Linear coupling-related pulse splitting in fiber lasers.

We demonstrate a unique pulse-splitting mechanism dominated by the linear coupling between two vector modes in a mode-locked fiber laser using polarization-maintaining fiber. As the linear coupling strength increases, the pulse experiences larger perturbations and manifests as stronger spectral sidebands. Correspondingly, the temporal pedestals possessing a higher intensity become untrapped and eventually evolve into a stable pulse. Such linear coupling-related pulse splitting is ubiquitous both in normal- and anomalous-dispersion regimes, fundamentally differing from that induced by the excessive nonlinear phase shift. Experimental observations fully sustain numerical results and provide a flexible approach to managing the number and energy of vector solitons.

Anomalous soliton trapping in net-normal dispersion lasers.

In a nonlinear optical system with birefringence such as fiber lasers, soliton trapping can be achieved when the fast (slow) component experiences blueshift (redshift) at normal dispersion to compensate for polarization-mode dispersion (PMD). In this Letter, we demonstrate an anomalous vector soliton (VS) whose fast (slow) component shifts to the red (blue) sides, which is opposite to traditional soliton trapping. It is found that the repulsion between the two components is induced by net-normal dispersion and PMD, while the attraction is ascribed to linear mode coupling and saturable absorption. The equilibrium of attraction and repulsion permits the self-consistent evolution of VSs circulating in the cavity. Our results indicate that the stability and dynamics of VSs are worth revisiting and studying in-depth, especially in lasers with complex configurations, despite it being a well-known object in nonlinear optics.

Self-consistent soliton evolution in single-two-mode fiber lasers.

Ultrafast few-mode fiber lasers have received increasing attention from basic research to practical applications due to their unique pulse performance and intriguing nonlinear dynamics. Here, we experimentally and numerically reveal the formation and evolution behaviors of a soliton in a mode-locked fiber laser composed of two-mode and single-mode fibers. The LP11 pulse walks away from the LP01 pulse in the two-mode fiber due to modal dispersion and then transforms into an auxiliary LP01 pulse after entering the single-mode fiber. After re-entering the two-mode fiber, the LP01 pulse excites the LP11 pulse via mode coupling; therefore, the LP11 pulse also consists of dominant and auxiliary pulses. Such a soliton fiber laser converges to an asymptotic steady state with unlocked spatial modes arising from the interplay between the strong modal dispersion and weak mode coupling.

Synchronous and asynchronous pulsating dual solitons in lasers.

Pulsating solitons are intriguing objects in laser physics and nonlinear science. Recently, emerging works on the pulsating multi-solitons have raised interest in interactions and synchronizations within multiple breathers. However, with their separation of the order of nanoseconds, the evolution and underlying dynamics of multiple pulsating solitons remain uncharted. In this work, we bring initial insights into the pulsating dual-soliton (PDS) with a separation of three orders of magnitude of the pulse duration. Chaotic, synchronous, and asynchronous pulsations are revealed to be controlled by the pump power. Specifically, two solitons can pulsate synchronously in the form of a frozen limit cycle. The asynchronous PDS at a high pump power brings the rotating limit cycle in the phase space. Unveiling the evolutionary dynamics of PDS, this work has potential in all-optical storage, signal encoding, and time division multiplexing communications.

Internal dynamics in bound states of unequal solitons.

The bound states (BSs) of solitons are found to have intriguing internal dynamics in ultrafast lasers. Here, we explore the binding mechanism and internal motions of asymmetric bound state (ABS) solitons constituted by unequal solitons at short-range with their tails directly overlapped. Experiments and simulations show that the periodic energy flux between two solitons, mediated by their overlapped tails, gives rise to a balanced separation and energy distribution across the ABS. The motion mechanisms of strong and weak solitons are discussed in detail. This work provides insights into the binding mechanism and internal dynamics of BSs.

Periodic attraction and repulsion within the tight-bound π-phase soliton molecule.

