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We examine moderate CO2 laser heating as a method of forming negative effective radius variations in conventional optical fiber. Fiber samples were subjected to focused CO2 laser radiation in the form of 100 ms pulses with power 0.36 to 1.3 W and thermalization time up to 2.2 s. Fiber temperatures reached 200 to 300 degrees Celsius in each pulse. The temper atures are significantly smaller than the glass transformation temperature, which is nearly 1500 °C. We show that short CO2 laser pulses may introduce negative variations up to 5 nm. The magnitude of the introduced variation increases roughly linearly for up to 6 pulses, and then fiber saturation is achieved. We reveal that it is possible to produce optical whispering gallery modes resonators in low-temperature heating mode and create a triangular-shape microresonator solely with the negative variations. The negative variations may also occur in peripheral areas of high-temperature heated fiber while introducing positive effective radius variations. To eliminate it, we propose low-temperature fiber annealing before high-temperature variations introducing. The negative variations may also be implemented to develop a new independent approach of shaping optical whispering gallery modes microresonators.

We present observation of ultra-narrow spectral modes in the generation of the random fiber laser by using a technique of optical heterodyning in real-time. We found that such modes have a spectral width below 3 MHz. The modes are spatially extended over the whole cavity and remain for tens of roundtrip times.

We propose a new mechanism for fully optical tuning and self-stabilization of microresonators based on optical fibers. We suggest using a fiber with core doped with rare earth active ions. Launching light into the core would result in heating and tuning the microcavity. We also show that utilizing a single laser for both pumping the micro-resonator and heating up the fibre creates a feedback for self-stabilization of the microcavity.

We directly observe ultra-narrow spectral modes in the generation of the random fiber laser by using a technique of optical heterodyning in real-time. These solitary spectral features appear slightly above the threshold of generation. We found that such modes could have a spectral width well below 3 MHz. These modes are spatially extended over the fiber laser length, and remain for tens of roundtrip times.

We show that radiation of random fiber Raman laser can be driven into pulsed operation via self-gain-switching. The regime is characterized by small time jitter. We also experimentally demonstrate the self-gain-modulation induced pulsed operation in random fiber laser occurs with switching of repetition rate, that occurs with the increase of the pump power. We present a simple formula that describes possible repetition frequencies.

We reveal that synthetic photonic lattices based on coupled fiber loops can realize deep neural networks for coherent manipulation of optical pulse trains, which could lead to advances in communications and real-time data analysis. We show the platform feasibility for an essential task of telecommunication signal equalization. We also demonstrated that utilizing all-optical Kerr nonlinearity within the system may open a path to complex signal manipulation.

Locking of a spectrum of a microresonator to the wavelength of a pumping laser is crucial to many applications, for instance in optical frequency comb generation [1] . A common approach is to utilise thermal locking that occurs due to energy dissipation within the micro-resonator when the pump laser wavelength lies on the blue wing of its resonance [2] . This, however, makes the feedback strength for compensation of perturbations in the mode spectrum dependent on the pump power, which may be inconvenient, for example, in low-power applications.

Optical neural networks (ONN) attracts a lot of attention based on their potential to offer orders-of-magnitude faster performance and lower energy consumption compared to purely electronic systems. There are strong advances in processing of spatially- and frequency-encoded optical data [1] . However the ONN development for coherent manipulation of optical pulse trains remains an open challenge, which could lead to advances in communications and real-time data analysis.

Since the first demonstration of the random fiber laser based on Rayleigh backscattering in optical fibers the field has undergone extensive development. Promising properties together with a simple laser design prompted the investigations of new regimes including, among others, pulse generation.

We investigate an impact of a small particle located on the surface of a fiber resonator on the dynamics of the whispering gallery modes (WGM) circulating near the fiber surface. We consider a single molecule of fibrinogen and bovine serum albumin that is put in contact with the surface of the fiber. We have studied both temporal dynamics of optical pulses propagating at the WGM as well as changes in the spectrum of WGM due to impact of the particle.

A stable single wavelength random fiber laser (RFL) with a partial-reflection random fiber grating (PR-RFG) for high temperature sensing is proposed and demonstrated for the first time, to the best of our knowledge. The PR-RFG is fabricated with the help of a femtosecond laser, with its highest reflection peak significantly higher than all other reflection peaks, which can ensure the stability of this filter-free RFL. Theoretical calculations also show that such a PR-RFG should be designed with reflectivity in the range of ∼ 30 % − 90 % to obtain one reflection peak significantly higher than other peaks. The threshold of this laser is only 6.4 mW. In addition, the RFL realizes temperature sensing in the range from 25°C to 500°C and has an optical signal-to-noise ratio of up to 70 dB.

