Because of the scientific successes of research using synchrotron radiation (SR) and the unique properties of SR beams, the design of optical elements for use in SR presents problems which are both challenging and important. This paper reviews the unique requirements imposed on SR optics and broadly discusses the research and development needed for their achievement.

A brief description of the nature and origins of synchrotron radiation is given with special reference to its geometrical optical properties and the use of storage rings as light sources. The geographical distribution of SR sources in the world is reviewed and some discussion of the level of experimental activity is given. Estimates of future levels of experimental activity are also made both for existing storage rings and those planned for the future. Calculations of the approximate number of mirrors and gratings that will be required are offered. Some general considerations are outlined showing how synchrotron radiation optical systems couple to the light source and indicating which parameters need to be maximized for best overall performance.*

Paraboloidal mirrors used at grazing incidence have come into widespread use as components of astronomical x-ray imaging systems (Wolter lenses). Although grazing incidence paraboloids by themselves have extremely narrow fields of view that are free of aberrations, if precise imaging is not required they are most useful as collimators and objectives for radiometric purposes. A recent design for the optics of a storage ring beam line grating/crystal (G/C) monochromator (1) uses two paraboloidal mirrors at small glancing angles (2°). One mirror is a collimator that provides parallel radiation for the G/C monochromator and the other is an objective that images the monochromatic beam at the exit slit. This optical system is designed to accept 6 mrad parallel to the orbital plane (horizontal) of the storage ring from a source approximately 1 mm x 0.5 mm, and to have a resolving power of 2000 or larger. Ray tracings showed that, under certain conditions, the system faithfully images the source. However, the imaging fidelity is wavelength dependent, when gratings are used in the monochromator, for reasons not immediately obvious. This paper investigates the aberrations of the beam line optical system to assess their effect on the image and to learn to what limits the horizontal acceptance angle can be increased without undue distortion of the slit image and serious impairment of the resolving power.

We numerically analyze at near-grazing incidence various configurations containing elliptical, toroidal, and the recently proposed common-axis rotationally symmetric anamorphic (CARSA) elements to determine the type and importance of aberrations inherent in each. CARS pairs of elliptical mirrors are shown to have negligible focusing errors for typical synchrotron source dimensions, and therefore comprise useful building blocks from which to construct grazing-incidence optical systems. To minimize aberrations, cylindrical, spherical, or toroidal elements should be avoided.

Design rules are discussed for stigmatic toroidal monochromators used in conjunction with toroidal mirrors. The phase balancing method and the brightness conservation law have been used to optimize coma balancing effects.

Recently, a design method has been proposed for grazing incidence synchrotron radiation mirrors (Nuclear Instruments and Methods, 175, 1980, 273-280) that emphasizes the conservation and development of symmetrical phase-space contours upon reflection. In this paper we will examine the difference between such surfaces and those of conventional synchrotron radiation mirrors by computing the degree and the nature of the phase-space distortions arising from each type of surface. The results will be of relevance in selecting optimal designs for reflecting optics for synchrotron radiation sources.

The history of optical fabrication is traced from Roman times to the 1900's to indicate the level of the art. This background serves as a reference for discussing the particular optical fabrication problems associated with grazing incidence optics. It is suggested that "bend and polish" techniques may be particularly applicable to the fabrication of vacuum ultraviolet and x-ray collimator optics.

Two large cylindrical reflectors used at near grazing incidence for Synchrotron radiation have been made by our firm. Each job required the development of new capability and the building or modification of machinery for grinding and lapping the surfaces.

A number of techniques may be employed to control the figure of a bent mirror. The accuracy of profile obtained depends on the accuracy of manufacture of both the mirror and the bending block. The profile should be verified by dimensional metrology, and the adoption of 'open' designs of bending block facilitates access for metrology. Open access to the bent mirror surface also facilitates in situ inspection by other techniques, such as interferometry and microscopy. A new design of bending block provides such access. The mounting for the bending block also maintains the pole of the mirror on a fixed axis regardless of mirror curvature, and the length of mirror illuminated is constant regardless of the angle of incidence set.