In the over-pumped dissipative system, the single pulse is prone to split into multi-soliton modes, among which the soliton molecule (SM) comprising two pulses has attracted much interest recently. In this Letter, the tight-bound SM with the π-phase-difference, a soliton pair predicted to be unstable observed in fiber lasers, is found to have oscillating separation with excellent stability. For the first time, to the best of our knowledge, we reveal the mechanism of the π-phase SM to circumvent the irreversible repulsion and the role of dispersive waves on the SM. During the periodic propagation, the destructive interference between solitons produces the repulsion while the dispersive waves give rise to the attractive force, leading to the dynamic oscillating behavior of the SM. The numerical simulation reproduces the experimental observation and offers panoramic insights into the nonlinear interactions between multiple components in dissipative systems.

Mode-locked and Q-switched mode-locked fiber laser based on a ferroferric-oxide nanoparticles saturable absorber.

We demonstrated an ultrafast erbium-doped fiber laser (EDFL) based on ferroferric-oxide (Fe3O4) nanoparticles as a saturable absorber (SA). The investigated SA was based on magnetic fluid deposited on the end face of a fiber ferrule connector. When the SA was inserted into an EDFL cavity, a stable 2.93 ps mode-locked pulse can be achieved by adjusting the intra-cavity polarization controller. The pulse had a central wavelength of 1572.39 nm and a 3 dB bandwidth of 1.39 nm. We also obtained Q-switched mode-locked pulses at 1593.4 nm. The repetition frequency and the temporal width of the Q-switched pulse envelope varied with the pump power. When the pump power reached 225 mW, the maximum average output power and the pulse envelope energy were up to 4.51 mW and 235.5 nJ. To the best of our knowledge, this is the first time that mode-locked and Q-switched mode-locked pulses have been obtained in a fiber laser based on Fe3O4 nanoparticles.

Dark solitons in the exploding pulsation of the bright dissipative soliton in ultrafast fiber lasers.

Soliton explosion is an extremely pulsating behavior of the bright dissipative soliton (DS) in ultrafast lasers. By numerical simulation, we find that the dark soliton (DAS) can coexist with the bright soliton during the exploding process. The collapsed temporal structure of the exploding soliton is induced by the DASs. We reveal the birthing, evolving, and decaying of the DASs inside the bright DS. The time-frequency analysis of the exploding soliton helps us better understand the temporal and spectral structures of the exploding soliton, which might be useful for real-time spectroscopy of the coexisting dark and bright solitons during the soliton explosion.

Stable and low-threshold random fiber laser via Anderson localization.

We report a stable and low-threshold Er-doped random fiber laser (RFL) based on a femtosecond-laser-inscribed random-distributed-grating array (RDGA) as the random feedback. The RDGA had a reflectivity of 93.5%, and its properties were numerically analyzed based on the transfer matrix method. The threshold of the laser was significantly reduced to 5.7 mW, and the linewidth was ~0.4 pm near the threshold as the Anderson localization effect existing in the RDGA significantly improved the laser quality factor (4 × 106). In addition, we propose a method to select RFL lasing modes by stretching a fiber grating filter used in the cavity with different axial strains. The center wavelength hardly drifted and the maximum jitter value of the peak power was less than 0.12 dB over 1 hour for the selected three lasing modes, which indicated that our laser operation was quite stable.

Pulsating soliton with broadened Kelly sidebands in an ultrafast fiber laser.

We experimentally observed a novel pulsating soliton state with broadened Kelly sidebands in an ultrafast laser. Through simulations, we found that the synchronized and unsynchronized resonant dispersive waves coexisted in the laser. The incoherent interaction between the unsynchronized dispersive waves and soliton resulted in oscillation of the intensity and central wavelength of the soliton. The frequencies of the Kelly sidebands oscillated with the soliton, resulting in their spectral broadening when measured by an optical spectrum analyzer. The numerical results agreed qualitatively well with the experiments. Our results are novel and useful for understanding the chaotic dynamics and designing of the ultrafast fiber laser.

Dynamics of soliton explosions in ultrafast fiber lasers at normal-dispersion.

