Holographic, or interference, gratings typically have a sinusoidal groove profile and are generated by the recording of an interference pattern onto a photoresist coated substrate. Holographic masters are subsequently reproduced in OEM quantities using the same high-fidelity replication processes that have been used for decades by the skilled technicians at Richardson Gratings with our ruled gratings. Holographic gratings typically have excellent wavefront flatness and can have high efficiency in a single plane of polarization. Holographic gratings display no ghosts, or periodic errors, as they are generated optically.
The range of useful diffraction efficiency is controlled by varying the modulation depth (the ratio of groove depth to groove spacing). The lower the modulation, the shorter the wavelength limit to which the grating can be used, but the peak efficiency may be lowered as well. We have found that three modulation levels are adequate for nearly all purposes. Since the grooves are symmetrical, they do not have a preferred blaze direction and hence the gratings carry no blaze arrows. Holographic gratings, like their ruled counterparts are most effective when used in the Littrow configuration.
In addition to sinusoidal grooves, it is possible to blaze, or modify the groove shape, in photoresist masters through the Sheridon Method and by Ion Etching.
Sheridon Gratings are recorded with the fringe fields inclined at a small angle with respect to the resist layer. This method leads to triangular grooves and a blaze performance very similar to a ruled grating, but these gratings are restricted to efficiency peaks near 250 and 210 nm. They have the same low stray light performance as gratings with sinusoidal grooves.
Blazed gratings are generated by bombarding holographically recorded masters with a beam of ions. This etching process alters the groove profile from sinusoidal to triangular, which often increase the peak efficiency of the grating.
Plano gratings offer many manufacturing flexibilities and can be mass produced extremely effectively through dicing or “sawing” processes. Dispersed light from plano gratings can be readily manipulated and tailored to any given instrument by use of focusing mirrors or lenses. Plano gratings offer the best in system resolution for a given clear aperture of light exposure.
Master holographic gratings as large as 180 mm in diameter are made routinely at Richardson Gratings. |