As noted above, the grating equation may be satisfied at a given angle by a number of wavelengths of different diffraction orders.
▲Figure 1-9:- Existence of diffraction orders
This can lead to problems when attempting to measure light in a given diffraction order, when the detection system is capable of sensing the wavelength in the next diffraction order etc.Order sorting is therefore required, and consists of the filtering of the monochromator input with long pass filters where higher diffraction orders might be present.This also leads to an explanation for measurement in first order. The wavelength of light that diffracts along the direction ofλ1 in order m+1, is λ1+△λ , where
Hence we define the free spectral range, the range of wavelengths over which overlapping of adjacent orders does not occur,
Diffraction grating production
Gratings found in monochromators are replicas based on master gratings.
Master diffraction gratings are produced by one of two means：
● Holographic exposure thenchemical etch of grooves
● Mechanical ruling of grooves
In the holographic technique a substrate is covered with a photoresist material whose properties change under light stimulation. Exposure to an interference pattern defines the grating outlay, chemical etching is then employed to selectively etch the substrate as a function of the photoresist.
This method produces almost sinusoidal grooves, but of very high surface quality.
The mechanical technique involves the mechanical inscribing, using a diamond tip in a “ruling engine” to define grooves on a metal substrate, a lengthy and difficult process.
This method yields very good, triangular grooves, resulting in gratings of very high efficiency. However, surface defects may have an impact in certain cases by introducing stray light into the monochromator.
▲Figure 1-10:- Groove shape obtained using ruling (upper) and holographic techniques
Replica gratings are resin casting of master gratings, on a glass substrate, which are then coated by a suitable metallic coating for the spectral range of use, such as aluminium.
Diffraction gratings may be produced on flat (plane) or non-flat (for example concave) substrates.
Diffraction grating efficiency
The efficiency of a grating is defined as the power of monochromatic light diffracted into a given order relative to that light incident.
In order to increase the efficiency of a grating at a given wavelength, the angle of the grooves is designed such that the specular reflection from the grating surface lies in the same direction as that wavelength in question.
This procedure is called blazing, the peak wavelength being the blaze wavelength.
▲Figure 1-11:- Example grating efficiency curves
The consideration of grating efficiency becomes more complicated when one considers polarised incident light, and in particular the case of TM polarised light in which case the electric field vector is perpendicular to the grooves, giving rise to anomalies, or abrupt changes in the grating efficiency curve.