High-efficiency gratings are desirable for several reasons. A grating with high efficiency is more useful than one with lower efficiency in measuring weak transition lines in optical spectra. A grating with high efficiency may allow the reflectivity and transmissivity specifications for the other components in the spectrometer to be relaxed. Moreover, higher diffracted energy may imply lower instrumental stray light due to other diffracted orders, as the total energy flow for a given wavelength leaving the grating is conserved (being equal to the energy flow incident on it minus any scattering and absorption).— Dr. Christopher Palmer37.
Measurements of the diffraction efficiency across the NUV-IR spectral range can be made, for example, using a highly automated spectrograph (automated efficiency checker – AEC) and detector on a movable arm at Richardson Gratings37. Grating efficiency measurements are generally performed with a double monochromator system (Fig.18). The first monochromator supplies monochromatic light derived from a tungsten lamp, mercury arc or deuterium lamp, depending on the spectral region involved. The grating being tested serves as the dispersing element in the second monochromator. Measurements can be made either in monochromator mode, using a fixed deviation angle, and the incidence angle varied to match the wavelength, or in spectrograph mode, in which the detector is moved to follow the spectrum vs. wavelength for a fixed incidence angle. Either the incidence angle or the wavelength is varied over a sufficient range to test the efficiency predictions. In the normal mode of operation, the output is compared with that from a high grade mirror coated with the same material as the grating. The efficiency of the grating relative to that of the mirror is reported (relative efficiency), although absolute efficiency values can also be obtained (either by direct measurement or through knowledge of the variation of mirror reflectivity with wavelength). Polarizers can be used to allow independent tests of the transverse electric (TE or P) and transverse magnetic (TM or S) polarizations.
Both transmission and reflection gratings can be tested. In order to prevent mechanical interference between the illuminating beam and the measurement arm, the illumination is slightly (a few degrees or less) out of plane. Studies of this effect with the code show that the effect of this small out of plane angle on efficiencies is in general very low, with a cosine-type behavior. Reproducibility in the measured efficiency is typically within 1%. For plane reflection gratings, the wavelength region covered is usually 190 nm to 2.50 mkm; gratings blazed farther into the infrared are measured in higher orders. Concave reflection gratings focus as well as disperse the light, so the entrance and exit slits of the second monochromator are placed at the positions for which the grating was designed (that is, concave grating efficiencies are measured in the geometry in which the gratings are to be used). Transmission gratings are tested on the same equipment, with values given as the ratio of diffracted light to light falling directly on the detector, (i.e., absolute efficiency).
The Fully Automated Ultraviolet Spectrographic Tester (FAUST)38 developed at DGEF optical facility at NASA/GSFC is rigid, easy to use automated reliable scatterometer capable to operate both in visible, UV and VUV environment (124 nm to 632.8 nm). The instrument is capable of measuring the bi-directional reflectivity over a wide scatter angular range, especially from the grazing region, such as the off-specular peak, which is particularly interesting in grating scatter measurements and provide a reliable reflectance data for numerical simulation and comparison of mathematical model of UV gratings scatter properties. The instrument has a dynamic range of over 11 orders of magnitude (in inverse steradian units), and a noise level of determined by Raleigh scattering in the air surrounding the sample and by electronic noise. The instrument is capable of very low angle (few arcseconds) and very wide-angle (up to 120 degrees) scatter measurements under zero to 90 degrees angle of incidence variations utilizing the same set-up. High dynamic range of the instrument allows measurements of the scattering properties of both very specular (mirror-like) and very rough surfaces with high accuracy. Long 2 – meter rotary arm yields an angular resolution of 0.001 degree over a 120 degrees range for reflectance measurements. The sample mount is capable of 3 arcseconds incident angle resolution and can hold parts up to 30 cm in diameter.
