The use of multimode fibers offers advantages in the field of communication technology in terms of transferable information density and information security. For applications using physical layer ...security or mode division multiplexing, the complex transmission matrix provides valuable knowledge about the communication channel. To measure the transmission matrix, the individual modes of the multimode fiber are excited sequentially at the input and a mode decomposition is performed at the output. Mode decomposition is usually performed using digital holography, which requires a reference wave and leads to high efforts. To overcome these drawbacks, a neural network is proposed, which performs mode decomposition with intensity-only camera recordings of the multimode fiber facet. Due to the high computational complexity of the problem, this approach was usually limited to a number of 6 modes. In this work, it is shown for the first time that by using a DenseNet with 121 layers it is possible to break through the hurdle of 6 modes. The training process is based on synthetic data. The advancement is demonstrated by a mode decomposition with 10 modes experimentally. A quantitative comparison of the proposed method with digital holography shows very good agreement. In addition, it is shown that the network can perform mode decomposition on a subset of 10 modes of a 55-mode fiber, which also supports modes unknown to the neural network. The smart detection using a DenseNet opens new ways for the application of multimode fibers in optical communication networks for physical layer security.
Few-mode fibers (FMFs) are promising for advancements in transmission capacity in classical and quantum communications. However, the inherent modal crosstalk limits the practical application of FMF. ...One reliable way to overcome this obstacle is the measurement of the complex transmission matrix (TM), describing the light propagation behavior of fiber. The TM can be obtained by performing mode decomposition (MD) of the spatial modes at the output of the fiber. MD techniques require the retrieval of both the amplitude and phase components of the detected light field, which is commonly done by using holography. However, the provision of a reference wave is highly unfavorable for the implementation of a holography-based MD in communication technology, especially for long fibers. Using deep neural networks to process intensity-only images, this drawback can be overcome. We introduce the mode transformer network, which can perform MD on 23 modes and has been trained offline using synthetic data. Experimentally, we demonstrate, for the first time, not only the measurement of complex TM of an FMF but also the inversion of the TM using a deep learning-based MD method. For mode transmission, we achieve an average fidelity of 97%. The short duration of the determination of TM allows for overcoming time-varying effects due to, e.g., mechanical stress or temperature fluctuations. The proposed reference-less calibration is promising for fiber communication with classical light and single photons, such as at quantum key distribution.
Wavefront shaping with spatial light modulators (SLMs) enables aberration correction, especially for light control through complex media, like biological tissues and multimode fibres. High-fidelity ...light field shaping is associated with the calculation of computer generated holograms (CGHs), of which there are a variety of algorithms. The achievable performance of CGH algorithms depends on various parameters. In this paper, four different algorithms for CGHs are presented and compared for complex light field generation. Two iterative, double constraint Gerchberg-Saxton and direct search, and the two analytical, superpixel and phase encoding, algorithms are investigated. For each algorithm, a parameter study is performed varying the modulator’s pixel number and phase resolution. The analysis refers to mode field generation in multimode fibre endoscopes and communication. This enables generality by generating specific mode combinations according to certain spatial frequency power spectra. Thus, the algorithms are compared varying spatial frequencies applied to different implementation scenarios. Our results demonstrate that the choice of algorithms has a significant impact on the achievable performance. This comprehensive study provides the required guide for CGH algorithm selection, improving holographic systems towards multimode fibre endoscopy and communications.
Space division multiplexing (SDM) is promising to enhance capacity limits of optical networks. Among implementation options, few-mode fibres (FMFs) offer high efficiency gains in terms of ...integratability and throughput per volume. However, to achieve low insertion loss and low crosstalk, the beam launching should match the fiber modes precisely. We propose an all-optical data-driven technique based on multiplane light conversion (MPLC) and neural networks (NNs). By using a phase-only spatial light modulator (SLM), spatially separated input beams are transformed independently to coaxial output modes. Compared to conventional offline calculation of SLM phase masks, we employ an intelligent two-stage approach that considers knowledge of the experimental environment significantly reducing misalignment. First, a single-layer NN called Model-NN learns the beam propagation through the setup and provides a digital twin of the apparatus. Second, another single-layer NN called Actor-NN controls the model. As a result, SLM phase masks are predicted and employed in the experiment to shape an input beam to a target output. We show results on a single-passage configuration with intensity-only shaping. We achieve a correlation between experiment and network prediction of 0.65. Using programmable optical elements, our method allows the implementation of aberration correction and distortion compensation techniques, which enables secure high-capacity long-reach FMF-based communication systems by adaptive mode multiplexing devices.