A cylindrical "bent" mirror configuration constructed for use at the SRS on surface EXAFS experiments is described. Date on its performance with an expanded laser beam, and with synchrotron radiation, are presented. This paper is a summary of the design criteria and initial results obtained with a bent cylindrical mirror used to focus the synchrotron radiation source at Daresbury Laboratory onto an experimental sample. It is used, in conjunction with a soft x-ray monochromator primarily for surface EXAFS. The principle behind its operation has been concisely explained by Howell and Horowitz¹ some time ago and the technique is well established. Thus this paper will concentrate on the geometry we chose and the method adopted for alignment.

Ideal optical elements can be approximated by bending flats or cylinders. This paper considers the applications of these approximate optics to synchrotron radiation. Analytic and raytracing studies are used to compare their optical performance with the corresponding ideal elements. It is found that for many applications the performance is adequate, with the additional advantages of lower cost and greater flexibility. Particular emphasis is placed on obtaining the practical limitations on the use of the approximate elements in typical beam-line configurations. Also considereC are the possibilities for approximating very long length mirrors using segmented mirrors.

After complete removal of all pits from a fine ground surface by optical polishing, a sub-system of fractures will remain which will cause scattering of either reflected or refracted radiation. This is especially critical in the case of incident ultraviolet radiation. This paper will show the cause and effect of this fracture system, if it is not totally eliminated.

The various theories of specular x-ray reflection at small grazing angles are briefly examined and compared with the reflectance of several very smooth mirror surfaces. Particular emphasis is given to examining the peak reflection efficiency and the harmonic rejection capability of x-ray mirrors operating in the hard x-ray domain. The graded index of refraction model of Croce and Nevot most closely matches the experimental curves. In this model the surface roughness is treated as a local variation in the refractive index. In experimental tests, platinum coated and float glass mirrors were observed to reflect x-rays with high efficiency up to 38 keV. The measured harmonic rejection capability of a float glass mirror and of a platinum coated mirror were found to be virtually identical. In particular, for a raw float glass mirror at a 3.5 milliradian angle of incidence, the mirror reflected with 85% efficiency at 8 keV and only 8% at 16 keV. For a platinum mirror at a 8 milli-radian angle, the corresponding reflectivities were 70% at 8 keV and 6% at 16 keV, Fresnel theory for a smooth platinum surface coating overestimates the reflectivity by a factor of about 30 for the 16 keV x-rays. Therefore, real platinum coated mirrors may be better harmonic rejectors than previously realized. In other experiments, changing the surface roughness of float glass by chemical etching altered the falloff rate beyond the knee of the reflection curve by factors as large as 2.5.

The nature of optical constants and their measurement by reflection or absorption techniques in the extreme ultraviolet and soft x-ray spectral region from 30 to 3000eV is discussed with emphasis on mirror design. Sources of optical constant data are mentioned and reflectance measurements for SiC and Kanigen between 40 and 200 eV are reported.

Multilayer x-ray reflectors which have layers that vary in thickness, either in depth or laterally along the surface, were prepared by sputtering. Alternating layers of W and C were deposited on flexible mica and smooth silicon-wafer substrates. X-ray diffraction properties of the multilayers were measured in the 1.54-8.34 A region. The mutlilayer on mica represents a dual-spacing depth-graded device. Diffraction from both the multilayer and mica was observed. A smoothly-depth-graded multilayer on silicon has a relatively-wide rocking curve which agrees well with diffraction theory. A laterally-graded multilayer on silicon has a bilayer thickness (d value) which varies linearly from 24.8 to 29.2 A in 6 cm.

The deposition of multilayer coatings for reflective filters in the extreme UV region is controlled by measuring the reflection of soft x-rays during the coating process. The x-ray source is aligned for every layer at the angle at which the maximas and minimas coincide with the peaks and valleys of the standing wave of the filter. The accuracy of the positioning between source and substrate has to be better than 1/50 degrees.

Chemical vapor deposited (CVD) silicon carbide (SiC) has been undergoing evaluation as a candidate mirror and mirror substrate material for several years. A summary of the test results is presented along with the material development work currently underway. The tests show that a super polished CVD-SiC surface is an outstanding mirror for some applications. For others, the preferred mirror is a composite produced by coating the polished CVD-SiC with an appropriate surface layer. An exceptional feature of the CVD SiC is its tolerance to cleaning, coating and recoating without damaging the super polish. Work to date has been on small, flat mirrors (less than 3" x 3"). For synchrotron radiation (SR) work, the need is for large mirrors (up to 10 x 40 inches) with aspheric figures. The current material development effort is concentrated on providing improved quality at the intermediate size of 41/2 x 22 inches. Future work will be devoted to establishing the technology of polishing and figuring, together with the scale-up to the 40 inch size.

