In recent years coherent diffraction imaging (CDI) has evolved into a mature technology. Thanks to its lensless nature, it allowed to bypass the limitations of X-ray optics. At the same time, laser development in combination with high harmonic generation (HHG) has pushed the coherent XUV photon flux to values comparable to 3rd generation synchrotron facilities, which enables lensless imaging experiments that were previously only possible at large-scale facilities. Furthermore, the intrinsic short pulse duration of HHG radiation has potential for imaging experiments down to attosecond time scales. In this contribution, we present our latest results on lensless imaging using a fiber laser driven HHG source at 92 eV. A high photon flux source is used for scanning coherent diffractive imaging (ptychography) demonstrating sub-50 nm resolution. Further, an extension to Fourier transform holography is shown, which enables to increase the useable bandwidth by a factor of five without sacrificing spatial resolution. This paves the way for combing high-resolution table-top lensless imaging with attosecond pump-probe experiments.
Intense, ultrafast laser sources with an operation wavelength beyond the well-established near-IR are valuable tools for exploiting the wavelength scaling laws of strong-field, light-matter interactions. Such laser systems enable the scaling of the phase matching photon energy cut-off in high-order harmonic generation, which allows for the generation of coherent soft X-ray radiation up to, and even beyond, the water window. Such laser-driven sources enable a plethora of subsequent applications. A number of these applications can significantly benefit from an increase in repetition rate. In that regard, ultrafast thulium-doped fiber laser systems (providing a broad amplification bandwidth in the 2 μm wavelength region) represent a promising, average-power scalable laser concept for driving high-order harmonic generation. These lasers are capable of delivering ~100 fs pulses with multi-GW peak power at hundreds of kHz repetition rate. In this work, we show that combining ultrafast thulium-doped fiber CPA systems with HHG in an antiresonant hollow-core fiber is a promising approach to realize high photon energy cut-off HHG from a compact setup. The realization is based on combining nonlinear pulse self-compression (leading to strong-field waveforms) and phase-matched high-order harmonic generation in a single antiresonant hollow-core fiber. In this demonstration, a photon energy cut-off of approximately 330 eV has been achieved, together with a photon flux >106 ph/s/eV at 300 eV. These results emphasize the great potential of exploiting the HHG wavelength scaling laws with 2 μm fiber laser technology. Improvements of the HHG efficiency, the overall HHG yield and further laser performance enhancements will be the subjects of our future work.
The development in optical manufacture, alignment and testing has enabled the increasing use of freeform surfaces in all kinds of optical systems. The demanding system requirements need the involvement of optical surfaces that is able to provide more degrees of freedom. For better and more efficient use of the freeform surfaces, the understanding of freeform surfaces from all perspectives is necessary. We therefore study the impact of optimization strategy and freeform location on typical optical systems for varying applications. The uses of different optimization strategies, as well as the choice of locations for placing one or two freeform surfaces are considered. Their respective impacts on the final system performance are analyzed according to different aberration constitutions. By concluding all findings, we present some general rules for using and optimizing freeform surfaces in real design work. In the end, a work flow that gives instructions on how to use freeform surfaces in system design is presented.
A hyperspectral imaging spectrometer covering the wavelength range from 420 nm to 1000 nm is designed for the purpose of monitoring Earth’s environmental change. It has an entrance slit length of 24 μm, f/# of 3, smile and keystone distortion smaller than 20% of the pixel pitch and a spectral resolution of 6.5 nm. We design and review thirteen systems including one Offner system, two Schwarzschild systems and ten TMA systems for such specifications. Freeform surface and aspheric surface are used in some of the systems to achieve the required system parameters. With all system performance being summarized and evaluated, advantages and disadvantages of three different system types are compared. We down select two systems for further fine adjustments and tolerancing analysis. Final systems with superior performance and detailed tolerancing analysis are given at the end.