Proper technical specifications are needed to ensure that the part supplied by the manufacturer meets the requirements of the customer. This is especially true for diffraction gratings, whose complete performance features may not be fully recognized. Documents that provide guidance in the specification of optical components, such as the ISO 10110 series ("Optics and optical instruments: Preparation of drawings for optical elements and systems"), do not clearly lend themselves to the specification of diffraction gratings. This chapter provide guidelines for generating clear and complete technical specifications for gratings.

Specifications should meet the following criteria.

• They should refer to measurable properties of the grating.

• They should be as objective as possible (avoiding judgment or interpretation).

• They should be quantitative where possible.

• They should employ common units where applicable (the SI system is preferred).

• They should contain tolerances..

A properly written engineering print for a diffraction grating will be clear and understandable to both the customer and the manufacturer.

All grating prints should contain, at a minimum, the following specifications.

1. Free Aperture. The free aperture, also called the clear aperture, of a grating is the maximum area of the surface that will be illuminated. The free aperture is assumed to be centered within the ruled area (see below) unless otherwise indicated. For configurations in which the grating will rotate, such as in a monochromator, it is important to specify the free aperture as the maximum dimensions of the beam on the grating surface (i.e., when the grating is rotated most obliquely to the incident beam). Also, it is important to ensure that the free aperture specifies an area that is completely circumscribed by the ruled area, so that the illuminated area never includes part of the grating surface that does not have grooves.

   The free aperture of the grating is that portion of the grating surface for which the optical specifications apply (e.g., Diffraction Efficiency, Wavefront Flatness or Curvature, Scattered Light – see below).

2. Ruled Area. The ruled area of a grating is the maximum area of the surface that will be covered by the groove pattern. The ruled area is assumed to be centered on the substrate face unless otherwise indicated. By convention, the ruled area of a rectangular grating is specified as "groove length by ruled width" – that is, the grooves are parallel to the first dimension; for example, a ruled area of 30 mm x 50 mm indicates that the grooves are 30 mm long.

   Most rectangular gratings have their grooves parallel to the shorter substrate dimension. For gratings whose grooves are parallel to the longer dimension, it is helpful to specify "long lines" to ensure that the grooves are made parallel to the longer dimension.

3. Substrate Dimensions. The substrate dimensions (width, length, and thickness) should be called out, as should their tolerances. If the grating is designed to be front-mounted, the substrate specifications can be somewhat looser than if the grating surface will be positioned or oriented by the precise placement of the substrate. Front-mounting a grating generally reduces its cost and production time (see Alignment below).

   A grating substrate should have bevels on its active face, so that it is easier to produce and to reduce chipping the edges while in use. Bevel dimensions should be specified explicitly and should be considered in matching the Ruled Area (above) with the substrate dimensions.^† For custom (special-size) substrates, certain minimum bevel dimensions may be required to ensure that the grating is manufacturable – please contact us for advice.

4. Substrate Material. The particular substrate material should be specified. If the material choice is of little consequence, this can be left to the manufacturer, but especially for applications requiring substrates with low thermal expansion coefficients, or requiring gratings that can withstand high heat loads, the substrate material and its grade should be identified. For transmission gratings, the proper specification of the substrate material should include reference to the fact that the substrate will be used in transmission, and may additionally refer to specifications for refractive index, inclusions, bubbles, striae, etc.

5. Nominal Surface Figure. Plane (flat) gratings should be specified as being planar; concave gratings should have a radius specified, and the tolerance in the radius should be indicated in either millimeters or fringes of red HeNe light (l = 632.8 nm) (a "wave" being a single wavelength, equaling 632.8 nm, and a "fringe" being a single half-wavelength, equaling 316.4 nm). Deviations from the nominal surface figure are specified separately as "wavefront flatness" or "wavefront curvature" (see below).

