Ruled Diffraction Gratings

The most vital component in the production of ruled diffraction gratings is the apparatus, called a ruling engine, on which master gratings are ruled. MKS has four ruling engines in operation, each producing a substantial number of high-quality master gratings every year. Each of these engines produces gratings with very low Rowland ghosts, high resolving power, and high efficiency uniformity.

Selected diamonds, whose crystal axis is oriented for optimum behavior, are used to shape the grating grooves. The ruling diamonds are carefully shaped by skilled diamond toolmakers to produce the exact groove profile required for each grating. The carriage that moves the diamond back and forth during ruling must maintain its position to better than a few nanometers for ruling times that may last from one day to several weeks.

The mechanisms for advancing the grating carriages on all MKS ruling are designed to make it possible to rule gratings with a wide choice of groove frequencies. The Diffraction Grating Catalog published by MKS the range of groove frequencies available.

In 1947 Bausch & Lomb acquired its first ruling engine from the University of Chicago; this engine was originally designed by Michelson in the 1910s and rebuilt by Gale. It underwent further refinement, which greatly improved its performance, and has produced a continuous supply of high-quality gratings of up to 200 x 250 mm ruled area.

The Michelson engine originally used an interferometer system to plot the error curve of the lead screw, from which an appropriate mechanical correction cam was derived. In 1990, this system was superseded by the addition of a digital computer servo control system based on a laser interferometer. The Michelson engine is unusual in that it covers the widest range of groove frequencies of any ruling engine: it can rule gratings as coarse as 32 grooves per millimeter (g/mm) and as fine as 5,400 g/mm.

The second ruling engine installed at MKS has been producing gratings since 1953, was originally built by the David W. Mann Co. of Lincoln, Massachusetts. Bausch & Lomb equipped it with an interferometric control system following the technique of Harrison of MIT. The Mann engine can rule areas up to 110 x 110 mm, with virtually no detectable ghosts and nearly theoretical resolving power.

While the lead screws of the ruling engines are lapped to the highest precision attainable, there are always residual errors in both threads and bearings that must be compensated to produce the highest quality gratings. The Mann engine is equipped with an automatic interferometer servo system that continually adjusts the grating carriage to the correct position as each groove is ruled. In effect, the servo system simulates a perfect screw.

The third ruling engine at MKS was built by Harrison and moved to Rochester in 1968. It has the capacity to rule plane gratings to the greatest precision ever achieved; these gratings may be up to 420 mm wide, with grooves (between 20 and 1500 per millimeter) up to 320 mm long. It uses a double interferometer control system, based on a frequency-stabilized laser, to monitor not only table position but to correct residual yaw errors as well. This engine produces gratings with nearly theoretical resolving powers, virtually eliminating Rowland ghosts and minimizing stray light. It has also ruled almost perfect echelle gratings, the most demanding application of a ruling engine.

The fourth MKS ruling engine was also built by Harrison in the 1960s and transferred in the 1970s to the Association of Universities for Research in Astronomy (AURA) in Tucson, Arizona, where it was installed to support the Kitt Peak National Observatory. At that time the original control system was upgraded to a solid-state system using integrated circuit technology. In 1995 the engine was acquired by and moved to Richardson Gratings. A laser-based control system was developed that uses two interferometers for translation and yaw correction. The ‘C’ engine has a grating carriage travel of 813 mm and can rule gratings with a groove length of 460 mm; this engine has ruled gratings up to 400 mm x 600 mm with good wavefront and efficiency characteristics, comparable to those of ‘B’ engine rulings.

Master gratings are ruled on carefully selected well-annealed substrates of several different materials. The choice is generally between BK-7 optical glass, special grades of fused silica, or a special grade of Schott ZERODUR^®. The optical surfaces of these substrates are polished to closer than λ/10 for green light (about 50 nm), then coated with a reflective film (usually aluminum or gold).

Compensating for changes in temperature and atmospheric pressure is especially important in the environment around a ruling engine. Room temperature must be held constant to within 0.01 °C for small ruling engines (and to within 0.005 °C for larger engines). Since the interferometric control of the ruling process uses monochromatic light, whose wavelength is sensitive to the changes of the refractive index of air with pressure fluctuations, atmospheric pressure must be compensated for by the system. A change in pressure of 2.5 mm of mercury (300 Pa) results in a corresponding change in wavelength of one part per million. This change is negligible if the optical path of the interferometer is near zero but becomes significant as the optical path increases during the ruling. If this effect is not compensated, the carriage control system of the ruling engine will react to this change in wavelength, causing a variation in groove spacing that are easily transmitted to the diamond. This may be done by suspending the engine mount from springs that isolate vibrations between frequencies from 2 or 3 Hz (which are of no concern) to about 60 Hz, above which vibration amplitudes are usually too small to have a noticeable effect on ruled master grating quality.

The actual ruling of a master grating is a long, slow and painstaking process. The set-up of the engine, prior to the start of the ruling, requires great skill and patience. The critical alignment requires the use of a high-power interference microscope, or an electron microscope for more finely spaced grooves.

After each microscopic examination, the diamond is readjusted until the operator is satisfied that the groove shape is appropriate for the particular grating being ruled. This painstaking adjustment, although time consuming, results in very "bright" gratings with nearly all the diffracted light energy concentrated in a specific angular range of the spectrum. This ability to concentrate the light selectively at a certain part of the spectrum is what distinguishes blazed diffraction gratings from all others.

Finished master gratings are carefully tested to be certain that they have met specifications completely. The wide variety of tests run to evaluate all the important properties include spectral resolution, efficiency, Rowland ghost intensity, and surface accuracy. Wavefront interferometry is used when appropriate. If a grating meets all specifications, it is then used as a master for the production of our replica gratings.

For over a century, great effort has been expended in keeping the spacing between successive grooves uniform as a master grating is ruled. In an 1893 paper, Cornu realized that variations in the groove spacing modified the curvature of the diffracted wavefronts  While periodic and random variations were understood to produce stray light, a uniform variation in groove spacing across the grating surface was recognized by Cornu to change the location of the focus of the spectrum, which need not be considered a defect if properly taken into account. He determined that a planar classical grating, which by itself would have no focusing properties if used in collimated incident light, would focus the diffracted light if ruled with a systematic 'error' in its groove spacing. He was able to verify this by ruling three gratings whose groove positions were specified to vary as each groove was ruled. Such gratings, in which the pattern of straight parallel grooves has a variable yet well-defined (though not periodic) spacing between successive grooves, are now called varied line-space (VLS) gratings. VLS gratings have not found use in commercial instruments but are occasionally used in spectroscopic systems for synchrotron light sources.