We found two kinds of soliton explosions based on the complex Ginzburg-Landau equation without nonlinearity saturation and high-order effects, demonstrating the soliton explosions as an intrinsic property of the dissipative systems. The two kinds of soliton explosions are caused by the dual-pulsing instability and soliton erupting, respectively. The transformation and relationship between the two kinds of soliton explosions are discussed. The parameter space for the soliton explosion in a mode-locked laser cavity is found numerically. Our results can help one to obtain or avoid the soliton explosions in mode-locked fiber lasers and understand the nonlinear dynamics of the dissipative systems.

High-order soliton evolution and pulse breaking-recovery in stretched ultrafast fiber lasers.

We present a new pulse regime in a stretched ultrafast fiber laser based on numerical simulations. The pulse breaking due to high-order soliton evolution in the passive fiber is recovered to a smooth pulse in the gain fiber with normal dispersion. The new pulse regime formed by the two nonlinear processes makes the ultrafast fiber laser generate ultra-broadband, ultrashort duration, high energy and large breathing ratio pulses. Our work gives insights into the nonlinear dynamics in fiber lasers and has potential for a better design of the stretched fiber lasers.

Continuous-wave-induced resonant spectral sidebands in soliton fiber lasers.

We observed the resonant sidebands similar to the Kelly sidebands, but caused by the coupling between continuous-wave (CW) and dispersive waves (DWs) in a mode-locked ring fiber laser for the first time, to the best of our knowledge. The coupling between the CW and DWs is a linear process, which is different from the previously observed parametric sub-sidebands. The regimes of the CW-induced sidebands are discussed. The experimental results agree both qualitatively and quantitatively with theoretical analysis. The results enrich the nonlinear dynamics of ultrafast fiber lasers and may have potential in optically controlling mode-locked lasers.

Spatio-spectral dynamics of the pulsating dissipative solitons in a normal-dispersion fiber laser.

For the first time, to the best of our knowledge, we observed the pulsating dissipative solitons in a mode-locked fiber laser at normal dispersion using the dispersive-Fourier transformation technology. The artificial saturable absorbers, as well as the birefringent filter formed by the nonlinear polarization rotation, make the polarization controller an effective component to adjust the laser state from stationary to pulsating. The pulsating dissipative solitons are accompanied with the spectrum breathing and oscillating structures due to the nonlinear pulse propagation. Our results can enhance the understanding of the pulsating solitons in the dissipative systems.

Efficient postprocessing technique for fabricating surface nanoscale axial photonics microresonators with subangstrom precision by femtosecond laser.

We demonstrated the subangstrom precise correction of surface nanoscale axial photonics (SNAP) micro-resonators by the femtosecond (fs) laser postprocessing technique for the first time. The internal stress can be induced by fs laser inscriptions in the fiber, causing nanoscale effective radius variation (ERV). However, the obtained ultraprecise fabrication usually undergoes multiple tries. Here, we propose a novel postprocessing technique based on the fs laser that significantly reduces the ERV errors and improves the fabrication precision without iterative corrections. The postexposure process is achieved at the original exposure locations using lower pulse energy than that in the initial fabrication process. The results show that the ERV is nearly proportional to the pulse energy of the postexposure process. The slope of the ERV versus the pulse energy is 0.07 Å/nJ. The maximum of the postprocessed ERV can reach 8.0 Å. The repeatability was experimentally verified by accomplishing the correction on three SNAP microresonators with the precision of 0.75 Å. The developed fabrication technique with fs laser enables SNAP microresonators with new breakthrough applications for optomechanics and filters.

Intensity modulated torsion sensor based on optical fiber reflective Lyot filter.