International audience; Breathing solitons are nonlinear waves in which the energy concentrates in a localized and oscillatory fashion. Similarly to stationary solitons, breathers in dissipative systems can form stable bound states displaying molecule-like dynamics, which are frequently called breather molecules. So far, the experimental observation of optical breather molecules and the real-time detection of their dynamics are limited to diatomic molecules, that is, bound states of only two breathers. In this work, the observation of different types of breather complexes in a mode-locked fiber laser: multibreather molecules, and molecular complexes originating from the binding of two breather-pair molecules or a breather pair molecule and a single breather is reported. The intermolecular temporal separation of the molecular complexes attains several hundreds of picoseconds, which is more than an order of magnitude larger than that of their stationary soliton counterparts and is a signature of long-range interactions. Numerical simulations of the laser model support the experimental findings. Moreover, nonequilibrium dynamics of breathing solitons are also observed, including breather collisions and annihilation. This work opens the possibility of studying the dynamics of many-body systems in which breathers are the elementary constituents.

We experimentally and numerically study the dynamics of whispering gallery modes slowly propagating within the cladding of an optical fiber near its end facet. We demonstrated that modes reflect from the fiber cleave. The reflection coefficient appears to reach 70% for the sample under study. Use of a facet provides a simple method to confine the axial propagation of the modes.

We showed that the microresonators of whispering gallery modes on the surface of standard optical fibers can be shaped by introducing negative radius variations of effective radius instead of positive. To do this, the sample of optical fiber is irradiated with CO2 laser and heated to temperatures that are well below the transformation temperature. The method reduces the influence of the heated spot to the surrounding silica and makes the modification method simpler. Combining both positive and negative modification also expands the range of axial shapes of microresonators that can be introduced with CO2 laser heating.

We demonstrate different types of breathing soliton complexes in a mode-locked fibre laser: multi-breather molecules, and molecular complexes arising from the binding of two breather-pair molecules or a breather-pair molecule and a single breather.

We present a systematic analysis of the stationary regimes of nonlinear parity-time(PT) symmetric laser composed of two coupled fiber cavities. We find that power-dependent nonlinear phase shifters broaden regions of existence of both PT-symmetric and PT-broken modes, and can facilitate transitions between modes of different types. We show the existence of non-stationary regimes and demonstrate an ambiguity of the transition process for some of the unstable states. We also identify the presence of higher-order stationary modes, which return to the initial state periodically after a certain number of round-trips., 8 pages, 5 figures

In the present Letter, we experimentally investigate extreme events in the time dynamics of the random distributed feedback fiber laser. We find that the probability of extreme events depends on the wavelength of the generated light. On spectrum tails, we register extreme events with intensity up to 50 times higher than the average generation power. Analysis of return times between successive rogue waves reveals their exponential distribution. Further investigation proves that the appearance of extreme waves in laser radiation obeys Poisson law. Characteristic radiation time varies from nanoseconds to tens of microseconds for most intense waves.

Random distributed feedback fibre lasers are well known type of fiber lasers where the optical feedback is organized via amplified Rayleigh scattering on random in space sub-micron refractive index inhomogenities1. Random distributed feedback fiber lasers found their applications in telecommunications and distributed sensing systems, as well as attracted considerable amount of interest from researches2. It is well-known that the generation spectrum of random distributed feedback fiber laser is a wide spectrum of typical width of 1 nm. It can be specifically tailored to demonstrate multiwavelength, tunable operation etc. However, the main features of the generation spectrum should be defined by the nature of the feedback itself. Usually the smooth bell-shaped spectrum is attributed to the incoherent nature of the feedback3. It is well known however that the Rayleigh scattering is an elastic scattering and should be resulted in the coherent feedback, which in turn leads to narrow features in the generation spectrum. Recently, narrow modes have been observed in the generation of the random distributed feedback fiber laser by means of scanning Fabry-Perot interferometer4. The spectral width of modes was about tens of picometers and was limited by the spectral resolution of the scanning interferometer. Those modes were attributed to the stimulated Brillouin scattering.

Photonic time-stretch techniques and their applications are reviewed. The approach enables the observation of signals that are otherwise too short or rapid for conventional measurement. Observing non-repetitive and statistically rare signals that occur on short timescales requires fast real-time measurements that exceed the speed, precision and record length of conventional digitizers. Photonic time stretch is a data acquisition method that overcomes the speed limitations of electronic digitizers and enables continuous ultrafast single-shot spectroscopy, imaging, reflectometry, terahertz and other measurements at refresh rates reaching billions of frames per second with non-stop recording spanning trillions of consecutive frames. The technology has opened a new frontier in measurement science unveiling transient phenomena in nonlinear dynamics such as optical rogue waves and soliton molecules, and in relativistic electron bunching. It has also created a new class of instruments that have been integrated with artificial intelligence for sensing and biomedical diagnostics. We review the fundamental principles and applications of this emerging field for continuous phase and amplitude characterization at extremely high repetition rates via time-stretch spectral interferometry.

