M74050
1/4M IMAGING SPECTROGRAPHS
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6.2 GRATING EFFICIENCY AND BLAZING
Efficiency and its variation with wavelength and spectral order are important characteristics of a
diffraction grating. For a reflection grating, efficiency is defined as the energy flow (power) of
monochromatic light diffracted into the order being measured, relative either to the energy flow of the
incident light (absolute efficiency) or to the energy flow of specular reflection from a polished mirror
substrate coated with the same material (relative efficiency). Efficiency is defined similarly for
transmission gratings, except that an uncoated substrate is used in the measurement of relative
efficiency.
High-efficiency gratings are desirable for several reasons. A grating with high efficiency is more useful
than one with lower efficiency in measuring weak transition lines in optical spectra. A grating with
high efficiency may allow the reflectivity and transmissivity specifications for the other components in
the spectrometer to be relaxed. Moreover, higher diffracted energy may imply lower instrumental
stray light due to other diffracted orders, as the total energy flow for a given wavelength leaving the
grating is conserved (being equal to the energy flow incident on it minus any scattering and
absorption).
Control over the magnitude and variation of diffracted energy with wavelength is called blazing, and
it involves the manipulation of the micro-geometry of the grating grooves. The energy flow distribution
(by wavelength) of a diffraction grating can be altered by modifying the shape of the grating grooves.
The blaze wavelength is the wavelength where the grating efficiency is enhanced by shaping the
grating grooves. Although holographic gratings are not shaped like ruled gratings, the peak grating
efficiency wavelength is also referred to as the blaze wavelength.
The choice of an optimal efficiency curve for a grating depends on the specific application. Often the
desired instrumental efficiency is linear; that is, the intensity of light transformed into signal at the
image plane must be constant across the spectrum. To approach this as closely as possible, the
spectral emissivity of the light source and the spectral response of the detector should be considered,
from which the desired grating efficiency curve can be derived. Usually this requires peak grating
efficiency in the region of the spectrum where the detectors are least sensitive.
In many instances, the diffracted power depends on the polarization of the incident light. For
completely unpolarized incident light, the efficiency curve will be exactly halfway between the P and
S efficiency curves. Anomalies are locations on an efficiency curve (efficiency plotted vs. wavelength)
at which the efficiency changes abruptly. These sharp peaks and troughs in an efficiency curve are
sometimes referred to as Wood's anomalies.
The efficiency curves shown are relative (not absolute) and were measured using an in-plane near
Littrow configuration. Please use the curves as a guide and not as absolute data. Grating diffraction
is dependent on the polarization of the radiation incident on the grating.
Software such as MonoTerm
and Oriel’s TracQ Basic may be configured to switch between gratings
at a specific wavelength. Typically, the most efficient grating is selected, so this switchover
wavelength would be where the two efficiency curves meet. To determine empirically the ideal
switchover wavelength, the output should be measured by an optical detector. Run a scan in the
crossover region using only Grating 1. Repeat the scan using only Grating 2. Where the detector
readings are closest is the optimal switchover wavelength. For a triple grating instrument, this
process can be repeated for Grating 2 and Grating 3.
If the selected grating’s efficiency has a sudden increase or decrease at a particularly critical
wavelength and the application demands extreme accuracy, it may be more desirable to select the
grating with the more gradual change in efficiency.