Generation of “perfect” vortex of variable size and its effect in angular spectrum of the downconverted photons
Abstract
The “perfect” vortex is a new class of optical vortex beam having ring radius independent of its topological charge (order). One of the simplest techniques to generate such beams is the Fourier transformation of the BesselGauss beams. The variation in ring radius of such vortices require Fourier lenses of different focal lengths and or complicated imaging setup. Here we report a novel experimental scheme to generate perfect vortex of any ring radius using a convex lens and an axicon. As a proof of principle, using a lens of focal length f = 200 mm, we have varied the radius of the vortex beam across 0.31.18 mm simply by adjusting the separation between the lens and axicon. This is also a simple scheme to measure the apex angle of an axicon with ease. Using such vortices we have studied noncollinear interaction of photons having orbital angular momentum (OAM) in spontaneous parametric downconversion (SPDC) process and observed that the angular spectrum of the SPDC photons are independent of OAM of the pump photons rather depends on spatial profile of the pump beam. In the presence of spatial walkoff effect in nonlinear crystals, the SPDC photons have asymmetric angular spectrum with reducing asymmetry at increasing vortex radius.
 Publication:

Scientific Reports
 Pub Date:
 February 2016
 DOI:
 10.1038/srep21877
 arXiv:
 arXiv:1601.03129
 Bibcode:
 2016NatSR...621877J
 Keywords:

 Physics  Optics