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Optical fibers and lenses are key components in many optical systems. Lenses used to couple light into the fiber or to tailor the output beam coming out from the fiber itself typically involve bulk optical schemes and complex packaging that limit the compactness and flexibility of the overall system. In the last years, new technological solutions based on the lab-on-fiber technology have been proposed to directly integrate optical components on the fiber end-face with a monolithic approach. In this review, we provide a detailed state of the art of the miniaturized lenses integrated on the end-face of optical fibers reported so far. The proposed devices have been divided into three main categories (refractive, diffractive and resonant including metasurfaces) according to their working principle. A critical discussion on the different categories of lenses is given, also providing a comparison in terms of ease of design and fabrication, application flexibility, compactness, scientific and technological maturity, and time to market.

This paper illustrates the use of spatial filtering with a horizontal slit near the source to enlarge the horizontal coherence in an experimental station and produce a diffraction‐limited round focus at an insertion device beamline for X‐ray photon correlation spectroscopy experiments. Simple expressions are provided to guide the optical layout, and wave propagation simulations confirm their applicability. The two‐dimensional focusing performance of Be compound refractive lenses to produce a round diffraction‐limited focus at 11 keV capable of generating a high‐contrast speckle pattern of an aerogel sample is demonstrated. The coherent scattering patterns have comparable speckle sizes in both horizontal and vertical directions. The focal spot sizes are consistent with hybrid ray‐tracing calculations. Producing a two‐dimensional focus on the sample can be helpful to resolve speckle patterns with modern pixel array detectors with high visibility. This scheme has now been in use since 2019 for the 8‐ID beamline at the Advanced Photon Source, sharing the undulator beam with two separate beamlines, 8‐ID‐E and 8‐ID‐I at 7.35 keV, with increased partially coherent flux, reduced horizontal spot sizes on samples, and good speckle contrast. [ABSTRACT FROM AUTHOR]

Beryllium is one of the most transparent materials to hard X‐ray radiation and, as a direct consequence, it is the main material for the fabrication of X‐ray refractive optics and instrumentation for synchrotron radiation sources and free‐electron laser facilities. However, it is known that almost all beryllium currently in use is polycrystalline material. In this paper, the influence of the microstructure of different beryllium grades on the optical properties of X‐ray refractive lenses is studied. The experiments were performed at the ESRF ID06 beamline in X‐ray coherent transmission microscopy mode in the near‐ and far‐fields. Two sets of refractive lenses made of beryllium O‐30‐H and IS‐50M grades with different internal microstructure were used. It was found that both beryllium grades have a strongly inhomogeneous structure, which inevitably produces speckle patterns under coherent illumination in imaging experiments. It was shown that fine‐grained beryllium O‐30‐H is better suited for imaging applications, whereas beryllium IS‐50M with a relatively large grain microstructure is more appropriate for focusing and collimation of X‐rays. A discussion on the requirements for X‐ray optical materials used at the third‐ and fourth‐generation synchrotrons is also presented. [ABSTRACT FROM AUTHOR]

Performance tests of parabolic beryllium refractive lenses, considered as X‐ray focusing elements in the future X‐ray free‐electron laser oscillator (XFELO), are reported. Single and double refractive lenses were subject to X‐ray tests, which included: surface profile, transmissivity measurements, imaging capabilities and wavefront distortion with grating interferometry. Optical metrology revealed that surface profiles were close to the design specification in terms of the figure and roughness. The transmissivity of the lenses is >94% at 8 keV and >98% at 14.4 and 18 keV. These values are close to the theoretical values of ideal lenses. Images of the bending‐magnet source obtained with the lenses were close to the expected ones and did not show any significant distortion. Grating interferometry revealed that the possible wavefront distortions produced by surface and bulk lens imperfections were on the level of ∼λ/60 for 8 keV photons. Thus the Be lenses can be succesfully used as focusing and beam collimating elements in the XFELO. [ABSTRACT FROM AUTHOR]

