Laser Dynamics Research Articles

Statistical dynamics of noise-like rectangle pulse fiber laser

The statistical dynamics of partially incoherent ultrafast lasers are complex and chaotic, which is significant for fundamental research and practical applications. We experimentally and theoretically reveal the statistical dynamics of the spectral evolutions and correlations in an incoherent noise-like rectangle pulse laser (NLRPL). Based on statistical histogram analysis, the probability distribution asymmetry of the spectral intensity fluctuation is decayed with the wavelength far away from the spectral peak due to the detection noise. The full-spectral correlation values indicate that the spectral similarity between two round trips is exponentially weakened as the round-trip offset increases. By studying the correlation map of spectral components, we find that the area of the high-correlation region is relevant to the pump power, which is reduced by increasing the pump power. The mutual information of the spectra demonstrates that two spectral components with symmetry about the spectral peak have a statistical dependence. Experimental observations and statistical properties can coincide well with theoretical numerical simulations. We reveal the pump-dependent spectral correlation of the NLRPL and provide multiple statistical methods for the characterizations of chaotic dynamics in incoherent light sources.

Dynamics and feedback sensitivity of vertical-cavity surface-emitting lasers with arbitrary optical feedback intensity

Vertical cavity surface emitting lasers (VCSELs) are the key laser sources in the high-performance computers (HPCs) and data centers (DCs). The explosive growth of data traffic in the HPCs and DCs requires increased bandwidths of VCSELs. In this paper, we deduce the Time-Delay (T-D) model considering the multiple round trips of light in VCSELs with an optical feedback cavity to enhance the modulation bandwidth. The T-D model is suitable for the VCSEL with an arbitrary optical feedback intensity. The influences of the round trip time m of light in the optical feedback cavity on the intensity-modulation responses with strong and weak optical feedback are studied. The feedback coefficients considering the multiple round trips of light in the optical feedback cavity and the dispersions of the distributed Bragg reflector are deduced, and are suitable for the arbitrary optical feedback intensity. The feedback coefficients are influenced by the value of m and the optical feedback intensity, and decrease as the value of m under arbitrary optical feedback intensity increases. The feedback coefficients under strong optical feedback are larger than that under weak optical feedback with the same value of m. The theories are useful for the high-speed VCSELs with optical feedback.

Charge Mobility and Strain Engineering in Two-Step MS-Grown MoS2/Seed Layer Heterointerface and Photo-Excitation Mechanism.

Two-dimensional (2D) materials have potential application and wide development prospects in photoelectron and spintronic devices. However, the properties of different growth conditions are challenging to study in the future. This, in turn, hinders further research into 2D materials and the manufacture of high-quality devices. A comprehensive understanding of the ultrafast laser spectroscopy and dynamics that take into account the substrate-transition metal dichalcogenide (TMD) interaction is lacking. Here, the strain effect is elucidated by systematically investigating the interfacial interaction between different substrates and MoS2. The strain and interface engineering of MoS2/seeds layer heterointerface and light-matter coupling are discussed in the Raman and photoluminescence spectra. The dramatic enhanced PL originates from the phase transition of MoS2 on different substrates and electron-hole pairs dissociated by exciton screening effect. Finite-difference time-domain simulation confirmed that the electric field, magnetic field, and polarization field of the heterojunction system changed after the strain was applied. In addition, based on the dependence of physical parameters of MoS2, the relative numerical changes of physical parameters of MoS2 films on different substrates as well as the photoelectric transfer, strain, and charge doping levels on the surface or interface will provide a direction for optimizing the selection of various devices.

Frequency modulations due to domain dynamics in terahertz quantum cascade lasers

The light output of quantum cascade lasers is strongly affected by electric field-domain oscillations if the laser shows electrical instabilities. This can result in a substantial broadening of the emission spectrum, which is investigated here by detailed simulations for a terahertz device. We explain how the light pulsation is affected by electrical injection conditions and external capacitances. Experimental data confirm that the oscillation frequencies are not essentially affected by circuit conditions, while a large external capacitance produces almost non-observable oscillations due to stabilization of the circuit.

