This study investigates the nonlinear effects on signal integrity in 16-QAM optical communication systems by focusing on received signal distributions without noise interference. Utilizing GPU-based simulations and analyzing “triplets” of consecutive signal points, we uncover that nonlinear interactions generate distinctive patterns in signal behavior, challenging the adequacy of standard Gaussian models. Our analysis employs the Gaussian Mixture Model (GMM), revealing that multi-component models offer a more accurate representation of signal distributions, highlighting the complexity of nonlinear effects. This research not only enhances our understanding of signal behavior under nonlinear conditions but also paves the way for future investigations into improving optical communication system design and reliability.
We introduce a new approach to reservoir computing (RC) in which single nonlinear device – semiconductor optical amplifier, replaces the entire nonlinear reservoir to perform computations. To study the performance of the proposed scheme, we use it for the benchmark prediction task of learning the Mackey-Glass chaotic attractor. Mildly chaotic attractor with tau = 17 and wilder chaotic behavior with tau = 30 are considered.
We report numerical and experimental studies of instabilities in a “noise-like pulse” dissipative soliton laser generating an output spectrum of 1000 nm bandwidth, and with two orders of magnitude variation in intracavity spectral width over one roundtrip. Simulations identify the origin of the laser instability as the sensitivity to noise of nonlinear soliton dynamics. Our experiments use real-time time and frequency domain measurements, and our simulations reproduce quantitatively both the full extent of intracavity supercontinuum broadening, as well as the probability distributions of temporal and spectral fluctuations, including rogue waves.
In this work, we demonstrate that the high-accuracy computation of the continuous nonlinear spectrum can be performed by using artificial neural networks. We propose the artificial neural network (NN) architecture that can efficiently perform the nonlinear Fourier (NF) optical signal processing. The NN consists of sequential convolution layers and fully connected output layers. This NN predicts only one component of the continuous NF spectrum, such that two identical NNs have to be used to predict the real and imaginary parts of the reflection coefficient. To train the NN, we precomputed 94035 optical signals. 9403 signals were used for validation and excluded from training. The final value of the relative error for the entire validation dataset was less than 0.3%. Our findings highlight the fundamental possibility of using the NNs to analyze and process complex optical signals when the conventional algorithms can fail to deliver an acceptable result.
We demonstrated how the nonlinear Fourier transform based on the Zakharov-Shabat spectral problem can be used to characterise coherent structures in dissipative systems. We consider as a particular, albeit important practical example model equation that is widely used to analyse laser radiation and demonstrate that dissipative solitons can be described by a limited number of degrees of freedom { discrete eigenvalues. Our approach can be applied for signal processing in a number of optical systems, from lasers to micro-resonators.
In the current work we demonstrate mode-locked fiber laser with automatic adjustment a coherence degree of the output pulses. As a source of the pulses we used 8-figure fiber laser with two amplifying fibers inside both loops of the laser cavity. Such configuration provides various pulsed regimes that have different degree of coherence from fully mode-locked single scale pulses to partly mode-locked double scale pulses. To search a pulsed regime with defined parameters we applied automatic genetic algorithm. To prove the feasibility of the genetic algorithm we applied it to find double – scale pulsed regimes with a fixed envelope duration of 50 ps and the contrast of the coherence peak in range of 0.02 – 0.5.
The work presents for the first time a comparative study of mode-locked figure-8 laser, in which two independently pumped active media are located either in the same or in different cavity loops. It is shown that the NALM2 configuration (both active media in the same cavity loop) delivers both higher average and peak radiation power. Flexibility of NALM/NALM2 technologies is further demonstrated for implementation of algorithmic electronically driven control over radiation mode-locking regimes. Also discussed are the results of experimental testing of electronic methods relying on NALM/NALM2 technologies for setting desired generation regimes.
