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 properties of light guiding mechanism and modes relating to the core refractive index in a hybrid photonic crystal fiber (PCF) are numerically investigated by the full-vectorial finite element method. The light guiding mechanism of core mode is the double-bandgap-guiding when the core refractive index is smaller than the effective index of the air-hole cladding in the hybrid-PCF. If the core refractive index is between the refractive indexes of the two kinds of hybrid-claddings, there is a co-operative action of the index-guiding and the bandgap-guiding on the guide mode in the core. Only the total internal reflection guiding occurs in the fiber when the core refractive index is greater than the effective index of the high-index rod cladding. Comparison of these guiding properties in the hybrid-PCF are also carried out. The results provide guidance for the design of hybrid photonic crystal fiber for special purposes.

Guidance characteristics correlated to the core refractive index in a hybrid photonic crystal fiber

Optical SHG properties of GaAs nanowires irradiated with multi-wavelength femto-second laser pulses

The theoretical and experimental study of an Yb-doped double clad large mode area photonic crystal fiber laser is reported. This fiber laser system uses one facet of the fiber directly as a cavity mirror， which is fine-polished to 0° angle after collapsing. Lasing is obtained with the feedback of this facet， and mode-locking is achieved by using a semiconductor saturable absorber as another cavity mirror and adding a grating-pair as the spectral filter. By adjusting the filtering conditions， continuous tuning from wide filtering condition to narrow filtering condition is achieved. Under the wide filtering condition， the system generates femtosecond pulses， the highest average power of which is 2.2 W， corresponding to 29.3 nJ pulse energy and 367 fs pulse duration; under the narrow filtering condition， it generates picosecond pulses， the highest average power of which is 4 W， corresponding to 53.3 nJ pulse energy and 1.14 ps pulse duration. In numerical simulation， the evolution and cavity-position-related characteristics of pulses are studied by using split-step Fourier method.

An Yb-doped large-mode-area photonic crystal fiber mode-locking laser with free output coupler

High energy dissipative soliton mode-locked fiber oscillator based on a multipass cell

High-precision distance measurement in a long range is critical for many advanced applications, such as satellite formation flying, free space optical communication and large scale machining. A 52 m absolute distance measurement in free space based on time of flight of femtosecond laser is demonstrated. The timing offset between target-reflected and the reference pulses is precisely characterized by balanced optical cross-correlation method. The balanced cross correlation signal is used for the feedback control of the cavity length and tightly locks the distance under test to multiple of pulse separation. As a result, the time of flight of the target-reflected pulse is determined by the repetition rate of the femtosecond laser, which effectively avoids the loss of timing resolution caused by direct access of pulse time-of-flight from photo-detection. In the experiment, a Yb -doped high repetition rate mode-locked fiber laser working at 1.04 μm is used as a femtosecond laser source, and a measurement precision of 12 nm is achieved in an average time of one second.

Hu Ming-Lie

Papers

Guidance characteristics correlated to the core refractive index in a hybrid photonic crystal fiber

Optical SHG properties of GaAs nanowires irradiated with multi-wavelength femto-second laser pulses

An Yb-doped large-mode-area photonic crystal fiber mode-locking laser with free output coupler

High energy dissipative soliton mode-locked fiber oscillator based on a multipass cell

Long range absolute distance measurement based on femtosecond laser balanced optical cross-correlation