We numerically study the propagation of 1-ps laser pulse in three tapered holey fibers （THFs）.The curvature indices of the concave,linear,and convex tapers are 2.0,1.0,and 0.5,respectively.The central wavelength,located in the normal dispersion regime,is 800 nm.The nonlinear coefficient of the THFs increases from the initial 0.095 m-1· W-1 to the final 0.349 m-1·W-1.Wave breaking accompanied by oscillatory structures occurs near pulse edges,and sidelobes appear in the pulse spectrum.With the increase in propagation distance z,the pulse shape becomes broader and the pulse spectrum flattens.A concave THF is advantageous to the generation of wave breaking and enables easier achievement of super flat spectra at short lengths.
This paper studies the generation of the dispersive wave (DW) in the normal dispersion regimes of the birefringent photonic crystal fiber (BPCF) fabricated in this work. The remarkable blue-shifted radiation is found to be generated when 30 fs pulses are input in the normal dispersion regime of the BPCF for the first time. The characteristics of the blue-shifted DW strongly depend on the polarization of the input pulse. As a result, two peaks appear in the blue-shifted region of the spectrum when the input pulses polarize along the slow axis of the BPCF. With the increase of the center wavelength of the initial input pulse, the difference between the wavelengths of the two peaks widens. The peak location in the spectrum can be explained by the phase matching condition between the DW and the input pulse. In addition, when the input polarization is set to an angle of 45° with respect to the principal axes of the BPCF, the cross-phase modulation and coherent coupling between two orthogonally polarized modes would result in pulse trapping in the BPCF. Accordingly, the DW shift toward short wavelength is restrained. The DW generation in the normal-dispersion regimes of BPCF can be controlled by the phase matching condition and polarization of the input pulse.
LI ShuGuang1,2, ZHENG Yi1, YIN GuoBing2, ZHANG Lei2, ZHOU GuiYao2 & HOU LanTian2 1 School of Science, Beijing Jiaotong University, Beijing 100044, China
A type of As2S3 chalcogenide glass mid-infrared dual-core photonic crystal fiber has been proposed. The dualcore photonic crystal fiber （PCF） consists of two asymmetric cores. The high polarization property and the coupling characteristics have been studied by using the finite dement method and mode coupling theory. Numerical results show that the birefringence at wavelength λ = 10 μm is up to 0.01386 and the coupling length can reach wavelength = 5 μm, 261 μm and 271.44 μm for x-polarized mode and y-polarized mode, respectively. It demonstrates that a 6.786-ram-long fiber can exhibit an extinction ratio of better than -10 dB and a bandwidth of 180 nm.
A polarization-dependent supercontinuum spectrum source of light from the UV to infrared region has been generated in our photonic crystal fiber with birefringence B=2.23×10 3.By tuning the polarization direction of the input pulse,it is found that the width of the supercontinuum spectrum changes dramatically with the input polarization directions.At the same time,we qualitatively explain the blue-shift peak and the red-shift peak in the experimental spectrum using phase matching conditions on dispersive waves,stoke waves and the pump wave.In addition,we also found that supercontinuum spectrum generation,to some extent,is dependent on the pump wavelength and average power of the pump.The spectrum is broadened with the increase of average power,but unchanged after average power reaches a certain value;when the pump wavelength is located in the anomalous dispersion and further away from the zero-dispersion wavelength,the spectrum of the supercontinuum is wider.
WANG XiaoYanLI ShuGuangHAN YingDU YingXIA ChangMingHOU LanTian
This paper proposes three kinds of tapered holey fibres with a multi-layer of holes whose pitch of air holes at the end of untapered and tapered are 5.8 μm and 1.8 μm. The central wavelength which locates in the anomalous dispersion region is 1.55 μm. An adaptive split-step Fourier method is numerically used to study the pulse propagation in tapered holey fibres. For the considered convex tapered holey fibre, at a wavelength of 1.55 μm, a compression factor of 136.7 can be achieved by initial width of 800 fs propagation through a length of 0.8 m. It demonstrates that in anomalous dispersion region, pulse can be compressed with the increase of nonlinearity coefficient and the decrease of dispersion coefficient.
The propagation of femtosecond laser pulses with wavelengths of 1550 nm, 1064 nm, 800 nm and 700 nm, respectively, which are in the normal dispersion region of the nano-structured photonic crystal fiber (N-PCF) with interesting broadband normal dispersion and highly nonlinear properties, is studied. For the effect of chirp variation mainly induced by group velocity dispersion (GVD) and self-phase modulation (SPM), after propagation over a short length, the wave breaking occurs. Namely, oscillatory structures are presented near pulse edges and sidelobes appear in the pulse spectrum. In the case of 800 nm, after the propagation of 20 mm, a super flat spectrum is obtained. The bandwidth of the super flat spectrum is associated with the dispersion length and the nonlinear length. By choosing N-PCF and laser pulse with appropriate parameters, a broadband super flat spectrum in a short length can be achieved.
A terahertz photonic crystal fibre （THz-PCF） is designed for terahertz wave propagation. The dispersion prop- erty and model birefringence are studied by employing the finite element method. The simulation result reveals the changing patten of dispersion parameter versus the geometry. The influence of the large frequency band of terahertz on birefringence is also discussed. The design of low loss, high birefringence THz-PCFs with zero dispersion frequency at 0.3 THz is presented.
This paper reports that, based on the electromagnetic scattering theory of the multipole method, a high-quality hollow beam is produced through a selectively liquid-filled photonic crystal fibre. Instead of a doughnut shape, a typical hollow beam is produced by other methods; the mode-field images of the hollow-beam photonic crystal fibre satisfy sixth-order rotation symmetry, according to the symmetry of the photonic crystal fibre （PCF） structure. A dark spot size of the liquid-filled photonic crystal fibre-generated hollow beam can be tuned by inserting liquid into the cladding region and varying the photonic crystal fibre structure parameters. The liquid-filled PCF makes a convenient and flexible tool for the guiding and trapping of atoms and the creation of all-fibre optical tweezers.