Strategies for accessing the multipulse regime of mode-locked fiber lasers

Important ongoing research on mode-locked fiber lasers aims at developing new types of multisoliton regimes, such as soliton molecules, molecular complexes, or soliton crystals. The on-demand generation of such multipulse structures is a major challenge, whereas experiments generally involve a tedious trial-and-error adjustment of the laser parameters. Here we present an approach based on a gradual and calibrated adjustment of the system configuration, which employs efficient parameter routes to reach well-defined multipulse regimes. Our numerical simulations show that once the mode-locking threshold is reached, we can adjust the laser parameters gradually to force the evolution of the laser dynamics towards multipulse structures with fewer distortion in their intensity profiles, which are accessible at reduced pump power levels.

Chaotic mode-competition dynamics in a multimode semiconductor laser with optical feedback and injection.

Photonic computing has attracted increasing interest for the acceleration of information processing in machine learning applications. The mode-competition dynamics of multimode semiconductor lasers are useful for solving the multi-armed bandit problem in reinforcement learning for computing applications. In this study, we numerically evaluate the chaotic mode-competition dynamics in a multimode semiconductor laser with optical feedback and injection. We observe the chaotic mode-competition dynamics among the longitudinal modes and control them by injecting an external optical signal into one of the longitudinal modes. We define the dominant mode as the mode with the maximum intensity; the dominant mode ratio for the injected mode increases as the optical injection strength increases. We deduce that the characteristics of the dominant mode ratio in terms of the optical injection strength are different among the modes owing to the different optical feedback phases. We propose a control technique for the characteristics of the dominant mode ratio by precisely tuning the initial optical frequency detuning between the optical injection signal and injected mode. We also evaluate the relationship between the region of the large dominant mode ratios and the injection locking range. The region with the large dominant mode ratios does not correspond to the injection-locking range. The control technique of chaotic mode-competition dynamics in multimode lasers is promising for applications in reinforcement learning and reservoir computing in photonic artificial intelligence.

Dynamic intrinsic spectrum reconstruction of narrow-linewidth laser based on band-extended frequency noise

Narrow-linewidth lasers have extensive applications due to advantages of low phase noise and high coherence. To explore the dynamics of narrow-linewidth lasers, the intrinsic spectrum is a kernel parameter which can show the dynamical coupling strength between the spontaneous emission and the stimulated emission in a laser cavity. Herein, we propose an intrinsic spectrum measurement method of narrow-linewidth lasers based on the band-extended frequency noise, which can improve the spectrum resolution. In the experiment, we successfully apply the method to the measure the linewidth compression dynamics of external-cavity distributed feedback lasers. The proposed measurement method can dynamically characterize the fast evolution process of the laser intrinsic spectrum with wide-range linewidth fluctuations, which will benefit for the dynamics research of narrow-linewidth lasers and tunable lasers.

Non-linear dynamics of multimode semiconductor lasers: dispersion-based phase instability and route to single-frequency operation.

Emission dynamics of a multimode broadband interband semiconductor laser have been investigated experimentally and theoretically. Non-linear dynamics of a III-V semiconductor quantum well surface-emitting laser reveal the existence of a modulational instability, observed in the anomalous dispersion regime. An additional unstable region arises in the normal dispersion regime, owing to carrier dynamics, and has no analogy in systems with fast gain recovery. The interplay between cavity dispersion and phase sensitive non-linearities is shown to affect the character of laser emission with phase turbulence, leading to regular self-excited oscillations of mode intensity, self-mode locking, and single-frequency emission stabilized by spectral symmetry breaking. Such physical behavior is a general phenomenon for any laser with a slow gain medium relative to the round trip time, in the absence of spatial inhomogeneities.