6. Wavefront Flatness or Curvature. This specification refers to the allowable deviation of the optical surface from its Nominal Surface Figure (see above). Plane gratings should ideally diffract plane wavefronts when illuminated by collimated incident light. Concave gratings should ideally diffract spherical wavefronts that converge toward wavelength-specific foci. In both cases, the ideal radius of the diffracted wavefront should be specified (it is infinite for a plane grating) and maximum deviations from the ideal radius should also be called out (e.g., the tolerance in the radius, higher-power irregularity in the wavefront). It is important to specify that grating wavefront testing be done in the diffraction order of use if possible, not in zero order, since the latter technique does not measure the effect of the groove pattern on the diffracted wavefronts. Deviations from a perfect wavefront are most often specified in terms of waves or fringes of red HeNe light. Generally, wavefront is specified as an allowable deviation from the nominal focus ("power") and allowable higher-order curvature ("irregularity").

7. Groove Spacing or Frequency. The number of grooves per millimeter, or the spacing between adjacent grooves, should be specified, but not both (unless one is subjugated to the other by labeling it as "reference"). For a grating whose groove spacing varies across the surface (e.g., an aberration-corrected concave holographic grating), the groove spacing (or frequency) is generally specified at the center of the grating surface.

8. Groove Alignment. Alignment refers to the angle between the groove direction and an edge of the grating substrate. Sometimes this angular tolerance is specified as a linear tolerance by stating the maximum displacement of one end of a groove (to an edge) relative to the other end of the groove. Generally a tight alignment specification increases manufacturing cost; it is often recommended that alignment be allowed to be somewhat loose and that the grating substrate dimensions not be considered for precise alignment but that the grating surface be oriented and positioned optically instead of mechanically (see comments in Substrate Dimensions above).

9. Diffraction Efficiency. Grating efficiency is generally specified as a minimum at a particular wavelength; often this is the peak wavelength (i.e., the wavelength of maximum efficiency). Occasionally efficiency specifications at more than one wavelength are called out.

   Either relative or absolute diffraction efficiency should be specified. Relative efficiency is specified as the percentage of the power at a given wavelength that would be reflected by a mirror (of the same coating as the grating) that is diffracted into a particular order by the grating (that is, efficiency relative to a mirror). Absolute efficiency is specified as the percentage of the power incident on the grating that is diffracted into a particular order by the grating.

   In addition to the wavelength and the diffraction order, grating efficiency depends on the incidence and diffraction angles a and b; if these angles are not explicitly stated, the standard configuration (namely the Littrow configuration, in which the incident and diffracted beams are coincident) will generally be assumed. Unless otherwise noted on the curves themselves, all Newport efficiency curves are generated for the near-Littrow conditions of use with eight degrees between the incident and diffracted beams: a – b = 2K = 8°.

   Generally diffraction gratings are polarizing elements, so that the efficiency in both polarizations should be considered:

   P-plane	TE	light polarized parallel to grooves
   S-plane	TM	light polarized perpendicular to grooves

   For each wavelength that has an efficiency specification, the following should be indicated: the wavelength, the efficiency (in percent), whether the efficiency specification is relative or absolute, the diffraction order, the polarization of the light, and the angles a and b. In some cases, the bandwidth of the exit slit in the spectrometer used to measure the grating efficiency may need to be called out as well.

Additional specifications are sometimes required based on the particular application in which the grating is to be used.

10. Blaze Angle. Although it is better to specify diffraction efficiency, which is a performance characteristic of the grating, sometimes the blaze angle is specified instead (or additionally). A blaze angle should be specified only if it is to be measured and verified (often done by measuring efficiency anyway), and a tolerance should be noted. In cases where both the diffraction efficiency and the blaze angle are specified, the efficiency specification should be controlling and the blaze angle specification should be for reference only.

11. Coating Material. Generally the Diffraction Efficiency specifications will dictate the coating material, but sometimes a choice exists and a particular coating should be specified. Additionally, dielectric overcoatings may be called out that are not implied by the efficiency specifications.

