D. Joyeux

Iterative algorithms for twin-image elimination in in-line holography using finite-support constraints

The quality of reconstructed images from in-line holograms can be seriously degraded by the linear superposition of twin images having the same information but different foci. Starting from the reconstructed field at the real image plane, we make use of the uncontaminated information contained in the out-of-focus wave (virtual image) outside the in-focus wave (real image) support, together with a finite-support constraint, to form an iterative procedure for twin-image elimination. This algorithm can reconstruct complex objects, provided that they are not recorded in very near-field conditions. For real objects additional constraints can be imposed, extending the algorithm application to very near-field conditions. The algorithm’s convergence properties are studied in both cases, and some examples are shown.

Soft-x-ray interferometer for measuring the refractive index of materials

We have designed and built a soft-x-ray interferometer to test the possibility of a direct measurement of the refractive index (i.e., the real part of the complex index) of materials in the soft-x-ray range. The interferometer is based on the Fresnel bimirror setup. It works near the zero path difference and requires only little spatial coherence. Plane mirrors at grazing incidence are the only optical elements. Interference fringes have been recorded at 4.8 nm, near the K edge of carbon. An index value could be obtained by measuring the fringe pattern shift between two such records, one without and one with a sample in one optical path. An estimation of the noise-limited accuracy in such an index determination shows that a few parts in 10−6 can be anticipated.

Optimization toward a high-average-brightness soft-x-ray laser pumped at grazing incidence

We report the near-field imaging characterization of a 10 Hz Ni-like 18.9 nm molybdenum soft-x-ray laser pumped in a grazing incidence pumping (GRIP) geometry with a table-top laser driver. We investigate the effect of varying the GRIP angle on the spatial behavior of the soft-x-ray laser source. After multiparameter optimization, we were able to find conditions to generate routinely a high-repetition-rate soft-x-ray laser with an energy level of up to 3 microJ/pulse and to 6x10(17) photons/s/mm2/mrad2/(0.1% bandwidth) average brightness and 1x10(28) photons/s/mm2/mrad2/(0.1% bandwidth) peak brightness.

Temporal coherence and spectral linewidth of an injection-seeded transient collisional soft x-ray laser

The temporal coherence of an injection-seeded transient 18.9 nm molybdenum soft x-ray laser was measured using a wavefront division interferometer and compared to model simulations. The seeded laser is found to have a coherence time similar to that of the unseeded amplifier, ~1 ps, but a significantly larger degree of temporal coherence. The measured coherence time for the unseeded amplifier is only a small fraction of the pulsewidth, while in the case of the seeded laser it approaches full temporal coherence. The measurements confirm that the bandwidth of the solid target amplifiers is significantly wider than that of soft x-ray lasers that use gaseous targets, an advantage for the development of sub-picosecond soft x-ray lasers.

Fourier-limited seeded soft x-ray laser pulse

We present what we believe to be the first measurement of the spectral properties of a soft x-ray laser seeded by a high-order harmonic beam. Using an interferometric method, the spectral profile of a seeded Ni-like krypton soft x-ray laser (32.8 nm) generated by optical field ionization has been experimentally determined, and the shortest possible pulse duration has been deduced. The source exhibits a Voigt spectral profile with an FWHM of 3.1+/-0.3 mA, leading to a Fourier-transform pulse duration of 4.7 ps. This value is comparable with the upper limit of the soft x-ray pulse duration determined by experimentally investigating the gain dynamics, from which we conclude that the source has reached the Fourier limit. The measured bandwidth is in good agreement with the predictions of a radiative transfer code, including gain line narrowing and saturation rebroadening.

Recent Progress on the Understanding of the Transient Ni‐like Ag X‐ray Laser at 13.9 nm at LULI facilities

This paper summarises our recent progress achieved in the characterisation and understanding of the Ni‐like Ag transient X‐ray laser pumped under traveling wave irradiation. We carried out two experiments at the LULI CPA laser facility. Several diagnostics of the plasma emission at the XRL wavelength or in the keV range indicate the presence of small‐scale spatial structures in the emitting XRL source. Single‐shot Fresnel interference patterns at 13.9 nm were successfully obtained with a good fringe visibility. For the first time we obtained plasma images with a high spatial resolution about 1 μm, showing the effects of pumping parameters on the X‐ray laser far‐field.

X-Ray Imaging of the Heating Zone of Non-Normal Incidence Pumped XRL Plasma

Soft x-ray emission, above 600 eV, from a grazing incidence pumped Ni-like Mo x-ray laser (GRIP-XRL) [1] plasma was investigated. Using a pinhole camera looking along the target surface, perpendicular to the direction of the XRL emission, spatially-resolved information was obtained with a resolution which was limited only by the pinhole size of 10 μm. The relative distance from the target surface to the plasma zone heated by th e picosecond pulse was investigated for different GRIP angles, energy ratios and delays between the plasma producing and the plasma heating pulses.

Spectral width of seeded and ASE XUV lasers: experiment and numerical simulations

We describe our recent progress in the investigation of the spectral properties of collisional XUV lasers, including both experimental measurements and numerical calculations. Using a wavefront-division, variable path-difference interferometer, we have characterized the temporal coherence and the spectral width of an injection-seeded transient XUV laser emitted at 18.9 nm from a Ni-like Mo plasma. Our results show that the temporal coherence of the beam is significantly increased by the injection-seeded operation, compared to the standard ASE mode, in agreement with detailed numerical simulations. Using the PPP code we have calculated the intrinsic linewidth of the same lasing line over a range of electron density and at temperatures that are relevant to transient collisional pumping. We discuss the relative contributions of homogeneous and inhomogeneous broadening to the overall profile.

Recent progress on the LASERIX facility

The LASERIX facility provides coherent and short soft x-ray beams for scientific applications. The beams are generated through high intensity laser interaction with matter using two different schemes, plasma based soft x-ray lasers, and high order laser harmonic generation. We describe in this communication the present status of the facility. The LASERIX beamtime has been recently opened to external users. We present two typical experiments performed in that context with the facility. The first one is dedicated to the fundamental study of the plasma based soft x-ray laser, whereas the second uses the existing beam to study irradiation induced dammage in DNA samples. We present also the development performed on the soft x-ray laser source to improve its stability and high repetition rate operation.

Temporal Coherence and Spectral Line Shape of a GRIP Transient X-Ray Laser

We have used a variable path-difference, wavefront-division interferometer to measure the temporal coherence of an unseeded Ni-like Mo GRIP X-ray laser. This quantity is inversely related to its spectral linewidth. We have investigated the role of several pump parameters on the inferred spectral width. Along this experiment, particular attention has been paid on the source stability