Diffraction efficiency of vortex beams in the region of 5-240 μm generated by a binary diffractive axicon
A Bessel beam with orbital angular momentum, or vortex beam, can be generated using diffraction from an axicon with spiral zones. In this case the Bessel function of the beam doesn’t depend on the wavelength of generation and is determined by the topological charge only. Hence, for vortex beam generation with a tunable radiation source, as the Novosibirsk free electron laser (NovoFEL), a diffractive axicon becomes a useful tool. In this work THz Bessel vortex beams with topological charges l = 1 and l = 2 were generated using the NovoFEL and binary phase axicons made from silicon. These axicons were manufactured for 141 μm so that a binary step is equal to π. Their diffraction efficiency is about 30%. Using Fourier filtration one can generate vortex Bessel beams in the full range of NovoFEL (5÷240 μm). In this paper a calculation of the diffraction efficiency of axicons with Fourier filtration in the NovoFEL generation range is given.