These instrument configurations (Fig.19), particularly designed for vacuum grating scatter testing are used for COS/HST VUV gratings characterization3. Light source is PtNe lamp; detector is multi-channel multi-anode array (MAMA) with CsTe cathode (Fig.20). Grating incidence angle fixed, diffracted angle scatter scanned through detector arm rotation. COS/HST grating efficiency measurements also were accomplished using this instrument configuration. Grating relative efficiency Rg(-1) / (Rm × Rrefl), where Rg(-1) is integrated light intensity at near specular flux at -1 grating order, Rm is integrated sample flat mirror reflection and Rrefl is sample mirror reflectance at given angle and wavelength. Mirror reflectance is measured independently by MacPherson spectrometer. Sample mirror are mounted alongside with grating. Mirror/grating positions and orientations are prerecorded and fine-tuned under vacuum using MAMA imaging detector.
The modular design of the FAUST scatterometer allows for fast and easy modification of the instrument set-up. The entire system, including data acquisition and analysis, is fully automated and controlled via PC computer by user-friendly LabView written program. The BRDF measurements of sample surfaces were performed using spectrally narrow laser light sources and VUV lamps. Various types of the detectors employed cover spectral range from 100 to 1100 nm. Scatter measurements, reduced to standard BRDF, of the instrument signature as well as VUV gratings efficiency measurements are presented to demonstrate the capability and performance of the instrument.
As a good example, the grating efficiency in this range is measured using a synchrotron light source. The NRL X24C beamline is attached to the NSLS X-ray ring at the Brookhaven National Laboratory as shown in Fig.21. The beamline’s monochromator40 has two elements that are scanned under computer control while keeping the entrance and exit slits fixed (Fig.22). Each monochromator element is selectable and can be a diffraction grating, a crystal, or a mirror. The various elements are mounted on two carousels that are rotated into position. For example, if dispersed EUV radiation is desired, the two selected elements would be a grating and a gold mirror. The grating and the mirror then move under computer control so that dispersed radiation of a known wavelength passes through the exit slit and into the calibration chamber. Gratings with 150 g/mm, 600 g/mm, and 2400 g/mm are available. The overall range of coverage of the gratings is approximately 12 Å to 2000 Å. The resolving power, spectral range, and flux delivered to the calibration chamber depend on the wavelength and the monochromator elements that are selected. The resolving power is approximately 400 when the 600 g/mm grating and a gold mirror are utilized. Various filters, mounted on a linear translation bar, may be moved into the beam for the purpose of suppressing higher-order radiation from the monochromator. The radiation is highly polarized (80-90%) with the electric field vector in the horizontal plane. This permits the study of the polarization properties of X-ray-EUV optics. The polarization properties impact the science that can be accomplished with solar and astrophysical spaceflight instruments. X24C is the world’s leading beamline for these studies. The minimum beam divergence is approximately 1 mrad in the vertical direction and adjustable over the range of 1 to 6 mrad in the horizontal direction (defined by a beamlimiting aperture). The size of the beam varies from 1 mm to 3 mm depending on the distance from the monochromator’s exit slit, the slit width, and the width of the beam-limiting aperture. Strict UHV beamline cleanliness and contamination procedures are followed. The typical base pressure in the beamline is 2 × 10-9 Torr or lower.
A photograph of the reflectometer, the photodiode chamber, and the larger instrument calibration chamber attached to the beamline is shown in Fig.23. Smaller instrument components such as gratings, filters, and sensors can be calibrated in the reflectometer and the photodiode chamber. Large components and complete instruments can be calibrated in the larger calibration chamber. The internal motions of the reflectometer are shown in Ref. 39. The small sample (such as a grating or filter) is mounted on the rotational axis and is precisely rotated to change the angle of incidence. The detector can be rotated in azimuthal angle about the sample and in altitude. The angular motions are performed under computer and are accurate to 0.05°. The entire reflectometer chamber can be rotated from the vertical orientation to the horizontal orientation to measure the sample response in the orthogonal polarization. The pressure in the reflectometer is typically less than 10-7 Torr. The photodiode chamber has a translational fixture that can hold up to six sensors. The sensor currents are measured by a precision Keithley electrometer and saved in a computer file. The sensor response can be related to the current from a silicon photodiode with absolute responsivity traceable to NIST.