A ground-based laser system for space debris cleaning requires pulse power well above the critical power for self-focusing in the atmosphere. Self-focusing results in beam quality degradation and is detrimental for the system operation. We demonstrate that, for the relevant laser parameters, when the thickness of the atmosphere is much less than the focusing length (that is, of the orbit scale), the beam transit through the atmosphere produces the phase distortion only. The model thus developed is in very good agreement with numerical modeling. This implies that, by using phase mask or adaptive optics, it may be possible to eliminate almost completely the impact of self-focusing effects in the atmosphere on the laser beam propagation.
We consider a model nondispersive nonlinear optical fiber channel with additive Gaussian noise at large SNR (signal-to-noise ratio) in the intermediate power region. Using Feynman path-integral technique we find the optimal input signal distribution maximizing the channel’s per-sample mutual information. The finding of the optimal input signal distribution allows us to improve previously known estimates for the channel capacity. We show that in the intermediate power regime the per-sample mutual information for the optimal input signal distribution is greater than the per-sample mutual information for the Gaussian and half-Gaussian input signal distributions.
Fibre lasers have been shown to manifest a laminar-to-turbulent transition when increasing its pump power. In order to study the dynamical complexity of this transition we use advanced statistical tools of time-series analysis. We apply ordinal analysis and the horizontal visibility graph to the experimentally measured laser output intensity. This reveal the presence of temporal correlations during the transition from the laminar to the turbulent lasing regimes. Both methods allow us to unveil coherent structures with well defined time-scales and strong correlations both, in the timing of the laser pulses and in their peak intensities.
We present the essential features of the dissipative parametric instability, in the universal complex Ginzburg- Landau equation. Dissipative parametric instability is excited through a parametric modulation of frequency dependent losses in a zig-zag fashion in the spectral domain. Such damping is introduced respectively for spectral components in the +ΔF and in the -ΔF region in alternating fashion, where F can represent wavenumber or temporal frequency depending on the applications. Such a spectral modulation can destabilize the homogeneous stationary solution of the system leading to growth of spectral sidebands and to the consequent pattern formation: both stable and unstable patterns in one- and in two-dimensional systems can be excited. The dissipative parametric instability provides an useful and interesting tool for the control of pattern formation in nonlinear optical systems with potentially interesting applications in technological applications, like the design of mode- locked lasers emitting pulse trains with tunable repetition rate; but it could also find realizations in nanophotonics circuits or in dissipative polaritonic Bose-Einstein condensates.
A diode-cladding-pumped mid-infrared passively Q-switched Ho3+-doped fluoride fiber laser using a reverse designed
broad band semiconductor saturable mirror (SESAM) was demonstrated. Nonlinear reflectivity of the SESAM was
measured using an in-house Yb3+-doped mode-locked fiber laser at 1062 nm. Stable pulse train was produced at a slope efficient of 12.1% with respect to the launched pump power. Maximum pulse energy of 6.65 μJ with a pulse width of
1.68 μs and signal to noise ratio (SNR) of ~50 dB was achieved at a repetition rate of 47.6 kHz and center wavelength of
2.971 μm. To the best of our knowledge, this is the first 3 μm region SESAM based Q-switched fiber laser with the
highest average power and pulse energy, as well as the longest wavelength from mid-infrared passively Q-switched
fluoride fiber lasers.
We report experimental study of vector solitons for the fundamental and harmonic mode-locked operation in erbiumdoper
fiber lasers with carbon nanotubes based saturable absorbers and anomalous dispersion cavities. We measure
evolution of the output pulses polarization and demonstrate vector solitons with various polarization attractors, including
locked polarization, periodic polarization switching, and polarization precession.