CVD-SiC has been identified as the leading mirror material for high energy synchrotron radiation because of its high K/a ratio and its ability to be super-polished to <10 A rms roughness. Technology already exists for depositing SiC over large areas (approximately 70 cm x 20 cm). The CVD process, substrate selection, and mirror design considerations are discussed.

The high thermal conductivity and low thermal expansion of silicon carbide make it an ideal mirror material for synchrotron radiation applications. Such mirrors must withstand absorption of a few kilowatts of x-ray flux without degradation or distortion. Silicon carbide has the added advantage of high reflectance in the vacuum UV. Many different polycrystalline forms of silicon carbide are available. The fabrication of mirrors in hot pressed, reaction bonded and chemical vapour deposited (CVD) silicon carbide is described, from manufacture of the initial blanks, through diamond grinding of the optical figure, to final polishing and installation in the synchrotron. The suitability of each of these materials as mirror substrates has been assessed from the point of view of crystal structure, polishability, optical figure stability, reflectance and vacuum compatibility. Chemical vapour deposited silicon carbide gives an excellent optical finish, the best polished sample produced so far having a reflectance three times that of gold at 584 R, but it is difficult to make in large flat pieces. The hot pressed and reaction bonded materials are much more easily obtainable and take a good enough polish for most applications. Several flat, spherical and cylindrical mirrors in CVD, hot pressed and reaction bonded silicon carbide are planned or have been in use as premirrors in the Synchrotron Radiation Source (SRS) at Daresbury. Their performance during machine operation and the future availability of silicon carbide mirrors is discussed.

The radiation resistance of silica and silica-based mirrors to synchrotron radiation is relatively low compared with that of a number of ceramic materials, of which silicon carbide has been studied most extensively, although other materials such as the hafnium carbides and nitrides have generally better X-ray reflective properties. The polishabilities of various types of silicon carbide, having microstructures which range from very homogeneous to a coarse porosity have been investigated, as well as that of one type of titanium nitride. All the materials are capable of being polished to a very high degree, and surface finishes of a few tenths of a nanometre peak to valley have been attained.

The optimization of a laminar groove grating was determined by the use of electromagnetic theory. It was found that the absolute efficiency of optimized laminar and sinusoidal groove gratings are about the same, and that triangular groove gratings show higher efficiency. However, in the examples studied, laminar gratings showed considerably less second and third order contamination than the equivalently optimized triangular grooved gratings.

An apparatus has been built at the LURE in view to permit reflectivity and efficiency measurements in the X-U.V. range for a large number of ontical systems*. The reflectometer exploits the opportunity offered at a synchrotron radiation center where a large choice of monochromators can be used like premonochromators delivering monochromatic light over the entire range from X-rays to U.V. This reflectometer has been designed to measure absolute reflectivity of mirrors, gratings and large 2d spacing structures usable in X-U.V. (natural and organic crystals, metallic multilayers, organic multilayers called Langmuir Blodgett layers). An example of tests on such structures are given in another paper of this sessionsl.Here we will be restricted to a general presentation of our reflectometer and applications to tests of new holographic gratings ion etched on glass substrate.

The performance of gratings and mirrors in the VUV is given for specific systems at HASYLAB. Metal mirrors and bent glass optics are considered first. The photon fluxes for a Seya-Namioka monochromator, a toroidal grating monochromator, and the FLIPPER plane grating monochromator are given.

X-ray and EXUV diffraction gratings for synchrotron radiation are made from a ho lographically recorded master by ionetching very shallow groove profiles into the super-smooth surface of a stable silica material. Both rectangular and saw-tooth grating profiles are produced. The gratings are evaluated using optical, contacting stylus and X-ray calibration techniques.

