Spatiotemporal mode-locking and dissipative solitons in multimode fiber lasers.

Multimode fiber (MMF) lasers are emerging as a remarkable testbed to study nonlinear spatiotemporal physics with potential applications spanning from high energy pulse generation, precision measurement to nonlinear microscopy. The underlying mechanism for the generation of ultrashort pulses, which can be understood as a spatiotempoal dissipative soliton (STDS), in the nonlinear multimode resonators is the spatiotemporal mode-locking (STML) with simultaneous synchronization of temporal and spatial modes. In this review, we first introduce the general principles of STML, with an emphasize on the STML dynamics with large intermode dispersion. Then, we present the recent progress of STML, including measurement techniques for STML, exotic nonlinear dynamics of STDS, and mode field engineering in MMF lasers. We conclude by outlining some perspectives that may advance STML in the near future.

Self-starting spatiotemporal mode-locking using Mamyshev regenerators.

Bridging multi-mode fibers and Mamyshev regenerators holds promise for pulse energy scaling in fiber lasers. However, initialization of a multi-mode Mamyshev oscillator remains a practical challenge. Here we report self-starting spatiotemporal mode-locking (STML) in a multi-mode Mamyshev oscillator without active assistance. The first initialized mode-locking is unstable, but stable STML can be attained by increasing the filter separation. Simulations verify the capability of reaching self-starting STML using Mamyshev regenerators and unveil the effect of filter separation on the self-starting ability.

Probabilistic shaping and neural network-based optimization for a nonlinear frequency division multiplexing system.

A joint scheme introducing probabilistic shaping (PS) at the transmitter and utilizing a neural network (NN) equalizer at the receiver is proposed to improve the performance of the b-modulated nonlinear frequency division multiplexing (NFDM) system. Through a numerical simulation, we demonstrate that PS plays a leading role for low launch power case, which improves the performance of the system effectively, while the NN equalizer's superiority appears in a high launch power region, whose main role is to weaken the correlation among subcarriers for improving system performance. The proposed scheme would enlighten the optimum modulation and detection schemes of the NFDM system.

Spatiotemporal Mode-Locking in Lasers with Large Modal Dispersion.

Dissipative nonlinear wave dynamics have been investigated extensively in mode-locked lasers with single transverse mode, whereas there are few studies related to three-dimensional nonlinear dynamics within lasers. Recently, spatiotemporal mode locking (STML) was proposed in lasers with small modal (i.e., transverse-mode) dispersion, which has been considered to be critical for achieving STML in those cavities because the small dispersion can be easily balanced. Here, we demonstrate that STML can also be achieved in multimode lasers with much larger modal dispersion, where we find that the intracavity saturable absorber plays an important role for counteracting the large modal dispersion. Furthermore, we observe a new STML phenomenon of passive nonlinear autoselection of single-mode mode locking, resulting from the interaction between spatiotemporal saturable absorption and spatial gain competition. Our work significantly broadens the design possibilities for useful STML lasers thus making them much more accessible for applications, and extends the explorable parameter space of the novel dissipative spatiotemporal nonlinear dynamics that can be achieved in these lasers.

Vector quartic solitons in birefringent fibers.

We theoretically investigated the vector properties of quartic solitons in a pure fourth-order-dispersion birefringent fiber and a quartic-dispersion-dominant mode-locked fiber laser. We found that, compared with scalar pure quartic solitons, a vector quartic soliton (VQS) in the birefringent fiber still preserves the Gaussian shape, except for the distinctions of reduced peak power, central frequency offset, slight frequency chirp, and mitigated oscillatory tails. We also demonstrated that pulse shaping in the mode-locked laser cavity could explicitly facilitate the formation of Kelly sidebands and distortion of oscillatory tails. Furthermore, dynamical evolutions of quartic group-velocity-locked and polarization-rotating vector solitons were obtained to enrich the nonlinear community of VQSs. We believe that our elaborate findings will bring insights into both the fundamental understanding and potential applications of VQSs.

Buildup dynamics of asynchronous vector solitons in a polarization-multiplexed dual-comb fiber laser.

Polarization-multiplexed dual-comb fiber lasers enable significant applications in dual-comb spectroscopy and optical sensing. However, the complexity of the underlying formation dynamics of dual-comb solitons has not been unveiled so far. Here, we capture the real-time spectral evolutions of both vector solitons from the initial fluctuations, with the help of the time-stretch dispersive Fourier transform technique. Both vector solitons experience the relaxation oscillation, quasi-mode-locking, beating dynamics, and mode locking, accompanying central wavelength shifts in opposite directions, which might be induced by the gain saturation during their buildup processes. Moreover, polarization-dependent gain in the gain fiber leads to the different buildup time of both vector solitons. Our findings open new perspectives for dual-comb buildup dynamics and might impact laser design for applications.

Carbon nanotube-synchronized dual-color fiber laser for coherent anti-Stokes Raman scattering microscopy.