Frequency-tunable microwave generation with parity-time symmetry period-one laser dynamics.

A novel frequency-tunable microwave signal generation method is proposed by incorporating parity-time (PT) symmetry in period-one (P1) laser dynamics in an optically injected semiconductor laser. In this method, P1 oscillation enables a large frequency tuning range and PT symmetry leads to excellent side-mode suppression and low phase noise. In an experimental demonstration, the side-mode suppression ratio reaches 58.4 dB and the phase noise is -126.2 dBc/Hz at 10 kHz offset when generating a 6.98 GHz signal, which are improved by 44.5 dB and 13.5 dB, respectively, compared with the previously reported optoelectronic oscillator-based P1 oscillation. By simply adjusting the optical injection strength, the frequency of the microwave signal generated by PT symmetry P1 dynamics is tuned from 5.07 GHz to 15.22 GHz, in which the phase noise is kept below 120 dBc/Hz at 10 kHz offset. The proposed method is expected to find applications in high-performance wireless communication and radar systems.

Forecasting the chaotic dynamics of external cavity semiconductor lasers.

Chaotic time series prediction has been paid intense attention in recent years due to its important applications. Herein, we present a single-node photonic reservoir computing approach to forecasting the chaotic behavior of external cavity semiconductor lasers using only observed data. In the reservoir, we employ a semiconductor laser with delay as the sole nonlinear physical node. By investigating the effect of the reservoir meta-parameters on the prediction performance, we numerically demonstrate that there exists an optimal meta-parameter space for forecasting optical-feedback-induced chaos. Simulation results demonstrate that using our method, the upcoming chaotic time series can be continuously predicted for a time period in excess of 2 ns with a normalized mean squared error lower than 0.1. This proposed method only utilizes simple nonlinear semiconductor lasers and thus offers a hardware-friendly approach for complex chaos prediction. In addition, this work may provide a roadmap for the meta-parameter selection of a delay-based photonic reservoir to obtain optimal prediction performance.

Deep neural network for modeling soliton dynamics in the mode-locked laser.

Integrating the information of the first cycle of an optical pulse in a cavity into the input of a neural network, a bidirectional long short-term memory (Bi_LSTM) recurrent neural network (RNN) with an attention mechanism is proposed to predict the dynamics of a soliton from the detuning steady state to the stable mode-locked state. The training and testing are based on two typical nonlinear dynamics: the conventional soliton evolution from various saturation energies and soliton molecule evolution under different group velocity dispersion coefficients of optical fibers. In both cases, the root mean square error (RMSE) for 80% of the test samples is below 15%. In addition, the width of the conventional soliton pulse and the pulse interval of the soliton molecule predicted by the neural network are consistent with the experimental results. These results provide a new insight into the nonlinear dynamics modeling of the ultrafast fiber laser.

Experimental study of single-shot polarization dynamics of dual dissipative solitons

The state of polarization provides an additional dimension to the pulse dynamics of dissipative solitons. However, due to lack of appropriate measuring instruments, the polarization evolution of dual pulse vector solitons is still unclear. Here, for an erbium-doped fiber laser mode-locked with carbon nanotubes, we characterize the single-shot polarization evolution of dual dissipative solitons in detail using a high-speed wave-resolved measurement technique. For the different birefringence distributions in the laser cavity, the coupling between the orthogonal components of the vector soliton has a great influence on its physical properties, which may lead to various physical images of the vector soliton. We observed typical polarization-locked vector solitons and polarization attractors in the dual soliton pulses. We also observed that the polarization dynamics of the lead and tail vector pulses have similar evolution dynamics on the Poincaré sphere, while they have a distinct phase difference. By virtue of the difference of the processing SOPs between the leading and trailing pulses, we resolve three kinds of two solitons pulses with distinct relative phase evolutions. Our results enrich the vector soliton dynamics in fiber lasers.

