In this work breathers are obtained in a hollow-core photonic crystal fiber (HC-PCF) for the first time. The nonlinear Schrödinger equation describing the propagation of pulses in a HC-PCF is investigated using the Hirota bilinear method and the auxiliary function method. Analytic breather solutions are derived by an appropriate choice of parameters. Dynamical behavior of breathers is exhibited, and the influences of different parameters on the characteristics of breathers are discussed. The presented results could be used in fiber lasers, nonlinear optics and Bose-Einstein condensates.

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The robustness and prolongation of multiple filamentation (MF) for femtosecond laser propagation in air are investigated experimentally and numerically. It is shown that the number, pattern, propagation distance, and spatial stability of MF can be controlled by a variable-aperture on-axis pinhole. The random MF pattern can be optimized to a deterministic pattern. In our numerical simulations, we configured double filaments to principlly simulate the experimental MF interactions. It is experimentally and numerically demonstrated that the pinhole can reduce the modulational instability of MF and is favorable for a more stable MF evolution.

In this contribution, we report the generation of 17-μJ mid-infrared (MIR) pulses with duration of 70 fs and bandwidth of 550 nm centered at 3.75 μm at 1-kHz repetition rate, by a two-stage femtosecond optical parametric amplifier utilizing 4H-silicon carbide crystal as the nonlinear medium. The crystal is selected as it processes orders of magnitude higher damage threshold than traditional MIR nonlinear crystals, and it supports extreme broad parametric bandwidth. With its distinguished features such as MIR central wavelength, ultra-broad bandwidth, self-stable carrier-envelope phase, and potential for energy scaling, this kind of MIR source holds promise for new approaches to extreme short isolated attosecond pulse generation as well as MIR spectroscopy applications. Mid-infrared (MIR) light sources in the molecular "fingerprint" region are of paticular importance in a number of disciplines, including industrial process monitoring, environmental monitoring, and molecular identification [1]. Driven by intense carrier-envelope phase-stable MIR pulses, high-order harmonic generation process exhibits higher cut-off energy, supporting shorter isolated attosecond pulses [2]. Broadband MIR light sources are also essential in optical coherence tomography [3,4], since broad spectrum leads to ultrahigh resolution and longer wavelength results in better penetration depth. As MIR light sources have widespread applications, several approaches are made to obtain laser radiations at this wavelength region. Lead-salt diodes exhibit two orders of magnitude lower Auger recombination rate compared with other materials, but they are hindered in applications by a low working temperature requirement, especially for continuous wave operation [5,6]. Unlike lead-salt lasers, the main nonradiative channel for quantum cascade lasers [7,8] is optical phonon emission rather than the Auger effect. That offers opportunities for room temperature operation. However, quantum cascade lasers suffer from their low wall plug efficiency, which sets limitation to applications such as portable sensors and infrared counter measurements [9]. Additionally, generation of lasers with wavelength longer than 3 μm based on solid-state lasers is limited by the multiphonon relaxation process in gain media at room temperature [10].Different from the laser radiation obtained by stimulated emission process mentioned above, nonlinear frequency conversion techniques as the third-generation femtosecond technology [11] have been developed to acquire MIR pulses. By utilizing virtual energy levels for amplification, they are free of cooling systems as no energy accumulates inside the gain media. Among these techniques, optical parametric amplification (OPA) [12][13][14][15][16][17][18], owing to its distinguished advantages including ultra-broad parametric bandwidth and ultrahigh gain per pass, has become an ideal way to generate MIR pulses, compared with other nonlinear processes [19][20][21][22][23][24][25].A series of nonlinear crystals are employed to serve in thes...

In this letter, long-distance interactions between optical solitons with an oscillating structure are investigated. Analytic two-soliton solutions for the variable-coefficient nonlinear Schrödinger equation are obtained. Different from the elastic interactions reported previously, the interaction solitons are accompanied with an oscillating structure during their interactions. Reasons for long-distance interactions are discussed, and influences of the corresponding parameters are analyzed. Those studies may provide a new insight into the soliton interactions.

We propose a novel kind of wide-range refractive index optical sensor based on photonic crystal fiber (PCF) covered with nano-ring gold film. The refractive index sensing performance of the PCF sensor is analyzed and simulated by the finite element method (FEM). The refractive index liquid is infiltrated into the cladding air hole of the PCF. By comparing the sensing performance of two kinds of photonic crystal fiber structures, a wide range and high sensitivity structure is optimized. The surface plasmon resonance (SPR) excitation material is chose as gold, and large gold nanorings are embedded around the first cladding air hole of the PCF. The higher order surface plasmon modes are generated in this designed optical fiber structure. The resonance coupling between the fundamental mode and the 5th order surface plasmon polariton (SPP) modes is excited when the phase matching condition is matched. Therefore, the 3rd loss peaks appear obvious red-shift with the increase of the analyte refractive index, which shows a remarkable polynomial fitting law. The fitnesses of two structures are 0.99 and 0.98, respectively. When the range of refractive indices is from 1.40 to 1.43, the two kinds of sensors have high linear sensitivities of 1604 nm/RIU and 3978 nm/RIU, respectively.

K 3 B 6 O 10 Cl (KBOC), a new nonlinear optical crystal, shows potential advantages for the generation of deep ultraviolet (UV) light compared with other borate crystals. In this paper we study for the first time the second harmonic generation (SHG) of a femtosecond Ti:sapphire amplifier with this crystal. Laser power is obtained to be as high as 220 mW at the central wavelength of 396 nm with a 1-mm-long crystal, and the maximum SHG conversion efficiency reaches 39.3%. The typical pulse duration is 83 fs. The results show that second harmonic (SH) conversion efficiency has the room to be further improved and that the new nonlinear crystal is very suited to generate the high efficiency deep ultraviolet laser radiation below 266 nm.

Hollow-core photonic crystal fibers (HC-PCFs) can be used for the supercontinuum generation. Because the design of HC-PCFs is flexible, the dispersion and nonlinear effects is variable, and the pulses in HC-PCFs can show different transmission characteristics. In this paper, the transmission of high-order solitons in HC-PCFs is studied. Through adjusting the group-velocity dispersion and nonlinear effects of HC-PCFs, we present the different transmission of high-order solitons, and analyze their characteristics. Results are conducive to the applications of HC-PCFs in nonlinear optics and ultrafast optics.