In this Letter, we present a passively synchronized dual-color fiber laser at 1.03 and 1.53 µm for coherent anti-Stokes Raman scattering (CARS) microscopy. This fiber laser consists of both Yb- and Er-doped laser cavities, combined by an ∼1.2m common branch with a single-walled carbon-nanotube-based saturable absorber. Stable passively synchronized mode-locked state is obtained within a total cavity length mismatch of 50 µm. We demonstrate the capability of the synchronized dual-color fiber laser for CARS spectroscopy in both low- and high-wavenumber regions, with a resolution of 6.6cm-1. Furthermore, a CARS image in the field of view 150×60µm2 of polystyrene at the Raman shift of 3041cm-1 has also been achieved. The results show the feasibility of our passively synchronized dual-color fiber laser to be employed as a stable and low-cost ultrafast laser source for CARS.

Nonlinear multimode interference-based dual-color mode-locked fiber laser.

We present a dual-color-soliton fiber laser at two different wavebands by nonlinear multimode interference. A saturable absorber (SA) with single-mode-multimode (MMF)-single-mode fiber structure is placed in the common branch shared by two sub-cavities. Saturable absorption effects can be simultaneously satisfied at 1.5 and 2 µm at a proper length of the MMF. Dual-color solitons can still remain, even by slightly tuning the length of the MMF. The periodical characteristic of this SA provides a flexible choice of MMF length, making it simple for simultaneous mode locking (SML) at two separate wavebands in practice. Our approach not only paves the way for SML at two or more wavebands by the MMF but also could lead to significant applications in pump-probe spectroscopy.

Free-running dual-comb fiber laser mode-locked by nonlinear multimode interference.

We present a simple and low-cost approach to generate dual combs from a single fiber-ring cavity based on nonlinear multimode interference. A single-mode fiber-graded-index multimode-single-mode fiber structure serves as an all-fiber saturable absorber for mode-locking. A pair of frequency combs with different comb spacing are generated by dual-wavelength and polarization-multiplexed mechanisms. Stable and simplified dual-comb spectroscopy is achieved using the proposed free-running fiber laser, with no need for complex phase-locking configurations. As a principle of concept, we demonstrate the dual-comb fiber laser for spectral determination of fiber Bragg grating (FBG)-based strain sensors, providing a low-cost alternative for interrogation of FBG-based sensing networks.

Multiple-soliton in spatiotemporal mode-locked multimode fiber lasers.

Experimental observations of spatiotemporal mode-locked multiple-soliton, including harmonic mode locking and multiple pulses, in multimode fiber (MMF) lasers are reported. Numerical simulations are conducted to investigate the nonlinear dynamics of multi-pulsing. The influences of cavity parameters on the spatiotemporal outputs are analyzed by simulations, which agree with the experimental observations qualitatively. This work would contribute to understanding the complex spatiotemporal nonlinear dynamics in MMF lasers.

Multiplexed static FBG strain sensors by dual-comb spectroscopy with a free running fiber laser.

We demonstrate a novel and compact FBG interrogation system for multiplexed static strain sensing with a free running mode-locked fiber laser. Multiplexed FBG array in cascading are interrogated by coherent dual-comb pulses generated from a single fiber laser. Dual-comb spectroscopy is achieved with the fiber laser to precisely detect the strain-induced spectral shifts of the FBG sensors. Multi-point strain measurements are performed to characterize the proposed system, where a large dynamic range of 520 µÎµ with 0.5 µÎµ resolution is achieved.

Observation of soliton molecules in a spatiotemporal mode-locked multimode fiber laser.

We report on the first experimental observation, to the best of our knowledge, of soliton molecules in a spatiotemporal mode-locked multimode fiber (MMF) laser. By adjusting the waveplates inside the cavity, not only the spatiotemporal mode-locking state with a stable single pulse but also soliton molecules are observed. Various soliton molecules, including soliton pairs, soliton triplets, and soliton quartets with different pulse separations, are achieved. Transition of different operation states with pump power is given. The results would be beneficial for further understanding of the nonlinear dynamics in spatiotemporal mode-locked MMF lasers.

Pulse dynamics of dual-wavelength dissipative soliton resonances and domain wall solitons in a Tm fiber laser with fiber-based Lyot filter.

We report on the first demonstration of dual-wavelength square-wave pulses in a thulium-doped fiber laser. Under appropriate cavity parameters, dual-wavelength dissipative soliton resonances (DSRs) and domain wall solitons (DWSs) are successively obtained. Meanwhile, dark pulses generation is achieved at the dual-wavelength DWSs region due to the overlap of the two domain wall pulses. The fiber-based Lyot filter, conducted by inserting PMF between an in-line PBS and a PD-ISO, facilitates the generation of dual-wavelength operation. The polarization-resolved investigation suggests that the cross coupling between two orthogonal polarization components in the high nonlinear fiber plays an important role in the square-wave pulses formation. The investigation may be helpful for further understanding the square-wave pulse formation and has potential in application filed of multi-wavelength pulsed fiber lasers.

