Broadband spectral characterization of the phase shift induced by population inversion in Ti:Sapphire

The spectral phase shift of broadband amplified pulses, induced by population inversion, was measured in Ti:Sapphire at different pump fluence values. The measurement was performed for two orthogonal polarization directions and at two different crystal temperatures of 296 K and 30 K. Zero shifts and sign changes were observed in the spectral phase, which are connected to the gain spectrum of the crystal. The electronic refractive index changes were also numerically calculated by the Kramers-Kronig theory. The results are highly important for achieving sub-10 fs pulse duration and phase stability in the next generation of Ti:Sapphire-based laser systems. © 2019 Optical Society of America

High-performance and high-power applications of nested anti-resonant nodeless hollow-core fibres

The use of hollow-core fibres (HCFs) for transmitting high-power, high-brightness laser light presents significant advantages over the current standard technology based on solid core fibres (SCFs). HCFs can deliver much higher optical power levels over longer distances while maintaining a near single-moded beam-quality. This approach has already shown promising results, and this thesis presents important findings on highly efficient coupling and delivery of such laser beams, as well as a relevant application that takes advantage of the hollow core. The studies conducted in this thesis cover a range of subjects. Initially, the coupling tolerances of an in-house developed, state-of-the-art Nested Antiresonant Nodeless Fibre (NANF) were examined and quantified. This led to the experimental verification of the fundamental limits governing free-space coupling of a near-Gaussian beam into the fibre’s fundamental mode. Exploring the opportunity of utilizing high average power lasers, the inevitable thermal load on the optical elements responsible for the input coupling was analysed. The effect of thermal lensing induced by the laser light on the coupling optics led to the degradation of the beam-quality above a certain characteristic threshold (in our experiments ~100W). The studies showed that using a high-purity lens pair over commercially available alternatives significantly decreased this parasitic effect, resulting in more efficient transmission and reduced thermalisation. Additionally, thermal lensing also modified the input spatial beam distribution, resulting in an increased excitation of unwanted higher order modes. Coupling light into these modes that exhibit greater amount of leakage increases the temperature of the fibre noticeably. A non-invasive technique to extract the excess light not coupled into the fundamental mode was designed and implemented. The decreased thermal load on the fibre coating in turn also reduced the possibility of thermal damage, allowing the further scaling of the coupled optical power. Based on the findings, coupling efficiency values of ~95% into NANFs are achieved and maintained at high average powers. This allowed the demonstration of the record delivery of 1 kW average optical power through 1km of HCF. In addition, the implementation of the improvements mentioned above allowed an enhanced setup to couple 2 kW of average optical power stably, as well as to deliver it beyond 10m efficiently. The results achieved in this thesis are limited by the equipment available, and simulations indicate that further scaling in power and transmitted length should be possible. By combining the efficient methods to couple light into HCFs with the low attenuation offered by NANF technology, the Thesis also examines the optical propulsion of micron sized particles inside the hollow core of such fibres. Initial experiments demonstrate the guidance of 10 μm diameter particles through 1m of fibre orientable in any spatial direction. By further upscaling the power and fibre length, this preliminary set of experiments indicates the possibility of a remote sensing solution, which can take advantage of the preservation of the high-quality optical beam to trap the microparticles over extended fibre lengths. Furthermore, extrapolations of the measurement results suggest the possibility, in future work, of accelerating such particles beyond 100m/s in air-filled HCFs

Dataset for: Limits of coupling efficiency into hollow-core antiresonant fibres

This data set supports the paper V. Zuba, H.C. Mulvad et al. &quot;Limits of Coupling Efficiency into Hollow-core Antiresonant Fibres&quot;, accepted for publication in IEEE Journal of Lightwave Technology.</span

Experimental investigation into optimum laser coupling efficiency into hollow-core NANFs

Over 94 % coupling efficiency is presented experimentally for a Gaussian-beam into an antiresonant hollow-core fiber optimized for 2nd order antiresonance guidance at 1064 nm, which demonstrates approximately 1.5% (0.06 dB) improvement over the 1st window counterpart

Limits of coupling efficiency into hollow-core antiresonant fibres

We theoretically explore the fundamental limits on the efficiency of coupling light into hollow-core antiresonant fibres. We study in particular the coupling of a free-space Gaussian beam to the guided modes of one of the most successful antiresonant fibre (ARF) geometries, the the nested antiresonantnodeless fibre (NANF). Through finite element simulations, we study the effect of the geometrical parameters of the fibre on the coupling efficiency, showing that coupling into the fundamental LP01-like mode is typically maximized around 96-98% when the incident beam waist is about 70% of the core diameter. We find that due to the nature of antiresonance guidance, higher coupling efficiencies are achieved for fibres operating in the second antiresonant window (or generally even-numbered windows) than in those operating in the fundamental antiresonant window (or generally odd numbered windows), although the difference between even and odd decreases with the order of the window. We verify this theoretical finding experimentally with precise measurements of coupling efficiency into two NANFs operating in first and second windows, respectively. Our results which consistently show a steady 1.4 percentage point higher coupling efficiency for the second window fibre imply that such fibers may be the most suitable candidates for applications such as laser delivery which require up to a few hundred meters of fiber

Reconstruction of attosecond pulses in the presence of interfering dressing fields using a 100 kHz laser system at ELI-ALPS

Attosecond Pulse Trains (APT) generated by high-harmonic generation (HHG) of high-intensity near-infrared (IR) laser pulses have proven valuable for studying the electronic dynamics of atomic and molecular species. However, the high intensities required for high-photon-energy, high-flux HHG usually limit the class of adequate laser systems to repetition rates below 10 kHz. Here, APT's generated from the 100 kHz, 160 W, 40 fs laser system (HR-1) currently under commissioning at the extreme light infrastructure attosecond light pulse source (ELI-ALPS) are reconstructed using the reconstruction of attosecond beating by interference of two-photon Transitions (RABBIT) technique. These experiments constitute the first attosecond time-resolved photoelectron spectroscopy measurements with attosecond pulses performed at 100 kHz repetition rate and one of the first experiments performed at ELI-ALPS in the framework of projects commissioning its newly installed technologies. These RABBIT measurements were taken with an additional IR field temporally locked to the extreme-ultraviolet APT, resulting in an atypical ω beating. We show that the phase of the 2ω beating recorded under these conditions is strictly identical to that observed in standard RABBIT measurements within second-order perturbation theory. This work highlights an experimental simplification for future experiments based on attosecond interferometry (or RABBIT), which is particularly useful when lasers with high average powers are used.ISSN:1361-6455ISSN:0368-3508ISSN:0953-4075ISSN:0022-370