Linear parabolic diamond refractive lenses are presented, designed to withstand high thermal and radiation loads coming from upgraded accelerator X-ray sources. Lenses were manufactured by picosecond laser treatment of a high-quality single-crystal synthetic diamond. Twelve lenses with radius of curvature at parabola apex R = 200 µm, geometrical aperture A = 900 µm and length L = 1.5 mm were stacked as a compound refractive lens and tested at the ESRF ID06 beamline. A focal spot of size 2.2 µm and a gain of 20 were measured at 8 keV. The lens profile and surface quality were estimated by grating interferometry and X-ray radiography. In addition, the influence of X-ray glitches on the focusing properties of the compound refractive lens were studied. [ABSTRACT FROM AUTHOR]

A quantitative analysis of the crucial characteristics of currently used and promising materials for X-ray refractive optics is performed in the extended energy range 8-100 keV. According to the examined parameters, beryllium is the material of choice for X-ray compound refractive lenses (CRLs) in the energy range 8-25 keV. At higher energies the use of CRLs made of diamond and the cubic phase of boron nitride (c-BN) is beneficial. It was demonstrated that the presence of the elements of the fourth (or higher) period has a fatal effect on the functional X-ray properties even if low- Z elements dominate in the compound, like in YB66. Macroscopic properties are discussed: much higher melting points and thermal conductivities of C and c-BN enable them to be used at the new generation of synchrotron radiation sources and X-ray free-electron lasers. The role of crystal and internal structure is discussed: materials with high density are preferable for refractive applications while less dense phases are suitable for X-ray windows. Single-crystal or amorphous glass-like materials based on Li, Be, B or C that are free of diffuse scattering from grain boundaries, voids and inclusions are the best candidates for applications of highly coherent X-ray beams. [ABSTRACT FROM AUTHOR]

A new prism-array lens for high-energy X-ray focusing has been constructed using an array of different prisms obtained from different parabolic structures by removal of passive parts of material leading to a multiple of 2π phase variation. Under the thin-lens approximation the phase changes caused by this lens for a plane wave are exactly the same as those caused by a parabolic lens without any additional corrections when they have the same focal length, which will provide good focusing; at the same time, the total transmission and effective aperture of this lens are both larger than those of a compound kinoform lens with the same focal length, geometrical aperture and feature size. This geometry can have a large aperture that is not limited by the feature size of the lens. Prototype nickel lenses with an aperture of 1.77 mm and focal length of 3 m were fabricated by LIGA technology, and were tested using CCD camera and knife-edge scan method at the X-ray Imaging and Biomedical Application Beamline BL13W1 at Shanghai Synchrotron Radiation Facility, and provided a focal width of 7.7 µm and a photon flux gain of 14 at an X-ray energy of 50 keV. [ABSTRACT FROM AUTHOR]

In this paper we discuss the fabrication of lenses made out of multi-component oxide glasses by using double-sided hot embossing to allow low-cost, low-volume replication of these lenses. We focus on the choice of the glass for both the stamps as well as for the molded component and we discuss the various process parameters of the hot embossing process that influences the lens characteristics. We also analyze the geometrical and optical characteristics of the lenses using state-of-the-art optical characterization tools. We find that lenses with radii of curvatures up to 8 mm for a diameter of 4.5 mm can be obtained under standard operation conditions (without vacuum) and that the stamp material should be further optimized in case lenses with higher radii of curvature need to be achieved. [ABSTRACT FROM AUTHOR]

In this paper we present results of measurements of fiber coupling efficiency for optical mini lenses fabricated from multi-component soft glasses by using low-cost hot embossing technique. We show that soft glasses prove to be a suitable material for fabricating various types of mini lenses to be used in the compact optical system in broadband range from visible up to infrared at around 5 μm. The fabricated elements are biconvex refractive lenses and Fresnel lenses. We discuss soft glass properties, the fabrication process and the results of quality measurements of the fabricated lenses by means of the modulation transfer function. Finally, we report on the use of these diffractive and refractive elements for light coupling to various types of multimode optical fibers from a laser diode. [ABSTRACT FROM AUTHOR]

