Solitons are stable localized wave packets that can propagate long distance in dispersive media without changing their shapes. As particle-like nonlinear localized waves, solitons have been investigated in different physical systems. Owing to potential applications in optical communication and optical signal processing systems, optical solitons have attracted intense interest in the past three decades. To experimentally study the formation and dynamics of temporal optical solitons, fiber lasers are considered as a wonderful nonlinear system. During the last decade, several kinds of theoretically predicted solitons were observed experimentally in fiber lasers. In this review, we present a detailed overview of the experimentally verified optical solitons in fiber lasers, including bright solitons, dark solitons, vector solitons, dissipative solitons, dispersion-managed solitons, polarization domain wall solitons, and so on. An outlook for the development on the solitons in fiber lasers is also provided and discussed.

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Recent progress of study on optical solitons in fiber lasers

Dissipative soliton evolution in passively mode-locked fiber lasers with large net-normal-dispersion and high nonlinearity is investigated numerically and confirmed experimentally. I have proposed a theoretical model including the nonlinear polarization evolution and spectral filtering effect. This model successfully predicts the pulse behaviors of the proposed laser, such as the multi-soliton evolution, quasi-rectangle-spectrum profile, trapezoid-spectrum profile, and unstable state. Numerical results show that, in contrast to the typical net- or all-normal-dispersion fiber lasers with the slight variation of the pulse breathing, the breathing ratios of the pulse duration and spectral width of our laser are more than three and two during the intra-cavity propagation, respectively. The nonlinear polarization rotation mechanism together with spectral filtering effect plays the key roles on the pulse evolution. The experimental observations confirm the theoretical predictions.

Numerical and experimental investigation of dissipative solitons in passively mode-locked fiber lasers with large net-normal-dispersion and high nonlinearity

Generation, optimization, and application of ultrashort femtosecond pulse in mode-locked fiber lasers

We report an $L$ -band passively mode-locked erbium-doped fiber ring laser using the nonlinear polarization rotation technique. The center wavelength of the fiber laser can be tuned from 1575.5 to 1603.6 nm. The 458.7-fs pulses at 1603.6 nm show an 8.6-MHz pulse repetition rate. To the best of our knowledge, it is the longest wavelength for which subpicosecond pulses have been achieved so far with the nonlinear polarization rotation technique.

$L$ -Band Femtosecond Fiber Laser Mode Locked by Nonlinear Polarization Rotation

This paper investigates the zero dispersion wavelength and dispersion slope control of hollow-core photonic bandgap fibres (PBGFs) by using a full-vector finite element method. By simulation we found that theoretically the zero dispersion wavelength can be tailored by respectively changing the rounded diameter of air holes, pitch, refractive index, normalized thickness of core rings, and hole diameter to pitch ratio. At the same time the tailoring of dispersion slope can also be realized by changing the rounded diameter of air holes or pitch or normalized thickness of core rings. To illustrate the reasonability of fibre designs, this paper also gives the variance of normalized interface field intensity which measures the scattering loss relatively versus wavelength for different designs. From the viewpoint of loss, varying the rounded diameter and the thickness of core ring could shift zero wavelength but it is difficult to get the required parameters within so tiny range in practical drawing of PBGFs, on the other hand, it is possible in practice to respectively alter the pitch and refractive index to shift zero wavelength. But varying hole diameter to pitch ratio is not worthwhile because they each induce large increase of loss and narrowness of transmission bandwidth. The zero dispersion wavelength can be engineered by respectively varying the rounded diameter of air holes, pitch, refractive index, and normalized thickness of core rings without incurring large loss penalties.

Zero dispersion wavelength and dispersion slope control of hollow-core photonic bandgap fibres

The eigenvalue equation associated with seven-core photonic crystal fiber was derived from coupled-mode equation. Mode properties were analyzed by its eigenvalues and eigenvector. Using the frequency domain finite difference method, the relationship between fiber structure and coupling coefficient was fully discussed through changing the wavelength, hole pitch and core distance. Mode shaping and higher-order supermode cut off was realized. The results were helpful to the fiber structure design and in-phase supermode selection.

Numerical analysis for structure optimization of seven-core photonic crystal fibers

It Is reported that the supercontinuum spectrum can be generated in holey microstructure fibers by femtosecond laser pulses with a 800nm center wavelength. Broadband continua extending from 440 to 890nm are generated in holey microstructure fibers. Based on the theory of scalar approximation, the effective refractive index of the cladding and effective area for the fundamental space-filling mode and chromatic dispersion of fibers are calculated. It is found that the holey microstructure fibers have specifically the ability of controlling dispersion and waveguide property. The mechanism of supercontinuum generation in holey microstructure fibers is explicated preliminarily. The theoretic analysis is in good accodance with the experimental result. It is, considered that supercontinuum broadband spectrum can be generated in holey microstructure fibers even if their cladding constitutes of random gas-line.

Supercontinuum generation in holey microstructure fibers by femtosecond laser pulses

The photo-induced insulator-metal transition for silicon-based VO 2 nanofilm is studied by THz time-domain spectroscopy (THz-TDS). Obvious variations of THz ray transmittance are observed before and after the CW laser beam exciting, and the conductivity of metallic-phased VO 2 film in the THz region is calculated in the thin film approximation. According to the measured results, the metallic-phased VO 2 film is characterized equivalently with Drude’s model, and complex conductivity, dielectric function and refractive index are acquired by the model. As an examination on the equivalent Drude model, numerical simulation based on the finite integral method in time domain is carried out. The results show that they are in good agreement with the experimental results. This work provides a reference for the study on phase transition of VO 2 nanofilm and its application in the THz region.

Photo-induced insulator-metal transition of silicon-based VO 2 nanofilm by THz time domain spectroscopy

A compact and highly stable high pulse energy passively mode-locked fiber laser is presented. A segment of Yb 3+ -doped double-clad large-mode-area fiber with extremely low nonlinearity is developed as the active medium. The self-starting mode-locked operation is achieved by the cooperation of nonlinear polarization evolution and semiconductor saturable-absorber mirror. The fiber laser generates laser pulses with the average power of 160mW at the repetition rate of 55.9MHz (corresponding to pulse energy of 3nJ), and the pulse duration is 10.6ps.

A mode-locked Yb 3+ -doped double-clad large-mode-area fiber laser

A dissipative-soliton mode locked laser based on Yb-doped single polarization large-mode-area photonic crystal fiber is demonstrated. The dissipative soliton dynamics is explored by numerical simulation. Compared with all normal dispersion fiber lasers, there are more pulse shaping mechanisms in a dissipative-soliton fiber laser, including the nonlinear absorption of SESAM, spectrum filtering of the chirped pulse and gain dispersion, which results in robust and stable operation. Among them, spectrum filtering plays a dominant role in mode-locking, it shapes pulse both in frequency domain and time domain and keeps the pulse duration below 1 ps in the cavity. For the first time, sub-1 ps pulse is directly generated in a dispersion compensation free cavity. The fiber laser directly generates 777 fs pulse at 1 W average power at a repetition rate of 514 MHz, corresponding to a single pulse energy of 20 nJ.

Hu Ming-Lie

Papers

Numerical analysis for structure optimization of seven-core photonic crystal fibers

Supercontinuum generation in holey microstructure fibers by femtosecond laser pulses

Photo-induced insulator-metal transition of silicon-based VO 2 nanofilm by THz time domain spectroscopy

A mode-locked Yb 3+ -doped double-clad large-mode-area fiber laser

Dissipative-soliton mode locked laser based on large-mode-area photonic crystal fiber