12. Scattered Light. Grating scattered light is usually specified by requiring that the fraction of monochromatic light power incident on the grating and measured a particular angle away from the diffracted order falls below a certain upper limit. The proper specification of scattered light would call out the test configuration, the polarization and wavelength of the incident light, the incidence angle, the solid angle subtended by the detector aperture, and the dimensions of the exit slit. Grating scatter is measured at Newport using red HeNe light.

13. Cosmetics. The cosmetic appearance of a diffraction grating does not correlate strongly with the performance of the grating, and for this reason specifications limiting the type, number and size of cosmetic defects are not recommended. Nevertheless, all Newport gratings undergo a rigorous cosmetic inspection before shipment.

14. Imaging Characteristics. Concave holographic gratings may be aberration-corrected, in which case they can provide focusing without the use of auxiliary optics. In these cases, imaging characteristics should be specified, generally by calling out the full width at half maximum intensity (FWHM) of the images.

15. Damage Threshold. In some instances, such as pulsed laser applications, diffracted gratings are subjected to beams of high power density that may cause damage to the delicate grating surface, in which case the maximum power per unit area that the grating surface must withstand should be specified.

16. Other specifications. Other specifications that relate to the functional performance of the grating should be called out in the print. For example, if the grating must perform in extreme environments (e.g., a satellite or space-borne rocket, high heat and/or humidity environments), this should be noted in the specifications.

Concave aberration-reduced gratings, often used in constant-deviation and flat-field spectrograph mounts (see Sections 7.5.3 and 7.5.5), have imaging properties that are tailored to the specific geometry of the spectrometer; that is, the grating recording coordinates g, r_C, d and r_D depend on the use coordinates a, r, b and r_C (all of these quantities are defined in Chapter 7). Consequently, a concave aberration-reduced grating requires additional specifications to be fully described.

The cases for constant-deviation monochromators and flat-field spectrographs are given separately below. In all cases, a clear optical schematic showing the quantities defined is highly recommended, especially to ensure that the definition of angles is understood.

17. Constant-deviation monochromator gratings. Concave holographic gratings used in constant deviation monochromators should have the following three parameters specified (see Figure 16-1) to be defined uniquely:
    • the distance r from the entrance slit to the grating center (often called the entrance arm distance),
    • the distance r' from the exit slit to the grating center (the exit arm distance), and
    • the angle 2K between these two arms (the deviation angle); alternatively, the half-deviation angle K may be specified provided it is clear which angle is called out.

Figure 16-1.   Constant-deviation monochromator geometry. The quantities that should be specified are the entrance arm distance r, the exit arm distance r', and the angle 2K between these arms.

18. Flat-field spectrograph gratings. Concave holographic gratings used in flat-field spectrographs require four parameters (see Figure 16-2):
    • the entrance arm distance r,
    • the angle a the entrance arm makes with the grating normal,
    • the distance r'_S from the grating center to the image (on the detector) of the shortest wavelength l_S in the spectrum, and
    • the obliquity angle F of the detector (as described in 2.3.2).

Figure 16-2.   Flat-field spectrograph geometry. The quantities that should be specified are the incidence angle a, the entrance arm distance r, the exit arm distance r'_S for the shortest wavelength in the spectrum to reach the detector, and the obliquity angle F of the detector. The detector is shown; the shortest and longest wavelengths l_S and l_L image at either end of the detector. [The distance r'_L from the grating center to the image of l_L is not shown.]

An alternative set of parameters for defining a flat-field spectrograph is the set of quantities a, r, l_H and b_H, where a and r are as above and

• the distance l_H is measured from the grating center to the line defined by the detector, such that these two lines are perpendicular, and
• the angle b_H is the angle the line l_H makes with the grating normal (see Figure 16-3).

Figure 16-3.   Alternative flat-field spectrograph geometry. A flat-field spectrograph can also be described uniquely by the following quantities: the incidence angle a, the entrance arm distance r, the distance l_H (the line from the grating center to the line defined by the detector, such that these two lines are perpendicular), and the angle b_H that the line l_H makes with the grating normal.

Converting from the parameter set in Figure 16-3 to that in Figure 16-2 can be accomplished using the formulas