We proposed and experimentally demonstrated a highly sensitive optical fiber torsion sensor based on a reflective Lyot filter for the first time to our knowledge. The reflective Lyot filter is constructed by inserting a section of polarization-maintaining fiber (PMF) between a fiber linear polarizer and a 3dB coupler based fiber loop reflector. Based on the intensity modulation, the proposed torsion sensor exhibits a high torsion sensitivity of up to 20.336 dB/rad, one order of magnitude higher than the achieved in state-of-the-art. In contrast, the temperature cross-sensitivity and strain cross-sensitivity of the proposed torsion sensor are low to -2.0 × 10-4 rad/°C and -6.39 × 10-6 rad/µÎµ, respectively, thus overcoming the cross-sensitivity problem resulting from temperature and strain. Moreover, we perform the theoretical simulation of the proposed torsion sensor, and the simulation result obtained agrees well with the experiment results, vividly confirming the viability of the fiber Lyot filter based torsion sensor. Such fiber Lyot filter may find potential applications of highly sensitive torsion sensors in the field of modern smart structure monitoring.

Numerical simulations of fast-axis instability of vector solitons in mode-locked fiber lasers.

We demonstrate the fast-axis instability in mode-locked fiber lasers numerically for the first time. We find that the energy of the fast mode will be transferred to the slow mode when the strong pump strength makes the soliton period short. A nearly linearly polarized vector soliton along the slow-axis could be generated under certain cavity parameters. The final polarization of the vector soliton is related to the initial polarization of the seed pulse. Two regimes of energy exchanging between the slow mode and the fast mode are explored and the direction of the energy flow between two modes depends on the phase difference. The dip-type sidebands are found to be intrinsic characteristics of the mode-locked fiber lasers under high pulse energy.

All-fiber passively mode-locked laser based on a chiral fiber grating.

A novel passively mode-locked all-fiber laser using a chiral fiber grating as a polarization-selective element is demonstrated for the first time, to the best of our knowledge. The chiral fiber grating serves as a key component to form an artificial saturable absorber to realize mode locking through nonlinear polarization rotation in the cavity. The laser generates stable short pulses with energy of 0.34 nJ, a fundamental repetition rate of 3.27 MHz, and an FWHM bandwidth of 28 nm. We also show that harmonic mode-locked pulse trains of different orders can be obtained by increasing the pump power.

The dissipative Talbot soliton fiber laser.

Talbot effect, characterized by the replication of a periodic optical field in a specific plane, is governed by diffraction and dispersion in the spatial and temporal domains, respectively. In mode-locked lasers, Talbot effect is rarely linked with soliton dynamics since the longitudinal mode spacing and cavity dispersion are far away from the self-imaging condition. We report switchable breathing and stable dissipative Talbot solitons in a multicolor mode-locked fiber laser by manipulating the frequency difference of neighboring spectra. The temporal Talbot effect dominates the laser emission state-in the breathing state when the integer self-imaging distance deviates from the cavity length and in the steady state when it equals the cavity length. A refined Talbot theory including dispersion and nonlinearity is proposed to accurately depict this evolution behavior. These findings pave an effective way to control the operation in dissipative optical systems and open branches in the study of nonlinear physics.

Phase-matching-induced near-chirp-free solitons in normal-dispersion fiber lasers.

Direct generation of chirp-free solitons without external compression in normal-dispersion fiber lasers is a long-term challenge in ultrafast optics. We demonstrate near-chirp-free solitons with distinct spectral sidebands in normal-dispersion hybrid-structure fiber lasers containing a few meters of polarization-maintaining fiber. The bandwidth and duration of the typical mode-locked pulse are 0.74 nm and 1.95 ps, respectively, giving the time-bandwidth product of 0.41 and confirming the near-chirp-free property. Numerical results and theoretical analyses fully reproduce and interpret the experimental observations, and show that the fiber birefringence, normal-dispersion, and nonlinear effect follow a phase-matching principle, enabling the formation of the near-chirp-free soliton. Specifically, the phase-matching effect confines the spectrum broadened by self-phase modulation and the saturable absorption effect slims the pulse stretched by normal dispersion. Such pulse is termed as birefringence-managed soliton because its two orthogonal-polarized components propagate in an unsymmetrical "X" manner inside the polarization-maintaining fiber, partially compensating the group delay difference induced by the chromatic dispersion and resulting in the self-consistent evolution. The property and formation mechanism of birefringence-managed soliton fundamentally differ from other types of pulses in mode-locked fiber lasers, which will open new research branches in laser physics, soliton mathematics, and their related applications.