The X-ray free electron laser (XFEL) is a novel laser source based on an accelerator system, which is capable of producing high-power and coherent x- rays. It is an extension of free electron lasers (FELs), in which a high energy electron beam moving in a periodic magnetic fi eld works as a laser medium. Although there is no theoretical limit on the lasing wavelength of FEL, the reflectivity of the cavity mirror sets a practical limit on shortening the available wavelength. In order to break the limit, a new FEL scheme called self-amplifi ed spontaneous emission (SASE) has been proposed, in which no optical cavity is required. This paper presents the principle of SASE FEL and its light source performances, together with the fundamental components in the SASE-based XFEL facility.

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The SPring-8 Angstrom Compact free-electron LAser (SACLA) has been open for user experiments since March 2012. SACLA is the world first compact XFEL facility aiming at a general-purpose machine spread widely, which was designed to make the facility size compact as much as possible with the state-of-the art technology. Success of SACLA actually stimulates lots of researchers who are eager to utilize XFEL for their experiments to build their own XFEL facilities. Construction of compact XFEL, which SACLA first showed how to realize, is now a hot topic all over the world and some of them are being investigated for budgeting. This article aims at providing basic knowledge on a SACLA system and toward which SACLA is going. For this purpose the article compares approaches to XFEL among three leading XFEL projects, LCLS, European XFEL and SACLA and then, describes features of SACLA in detail.

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We summarize the scientific cases and beamline technology for utilizing X-ray Free-Electron Lasers (XFELs). The scientific targets are classified into two categories: observation with ultrahigh spatiotemporal resolution and the creation of extreme states. As a representative of the former, the coherent diffraction imaging (CDI) method reveals the structure of non-crystalline samples with a single-shot exposure of the XFEL pulse. For the latter, nonlinear interactions between matter and short-wavelength radiation have been investigated. Combining optical lasers and XFEL with a pump and probe technique is a powerful tool for studying the ultrafast dynamics of materials. The state-of-the-art beamline technologies developed at SACLA are also introduced.

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Coherent X-ray diffraction imaging (CXDI) is a lens-less imaging technique that visualizes the structures of non-crystalline particles with micro- to sub-micrometer dimensions. In CXDI experiments, single spatially isolated particles are irradiated by X-ray beam with high transverse coherence. The projection images of the particles along the incident X-ray direction are then directly reconstructed from the diffraction amplitude using a phase retrieval algorithm. We recently developed a sample preparation system operated under a moisture controlled condition and a diffractometer to conduct cryogenic CXDI experiments at SPring-8 and the X-ray free electron laser (XFEL) facility called SACLA. Here we briefl y introduce the theoretical background and show the preliminary experimental data from samples prepared by the humidity-control system collected with the diffractometer at BL29XU of SPring-8 and the BL3 of SACLA.

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The advantages of SPring-8 Compact SASE Source as a light source for spectroscopic measurements in the extreme ultraviolet (EUV) wavelength region are introduced by referring to our recent study of nonlinear photoionization processes of He, in which the absolute two-photon ionization cross sections of He at four different wavelengths in the 54～62 nm region were determined using intense pulses of the freeelectron laser (FEL). In addition, our recent effort to generate intense full-coherent EUV light pulses are introduced, in which significant amplification of the 13th harmonic of ultrashort laser pulses at 800 nm was achieved by FEL seeded with the 13th harmonic.

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We have developed a precision fabrication method to realize an X-ray mirror with nearly atomic smoothness and shape accuracy of close to 1 nm peak to valley. A set of KB (Kirkpatric-Baez) mirrors for focusing the Japanese XFEL (X-ray free electron laser) named SACLA (SPring-8 angstrom compact free electron laser) to a micrometer spot size was developed and installed in the experimental hatch of SACLA to enable XFEL experiments to be carried out by the public. The spot size achieved was 0.9 × 1.1 μm² which was theoretically predicrted under the diffraction-limited condition. Another set of focusing mirrors is currently under development to enable the sub-50 nm focusing of SACLA, in which two-step focusing will be employed to realize nanofocusing with enough working distance such as several hundreds of millimeters.

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Present x-ray free electron lasers provide mJ-level x-ray pulses with femtosecond duration. New nonlinear optical phenomena, similar to optical frequency lasers, are expected. In this article, x-ray interaction physics and possible applications are reviewed and discussed. Especially, we discuss current and near-future experiments of nonlinear x-ray interactions with a solid target.

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We demonstrated an all-optical wavelength conversion and multicasting characteristics based on an alloptical triode using two negative feedback InGaAsP-InP semiconductor optical amplifiers. The output modulation degree improved from 30 to 60% using fiber Bragg gratings. The wavelength dependence of the conversion characteristics showed an output modulation degree that exceeded 60% in the range of 1525 to 1543 nm. However, the output modulation degree decreased in longer wavelength from 1557 nm. We performed wavelength conversions using four control signals and measured the bit error rate characteristics of 2.5 Gbps for each wavelength. Our results show that this device can realize alloptical multicasting using four channels at the same time.

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