Features of Phase Wave Front Binary Encoding and Their Potential Utilization for Alignment Purposes

We make use of binary encoding of a phase wave front for the generation of diffractive elements. Binary encoding enables to introduce a constant phase shift between different diffraction orders. We describe the main features of phase wave front encoding and then suggest to apply them for a design of a diffractive alignment element. The proposed positioning tool is a modified version of the hyperbolic zone plate. The contribution describes properties of binary encoding of phase wave fronts. Binary encoded elements generate different diffractive orders. Usually it is a drawback leading to the substantial limitation of diffraction efficiency. The paper puts the particular attention to a possibility of introducing an arbitrary mutual phase shift between conjugate diffractive orders. This interesting feature of binary encoding was not distinctly pointed out till now, to our knowledge. In a particular case of phase binary encoding the phase shift between +1 and -1 orders can give rise to their destructive interference. We suggest to utilize this interference for alignment applications. The proposed positioning tool is a modified hyperbolic zone plate. It forms in its focal plane diffractive field with the characteristic central dark spot. According to last investigations such diffractive structures are particularly promising for alignment purpose. The phase wave front             Φ =       → → r i r U exp can be encoded in a diffractive element by means of the periodical complex function             Φ → r T with a period of π 2 of a phase argument ) (r Φ . Module of the transmittance values fulfill the inequality: