Orbital angular momentum (OAM) represents an additional degree of freedom of a light beam, to be added to the standard ones (e.g. polarization, wavelength) [1-2]. Previously, we have demonstrated an integrated device for OAM generation, in which the Whispering Gallery mode (WGM) of the micro-ring resonator to a propagating OAM mode by using an angular grating embedded within the resonator structure [3]. This integrated approach allows compact device footprint and OAM mode order selectivity with injection wavelength, which is highly desirable for telecommunications systems. To bring OAM closer to actual system implementations, several groups around the world are working on the development of optical fibres for propagation of multiplexed OAM modes. In order to achieve efficient coupling between OAM beams and OAM fibres, it is important to obtain the information of the divergence angle of the OAM beams which should be compatible with the N.A. of the fibres. In this work, we use Fourier image inspection [4] of the emission of the device to analyse the divergence angles of the OAM beams, as shown in Fig.1.
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