Dammann grating array generators in amorphous silicon nitride

The idea of using a computer-optimized binary-phase grating as a beamsplitter came from Dammann and Gortler in 1971; hence these holographic optical elements are often referred to as Dammann gratings. As a result of the growing interest toward optical computing in recent years, there has been extensive research both in finding optimal Dammann grating structures by computer optimization, and in developing lithographic fabrication methods that would meet the rather severe accuracy requirements. In this paper we first give expressions for the power spectrum of a Dammann grating and present some recent advances in optimisation of the grating structure by stochastic algorithms. We then proceed to our experimental results in the fabrication of these elements in silicon nitride. Conventional photolithographic and electron-beam lithographic techniques are employed to construct the amplitude mask, which is converted into a phase grating using contact copying and chemical etching of the silicon nitride thin films.