This paper presents a new concept for the study of spatio-temporal generation regimes in which several time scales coexist in the radiation (one of which is related to the cavity round-trip time). The essence of the concept is that the time dependence of the intensity is investigated in two dimensions, one of which corresponds to the evolution over successive cavity round trips. It is shown that fiber lasers of various types, e.g., quasicontinuous lasers and passively and actively mode-locked pulsed lasers, have a variety of spatio-temporal generation regimes. The possibility of experimental detection of localized structures, including solitons, in fiber laser radiation is demonstrated. The prospects of the proposed approach are discussed.

Raman fiber lasers are known for a various types of generation regimes based on their stochasticity properties. Usually, Raman fiber lasers generate in so-called turbulent generation regime characterized by almost stochastic temporal dynamics and wide generation spectrum. However, under specific conditions, one can achieve a so-called laminar generation regimes with largely suppressed temporal intensity fluctuations and narrow generation spectrum consisting of highly correlated longitudinal modes. From other side, by an introducing a concept of PT-symmetry, one can achieve generation regimes different by their parity-time-symmetry properties. In coupled Raman fiber lasers PT-symmetry could be introduced as well. In this manuscript we consider an interplay between PT-symmetric generation properties and temporal properties of the generation. We show that various turbulent and laminar generation regimes could have different PT-symmetric properties.

We show that whispering gallery modes on the surface of optical fibers can be reflected from the optical fiber facet. Due to reflection, a locking effect occurs near the optical fiber facet for the whispering gallery modes with an axial component of the wave vector. We obtained an analytical expression for determining the resonant wavelengths. We also experimentally measured the phase velocity of the optical pulse propagating in whispering gallery modes and reflecting from the fiber facet.

The concept of parity-time (PT) symmetry in optics attracts great deal of attention. Recently, a parity-time symmetry-breaking laser consisted of two coupled microrings has been experimentally demonstrated [1, 2]. A single-mode operation in an initially multimode system was achieved through stronger mode discrimination close to a PT-symmetry breaking transition. Further on, a single transverse mode operation in a system of coupled microring lasers was demonstrated near the exceptional point [3]. A possible route to a PT symmetry-based mode-locking has been theoretically discussed in paper [4].

Spectral correlations in fiber lasers attract a lot of attention during the last decade. Being formed due to fundamental interactions of light propagating through the cavity, they directly influence the general radiation statistical properties. In particular, study of inner correlations may reveal the physics of generation of radiation in random distributed feedback laser (RDFL).

Among the various types of microresonators, cylindrical microresonators based on optical fibers turned out to be a promising object for future applications. It was shown that the manufacturing accuracy of standard telecommunication optical fibers and the quality of their surface are high enough to excite high-quality whispering gallery (WGM) modes in their cladding [1]. Microresonators of this type are distinguished by low cost and reliability at a sufficiently high quality factor. In addition, the ability to control the WGM propagation along the fiber axis by introducing controlled radius variations allows creating fundamentally new photonic devices based on microresonators, such as delay lines [2], optical fluid devices, or others [3]. Here we study the features of the propagation of optical pulses within whispering gallery modes of a cladding of an optical fiber. We experimentally demonstrated the possibility to transmit pulses as far as several millimeters along the fiber axis and have measured their propagation speed. We have also utilized the pulse excitation for time domain reflectometry to measure a distance between the excitation point and the fiber cleave.

In the last few decades localization of light has drawn considerable attention. Anderson localization was originally formulated for electrons in disordered lattices [1], but now this effect is being theoretically and experimentally demonstrated also for light in various photonic systems: in disordered two-dimensional photonic lattices [2], in synthetic photonic lattices [3] and even in biological nanostructures of native silk [4].

A synthetic photonic lattice (SPL) is a re-configurable test-bed for studying the dynamics of one-dimensional mesh lattices including the photonic implementations of discrete time quantum walks. Unlike other realizations of photonic lattices, SPL possesses easy and fast control of lattice parameters. Here we consider disordered SPL where the coupling ratio between the two fiber loops realizing the lattice is random but does not change between the round trips. We obtain a new analytical result for the localization length (inverse Lyapunov exponent) for a practical case of weak coupling disorder. We also numerically study the dynamics of the pulse train circulating within the loops and observe that despite delocalization transition at the band center the pulse spreading is arrested even at small values of the disorder.

