Bulletin of the Lebedev Physics Institute最新文献

Compensation for nonlinear signal distortions is a key challenge for ensuring high throughput and increasing the range of modern fiber-optic communication systems. Nonlinear effects significantly limit data transmission quality, especially with increasing signal power and propagation over long distances. Therefore, the development of accurate, robust, and computationally efficient compensation methods incorporating modern machine learning approaches is particularly relevant in optical telecommunications engineering. The paper presents a comparative analysis of efficient nonlinear distortion compensation algorithms in fiber-optic communication systems using machine learning methods. Modifications of the classical digital backpropagation (DBP) method are discussed: learned DBP (LDBP), enhanced DBP (EnDBP), a perturbation-based model (PBM), a hybrid PB-DBP scheme, and an approach using convolutional neural networks (CNNs). Using experimental data from a laboratory prototype of a 2000-km-long optical transmission line, the capabilities of parameter learning are demonstrated and the effectiveness of the methods in terms of compensation accuracy and computational complexity is compared. The methods demonstrate a significant increase in signal-to-noise ratio and allow a balance between accuracy and load on the digital signal processing module to be optimized.

We propose using the nonlinear Fourier transform (NFT) not as a method for integrating equations but as a tool for studying the properties of localized coherent structures in dissipative nonlinear systems. The effectiveness of using a discrete nonlinear spectrum for quantitatively describing the dynamics of optical pulses is demonstrated, even when the original model is not integrable. The application of this method is substantiated using the solution to the cubic Haus–Ginzburg–Landau equation (HGLE), which describes the generation of optical pulses from noise in passively mode-locked lasers. It is shown that stabilization of the discrete nonlinear spectrum serves as a reliable indicator of stable soliton generation. Additionally, algorithms for tracing individual discrete eigenvalues are proposed, including a machine-learning-based algorithm, which expands the toolbox for analyzing and automating the processing of data obtained using NFT.

The numerical simulation results for optical trapping of silicon nanoparticles using lensed single-mode fibers at a wavelength of 1550 nm are presented. A single lensed fiber is shown to provide stable trapping of particles with a radius of up to 100 nm, whereas for larger particle sizes trapping becomes unstable due to increasing scattering force. An optical tweezers design based on two opposing lensed fibers is proposed, which makes it possible to expand significantly the size range of trapped particles. The sensitivity of the trapping force to fiber misalignment is analyzed. The obtained results can be used in the design of compact fiber optical tweezers for manipulating dielectric nanostructures and micro-objects in microfluidic systems.

The results of a theoretical study of all-glass microstructured optical fibers (MOFs) with leakage channels, containing two layers of round fluorine-doped silica elements in the cladding and doped cores 20 µm in diameter with different reduced refractive indices, are presented. Numerical analysis of the properties of these MOFs was performed using the finite-element method (FEM). The leakage losses of fundamental and higher modes in the spectral range from 0.79 µm to 1.35 µm for bent MOF at a bending radius of 8 cm are calculated. It is shown that the considered MOF design provides efficient single-mode operation in the spectral range from 0.99 to 1.28 µm. The leakage losses at a wavelength of 1.05 µm are of no more than 0.056 dB/m for fundamental modes and no less than 12.3 dB/m for higher modes.

A narrow-linewidth erbium-doped fiber laser with random distributed feedback (RDFB) was demonstrated. A record-short 2-mm RDFB sample was fabricated using femtosecond inscription technique of disordered structures directly within the fiber. Despite its compact size, this structure provides a reflectivity equivalent to ~10 km of standard single-mode fiber. In a ring cavity configuration, the laser showed stable single-frequency operation over the entire pump power range. The central lasing wavelength was 1549.8 nm, the emission linewidth at half maximum was 0.6 kHz, and the maximum output power was 10.1 mW.

The work presents a numerical and experimental study of pulse-to-pulse interactions occurring due to gain depletion and recovery in a soliton polarization-maintaining fiber laser. Specifically, it is demonstrated that the direction and magnitude of the interaction forces depend significantly on the stage of the combined evolution of the soliton and the dispersive wave occurring in the laser’s gain fiber. Numerical simulations demonstrate that by varying the position of the saturable absorber (SA) within the resonator, it is possible to control the soliton interaction occurring due to gain depletion and recovery. In particular, placing the SA immediately after the gain fiber enhances the pulse-to-pulse attraction, transforming the laser into a generator of coupled solitons or soliton bunches. Conversely, by optimizing the distance between the SA and the gain fiber, it is possible to enhance the repulsive interaction and ensure optimal conditions for stable harmonic mode locking with multi-GHz pulse repetition rates. The numerical simulation results are consistent with the experimental data.

A directional coupler (DC) architecture consisting of curved single-mode waveguides with a structured depressed cladding was proposed. The DCs were written in the bulk of industrial borosilicate K8 glass using a femtosecond laser beam. The dependence of the optical power fraction transferred between the DC arms in the telecommunication wavelength range on the length of the coupling section was experimentally and theoretically studied. The maximum fraction of transferred power was 56% and was found to be limited by the asymmetry of the DC arms. The modal propagation loss was as low as 0.8 dB/cm.

The paper addresses the problem of increasing the soliton fiber-optic line throughput limited by nonlinear effects in optical fiber. An approximate analytical description of the pulse dynamics in a passive communication line medium is developed, and a combined quasi-analytical method for compensating for nonlinear distortions at the receiver is proposed based on this description. The proposed approach enables a transition from a phase modulation format to an amplitude‒phase modulation format for a soliton signal, leading to an increase in the throughput of the systems in question due to constellation compression.

A highly Yb-doped polarization-maintaining fiber has been developed. Optimization of the aluminophosphosilicate core composition made it possible to achieve a high gain (comparable with that of phosphate glasses), simultaneously with high pump-to-signal conversion efficiency. Codoping the core with germanium oxide provided high photosensitivity and made it possible to record highly reflective fiber Bragg gratings directly in the active fiber. Single-frequency fiber lasers have been designed based on the developed fiber, which generate in a single polarization mode with a record-short cavity length, equal to 14 mm. The pump-to-signal conversion efficiency was shown to be 22% for a polarized single-frequency laser at a wavelength of 1064 nm and 17% for a polarized single-frequency laser at 976 nm.

The latest advances in utilizing a novel method for generating gigahertz-repetition-rate pulses in fiber lasers based on semiconductor optical amplifiers are reviewed. This method, based on self-sustained cross-gain modulation in lasers with negative optical feedback, does not require active modulation or the use of saturable absorbers and provides stable generation of subnanosecond pulses. The applicability of the approach has been demonstrated experimentally at repetition rates up to 9.77 GHz. The possibility of broad wavelength tuning in a wide range from 1480 to 1556 nm with preserved pulsed operation mode and control of pulse repetition frequency from 1.85 to 6.2 GHz is also demonstrated. The physical principles underlying the method and its advantages for practical applications in telecommunications, metrology, and spectroscopy are discussed.