirrors with 600 mm radius of curvature, two plane mirrors and a round trip length of 1.2 m, demonstrating a stable power enhancement of near-infrared laser light by a factor of 2000. Beam radii of 5.7 mm × 2.6 mm (sagittal × tangential 1/e(2) intensity radius) on all mirrors are obtained. We propose a simple all-reflective ellipticity compensation scheme. This will enable a significant increase of the attainable power and intensity levels in enhancement cavities.

Preferential gain photonic-crystal fiber for mode stabilization at high average powers

We report on the design and experimental investigation of a preferential gain photonic-crystal fiber with a mode-field diameter of 47 µm. This few-mode fiber design confines the doping of Ytterbium-ions just to the center of the core and, therefore, promotes fundamental mode operation. In a chirped-pulse amplification system we extracted up to 303 W of average power from this fiber with a measured M2 value of 1.4.

Multi-gigawatt ultrashort pulses at high repetition rate and average power from two-stage nonlinear compression.

We present simple and compact (1.5m x 0.5m footprint) post-compression of a state-of-the-art fiber chirped pulse amplification system. By using two stage nonlinear compression in noble gas filled hollow core fibers we shorten 1 mJ, 480 fs, 50 kHz pulses. The first stage is a 53 cm long, 200 µm inner diameter fiber filled with xenon with subsequent compression in a chirped mirror compressor. A 20 cm, 200 µm inner diameter fiber filled with argon further broadens the spectrum in a second stage and compression is achieved with another set of chirped mirrors. The average power is 24.5 W/19 W after the first/second stage, respectively. Compression to 35 fs is achieved. Numerical simulations, agreeing well with experimental data, yield a peak power of 5.7 GW at a pulse energy of 380 µJ making this an interesting source for high harmonic generation at high repetition rate and average power.

Enhancement cavities for zero-offset-frequency pulse trains

The optimal enhancement of broadband optical pulses in a passive resonator requires a seeding pulse train with a specific carrier-envelope-offset frequency. Here, we control the phase of the cavity mirrors to tune the offset frequency for which a given comb is optimally enhanced. This enables the enhancement of a zero-offset-frequency train of sub-30-fs pulses to multi-kW average powers. The combination of pulse duration, power, and zero phase slip constitutes a crucial step toward the generation of attosecond pulses at multi-10-MHz repetition rates. In addition, this control affords the enhancement of pulses generated by difference-frequency mixing, e.g., for mid-infrared spectroscopy.

Balancing of thermal lenses in enhancement cavities with transmissive elements

Thermal lensing poses a serious challenge for the power scaling of enhancement cavities, in particular when these contain transmissive elements. We demonstrate the compensation of the lensing induced by thermal deformations of the cavity mirrors with the thermal lensing in a thin Brewster plate. Using forced convection to fine-tune the lensing in the plate, we achieve average powers of up to 160 kW for 250-MHz-repetition-rate picosecond pulses with a power-independent mode size. Furthermore, we show that the susceptibility of the cavity mode size to thermal lensing allows highly sensitive absorption measurements.

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.