Formation of coherent structures and patterns from unstable uniform state or noise is a fundamental physical phenomenon that occurs in various areas of science ranging from biology to astrophysics. Understanding of the underlying mechanisms of such processes can both improve our general interdisciplinary knowledge about complex nonlinear systems and lead to new practical engineering techniques. Modern optics with its high precision measurements offers excellent test-beds for studying complex nonlinear dynamics, though capturing transient rapid formation of optical solitons is technically challenging. Here we unveil the build-up of dissipative soliton in mode-locked fibre lasers using dispersive Fourier transform to measure spectral dynamics and employing autocorrelation analysis to investigate temporal evolution. Numerical simulations corroborate experimental observations, and indicate an underlying universality in the pulse formation. Statistical analysis identifies correlations and dependencies during the build-up phase. Our study may open up possibilities for real-time observation of various nonlinear structures in photonic systems.

This work investigates a concept of coupled fiber lasers exhibiting PT-symmetry and a PT-transition between PTsymmetric and PT-broken lasing states. We consider a system operated via Raman gain comprising two fiber loops (ring cavities) connected to each other by means of two fiber couplers with adjustable phase shift between them. By changing the phase shift or/and amplification (loss) in fiber loops, one can switch between generation regimes, realizing either PTsymmetric or PT-broken solution. In the PT-symmetric lasing regime, equal powers are generated in both cavities despite only active one is pumped. We make theoretical and numerical description of the proposed coupled fiber lasers starting with the simple discrete matrix model taking into account coupling, phase delays, gain (which is assumed to be saturated), losses and nonlinear phase shift. We show how the PT-transition is affected by self-phase modulation inside the fiber cavity and investigate requirements that should be met in order to observe PT-transition experimentally despite Kerr effect that violates exact symmetry conditions. In particular, we show that PT-transition may be observable only near lasing threshold. Further on we adopt more sophisticated model based on Nonlinear Schrodinger equation for PT fiber laser. Taking into account quasi-CW polychromatic radiation with typical spectral bandwidth of fiber Raman lasers, chromatic dispersion and Kerr nonlinearity, we demonstrate both PT-symmetric and PT-broken lasing in a fiber laser.

A type of a random laser with a feedback organized by a number of randomly distributed fiber Bragg gratings (FBG) can demonstrate single-mode, or low threshold operation. Nevertheless, the spectrum of such lasers can be unstable. Here we study temporal variations of the spectra of random fiber laser with a set of Bragg gratings and show that they depend on the level of surrounding noise.

In present paper correlations between different parts of spectrum of a fiber laser with randomly distributed feedback (RDFL) were experimentally measured directly. Implemented statistical analysis demonstrate weak cross-correlations between different lines in generation spectrum. These correlations were vizualized by plotting 2-D probability density functions. Linear correlation coefficient (Pearson coefficient) was calculated for each pair of spectrum lines.

Fiber lasers operating via Raman gain or based on rare-earth-doped active fibers are widely used as sources of CW radiation. However, these lasers are only quasi-CW: their intensity fluctuates strongly on short time scales. Here the framework of the complex Ginzburg-Landau equations, which are well known as an efficient model of mode-locked fiber lasers, is applied for the description of quasi-CW fiber lasers. The vector Ginzburg-Landau model of a Raman fiber laser describes the experimentally observed turbulent-like intensity dynamics, as well as polarization rogue waves. Our results open debates about the common underlying physics of operation of very different laser types - quasi-CW lasers and passively mode-locked lasers. Fiber lasers operating via Raman gain or based on rare-earth-doped active fibers are widely used as sources of CW radiation. However, these lasers are only quasi-CW: their intensity fluctuates strongly on short time scales. Here the framework of the complex Ginzburg-Landau equations, which are well known as an efficient model of mode-locked fiber lasers, is applied for the description of quasi-CW fiber lasers. The vector Ginzburg-Landau model of a Raman fiber laser describes the experimentally observed turbulent-like intensity dynamics, as well as polarization rogue waves. Our results open debates about the common underlying physics of operation of very different laser types - quasi-CW lasers and passively mode-locked lasers.

A temporal variation of a spectrum of excited modes in a continuously pumped erbium-doped random fiber laser (RFL) based on randomly distributed Bragg gratings is studied. The developed phenomenological theoretical model assumes a hard excitation mechanism of the eigenmodes’ instability. The model qualitatively explains peculiarities of the spectrum variation observed experimentally.