Andreas Vernaleken

Single-pass high-harmonic generation at 20.8 MHz repetition rate

We report on single-pass high-harmonic generation (HHG) with amplified driving laser pulses at a repetition rate of 20.8 MHz. An Yb:YAG Innoslab amplifier system provides 35 fs pulses with 20 W average power at 1030 nm after external pulse compression. Following tight focusing into a xenon gas jet, we observe the generation of high-harmonic radiation of up to the seventeenth order. Our results show that state-of-the-art amplifier systems have become a promising alternative to cavity-assisted HHG for applications that require high repetition rates, such as frequency comb spectroscopy in the extreme UV.

Non-collinear high harmonic generation: a promising outcoupling method for cavity-assisted XUV generation.

We present first experimental results of our investigation of non-collinear high harmonic generation (NCHHG) with a chirped pulse amplification system. Collimated high harmonic radiation of higher than 9th order is observed along the bisector of two fundamental beams crossing in a xenon gas target. The obtained results show that cavity-assisted non-collinear high harmonic generation is a promising candidate for efficient generation and outcoupling of extreme ultraviolet (XUV) radiation.

Carrier-envelope frequency stabilization of a Ti:sapphire oscillator using different pump lasers

Summary form only given. Among the various femtosecond laser oscillators that are commonly used as optical frequency combs in a wide range of fields, Ti:sapphire oscillators offer the advantage of operating at the highest repetition rates, supporting the shortest pulses with octave-spanning spectra directly from the laser, and exhibiting very low residual frequency noise due to the high quality factor of their resonators. However, they require a pump laser with excellent beam quality emitting around 532nm and show inferior performance when operated with nonideal pump laser. Recently, several cost-efficient alternative pump lasers with smaller footprints than the most commonly used bulky and costly frequency-doubled single-longitudinal-mode (SLM) diode-pumped solid-state (DPSS) lasers became commercially available.In an initial set of measurements [1], we characterized and compared the performance of a carrier-envelope frequency stabilized femtosecond Ti:sapphire oscillator when pumped by four different pump lasers: a frequency-doubled multi-longitudinal-mode (MLM) DPSS laser with active noise cancellation (Lighthouse Photonics Sprout G-10W NET), a frequency-doubled MLM optically pumped semiconductor laser (OPSL, Coherent Verdi G5) and two frequency-doubled SLM DPSS lasers (Coherent Verdi V10, Coherent Verdi V5). The carrier-envelope frequency f0 of the oscillator was stabilized by means of the f-to-2f self-referencing technique, and the residual phase noise of the stabilized oscillator was measured with a second identical f-to-2f interferometer outside the feedback loop. The best performance of the oscillator obtained with each pump laser is shown in Fig. 1. We found an integrated rms residual phase jitter of below 160mrad for all tested pump lasers, which is less than 1/40 of an optical cycle and amounts to an rms timing jitter of less than 70as.The relative intensity noise (RIN) of the pump lasers was also measured. While showing certain signatures that could be traced in the residual phase jitter, the RIN was found to be low enough to enable stabilization of f0 in all cases. We found both SLM and both MLM pump lasers under test to be suitable for carrier-envelope frequency stabilization. More recent results obtained with additional pump sources that were not available in the first study will be presented.

Multi-pass-cell-based nonlinear pulse compression to 115 fs at 7.5 µJ pulse energy and 300 W average power

We demonstrate nonlinear pulse compression by multi-pass cell spectral broadening (MPCSB) from 860 fs to 115 fs with compressed pulse energy of 7.5 µJ, average power of 300 W and close to diffraction-limited beam quality. The transmission of the compression unit is >90%. The results show that this recently introduced compression scheme for peak powers above the threshold for catastrophic self-focusing can be scaled to smaller pulse energies and can achieve a larger compression factor than previously reported. Good homogeneity of the spectral broadening across the beam profile is verified, which distinguishes MPCSB among other bulk compression schemes.

Power Scaling Limitations for Cavity-Assisted High-Harmonic Generation

Harmonics up to order 21 generated in Xe are coupled out of a 78-MHz enhancement cavity with a fused silica Brewster plate. An irreversible degradation of the plate, accelerated by the generated harmonics, is observed.

XUV frequency combs

While being the most precise measurement tool in physics, high precision laser spectroscopy is still limited to wavelengths in the range between the infrared and the near ultraviolet. The generation of XUV frequency combs might be a route to extend optical frequency metrology into extreme ultraviolet (XUV) spectral region where many elements have fundamental transitions. The method of choice for XUV frequency comb generation has been cavity-assisted high harmonic generation, where an infrared frequency comb is converted into the XUV inside a femtosecond enhancement cavity at the full repetition rate of the oscillator. Our recent efforts have been directed towards a significant improvement of the average power of XUV combs. To this end, we experimentally investigated the process of non-collinear high harmonic generation (NCHHG) and proved it to be useful as a combined method for efficient generation and outcoupling of XUV radiation. Also, we developed a high repetition rate single-pass amplifier which has the potential to boost the available power for intracavity HHG.

375-W 37.5-μJ less than 170 fs laser system utilizing nonlinear pulse compression in fused silica

We present a novel, robust and efficient scheme for nonlinear pulse compression at highest average powers, which is based on self-phase modulation in a bulk nonlinear medium placed inside a multi-pass-cell (MPC). The scheme is suitable for the compression of sub-ps pulses with peak powers exceeding the threshold for catastrophic self-focusing of the nonlinear medium. Experimentally, we compress the output pulses from an Yb:YAG-Innoslab amplifier from 850 fs to <170 fs at 375 W of output power, 37.5 μJ pulse energy and almost diffraction-limited beam quality using fused silica as the nonlinear medium. The efficiency of the compression unit exceeds 90%.