Replicated nickel gratings are being developed which show good potential as damage resistant gratings for use in high energy lasers, and in monochromators for synchrotron radiation

Mechanical ruling of diffraction gratings is an art with a 150 year history and an indefinite figure. It is by far the most versatile of ruling methods available. It leads to the maximum attainable efficiencies over most of the electromagnetic spectrum and for a whole class of gratings represents the only approach to producing them. Since alternative methods have recently became feasible, and often very useful, it is appropriate to look at what the currnet boundaries are in terms of the various attributes that define a grating and its performance.

In this paper a novel scheme is proposed to measure and monitor the quality of the output beam from a grazing incidence monochromator using transmission gratings as a second dispersive system. With this method it has been possible both to characterize the efficiency and performance of the grazing incidence beam line optics and to find optimum alignment procedures to achieve the best performance of a high resolution, high throughput reflection optics system. In particular, it has been found possible, through a careful alignment procedure to achieve a transmitted intensity distribution with only minor contributions of higher harmonics.

The manufacturing tolerances for X-ray mirrors used for synchrotron radiation are demanding, in that departures from the theoretical surface may not exceed a few 1/10 μm, and the small scale roughness should not exceed 1 nm peak to valley. Both large and small scale deviations are measured with contacting probe instruments. X-ray mirrors must be stable and radiation resistant in intense synchrotron beams. Alternatives to silica glass include electroless nickel and silicon carbide. These materials can be finished to yield a small scale roughness of no more than a few 1/10 nm peak to valley.

Angle resolved scattering of synchrotron radiation incident on optical surfaces has been measured in the energy range from 75-750 eV. The scattering was measured in the plane of incidence, typically from 3 degrees to at least 40 degrees from the specular direction, with an angle of incidence of 84 degrees from the surface normal. The incident radiation was polarized either parallel or perpendicular to the plane of incidence, but the polarization state of the scattered radiation was not measured. Polished and diamond-turned optical surfaces were studied. The diamond-turned surfaces were turned with an interrupted cut, and measurements were taken with the direction of the grooves both parallel and perpendicular to the plane of incidence. The three diamond-turned surfaces were Cu, electroless Ni on Cu, and electroplated Au on electroless Ni on Cu. The polished samples included evaporated Au on a sapphire substrate.

This paper discusses the effects of surface topographic scattering on the performance of optical elements and illustrates the results in terms of the focussing mirrors of the Brookhaven plane-grating monochromator. Performance figures are described in terms of the power spectral density of the surface roughness, where the contributing range of surface spatial wavelengths, d, lies between the wavelength of the operating radiation and the aperture of the part: from nanometers to centimeters. This is considerably greater than the range included in "total-integrated scatter" or "TIS" measurements involving visible light at normal incidence: d = 0.65 to 12 pm. In order to explore the contributions of longer surface wavelengths we have measured the profiles of several optical-quality surfaces -- polished, diamond-turned and turned-and-polished -- with a mechanical stylus gauge, and have estimated their power spectral densities over the range d = 2 to 500 pm. The power spectra generally increase with increasing wavelength, although the spectral shapes vary widely. Contributions from d > 12 pm are significant in all of the cases examined, and are completely dominant in several cases. These preliminary results show that any useful characterization of surface finish must involve the power spectral density of its surface roughness over an extended range of surface wavelengths, including the critical long-wavelength region. A general discussion of measurement techniques and performance figures is given to provide a rational basis for the systematic study of these issues.

Techniques for manufacturing grazing incidence optics have been developed by Applied Optics Center during the past few years. Techniques include diamond machining and low scatter finishing. In testing grazing incidence optics for surface figure, slope error can be determined interferometrically or preferably, with a non-contact profilometer developed by Applied Optics Center

New technologies are continually being developed and applied to machine tools which improve the quality and c pability of single point machining. The development of the single cryst diamond tool and recent improvements in vibration control have made possible the generation of optical quality surface finishes. The marriage of this surface finish quality with the tool path Flexibility of numerical control machine tools makes feasible the fabrication of many needed optical components. These components could not be fabricated by conventional lapping and polishing because of inadequate figure control and high cost. This report is intended to familiarize the reader with the basic requirements necessary for machining precision optical components. In addition, a brief description of capabilities existing at this facility is presented.

Single-point diamond turning machines have made it possible to fabricate new optical components. However, new problems have arisen in the areas of polishing and testing. This paper presents an overview of past and present polishing and testing practices and offers suggestions for future research.