Self-synchronized temporal-spectral characterization system for revealing ultrafast fiber laser dynamics

Due to the electronic bottleneck limited real-time measurement speed of common temporal-spectral detection and the particle-like nature of optical soliton enabled nonrepeatable transient behaviors, capturing the ultrafast laser pulses with unknown times of arrival and synchronously characterizing their temporal-spectral dynamic evolution is still a challenge. Here, using the Raman soliton frequency shift based temporal magnifier and dispersive Fourier transform based spectral analyzer, we demonstrate a self-synchronized, ultrafast temporal-spectral characterization system with a resolution of 160 fs and 0.05 nm, and a recording length above milliseconds. The synchronized nonlinear process makes it possible to image full-filled temporal sub-picosecond pulse trains regardless of their arrival times and without extra pump lasers and photoelectric conversion devices. To demonstrate the significance of this improvement, a buildup dynamic process of a soliton laser with a complex breakup and collisions of multisolitons is visually displayed in the spectral and temporal domains. The soliton dynamic evolution processes observed by our characterization system are in one-to-one correspondence with the numerical simulation results. We believe this work provides a new multidimensional technique to break the electronic bottleneck to gain additional insight into the dynamics of ultrafast lasers and nonlinear science.

Analysis of temporal structure of laser chaos by Allan variance.

Allan variance has been widely utilized for evaluating the stability of the time series generated by atomic clocks and lasers, in time regimes ranging from short to extremely long. This multiscale examination capability of the Allan variance may also be beneficial in evaluating the chaotic oscillating dynamics of semiconductor lasers- not just for conventional phase stability analysis. In the present study, we demonstrated Allan variance analysis of the complex time series generated by a semiconductor laser with delayed feedback, including low-frequency fluctuations (LFFs), which exhibit both fast and slow dynamics. While the detection of LFFs is difficult with the conventional power spectrum analysis method in the low-frequency regime, the Allan variance approach clearly captured the appearance of multiple time-scale dynamics, such as LFFs. This study demonstrates that Allan variance can help in understanding and characterizing diverse laser dynamics, including LFFs, spanning a wide range of timescales.

Quantification of dissipative effects in a complex Ginzburg-Landau equation governed laser system by tracing soliton dynamics.

The concept of dissipative solitons has provided new insight into the complex pulse dynamics in mode-locked lasers and stimulated novel laser cavity designs. However, most of these studies are restricted to qualitative regimes, because it is difficult to quantify dissipative effects in a mode-locked laser. Meanwhile, the quantification of dissipative effects is a general problem that can be also encountered in other dissipative systems. In this paper, we demonstrate a method for quantifying dissipative effects in a mode-locked laser based on analyzing the soliton dynamics traced by time-stretch dispersive Fourier transform. As a result, we are able to quantitatively reproduce the evolution of the pulse that seeds mode-locking through simulations and gain a deeper understanding of the whole process. The obtained physical picture of mode-locking allows us to propose a simple method to quantify the energy threshold for mode-locking buildup and the stability of mode-locked states. A parameter is introduced to evaluate mode-locking conditions, which can serve as a criterion for designing mode-locked lasers. This work opens up new possibilities in the diagnosis and improvement of mode-locked lasers and studies of soliton physics.

Spatiotemporal lasing dynamics in a Limaçon-shaped microcavity.

Limaçon-shaped microdisk lasers are promising on-chip light sources with low lasing threshold and unidirectional output. We conduct an experimental study on the lasing dynamics of Limaçon-shaped semiconductor microcavities. The edge emission exhibits intensity fluctuations over a wide range of spatial and temporal scales. They result from multiple dynamic processes with different origins and occur on different spatiotemporal scales. The dominant process is an alternate oscillation between two output beams with a period as short as a few nanoseconds.

