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Vortex Beams (OAM)

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Optical beams carrying OAM typically exhibit helical wavefronts with a Poynting vector component in the azimuthal direction, which results in orbital angular momentum of per lh/2(pi) photon in the direction of beam propagation, l being the intrinsic charge of the OAM beam. The pitch and handedness of the helix determines the intrinsic charge type (positive/negative) of the OAM beams. 

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Generation

Here, we experimentally demonstrate an all-fiber technique for the excitation of an OAM mode using a fused fiber coupler. Our approach is based on mode-selective coupling between different modes in two dissimilar fibers. The phase matching condition is studied using the COMSOL Multiphysics® eigen mode solver to estimate the required ratio between the cladding diameter of the SMF and that of the few mode fiber. 

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Propagation

We use modified polarizing Sagnac interferometer to generate composite OAM beams. We analyze the propagation dynamics of the composite beam with a perturbation. A knife edge is placed immediately after the lens to block a part of the beam as shown in image and the beam profile is captured along the propagation path. It is clearly evident that the truncated beam regained its original intensity structure in the Rayleigh range.To understand the above phenomenon, we experimentally observed the propagation dynamics of the two orthogonal components of the composite beam. It is clearly observed that the +l and -l charge vortex beams rotate in opposite directions, but at the same rate. At the Rayleigh range, the two vortex beams appear at diametrically opposite orientation and the “self-healing” behaviour is observed.

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Purity Estimation

Purity estimation is an important aspect that needs careful attention while investigating the generation as well as propagation of OAM beams. In our work, we address this need through the demonstration of an optical correlation technique that incorporates both amplitude as well as phase structures of LG modes to accomplish the modal decomposition. In modal decomposition method, any scalar light beam (U) can be represented by a superposition of LGl,p modes with corresponding complex weights Wl,p. The complex weight of a mode is calculated by optically correlating the input beam with the complex conjugate of the mode.

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Reference :

Srinivas Pachava, Raghu Dharmavarapu, Anand Vijayakumar, Sruthy Jayakumar, Amogh Manthalkar, Awakash Dixit, Nirmal K. Viswanathan, Balaji Srinivasan, Shanti Bhattacharya,Generation and decomposition of scalar and vector modes carrying orbital angular momentum: a review,Optical Engineering,(2019).

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11.gif

​l = 2, sigma plus

13.gif

l = -2, sigma plus

oam1.png
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