Holographic diffraction gratings

Diffraction gratings formed by recording an interference pattern in a suitable light-sensitive medium (commonly called holographic diffraction gratings) have replaced conventional ruled gratings for many applications. While Burch and Palmer [1961] first showed that transmission gratings could be made by photographing interference fringes using silver halide emulsions, it was the use of photoresist layers coated on optically worked blanks which finally led to the production of spectrographic gratings of high quality [Rudolph & Schmahl, 1967; Labeyrie & Flamand, 1969]. After processing, the photoresist layer yields a relief image (see section 7.3) which can be coated with an evaporated metal layer and used as a reflection grating.

Holographic gratings have several advantages over ruled gratings. Besides being cheaper and simpler to produce, they are free from periodic and random errors and exhibit much less scattered light. In addition, it is possible to produce much larger gratings of finer pitch, as well as gratings on substrates of varying shapes, and gratings with curved grooves and varying pitch. This makes it possible to produce gratings with unique focusing properties and opens up the possibility of new designs of spectrometers [Namioka, Seya & Noda, 1976].

Against this, their main disadvantage is that the groove profile cannot be controlled as easily as in ruled gratings. While even a sinusoidal profile can give high diffraction efficiencies for small grating spacings (≈ λ) [Loewen, Maystre, McPhedran & Wilson, 1975], it is usually necessary to produce a triangular groove profile for maximum diffraction efficiency.