For the first time, single-crystal diamond planar refractive lenses have been fabricated by laser micromachining in 300 µm-thick diamond plates which were grown by chemical vapour deposition. Linear lenses with apertures up to 1 mm and parabola apex radii up to 500 µm were manufactured and tested at the ESRF ID06 beamline. The large acceptance of these lenses allows them to be used as beam-conditioning elements. Owing to the unsurpassed thermal properties of single-crystal diamond, these lenses should be suitable to withstand the extreme flux densities expected at the planned fourth-generation X-ray sources. [ABSTRACT FROM AUTHOR]

The spatial structure of a beam focused by a planar refractive lens and Bragg diffracted from perfect silicon crystals was experimentally studied at the focal plane using a knife-edge scan and a high-resolution CCD camera. The use of refractive lenses allowed for a detailed comparison with theory. It was shown that diffraction leads to broadening of the focused beam owing to the extinction effect and, for a sufficiently thin crystal, to the appearance of a second peak owing to reflection from the back surface. It was found that the spatial structure of the diffracted beam depends on whether the crystal diffracts strongly (dynamically) or weakly (kinematically). The results help to understand the physical origin of the diffracted intensity recorded in a typical microbeam diffraction experiment. [ABSTRACT FROM AUTHOR]

Abstract: A tremendous progress in X-ray optics development was made in the past decade. Progress has been driven by the unique properties of X-ray beams produced by third generation synchrotron sources. The very low emittance coupled with high brilliance allows one to develop efficient focusing devices for new X-ray microscopy techniques. This article provides an overview of the state-of-the-art in micro-focusing optics and methods for hard X-rays. The main emphasis is put on those methods which aim to produce submicron and nanometer resolution. These methods fall into three broad categories: reflective, refractive and diffractive optics. The basic principles and recent achievements are discussed for all optical devices. To cite this article: A. Snigirev, I. Snigireva, C. R. Physique 9 (2008). [Copyright &y& Elsevier]

Abstract: X-ray compound refractive lenses (CRL) are becoming a widespread tool for the generation of microfocus spot sizes at synchrotron beamlines. The calculation of their performance by means of ray-tracing is useful for a rapid estimation of flux, resolution and focusing properties achievable in a beamline, when other optics are present, or simply to study the lens acceptance and focusing in the presence of a particular bending magnet, wiggler or undulator X-ray source. The ray-tracing method presented in this paper has been used to calculate the efficiency of beryllium CRL''s using, for the instrument layout, realistic source size and divergence, and usual optics like perfect crystal monochromators. It is shown that the intensity transmitted by the lens, the effective aperture and the gain are in good agreement with analytical formulas. Additional information provided when running the program are the precise shape of beam at the focus, and at any position along the optical axis. For instance the intensity distribution at the CRL entrance and exit planes allows a comparison between the effective and the geometrical apertures. Finally, the method provides a precise value for the lens focal distance, which depends on the CRL length. [Copyright &y& Elsevier]

Silicon saw-tooth refractive lenses have been in successful use for vertical focusing and collimation of high-energy X-rays (50–100 keV) at the 1-ID undulator beamline of the Advanced Photon Source. In addition to presenting an effectively parabolic thickness profile, as required for aberration-free refractive optics, these devices allow high transmission and continuous tunability in photon energy and focal length. Furthermore, the use of a single-crystal material ( i.e. Si) minimizes small-angle scattering background. The focusing performance of such saw-tooth lenses, used in conjunction with the 1-ID beamline's bent double-Laue monochromator, is presented for both short (∼1:0.02) and long (∼1:0.6) focal-length geometries, giving line-foci in the 2 µm–25 µm width range with 81 keV X-rays. In addition, a compound focusing scheme was tested whereby the radiation intercepted by a distant short-focal-length lens is increased by having it receive a collimated beam from a nearer (upstream) lens. The collimation capabilities of Si saw-tooth lenses are also exploited to deliver enhanced throughput of a subsequently placed small-angular-acceptance high-energy-resolution post-monochromator in the 50–80 keV range. The successful use of such lenses in all these configurations establishes an important detail, that the pre-monochromator, despite being comprised of vertically reflecting bent Laue geometry crystals, can be brilliance-preserving to a very high degree. [ABSTRACT FROM AUTHOR]