Ultrasonic welding is a universal, clean, and secure alternative joining method. Alternative joining technologies are used increasingly to fulfill specific functional requirements of the seam, such ...as fluid impermeability to achieve positive bonding during the assembly of technical textiles. In this research, the effect of important ultrasonic welding parameters on peel strength and weld seam thickness was investigated for flexible and lightweight textile material, which is a valuable innovative hybrid textile for technical applications like architectural, construction, and protective textiles. Three main welding parameters with three different levels were selected based on the preliminary test results of 6 and 12 mm welding widths, and a superimposed type of seam was applied. Light scanning microscopic images were used to examine the effective weld locations and their morphology at the joining interface. The parametric influence of ultrasonic welding technique on-seam quality and their tendencies in the relationship were analyzed. Optimized peel strength yielding parametric levels were also assessed numerically. The result shows that the optimal peel strength value was obtained at a welding speed of 2.318 m/min, power of 119.382 W, and pressure force of 349.729 N for a 12 mm welding width. The weld seam thickness had an inverse relationship with the peel strength, and a higher amount of thickness was reduced to 12 mm welding width than 6 mm. Microscopic cross-sectional image of weld seam indicated that a compressed yarn between the coating material at higher welding power and pressure force in lower welding speed. A nonlinear quadratic numerical model was developed to predict the peel strength, and their results were close to the regressed diagonal line against the actual points. The statistical analysis was carried out to show the significant effect of process parameters on peel strength, whereby the obtained results were statistically significant.
Multimode fibers (MMF) are promising candidates to increase the data rate while reducing the space required for optical fiber networks. However, their use is hampered by mode mixing and other ...effects, leading to speckled output patterns. This can be overcome by measuring the transmission matrix (TM) of a multimode fiber. In this contribution, a mode-selective excitation of complex amplitudes is performed with only one phase-only spatial light modulator. The light field propagating through the fiber is measured holographically and is analyzed by a rapid decomposition method. This technique requires a small amount of measurements N, which corresponds to the degree of freedom of the fiber. The TM determines the amplitude and phase relationships of the modes, which allows us to understand the mode scrambling processes in the MMF and can be used for mode division multiplexing.
Multimode fibers are regarded as the key technology for the steady increase in data rates in optical communication. However, light propagation in multimode fibers is complex and can lead to ...distortions in the transmission of information. Therefore, strategies to control the propagation of light should be developed. These strategies include the measurement of the amplitude and phase of the light field after propagation through the fiber. This is usually done with holographic approaches. In this paper, we discuss the use of a deep neural network to determine the amplitude and phase information from simple intensity-only camera images. A new type of training was developed, which is much more robust and precise than conventional training data designs. We show that the performance of the deep neural network is comparable to digital holography, but requires significantly smaller efforts. The fast characterization of multimode fibers is particularly suitable for high-performance applications like cyberphysical systems in the internet of things.
Multimode fibers hold great promise to advance data rates in optical communications but come with the challenge to compensate for modal crosstalk and mode-dependent losses, resulting in strong ...distortions. The holographic measurement of the transmission matrix enables not only correcting distortions but also harnessing these effects for creating a confidential data connection between legitimate communication parties, Alice and Bob. The feasibility of this physical-layer-security-based approach is demonstrated experimentally for the first time on a multimode fiber link to which the eavesdropper Eve is physically coupled. Once the proper structured light field is launched at Alice's side, the message can be delivered to Bob, and, simultaneously, the decipherment for an illegitimate wiretapper Eve is destroyed. Within a real communication scenario, we implement wiretap codes and demonstrate confidentiality by quantifying the level of secrecy. Compared to an uncoded data transmission, the amount of securely exchanged data is enhanced by a factor of 538. The complex light transportation phenomena that have long been considered limiting and have restricted the widespread use of multimode fiber are exploited for opening new perspectives on information security in spatial multiplexing communication systems.
When fiber-reinforced plastic (FRP) components are designed, it is very important to ensure that textiles are formed into complex 3D geometries without folds, and that the reinforcing structure is ...oriented appropriately. Most research in this context is focused on finite element (FE) forming simulations and the required characterization of textile reinforcements. However, the early stage of the design of FRPs, where kinematic draping simulations are used, is barely considered. In particular, the need for a critical shear angle for the execution and evaluation of kinematic draping simulations is often neglected. This paper presents an extended picture frame test stand with an optical device recording shear-induced deformations with the help of a laser line emitter. Associated hardware and software for detecting and quantifying the fold formation during a picture frame test were developed. With the additional recorded information, a material-specific critical shear angle can be determined, material behaviors can be compared, and FE-based simulation methods can be evaluated. This innovative test stand and the associated software tools will help engineers to decide on suitable materials and improve transparency in the early stages of the design process.
This paper presents preliminary experimental results on the influence on the aerodynamic drag of a cylinder from the cylinder type (i.e., rigid or soft) and its textile surface. Both a rigid cylinder ...and a soft-body cylinder, with a gelatin layer, each with five different textile surfaces were measured in the wind tunnel using force measurement technology. The drag coefficient was determined for several Reynolds numbers. The study shows that the elasticity of a cylinder has a significant influence on the drag force and the airflow type. However, the influence of the soft-body cylinder depends on the respective fabric. With the given measurements, no exact statements can yet be made to quantify the influence. This influence must be studied independently and in conjunction with the textile surface in order to gain understanding of the overall system of airflow, textile and elastic body.
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