Holographic grating can provide much higher angular dispersion due to the ability to write variations with very small periods, perform better in the ultraviolet end of the spectrum, and can be written onto curved surfaces to provide focusing capabilities in addition to the angular dispersion. Many processes can produce holographic gratings in a variety of materials, though the most common type found in spectrometers consists of a glass substrate exposed to interfering ultraviolet beams. A holographic grating is created by interference of two laser beams within a responsive material which results in a periodic variation in the refractive index of the material.
A ruled grating consists of a material into which a large number of parallel grooves are etched and then coated with a highly reflective material such as gold. Two different types of gratings can be used for spectroscopic applications. Both the period and geometry of the grating must be carefully selected in order to meet the needs of your spectroscopic application. The geometry of the periodic variation strongly influences the available efficiency. The efficiency of the diffraction grating determines the amount of optical power available to the DMD (in the Nano) or detector array as a function of wavelength. The period of the variation in the optical property of the grating strongly determines the available angular dispersion.
The change in output angle as a function of wavelength, called the angular dispersion, plays an important role in determining the wavelength resolution of the spectrometer. The diffraction grating separates the wavelength components of the light by directing each wavelength into a unique output angle. A diffraction grating consists of a material containing a periodic variation in one of its optical properties. After light passes through the slit, a set of lenses or mirrors collimates the light (makes all light rays parallel to the optical axis) and delivers the light to the diffraction grating.
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