Optical element to generate zero-order quasi-Bessel beam with “focal length”

Abstract

An optical element has been invented to generate a zero-order quasi-Bessel beam with a certain distance to the element, which does not exist in the zero-order quasi-Bessel beam by using a traditional axicon. The cross section of designed element is an isosceles triangle whose equal sides are circumscribed by two semi-ellipses. Using a well-developed three-dimensional (3D)-printing technique, we have fabricated a series of elements working at terahertz (THz) frequency. Both simulated and experimental results clearly show that there is a certain distance between the generated quasi-Bessel beam and this element. A physical analysis based on geometric optics theory is performed to explain the obtained results. Because it is a refractive transmitted optical element, we propose that it can be also realized at another frequency band if the relevant processing techniques are available.

Diffractive deformed axicon to generate terahertz diffraction-free beam

“…The beam generated at the critical point of total reflection C intersects the symmetry axis of the element at F. The DFB is not generated until beyond point F, before which a hollow halo is formed. This phenomenon is similar to the findings of related study [7] . In theory, the compressed diffractive element should have a similar beam effect as the original axicon, and in experiments it is found that the focal length of the diffractive element produces a DFB that is shortened.…”

“…The order of the devices in the experiment is: a terahertz emitting source working at terahertz (THz) frequency, a chopper, a collimating lens, the axicon, and a terahertz detector. Figure 3 In comparison with the existing study [7] , the thickness of the optical device is compressed by 76%, and furthermore, the diffraction-free distance of the outgoing light based on the diffractive deformed axicon increases by 105.36 mm, and the central spot diameter increases by 10.52 mm. The extension of the diffraction-free distance is beneficial for the practical application of this beam.…”

“…Therefore, researchers have proposed a series of combinations of axicons as well as the design of new deformed ones [6] . One of them, an elliptic axicon [7] obtained by rotating a combination of the triangle and the semi-ellipse, has been experimentally verified to produce a Bessel beam whose starting point is no longer close to the device with a diffraction-free length of 293 mm. However, due to the introduction of a semi-ellipse in the geometric design, this axicon tends to have a certain thickness, which is unfavorable in practical experiments.…”