Integrated optical modulator manipulating the polarization and rotation handedness of Orbital Angular Momentum states

Abstract

Recent studies demonstrated that the optical channels encoded by Orbital Angular Momentum (OAM) are capable candidates for improving the next generation of communication systems. OAM states can enhance the capacity and security of high-dimensional communication channels in both classical and quantum regimes based on optical fibre and free space. Hence, fast and precise control of the beams encoded by OAM can provide their commercial applications in the compatible communication networks. Integrated optical devices are good miniaturized options to perform this issue. This paper proposes a numerically verified integrated high-frequency electro-optical modulator for manipulation of the guided modes encoded in both OAM and polarization states. The proposed modulator is designed as an electro-optically active Lithium Niobate (LN) core photonic wire with silica as its cladding in a LN on Insulator (LNOI) configuration. It consists of two successive parts; a phase shifter to reverse the rotation handedness of the input OAM state and a polarization converter to change the horizontally polarized OAM state to the vertically polarized one. It is shown that all four possible output polarization-OAM encoded states can be achieved with only 6 V and 7 V applied voltages to the electrodes in the two parts of the modulator.In the past decades, interests in exploiting Orbital Angular Momentum (OAM) as a new degree of freedom for encoding the information in optical communication channels have been enhanced 1 . Utilizing this technique in classical communication based on both optical fibre 2 and free space 3-7 , and also in quantum communication 8-10 demonstrated a promising increased data transmission rate for the future networks. In addition, secure transmission is one of the concerns of communication systems. Besides the classical cryptography techniques, Quantum Cryptography (QC) improved the security between authorized partners connected by a quantum channel. Quantum Key Distribution (QKD) protocols are used intensively for approaches of QC. As an example, BB84 QKD protocol employs four states belonging to two conjugate bases (such as horizontal, vertical, left circular and right circular polarization states) for data encoding 11,12 . By combining polarization with OAM, generation of a rotation invariant qubit is possible. In this case, QKD is proved to be independent of alignment 13,14 . This toolbox can be a good choice for free-space optical and quantum communication with moving objects, such as satellites or flying platforms. The rotation invariant qubit in QKD has been operated experimentally over a distance of 210 m 15 . Moreover, the possibility of using OAM together with polarization for encoding the high-dimensional QKD states improves noise resistance and increase the content of information carried by each photon 16 . Carrier modes with phase term of exp(ilφ) have OAM corresponding to lℏ; per photon. In this term l (l = 0, ±1, ±2, …) is the topological charge of the mode and its sign determines the rotation handedness...

Integrated all-optical manipulation of orbital angular momentum carrying modes via enhanced electro-optic Kerr effect

“…According to ref [38], such a phase compensation is electro-optically attainable. The proposed modulator of ref [38] was designed for manipulation of (e. g. TE polarized) OAM= ±1 modes which are decomposed into two guided ones and so included one phase shifter to induce the desired relative phase between T E 01 and T E 10 modes. Since at least two relative phases associated to |l| = 2 OAM modes have to be satisfied simultaneously, the proposed scheme in ref [38] is not proportional.…”

“…The proposed modulator of ref [38] was designed for manipulation of (e. g. TE polarized) OAM= ±1 modes which are decomposed into two guided ones and so included one phase shifter to induce the desired relative phase between T E 01 and T E 10 modes. Since at least two relative phases associated to |l| = 2 OAM modes have to be satisfied simultaneously, the proposed scheme in ref [38] is not proportional. The improved modulator in this paper, thus, consists of two phase shifters to satisfy two required relative phases between three decomposed guided modes which are adequate for generation of desired OAM= ±2 modes.…”

“…More specifically, some integrated optical modulators have been theoretically and experimentally investigated to emit, (de)multiplexe, switch, receive, detect, and sort OAM carrying modes [26][27][28][29][30][31][32][33][34][35][36][37]. Even more, an integrated optical modulator has been proposed to alter the polarization and rotation handedness of |l| = ±1 OAM carrying modes via manipulation of their decomposed guided modes by linear electrooptic effect [38]. However, although higher (|1| > 1) order OAM carrying modes impressively enable to provide even more enhanced capacity and improved security communication systems, but their integrated optically manipulation is not operated yet, to the best of our knowledge.…”

“…This paper proposes a versatile-designed integrated optical configuration enabling all-optically exchange of the spatial OAM modes with l = ±2 via electro-optically manipulation of their decomposed guided modes. Similar to the introduced basic device of reference [38], the principal outline of the improved proposed modulator schematically displayed in figure 1 1). Whereas the expressed effect can be utilized over diverse wavelength and polarization states, the novel proposed device enables to compatibly operate in practical fiber-based and freespace MDM and QKD communication systems and at the following of any desired polarization and wavelength dependent modulators (such as wavelength and polarization converters) which employ higher order OAM= ±2 modes.…”

Characterizing High-Quality High-Dimensional Quantum Key Distribution by State Mapping Between Different Degrees of Freedom

“…The free-space EOM could hardly achieve higher modulation rate as the high half-wave voltage required. Though there is still a long way to make them practical, the integrated optical waveguides [52] and modulators [53,54] may be a potential solution to high-speed modulation of OAM photons.…”