Xiquan Fu

Noise-like pulse in a gain-guided soliton fiber laser

We report on the operation of a passively mode-locked fiber ring laser made of purely positive dispersion fibers and mode-locked by using the nonlinear polarization rotation technique. It was experimentally found that apart from the gain-guided soliton operation the laser can also emit a kind of noise-like pulse. We show numerically that the noise-like pulse emission is caused by the peak power clamping effect of the laser cavity on the gain-guided soliton.

Wide spectral and wavelength-tunable dissipative soliton fiber laser with topological insulator nano-sheets self-assembly films sandwiched by PMMA polymer

Topological insulators have been theoretically predicted as promising candidates for broadband photonics devices due to its large bulk band gap states in association with the spin-momentum-locked mass-less Dirac edge/surface states. Unlike the bulk counterpart, few-layer topological insulators possess some intrinsic optical advantages, such as low optical loss, low saturation intensity and high concentration of surface state. Herein, we use a solvothermal method to prepare few-layer Bi₂Te₃ flakes. By sandwiching few-layer Bi₂Te₃ flakes with polymethyl methacrylate (PMMA) polymer, a novel light modulation device had been successfully fabricated with high chemical and thermal stabilities as well as excellent mechanical durability, originating from the contribution of PMMA acting as buffer layers that counteract excessive mechanical bending within the fragile Bi₂Te₃ flakes. The incorporation of the as-fabricated PMMA-TI-PMMA as saturable absorber, which could bear long-term mechanical loadings, into the fiber laser cavity generated the stable dissipative soliton mode-locking with a 3-dB spectral bandwidth up to 51.62 nm and tunable wavelength range of 22 nm. Our work provides a new way of fabricating PMMA-TI-PMMA sandwiched composite structure as saturable absorber with promising applications for laser operation.

Observation of central wavelength dynamics in erbium-doped fiber ring laser.

We report on the observation of central wavelength dynamics in an erbium-doped fiber ring laser by using the nonlinear polarization rotating technique. The evolution of central wavelength with the laser operation state was observed experimentally. Numerical simulations confirmed the experimental observation and further demonstrated that the dynamics of wavelength evolution is due to the combined effects of fiber birefringence, fiber nonlinearity, and cavity filter.

Trapping and controlling the dispersive wave within a solitonic well.

We have numerically studied the effect of mutual interactions between soliton and dispersive waves and the possibility to create a solitonic well consisting of initial twin-solitons moving away from each other to trap the incident dispersive wave. Different from the case of the solitonic cage formed by the velocity-matched twin-solitons, the intense dispersive wave can break up into small pulses, which are almost completely trapped within the solitonic well. Moreover, the corresponding spectrum of the trapped dispersive wave can be narrowed firstly and then expanded, and a new dispersive wave can be generated as the twin-solitons collision occurred. By adjusting either the peak power or temporal width of incident dispersive wave, both the intensity of the collision-induced dispersive wave and the position where it is generated can be controlled.

Dressed dynamics of two time-reversed shapes of Airy pulses in a relaxing nonlinear medium

We investigate the dressed dynamics of two kinds of asymmetric Airy pulses with time-reversed shapes in a relaxing nonlinear medium. Airy tails act as low intensity “dressing” pulses, refuel the Airy main lobe, and modify propagation dynamics of Airy pulses. It is demonstrated that a soliton sheds from the main lobe of Airy pulses and the spectrum maximum intensity is periodically manifested due to the effect of self-phase modulation. The intrinsically asymmetric nature of Airy pulses is revealed through the nonlinear generation of a Raman soliton self-frequency shift and the generation of static solitons driven by the soliton fission processes and the interactions with dressing Airy tails. The resulting Raman-induced frequency shift can be controlled by using time-reversed Airy pulses and varying truncation coefficients. Numerical results show that the frequency shift can be enhanced with a larger input peak power as well.

Role of chirp in spatiotemporal modulation instability of broadband pulsed laser

We report on the theoretical derivation and experimental demonstration of the influence of chirp on the spatiotemporal modulation instability in a nonlinear Kerr medium. Based on a derived (3 + 1)-dimensional nonlinear Schrodinger equation for chirped laser pulse propagation, we obtain the expression for the instability gain spectrum of a chirped laser pulse by using a standard linear stability analysis. It is shown that the pulse chirp does not affect the instability region and the fastest growth frequency. In the experiment, we observe the nonlinear growth of spatial stripes with different modulation frequency due to the small-scale self-focusing. We also obtain the gain spectra of the laser pulses with different values of pulse chirp. It is found that, for the condition that the contributions of nonlinearity (Nnl) are equal, the experimental results are in agreement with the theoretical predictions.

Effect of gain bandwidth on the amplification of ultrabroad bandwidth pulse in an erbium-doped nonlinear amplifying fibre loop mirror

The amplification of a ultrabroad bandwidth pulse in an erbium-doped nonlinear amplifying fibre loop mirror (ED-NALM) in the case of pulses with spectral bandwidth comparable to the gain bandwidth is investigated. Based on the numerical simulation of a modified nonlinear Schrodinger equation, it is shown that the amplifier will distort the pulse spectrum and limit the efficiency of amplification. However, the distortion can be minimized by choosing the corresponding optimal loop length for different gain bandwidth. In addition, the performance of ED-NALM is sensitive to variation in gain bandwidth when the pulse is initially frequency chirped, while it is slightly sensitive to variation in gain bandwidth when the input pulse has a different peak power at the optimal loop length.

Dynamics of finite energy Airy beams modeled by the fractional Schrödinger equation with a linear potential

The dynamics of finite energy Airy beams modeled by the fractional Schrodinger equation (FSE) with a linear potential are numerically investigated. Different from the propagation properties of Airy beams described by the standard Schrodinger equation, the splitting phenomenon, which is presented under the FSE without potential, is influenced by the quadratic chirp and the Levy index. As the linear potential is considered, the periodic evolution of Airy beams is shown, and the period is inversely proportional to the linear potential coefficient. The beam width can undergo an abrupt decrease or increase depending on the sign of potential coefficient. The beam deviation distance increases with the beam width greatly changed if the Levy index increases. Moreover, the quadratic chirp does not influence the evolution period but the intensity distribution. In addition, the intriguing properties are analytically clarified. All these features confirm the promising applications of Airy beams in optical manipulation and optical switch.

Multi-Solitons Shedding From Truncated Airy Beam in Nonlocal Nonlinear Media

Based on the nonlinear Schrödinger model, the dynamics of truncated Airy beam in nonlocal nonlinear media is investigated. The results show that more spatial solitons are shed from Airy beam in the nonlocal media than the case in the local media. The more shedding solitons can be obtained by increasing the initial intensity or decreasing the truncated coefficient, and these solitons have larger interval because of the nonlocal effect. The shedding multiple spatial solitons of Airy beam can be used as a kind of spatial beam splitter.

Generation of ring-shaped beams by a graded-index plasma lens

We experimentally demonstrate the generation of ring-shaped beams using a plasma channel with Gaussian profile. The plasma channel is produced when an intense femtosecond pulse propagates in carbon disulfide. The refractive index of the plasma is similar to that of a graded-index lens. Due to the existence of critical plasma density, the beam from the He–Ne laser cannot pass through the center region of the plasma and is refracted in the periphery. The dark spot size of the ring-shaped beams can be controlled easily by a femtosecond pulse. We also show the propagation of ring-shaped beams in free space.