A Fresnel-like X-ray lens can be constructed by a triangular array of identical prisms whose base corresponds to the 2π-shift length. Each column of prisms is progressively shifted from the optical axis by an arbitrary fraction of the prism height. Similarly to the multi-prism lens, quasi-parabolic profiles are formed by a superposition of straight-line segments. The resulting projected lens profile is approximately linear with a Fresnel-lens pattern superimposed on it to provide the focusing. This geometry exhibits a significantly larger effective aperture than conventional parabolic refractive lenses. Prototype lenses were fabricated by deep reactive ion etching of silicon. These one-dimensionally focusing lenses were tested at a synchrotron beamline and provided focal line-widths down to 1.4 μm FWHM and an intensity gain of 39 at a photon energy of 13.4 keV. Fabrication imperfections gave rise to unwanted interference effects resulting in several intensity maxima in the focal plane. The presented design allows the focal length to be shortened without decreasing the feature size of the lens. Furthermore, this feature size does not limit the resolution as for real Fresnel optics. [ABSTRACT FROM AUTHOR]

Abstract: X-ray compound refractive lenses (CRLs) are successfully applied in many experiments with synchrotron radiation, especially, at the third generation SR sources. At the same time, advantages of CRL employment at SR sources of previous generations with larger source sizes are not so obvious, and it seems to be useful to reveal the conditions and the experiments in which application of CRLs could be effective. With this purpose, first study of SR beam focusing by plastic planar lenses with a parabolic profile in the photon energy range from 10.4 to 14keV was carried out at the VEPP-3 storage ring. Three different experimental techniques were applied to measure intensity distribution in the focal line. The measured focused beam line was 17–18μm and the real gain in intensity was about 7. The obtained values differ from the expected calculated ones by a factor of about 2. The possible reasons of this discrepancy and potential employment of CRLs in real SR experiments at the VEPP-3 are discussed. [Copyright &y& Elsevier]

Compound refractive lenses (CRLs) are effective for collimating or focusing high-energy X-ray beams (50-100 keV) and can be used in conjunction with crystal optics in a variety of configurations, as demonstrated at the 1-ID undulator beamline of the Advanced Photon Source. As a primary example, this article describes the quadrupling of the output flux when a collimating CRL, composed of cylindrical holes in aluminium, is inserted between two successive monochromators, i.e. a modest-energy-resolution premonochromator followed by a high-resolution monochromator. The premonochromator is a cryogenically cooled divergence-preserving bent doubleLaue Si(111) crystal device delivering an energy width ΔE/E ... 10-3, which is sufficient for most experiments. The high-resolution monochromator is a four-reflection flat Si(111) crystal system resembling two channel-cuts in a dispersive arrangement, reducing the bandwidth to less than 10-4, as required for some applications. Tests with 67 and 81 kev photon energies show that the highresolution monochromator, having a narrow angular acceptance of a few microradians, exhibits a fourfold throughput enhancement due to the insertion of a CRL that reduces the premonochromatized beam's vertical divergence from 29 µrad to a few microradians. The ability to focus high-energy X-rays with CRLs having long focal lengths (tens of meters) is also shown by creating a line focus of 70-90 µm beam height in the beamline end-station with both the modest-energyresolution and the high-energy-resolution monochromatic X-rays. [ABSTRACT FROM AUTHOR]

For many X‐ray microprobe experiments it is desirable to be able to vary the beam size: using large beams for survey scans and a small beam for the final measurements. Beryllium refractive lenses were found to be a simple and controllable method for enlarging the focus in a Kirkpatrick–Baez‐based microprobe. They can provide variable spot size, can be quickly inserted or removed and do not move the beam center on the sample. [ABSTRACT FROM AUTHOR]