We report on the experimental demonstration of enhanced-frequency-chirping (EFC) in a multipass cell (MPC) based post-compression scheme through dispersion engineering. The transfer of the nonlinear interaction into the EFC-regime facilitates a smooth spectrum void of strong modulations that are present in regular self-phase-modulated spectra. The resulting 32 fs pulses exhibit minimal side features with more than 96% of energy contained in the temporal main feature. The experiments were performed with a mJ-class Yb:fiber laser with repetition rate of 50kHz at 70 W of average power. The results show the extended capabilities for dispersion-tailored pulse propagation in MPCs.
Within this work we demonstrate the efficient nonlinear temporal compression of mJ pulses emitted by an ultrafast thulium-doped fiber laser system. For spectral broadening, a krypton and helium filled Herriott-type multi-pass cell with broadband dielectric mirrors is employed. The input pulses with 1,78 mJ and 85 fs are spectrally broadened and subsequently compressed utilizing fused silica plates revealing a pulse duration below 29 fs while featuring an overall transmission of 91%. In addition to the preservation of the input beam quality, the system exhibits a shot-to-shot noise ratio of less than 1.2% as well as an excellent long-term power stability with fluctuations below 1% over a time span of 2 hours.
The presented results demonstrate the advantageous properties of the multi-pass cell approach: High efficiency and high transversal beam quality at high average power, not only for conventional ultrafast ytterbium-based laser systems at 1 µm wavelength, but also in the mid-infrared regime. We believe that this system, delivering an average power above 162 W and sub-5-cycle pulse duration, provides a promising working point for following secondary source experiments like THz- or high harmonic generation.
Few-cycle laser systems in the short-wavelength infrared (SWIR) region (from 1.4 μm to 3 μm) with high pulse energy and high average power have become increasingly impactful as driving sources for THz, midinfrared and soft X-ray generation with numerous subsequent applications. Compared with the well-established near-IR region, the longer wavelength enables higher conversion efficiency in the THz and midinfrared generation, and is considered as a favorable tradeoff between pushing the high harmonic generation phase matching cut-off up to the water window region (300 eV – 500 eV) while maintaining reasonable single emitter efficiency. In this contribution, we present our first results of the nonlinear post compression of a high-power ultrafast thulium-doped fiber laser output in a 1.05 m long rod-type hollow-core fiber filled with ~3 bar argon. With around 90 fs input pulse duration (FWHM) and 180 W input average power, the nonlinear compression stage delivers compressed pulses with a duration of 10.2 fs (FWHM), an average output power of 132 W. This corresponds to 1.3 mJ pulse energy at a repetition rate of 101 kHz and 1.64 cycles at a central wavelength of 1.87 μm. With this, we estimate a compressed pulse-peak power of about 80 GW, with an energy content of ~66% in the main feature. It is the highest average power mJ-class few-cycle source in the SWIR region reported to date. Featuring a unique combination of peak- and average- power with less than 2 cycle pulses, this laser source is highly interesting for nonlinear frequency conversion addressing THz, midinfrared and soft X-ray region.
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