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International Conference of Optical Imaging and Measurement (ICOIM)

Fiber coupled system design for quantum secure measurement and control communication

Luhe Zhang, Zhenchong Xing, Wei Min Wang, Zheng Zhang

Published Oct 20, 2023• 0 citations
Publisher

Abstract

A large field of view coupling optical system was designed and optimized using PW algorithm to address the high fidelity space single mode fiber coupling problem in quantum secure communication with large field of view angle reception. A coupling system consisting of four spherical mirrors was designed using Zemax optical design software. After aberration test, the coupling system can realize important roles in the fields of unmanned system quantum communication, optical information processing and fiber optic imaging.

DOI
10.1109/icoim60566.2023.10491561

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A large field of view coupling optical system was designed and optimized using PW algorithm to address the high fidelity space single mode fiber coupling problem in quantum secure communication with large field of view angle reception. A coupling system consisting of four spherical mirrors was designed using Zemax optical design software. After aberration test, the coupling system can realize important roles in the fields of unmanned system quantum communication, optical information processing and fiber optic imaging.
The coupling efficiency between free-space optics and fiber optics is highly susceptible to environmental disturbances (e.g., beam displacement, angular jitter, and wavefront aberrations), which has become a critical bottleneck in high-precision optical transmission systems. To address this issue, a cascaded adaptive feedback control system integrated into the optical transmission path is proposed. Targeting the dual requirements of "rapid response" and "long-term stability" in practical optical transmission, a power-feedback-based ramp-up algorithm is developed to simultaneously meet these needs. The algorithm dynamically adjusts piezoelectric-driven reflective mirrors to achieve multi-dimensional autonomous beam alignment and real-time optimization of coupling states. The system is validated on a rubidium Raman photon source experimental platform, demonstrating its applicability to demanding quantum and sensing platforms. Experimental results show that the system increases multimode (single-mode) optical coupling efficiency from below 1% to over 80% (70%) within 10 s (20 s) and enters a long-term stable transmission state after 65 s (70 s). Additionally, real-time link monitoring effectively suppresses coupling attenuation caused by beam jitter, achieving a balance between response speed and transmission stability without the need for independent mode optimization. This scheme provides practical technical support for efficient coupling in high-precision optical transmission systems such as laser communication and optical sensing.
Quantum Key Distribution (QKD) systems will play an important role in future networks for secure data communication. In order to provide a global coverage of a future QKD service, satellites are needed to bridge large distances. These satellite systems must be cost efficient to facilitate deployment since many network nodes will be needed. The CubeSat standard is frequently used for New Space projects as a versatile platform for satellite payloads. It is also chosen as a baseline for the construction of the system described in this paper. The DLR Institute of Communications and Navigation develops optical free space communication systems for scientific research in classical and quantum communications. A previous development of a 1/3 U CubeSat communication terminal for up to 100 Mbit/s downlinks is adapted to be used for quantum communications tests with multiple transmitters. Since the original laser terminal was designed for C-band wavelengths, a redesign of the optical system is needed to achieve polychromatic performance for the three sources. The optical system consists of a fiber collimator, a beam steering system and an afocal telescope. Most important requirement of the latter is a similar magnification for all wavelengths to ensure concentric beams pointing to the OGS. Scenarios in which the system is optimized either for lowest divergences or a similar magnification of the telescope are demonstrated in Figure 1. As the afocal telescope is used bidirectionally, it also needs to be optimized for the incident wave-front of the beacon from the OGS with respect to the beam shape at the four quadrant detector. These parameters are important for a correct pointing control. A fiber-based wavelength division multiplexer (WDM) is used for combining signals from three sources into one output fiber. As shown in the block diagram in Figure 2, it is based on cascaded thin film interference filters which are coupled to the fibers. Especially the propagation of 850 nm and 1550 nm signals in one single mode fiber is critical. Therefore the types of optical fibers were selected with respect to the bend loss, single mode propagation behavior, polarization integrity and optimal cladding diameter for the production.
Satellite-based quantum communication is a promising approach for realizing global-scale quantum networks. For free-space quantum channel, single-mode fiber coupling is particularly important for improving the signal-to-noise ratio of daylight quantum key distribution (QKD) and compatibility with standard fiber-based QKD. However, achieving a highly efficient and stable single-mode coupling efficiency under strong atmospheric turbulence remains experimentally challenging. Here, we develop a single-mode receiver with an adaptive optics (AO) system based on a modal version of the stochastic parallel gradient descent (M-SPGD) algorithm and test its performance over an 8 km urban terrestrial free-space channel. Under strong atmospheric turbulence, the M-SPGD AO system obtains an improvement of about 3.7 dB in the single-mode fiber coupling efficiency and a significant suppression of fluctuation, which can find its applications in free-space long-range quantum communications.
Optimising the interconnection between free-space and fibre links will be necessary for future quantum communication networks. In daylight free-space quantum communication based on direct detection, the required Field Of View (FoV) of the receiver system needs to be minimised to reduce solar background noise coupling into the detectors. Reducing the FoV requires minimising beam wander effects caused by atmospheric turbulence through active optics. We implement a fine tracking system designed to correct tip and tilt wavefront aberrations, using two feedback loops; each of them consisting of a quadrant detector and a fast steering mirror for stabilising the beam in the whole optical axis of the receiver. We test the performance of the tracking system with different optical fibres in order to evaluate the reduction in the quantum bit error rate (QBER) caused by solar background noise. A reduction of 75% for single mode fibre was obtained, and 45% reduction for a 25 µm core diameter fibre, both cases for strong turbulence (Cn2~10-12 – 10-13 m-2/3) and 100 m propagating channel. These results look promising for enabling free-space Quantum Key Distribution (QKD) in wireless networks for realistic/adverse conditions such as daylight and strong turbulent regimes.
The effects of different kinds of communication channels on the key indexes such as bit error rate and communication rate of quantum communication system, represented by quantum secure communication system, are systematically studied. For the actual channel, the transformation of quantum states caused by physical effects such as loss, dispersion and polarization mode dispersion in optical fiber channel is analyzed. With building in the spatial channel noise experimental data under different weather conditions, the mathematical model is established, and the software is designed and implemented.
Secure communication became extremely important in the Information Age. Quantum communication protocols have been developed to provide absolutely secure transmission of information. Historically, quantum key distribution (QKD) uses the two-dimensional polarization basis to encode information and the key element of long-distance communication is single-mode optical fiber. However, it intrinsically limits the information content to 1 bit per photon. Nowadays, significant effort is devoted to increase the data carrying capacity of a single quantum secure optical line. Large-core multimode fibers support a multitude of transverse optical modes and can potentially transfer information at much higher density.