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...
A combination of orbital angular momentum (OAM) and spin angular momentum (SAM) degrees of freedom for a single photon leads to enhanced transmission rates and security in high-dimensional quantum key distribution (QKD) systems. Therefore, the actual commercial QKD sources require accurate and fast generation of encoded states. This paper proposes a schemed and numerically verified integrated high-speed and low-loss electro-optical modulator for the manipulation of modes encoded in OAM-SAM states. The modulator is designed as an electro-optically active lithium niobate on insulator photonic wire configuration. The obtained results indicate the high purity (>92%) and low quantum bit error rate (∼8%) generation of encoded states for 4D-QKD with structured photons via low applied voltage values not more than 9 volts.
The mode division multiplexing (MDM) technique using higher order orbital angular momentum (OAM) carrying modes through a channelized bandwidth provides enhanced capacity communication systems. Furthermore, encrypted channels via OAM based high-dimensional quantum key distribution (QKD) improve the transmission rate and security. In such applications, mode-selective manipulation of the spatially distributed (here, OAM) modes is a significant function to implement MDM and QKD networks. This paper proposes a novel versatile-designed chip which is configured in a Ycut periodically poled lithium niobate (PPLN) photonic wire. This integrated optical device acts as spatial mode converter for data modulated on higher order OAM = ±2ћ modes. The conversion is based on an enhanced electro-optic Kerr effect via phase-mismatched cascaded polarization coupling interactions of decomposed guided modes in two successive PPLN sections. The high-purity (91%) and low-voltage (<14.6 V) of the proposed device enable its operation in compatibility with applicable commercial modulators in OAM-based MDM and QKD systems.
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