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Communications Physics

Partial coherence and multi-channel vortices enhance high-security free-space optical communication

Yanghong Li, Weiqing Lin, Wenxuan Li, Zitong Wu, Dongmei Deng

Published Dec 9, 2025• 3 citations

Abstract

Vortex beams (VBs) shift keying is a digital communication technique that encodes data by rapidly switching or shifting the orbital angular momentum (OAM) state of a light beam, improving the performance of free-space optical communication. However, current VB-based shift keying secure systems still face several issues, such as limited capacity, the need for model retraining during key renewal and in some cases, vulnerability to ciphertext-only or known-plaintext attacks. In this article, we address these critical challenges by integrating VB array, phase encoding, spatial coherence modulation and deep learning techniques. By utilizing 9 spatial channels each with 16-ary phase differences in the proof-of-principle experiments, the information content carried by a single symbol reach 36 bits. Moreover, a double-encryption transmission scheme is proposed, leveraging the coherence structure and spatial reading trajectory of the VB array. This method has been experimentally verified to provide ultra-high security at the physical layer, effectively resisting both ciphertext-only and known-plaintext attacks. The corresponding bit error rates and pixel error rates can highly reach at least 3.7 × 10−1 and 8.6 × 10−1, respectively. Our work offers the potential for next generation VB-array-based high security FSO communication. Vortex-beam-based shift keying secure systems enhance secure digital communications, while it faces challenges such as limited capacity and vulnerability to attacks. This paper presents a double encryption scheme leveraging coherent structures and spatial reading trajectories whichenables key updates without retraining, and maintains high security.

DOI: 10.1038/s42005-025-02441-2

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Orbital angular momentum (OAM), a new dimension of photons, has potentials in lots of domains as high-dimensional data coding/decoding. Here we experimentally demonstrate a free-space data transmission system based on 8 bits multi-state OAM shift keying, where multiplexed optical vortices containing 8 various OAM states are employed to constitute 8 bits binary symbols. In the transmitter, the data coding of OAM shift keying is realized by switching a series of special-designed holograms. And in the receiver, the decoding is done by a single Dammann vortex grating along with image processing. We experimentally transmit data, including a gray-scale image, in free-space for 10 meters, showing zero bit-error-rate. The demonstrated results indicate a wide prospect for the future high-dimensional large data rate optical security communications.

In recent years, optical information processing has become increasingly important due to its ability to handle large amounts of data efficiently. Amplitude, phase, spatial frequency, wavelength, and polarization are the physical dimensions used for information encoding into the light beam. Information encoding using orbital angular momentum (OAM) carried by a vortex beam is gaining interest in this regard. Such beams provide high-dimensional orthogonal states for encoding and have shown more stability in turbulent media than Gaussian beams, especially in the case of free space propagation. This study demonstrates an image encryption method utilizing an array of OAM light modes. The approach uses a non-interferometric intensity recording-based encoding scheme, which offers ease of implementation. Quick response codes have been utilized for encoding to avoid information loss from non-uniform distribution of light and aberrations. The encryption scheme is highly secure because the encrypted light beam contains only partial information about the object, preventing complete information retrieval without proper key information.

Information security is a critical challenge in free-space optical (FSO) communications. Several methods have been applied to prevent eavesdropping during information transmission. We propose four operations: addition, subtraction, multiplication, and division of the orbital angular momentum (OAM) mode to achieve security in the encoding and decoding of information transmission for optical communications. The transmitted information is decoded by the addition, subtraction, multiplication, and division of the OAM modes according to the values of the topological charges at different diffraction orders of the vortex beam generated by the vortex Dammann grating. The results show that correct information can be obtained when the four operations of the OAM modes are applied. This method can be used to prevent eavesdropping during information transmission in FSO communication systems.

Vortex beams carrying orbital angular momentum (OAM) modes hold the promise of facilitating high-dimensional symbol mappings for information encryption. However, challenges persist in achieving both high-security and high-capacity encryption with OAM modes, mainly due to limitations in encoding degrees of freedom and compatible decoding methods. Herein, we present an encryption scheme that integrates truncated OAM modes with phase encoding. Truncated OAM modes are hybrid modes that emerges from mode dispersion caused by the absence of amplitude modulation during generation. By combining the generation function with phase encoding, we can modulate the mode dispersion, creating a discrete set of truncated modes. This introduces a phase degree of freedom for encoding information onto the mode spectrum, achieving high-security encryption. The exponential expansion of the phase factors enables the construction of higher-order symbol maps, thereby increasing encryption capacity. As a proof-of-concept, we implemented an optical information encrypting link using 4 OAM modes and 32-order phase encoding, resulting in over 3×104 modulation orders. The neural network successfully decoded the encryption information with 100% accuracy. This scheme offers the potential to incorporate additional OAM modes in the generating function and further partition the phase encoding, enabling even higher-dimensional symbol mapping. These advancements open up new possibilities for advancing high-security and high-capacity information encryption based on OAM modes.

Optical encryption provides strong physical-layer security but is limited by the slow response of spatial light modulators. We propose and experimentally demonstrate a spatiotemporal noise chaffing system inspired by the “chaffing and winnowing” principle for ultrahigh-speed temporal encryption. By exploiting the symmetric spatial properties and orthogonality of conjugated orbital angular momentum (OAM) states, high-speed temporal signals (“wheat”) and spatial noise (“chaff”) are simultaneously encoded. This mechanism suppresses information leakage by degrading the temporal signal-to-noise ratio while enabling authorized recovery. Furthermore, a variable-weight multimodal OAM (VW-multimodal OAM) scheme combined with a multimodal generation neural network (MGNN) exponentially expands the key space beyond 10¹⁰. Experimentally, a record secure transmission rate of 1.25 Tbps per mode is achieved in an eight-channel wavelength-division-multiplexed coherent link. The product of rate and key space surpasses existing methods by five orders of magnitude, establishing a new photonic-security paradigm for future ultrafast and secure communication networks. Optical encryption provides strong data protection but has been limited by the modulating bandwidth of spatial devices. Here, the authors demonstrate a spatiotemporal noise chaffing system using conjugated orbital angular momentum modes to achieve terabit-per-second secure transmission and expand the encryption key space beyond 10¹⁰.

A free-space communication scheme is proposed based on orbital angular momentum (OAM) states and pulse position modulation (PPM). It can not only improve the channel capacity, but also be suitable for single-photon detection to realize the communication in the case of ultra-weak light. A parallel coding method is proposed to realize high-capacity and low bit error rate (BER) communication under high background. Theoretical simulation and experimental results show that the scheme can effectively suppress the background noise. The BER is as low as 1.27 × 10

^{-5}

under the condition of 16 OAM states, 16 single-photon detectors (SPDs) coincidence, and the background noise of 10

^{7}

counts per second. The BER performance is an order of magnitude better than the traditional PPM, and the channel capacity is increased by 12 times. This work provides an effective solution for deep space high-speed and low BER communication.