M77781A
MS257
™ USB/RS232
MONOCHROMATOR AND SPECTROGRAPH
66
Figure 26:
Polarization and Efficiency
Polarization
The diffraction efficiency of a grating usually depends on the polarization of the radiation incident I (on the
grating (See Figure 26). This figure shows the effect of polarization on the grating efficiency of a 1200
I/mm diffraction grating.
Ruled Gratings
The grating grooves can be ruled by a mechanical ruling engine, or etched using a holographic process.
Ruled gratings technology lends itself to having a blaze angle formation, to offer the highest efficiency at
the blaze wavelength. They are widely thought to cause a great degree of light scattering, but this is no
longer true. New ruled gratings use interferometrically controlled ruling engines. This produces very
accurate grooves, and very little stray light.
Holographic Gratings
Holographic gratings are created using a interferometric hologram and an etching process. They have the
reputation for being the
“best” gratings, but this is not necessarily the case. Sinusoidally grooved gratings
produce very little scattered light but have low flat efficiency curves, a though they are generally quite
broad. The blazed holographic gratings use etching during the interferometric process, or an ion gun to
form a blaze angle in a secondary process. The former does not produce strong blazing. While
holographic gratings produce high efficiencies at the blaze wavelengths, there is also an increase in stray
light due to the formation of microstructure along the edges of the grooves. Oriel Instruments offers both
rules and holographic gratings to provide the best combination of blaze, efficiency and low light scatter for
different wavelength ranges.