Dynamics of Hermite–Gaussian laser beam in plasma and terahertz generation

In the present work, the dynamical study of a Hermite–Gaussian (HG) laser beam propagating through plasma with relativistic nonlinearity has been carried out. Furthermore, the dependency of THz generation on the HG laser dynamics has also been investigated. Self-focusing of laser beam inside plasma is achieved due to the modification in electron’s rest mass due to their motion at relativistic velocities. The coupled differential equations obtained using the method of moment determines the variation in spot size of laser beam and have been solved numerically. The laser beam propagating through plasma excites the plasma wave and the nonlinear coupling of HG beam and plasma wave generates THz radiation. It has been observed that THz generation strongly depends upon the variation of spot size, TEM modes, plasma density and intensity of laser beam. Furthermore, the self-trapping of HG laser beam has also been studied.

Theoretical and experimental investigations of dispersion-managed, polarization-maintaining 1-GHz mode-locked fiber lasers.

High-repetition-rate (up to GHz) femtosecond mode-locked lasers have attracted significant attention in many applications, such as broadband spectroscopy, high-speed optical sampling, and so on. In this paper, the characteristics of dispersion-managed, polarization-maintaining (PM) 1-GHz mode-locked fiber lasers were investigated both experimentally and numerically. Three compact and robust 1-GHz fiber lasers operating at anomalous, normal, and near-zero dispersion regimes were demonstrated, respectively. The net dispersion of the linear cavity is adjusted by changing types of PM erbium-doped fibers (EDFs) and semiconductor saturable absorber mirrors (SESAMs) in the cavity. Moreover, the long-term stability of the three mode-locked fiber lasers is proved without external control. In order to better understand the mode-locking dynamics of lasers, a numerical model was constructed for analysis of the 1-GHz fiber laser. Pulse evolution simulations have been carried out for soliton, dissipative-soliton, and stretched-pulse mode-locking regimes under different net dispersion conditions. Experimental results are basically in agreement with the numerical simulations.

Single-cavity dual-comb fiber lasers and their applications

Single-cavity, dual-comb lasers are those specially designed mode-locked lasers that can emit more than one, asynchronous ultrashort pulse trains with stable repetition frequency difference between them. Unlike the long-studied, widely-used femtosecond lasers generating one stable pulse train, systematic investigation on them and their potential dual-comb applications only began, based on the fiber laser platform, around a decade ago, despite sporadic and limited reports of similar lasing phenomena since the beginning of the mode-locked laser studies. From a historic perspective, the birth of this novel technology is the lucky outcome of the timely collision of perpetual search for novel pulsing laser dynamics and concerted pursuit of open-minded solutions for out-of-lab dual-comb systems in the 2010s. In this review article, first, the current schemes to implement single-cavity dual optical frequency comb fiber lasers and their applications are summarized, based on the concept of multiplexed mode-locked lasers. The characteristics of reported single-cavity, dual-comb fiber lasers are discussed as well as their applications in spectroscopy, ranging, Terahertz (THz) spectroscopy, and asynchronous optical sampling (ASOPS). Finally, the more recent development of single-cavity, multi-comb lasers is presented.

Creeping and erupting dynamics in a pure-quartic soliton fiber laser.

Pure-quartic solitons (PQSs) are gradually becoming a hotspot in recent years due to their potential advantage to achieve high energy. Meanwhile, the fundamental research of PQSs is still in the fancy stage, and exploring soliton dynamics can promote the development of PQSs. Herein, we comprehensively and numerically investigate the impact of saturation power, small-signal gain, and output coupler on PQS dynamics in passively mode-locked fiber lasers. The result indicates that altering the above parameters makes PQSs exhibit pulsating or creeping dynamics similar to traditional solitons. Moreover, introducing an intra-cavity filter combined with intra-cavity large fourth-order dispersion makes PQSs go through stationary, pulsating to erupting. That is, the intra-cavity filter changes PQS dynamics. These findings provide new insights into PQS dynamics in fiber lasers.