As shown recently, a long telecommunication fibre may be treated as a natural one-dimensional random system, where
lasing is possible due to a combination of random distributed feedback via Rayleigh scattering by natural refractive
index inhomogeneities and distributed amplification through the Raman effect. Here we present a new type of a random
fibre laser with a narrow (~1 nm) spectrum tunable over a broad wavelength range (1535-1570 nm) with a uniquely flat
(~0.1 dB) and high (>2 W) output power and prominent (>40 %) differential efficiency, which outperforms traditional
fibre lasers of the same category, e.g. a conventional Raman laser with a linear cavity formed in the same fibre by adding
point reflectors. Analytical model is proposed that explains quantitatively the higher efficiency and the flatter tuning
curve of the random fiber laser compared to conventional one. The other important features of the random fibre laser like
"modeless" spectrum of specific shape and corresponding intensity fluctuations as well as the techniques of controlling
its output characteristics are discussed. Outstanding characteristics defined by new underlying physics and the simplicity
of the scheme implemented in standard telecom fibre make the demonstrated tunable random fibre laser a very attractive
light source both for fundamental science and practical applications such as optical communication, sensing and secure
transmission.
The modulation instability (MI) in optical fiber amplifiers and lasers with anomalous dispersion leads to CW beam
breakup and the growth of multiple pulses. This can be both a detrimental effect, limiting the performance of
amplifiers, and also an underlying physical mechanism in the operation of MI-based devices. Here we revisit the
analytical theory of MI in fiber optical amplifiers. The results of the exact theory are compared with the previously
used adiabatic approximation model, and the range of applicability of the latter is determined. The same technique is
applicable to the study of spatial MI in solid state laser amplifiers and MI in non-uniform media.
We experimentally demonstrate a Raman fiber laser based on multiple point-action fiber Bragg grating (FBG)
reflectors and distributed feedback via Rayleigh scattering in a ~22 km long optical fiber. Twenty two lasing lines with
spacing of ~100 GHz (close to ITU grid) in C-band are generated at Watts power level. In contrast to the normal cavity
with competition between laser lines, the random distributed feedback cavity exhibits highly stable multiwavelength
generation with a power-equalized uniform distribution which is almost independent on power. The current set up
showing the capability of generating Raman gain of about 100-nm wide giving the possibility of multiwavelength
generation at different bands.
Different generation modes of all-positive-dispersion all-fibre Yb laser mode-locked due to effect of non-linear
polarization evolution are investigated. For the first time we realized in the same laser both generation of single
picoseconds pulse train and a newly observed lasing regime where generated are picosecond wave-packets, each being a
train of femtosecond sub-pulses. Using both experimental results and numerical modeling we discuss in detail the
mechanisms of laser mode-locking and switching of generation regimes and show a strong dependence of output laser
characteristics on configuration of polarization controllers. A good qualitative agreement between experimental results
and numerical modeling is demonstrated.
We present results on characterization of lasers with ultra-long cavity lengths up to 84km, the longest cavity ever reported. We have analyzed the mode structure, shape and width of the generated spectra, intensity fluctuations depending on length and intra-cavity power. The RF spectra exhibit an ultra-dense cavity mode structure (mode spacing is 1.2kHz for 84km), in which the width of the mode beating is proportional to the intra-cavity power while the optical spectra broaden with power according to the square-root law acquiring a specific shape with exponential wings. A model based on wave turbulence formalism has been developed to describe the observed effects.
A theoretical model is developed to describe the propagation of ultra-short optical pulses in fiber transmission systems in the quasi-linear regime, with periodically inserted in-line lumped nonlinear optical devices. Stable autosoliton solutions are obtained for a particular application of the general theory.
We present an analysis of the performance of backward-pumped discrete Raman amplifier modules designed for simultaneous amplification and dispersion and/or dispersion slope compensation, both in single-channel and in multi-channel systems. Optimal module parameters are determined within a realistic range of pump and signal powers.
In this paper we consider an effective variational description for the optical pulse evolution in fiber links with dispersion management (varying dispersion). Instead of solving a nonlinear partial differential equation with varying coefficients, one can gain some important information about the features (e.g. breathing solitons) by an analysis of two ordinary differential equations. This approach provides a clear physical picture of the pulse evolution in a transmission line under the combined actions of nonlinearity, varying dispersion, fiber loss, and periodic amplification. An optical pulse propagation along a transmission line must be periodically reproduced.
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