When ordering a concave grating, please use the following example format:
C 1800 Ø63.5xThk12.5 190-400 (TM+TE)/2 (-1) deviation angle 10°, RoC -750 mm
- C stands for Concave grating
- 1800 is the groove density (groove frequency) in grooves/mm
- W is the blank dimension in mm parallell with the grating grooves
- H is the blank dimension in mm perpendicular to the grating grooves
- Ø is the diameter in mm
- Thk is the blank centre thickness in mm
- 190-400 nm is the desired optimisation range. A specific wavelength or range with peaked wavelength can also be specified
- (TM+TE)/2 (-1) is desired polarisation state and diffraction order the grating should be optimised for. TE or TM can also be specified
- Constant deviation angle 10° ( |α°- β°| ) is the configuration the grating should be optimised for. Constant incidence angle α° can also be specified
- Standard tolerances on W, H, Dia: ± 0.2 mm Thk ± 0.5 mm. CA > 90 % of blank diameter.
Currently stocked concave grating substrates:
|Size [mm]||Thk [mm]||RoC [mm]||Material|
|40 x 45||7||-225||Pyrex|
|50 x 50||7||-225||Pyrex|
Concave Gratings Rowland Type
This type of grating is the holographic counterpart of the classically ruled concave grating, invented by Rowland in the 19th century. The Rowland grating has straight grooves which are equally spaced along a chord of the concave surface.
These gratings are preferably used in mountings based on the Rowland circle, i.e. a circle with a diameter equal to the radius of curvature of the grating. If the entrance slit is located on the Rowland circle, then the spectral focus will also be on this circle.
If R is the radius of curvature, and α and β are the angles of incidence and diffraction, respectively, then
LA = Rcosα
LB = Rcosβ
where LA and LB are the distances from grating centre to the entrance slit and spectral focus.
The aberrations (except astigmatism) are small for all points on the Rowland circle, making the gratings useful for wide spectral ranges.
Since all diffracted orders are focused on the Rowland circle, it is possible to make measurements on different orders simultaneously.
Extremely low stray light
Spectrogon’s Rowland holographic gratings are fabricated with the same proprietary technique as our low stray light plane gratings.
The sinusoidal groove profile of holographic gratings gives a better overall performance than a triangular “blazed” profile. Though a blazed concave grating may have higher efficiency at the grating centre, other points on the surface are “off blaze”, giving lower efficiency.
Accurate groove frequency
The groove frequency (at the grating centre) is accurate within ±0.2 grooves/mm of the nominal value. The gratings are holographically exposed using optically flat wavefronts, ensuring straight and equispaced grooves.