Hybrid OAM-Amplitude multiplexing and demultiplexing of incoherent optical states

Highlights

• Local demultiplexing of channels encoded in the orbital angular momentum.

• Demultiplexing of mutually incoherent beams in free space optical communication links.

• High-density transmission in long-distance communication systems.

Abstract

We propose multiplexing and demultiplexing of Orbital Angular Momentum (OAM) states realized with incoherent beams to increase the effective transmission area in a free space optical communication link. The overlapped states of the beams have been effectively distinguished using a hybrid OAM-Amplitude scheme exploiting only a small portion of the entire wavefront at the receiver. The method has been theoretically formalized and the transmission experimentally tested up to 1 Mbit/s. The proposed method is suitable for high density transmission in long-distance free space optical communication systems.

Introduction

The discovery of radiation carrying orbital angular momentum at the end of the twentieth century [1], [2], [3], [4], [5] has had a huge impact in the field of radio and optical telecommunications [6], [7], [8]. The additional degree of freedom introduced by the momentum quantization of photons $L_z=ħl$ (where $l$ is an integer known as the topological charge and $ħ=h/2\pi$, with $h$ the Planck constant) allows to encoding additional information in a communication system intrinsically limited by the frequency band. Different approaches to demultiplex information by exploiting different OAM states both in free space [9], [10], [11] and in fiber [12], [13] have been proposed. These methods generally use the entire wavefront of the radiation to discriminate the states and hence to separate the original channels. The advantage of using the whole wavefront lies in the fact that demultiplexing of states can be achieved with extremely efficient approaches both with refractive [14] and diffractive [15] optics.

Despite the multiplexing and demultiplexing of OAM beams having been extensively explored, locally discriminating OAM states by accessing only a small portion of the entire wavefront is still attracting great interest. This approach becomes essential when the receiver is at large distances from the transmitter and the beam size increases due to the intrinsic divergence of the photon beam, until it exceeds that of the receiver. Local detection methods exploiting double slits have been proposed in previous works to measure the topological charge by using a composite vortex [16] or the intrinsic Gaussian curvature of the OAM radiation [17], [18]. In a more recent work, [19] a phase-locked two-arms interferometer has been proved to be effective in discriminating OAM states of a coherent beams, even by accessing a small portion of the beam only. In the present work, we show that a similar scheme can be used to distinguish OAM states of mutually incoherent beams to avoid amplitude modulations typically generated by superimposed coherent states. This novel approach doubles the effective transmission area preserving the possibility of multiplexing and demultiplexing independent channels. Moreover, with the proposed approach, the incoherent sources can be modulated independently for each channel in order to make the communication link compatible with high-speed modulation systems, much beyond the limit imposed by the spatial light modulator used in [19]. We introduce the proposed method in Section 2 with an extensive formalization of a hybrid OAM-Amplitude scheme for fully incoherent sources. The experimental setup for multiplexing and demultiplexing of two independent channels is described in Section 3, while in Section 4 we discuss our results. Finally, we collect our conclusions in Section 5.