Quantized pulse separations of phase-locked soliton molecules in a dispersion-managed mode-locked Tm fiber laser at 2 µm.

We report, for the first time to the best of our knowledge, the discovery of quantized pulse separations between phase-locked soliton pairs in a thulium-doped fiber laser at the wavelength range of 2 µm. The soliton pairs are formed by the nonlinear binding interaction between the solitons and dispersive waves, and the two lowest-order Kelly sidebands drive the interaction in these soliton pairs and fix the pulse separations of the soliton pairs to a discrete set of values. Our experimental observations are well confirmed by numerical simulations.

Generation of wavelength-tunable soliton molecules in a 2-µm ultrafast all-fiber laser based on nonlinear polarization evolution.

We report on the experimental observation of stable single solitons and soliton molecules in a 2-µm thulium-holmium-doped fiber laser mode-locked through the nonlinear polarization evolution technique within an anomalously dispersive cavity. Single 0.65 nJ solitons feature a 7.3 nm spectral FWHM and 540 fs temporal duration, yielding a time-bandwidth product close to the Fourier-transform limitation. Under the same pumping power of 740 mW, stable out-of-phase twin-soliton molecules, featuring a temporal separation of 2.5 ps between the two ∼700 fs pulses, are generated in a deterministic way, while the central wavelength of the soliton molecules can be tuned from 1920 to 1940 nm. Finally, we present strong experimental evidence of vibrating soliton molecules.

Soliton rains in a normal dispersion fiber laser with dual-filter.

We demonstrate an experimental observation of soliton rains in an all normal dispersion Yb-fiber laser. The cavity consists of a narrow bandwidth filter and a birefringent plate (BP) filter. Soliton rain is obtained in the weak mode-locking regime, while multisolitons behaving as soliton bunches and harmonic mode-locking under strong mode-locking are also observed. Distinctive multisoliton interactions are observed via changing the pump power and adjustment of the waveplates as well as the BP. To the best of our knowledge, this is the first demonstration of soliton rains in normal dispersion fiber lasers.

Enhancement of cascaded four-wave mixing via optical feedback.

A scheme is proposed to enhance the cascaded four-wave mixing (CFWM) generation, by introducing an optical feedback to the input port. Experimental and numerical results show that more efficient CFWM generation can be obtained by the scheme. The number of CFWM products is increased, as well as the powers of most CFWM products are improved.

Polarization dynamics in dissipative soliton fiber lasers mode-locked by nonlinear polarization rotation.

We numerically studied the polarization dynamics in dissipative soliton lasers mode-locked by nonlinear polarization rotation (NPR). It was found that the polarization states of the intracavity dissipative soliton vary with time across the pulse. Depending on output coupling ratios, the polarization states of the pulse peak before the polarizer can be either nearly circular or nearly linear polarizations. The polarization dependent component in NPR is found to play a role of spectral filter under high and medium output coupling. However, NPR may work as a weak optical limiter under low output coupling, when additional spectral filtering is necessary to maintain steady mode-locking state.

Surfactantless photochemical deposition of gold nanoparticles on an optical fiber core for surface-enhanced Raman scattering.

Surface-enhanced Raman scattering (SERS) probes based on gold nanoparticles modifying the core of the optical fiber were made by a surfactantless photochemical deposition method. The growth kinetics and shape evolution of gold nanoparticles depending on different experimental conditions were studied. It was found that, under the condition of detectable gold nanoparticle deposition, increasing the concentration of chloroauric acid (HAuCl(4)) was not conducive to the deposition whereas increasing the concentration of sodium citrate (Na(3)Ct) would speed up the deposition. By controlling the concentration of the reaction solution and irradiation time, we obtained fused spherical-like, spherical, and flowerlike gold nanoparticles. To test the SERS activity of the probes, the SERS spectra of a rhodamine 6G aqueous solution were recorded in direct detection mode and remote mode. We have also developed a new approach to improving the SERS sensitivity when detecting in remote mode.

Suppression of continuous lasing in a carbon nanotube polyimide film mode-locked erbium-doped fiber laser.

We demonstrated an erbium-doped mode-locked fiber laser using a single-walled carbon nanotube-dispersed polyimide (SWNT-PI) film. Different mode-locking operations were compared and analyzed utilizing SWNT-PI films with different concentrations (2, 1, and 0.25 wt.%, respectively). It was found that the continuous single-pulse mode-locking operation was often accompanied by a continuous wave oscillation part for the 1 and 0.25 wt.% SWNT-PI films, whereas the 2 wt.% SWNT-PI film presented the most excellent mode-locking performance, thanks to sufficient modulation depth. Using the 2 wt.% SWNT-PI film, a stable pulse train with a pulse width of 840 fs and a repetition rate of 15.3 MHz was achieved. The average output power was 0.33 mW at the pump power of 155 mW under an output coupling ratio of 10%. Operational performance of the laser cavity when employing the 2 wt.% SWNT-PI film was also demonstrated.