Differences Between Ruled and Holographic Gratings

The efficiency curves of ruled and holographic gratings generally differ considerably, though this is a direct result of the differences in groove profiles and not strictly due to method of making the master grating. For example, holographic gratings made using the Sheridon method have nearly triangular groove profiles, and therefore have efficiency curves that look more like those of ruled gratings than those of sinusoidal-groove holographic gratings.

There exist no clear rules of thumb for describing the differences in efficiency curves between ruled and holographic gratings; the best way to gain insight into these differences is to look at representative efficiency curves of each grating type. The paper by Loewen et al. on which this chapter is based contains even more efficiency curves, and the book Diffraction Gratings and Applications by Loewen and Popov has an extensive collection of efficiency curves and commentary regarding the efficiency behavior of plane reflection gratings, transmission gratings, echelle gratings and concave gratings.

Since holographic gratings do not involve burnishing grooves into a thin layer of metal, the surface irregularities on its grooves differ from those of mechanically ruled gratings. Moreover, errors of ruling, which are a manifestation of the fact that ruled gratings have one groove formed after another, are nonexistent in interferometric gratings, for which all grooves are formed simultaneously. Holographic gratings, if properly made, can be entirely free of both small periodic and random groove placement errors found on even the best mechanically ruled gratings. Holographic gratings may offer advantages to spectroscopic systems in which light scattered from the grating surface is performance-limiting, such as in the study of the Raman spectra of solid samples, though proper instrumental design is essential to ensure that the performance of the optical system is not limited by other sources of stray light.

While holographic gratings generally exhibit lower scattered light than early ruled gratings, modern control systems and improved master coatings have led to ruled masters whose replicas exhibit scattered light as low as that of replicas of holographic masters. Some commercially-available Raman spectrometers now use ruled gratings, since their scattered light properties are suitable even for such a demanding application.

The groove profile has a significant effect on the light intensity diffracted from the grating. While ruled gratings may have triangular or trapezoidal groove profiles, holographic gratings usually have sinusoidal (or nearly sinusoidal) groove profiles (see Figure 4-5). A ruled grating and a holographic grating, identical in every way except in groove profile, will have demonstrably different efficiencies (diffraction intensities) for a given wavelength and spectral order. Moreover, ruled gratings are more easily blazed (by choosing the proper shape of the burnishing diamond) than are holographic gratings, which are usually blazed by ion bombardment (ion etching). Differences in the intensity diffracted into the order in which the grating is to be used implies differences in the intensities in all other orders as well; excessive energy in other orders usually makes the suppression of stray light more difficult.

The distribution of groove profile characteristics across the surface of a grating may also differ between ruled and holographic gratings. For a ruled concave grating, the facet angles are not aligned identically, and the effective blaze wavelength varies from one side of the grating to the other. A holographic grating, on the other hand, usually demonstrates much less variation in efficiency characteristics across its surface. Gratings have been ruled by changing the facet angle at different places on the substrate during ruling. These so-called "multipartite" gratings, in which the ruling is interrupted and the diamond reoriented at different places across the width of the grating, demonstrate enhanced efficiency but do not provide the resolving power expected from an uninterrupted ruling (since each section of grooves may be out of phase with the others).

The number of grooves per millimeter for ruled and holographic gratings can vary over a very wide range. Gratings of both types can be made with very coarse groove patterns – as low as 30 g/mm for ruled gratings and as low as 1 g/mm for holographic gratings. As an upper limit, both holographic and ruled gratings have been produced with groove densities up to 10,000 grooves per millimeter.

Classical ruled plane gratings, which constitute the vast majority of ruled gratings, have straight equally-spaced grooves. Classical ruled concave gratings have unequally spaced grooves that form circular arcs on the grating surface, but this groove pattern, when projected onto the plane tangent to the grating at its center, is still a set of straight equally spaced lines. [It is the projected groove pattern that governs imaging.] Even ruled varied line-space (VLS) gratings do not contain curved grooves, except on curved substrates. The aberration reduction possible with ruled gratings is therefore limited to that possible with straight grooves, though this limitation is due to the mechanical motions possible with present-day ruling engines rather than with the burnishing process itself.

Holographic gratings, on the other hand, need not have straight grooves. Groove curvature can be modified to reduce aberrations in the spectrum, thereby improving the throughput and spectral resolution of imaging spectrometers. A common spectrometer mount is the flat-field spectrograph, in which the spectrum is imaged onto a flat detector array and several wavelengths are monitored simultaneously. Holographic gratings can significantly improve the imaging of such a grating system, whereas classical ruled gratings are not suitable for forming well-focused planar spectra without auxiliary optics.

The interference pattern used to record holographic gratings is not dependent on the substrate shape or dimension, so gratings can be recorded interferometrically on substrates of low ƒ/number more easily than they can be mechanically ruled on these substrates. Consequently, holographic concave gratings lend themselves more naturally to systems with short focal lengths. Holographic gratings of unusual curvature can be recorded easily; of course, there may still remain technical problems associated with the replication and testing of such gratings.

The substrate shape affects both the grating efficiency characteristics its imaging performance.

• Grating efficiency depends on the groove profile as well as the angle at which the light is incident and diffracted; for a concave grating, both the groove profile and the local angles vary with position on the grating surface. This leads to the efficiency curve being the sum of the various efficiency curves for small regions of the grating, each with its own groove profile and incidence and diffraction angles.
• Grating imaging depends on the directions of the diffracted rays over the surface of the grating, which in turn are governed by the local groove spacing and curvature (i.e., the groove pattern) as well as the local incidence angle. For a conventional plane grating used in collimated light, the groove pattern is the same everywhere on the grating surface, as is the incidence angle, so all diffracted rays are parallel. For a grating on a concave substrate, though, the groove pattern is generally position-dependent, as is the local incidence angle, so the diffracted rays are not parallel – thus the grating has focal (imaging) properties as well as dispersive properties.

While ruled master gratings can generally be as large as 320 x 420 mm, holographic master gratings are rarely this large, due to the requirement that the recording apparatus contain very large, high-quality lenses or mirrors, and well as due to the decrease in optical power farther from the center of the master grating substrate.

A ruled master grating is formed by burnishing each groove individ-ually; to do so, the ruling diamond may travel a very large distance to rule one grating. For example, a square grating of dimensions 100 x 100 mm with 1000 grooves per millimeter will require the diamond to move 10 km (over six miles), which may take several weeks to rule.

In the fabrication of a master holographic grating, on the other hand, the grooves are created simultaneously. Exposure times vary from a few minutes to tens of minutes, depending on the intensity of the laser light used and the spectral response (sensitivity) of the photoresist at this wavelength. Even counting preparation and development time, holographic master gratings are produced much more quickly than ruled master gratings. Of course, an extremely stable and clean optical recording environment is necessary to record precision holographic gratings. For plane gratings, high-grade collimating optics are required, which can be a limitation for larger gratings.