Generation of optical vortices in multihelical optical fibers

It is theoretically demonstrated that multihelical fibers are capable to change the topological charge of the incoming field by l units in the transmitted and reflected light. The magnitude of the change in the topological charge coincides with the number of coaxial helicoids forming the fiber core. This can be used in designing generators of optical vortices (OVs) from Gaussian beams: broadband ones in transmitted light and narrowband ones in reflected light.     

Twisted clad microstructured optical fibers: revisited

The paper discusses confinements corresponding to low-order sustained hybrid modes in twisted clad microstructured optical fiber with twists in the form of conducting sheath helix structure introduced at the core–clad interface and inclined at certain angles. Dispersion relations for such fiber structures are deduced under strict electromagnetic boundary conditions. Varying the angle of pitch (of the introduced conducting sheath helix), investigations in respect of the dependence of power confinements due to the existing low-order hybrid modes have been carried out. The obtained results indicate the usefulness of such microstructured fibers for attenuation or amplification of power in the guide.     

Filter of optical vortices: highly twisted high-birefringence optical fibers

Modes and bandgap structure of highly twisted high-birefringence weakly guiding fibers are studied in the scalar approximation. It is shown that within the gap the system can serve as a filter of circular optical vortices.     

Propagation of optical vortices in coiled weakly guiding optical fibers

The structure of the l = 1 modes of regularly coiled weakly guiding optical fibers with round cross section is determined. The l = 1 modes are shown to be represented by two right-and left-hand polarized stable optical vortices, as well as by TE and TM modes. The polarization corrections to the scalar propagation constant are found. The stability of an optical vortex transmitted along a coiled fiber is estimated. The parameters for optimal coiling at which the transmission of vortices along the fibers is the most stable with respect to fiber perturbations are determined.     

Free-space propagation of second harmonic beams carrying optical vortices

A vorticity of free propagating SH beam produced in nonlinear crystal by combined beam composed of two coaxial Laguerre–Gaussian vortex beams is analyzed. It is revealed that the vortical structures of SH beam in the near and far fields are different when two vortex beams are carrying vortices with topological charges of different signs. The diffraction of SH beam under free propagation is investigated. A qualitative agreement of the theoretical predictions with experimental results is obtained.     

Quasi-soliton propagation in dispersion-managed optical fibers

Combination of programmed chirp and dispersion profiles produced a solitonlike nonlinear pulse with a stationary structure (quasi-soliton). The quasi-soliton has more attractive properties than those of a soliton because of its reduced interaction and smaller peak power than a soliton and allows ~100-Gbit / s transmission over a 125-dispersion distance with reasonable power.     

Optical vortices in anisotropic optical fibers with torsional stress

Analytical expressions for the higher-order modes with the azimuthal number equal to unity and for corresponding propagation constants of optical fibers with linear anisotropy of the fiber material and a circular anisotropy induced by torsional mechanical stress been obtained at practically important relationships between fiber parameters. The possibility of stable propagation of optical vortices in these fibers and the dependence of characteristics of sustained optical vortices on the fiber parameters are demonstrated.     

Dynamic behaviors of optical vortices in dual-core photonic crystal fibers

Novel evolution dynamics of optical vortices propagating in a dual-core photonic crystal fiber (PCF) is investigated, which can be explained well by using the equivalent dipole mode superposition principle. Thanks to the coupling between the two cores of PCF, exciting vorticity splitting and topological charge-flipping are achieved by inducing a vortex beam into one of the two cores of the PCF. What is more, the evolutions of two vortices located in each core separately can be controlled by means of modulating the initial phase difference of them. Our results may offer possibilities for applications of optical vortices, orbital angular momentum modulations, as well as optical communications.     

Wavelength dependent propagation in optical multimode fibers

Under various assumptions concerning the wavelength dependence of the refractive index of a fiber whose radial dependence is not an “a priori” known function of r, a method is described to obtain a closed form expression for the flight time of a ray in a multimode fiber. Consequently, the refractive index n(r) is shown to be the solution of a differential equation. Thus, for these selected profiles, we avoid the problem of the high precision required in the calculation of the flight time of a ray, a problem that arises frequently if the impulse response is to be reconstructed from a large number of rays, and transpose it to the determination of a single function of r, namely the refractive index.     

Optical control of vortices in dense media of gas-filled optical fibers

The problem of the formation of optical vortices in a medium composed of three-level atoms is analyzed with allowance for local-field effects in off-resonance Raman regimes of ??-scheme interaction. The possibility of optical control of the vortex dynamics without disturbing their stability is demonstrated in the optically dense medium of a hollow fiber filled with ⁸⁷Rb vapor.     

Topological charge dependent propagation of optical vortices under quadratic phase transformation

We make optical vortices of different topological charge and diffract them through a quadratic phase mask using the same spatial light modulator. This phase mask shows the diffraction in which the positive diffracted order has different dynamics than the negative diffracted order. The diffraction pattern and its orientation depend on the charge of the vortex as well as its sign. The experimental results are verified with exact analytical results.     

Reverse propagation of femtosecond pulses in optical fibers

We present a numerical technique for reversing femtosecond pulse propagation in an optical fiber, such that given any output pulse it is possible to obtain the input pulse shape by numerically undoing all dispersion and nonlinear effects. The technique is tested against experimental results, and it is shown that it can be used for fiber output pulse optimization in both the anomalous and normal dispersion regimes.     

Nonlinear control of the propagation of optical pulses in doped optical fibers

Results of study of nonlinear processes occurring during the propagation of light pulses in optical fibers doped with atoms of rare-earth elements under conditions of atomic coherence and interference are presented. For a three-level Λ scheme of interaction, the linear (χ⁽¹⁾) and nonlinear (χ⁽³⁾) susceptibilities of such a medium are calculated. It is shown that the coefficients of Kerr nonlinearity and nonlinear absorption can reach extremely large values and can be negative. The competition between linear and nonlinear processes in the Λ scheme allows one to obtain compensation regimes when the coefficients of dispersion or absorption of the optical fiber material vanish. The efficient control of the optical properties of such a system over wide limits proves to be possible owing to variations in the parameters of light pulses at the input of the medium. The necessary conditions for realizing regimes with “slow” light, as well as for self-compression of a probing pulse on ultimately small spatial scales in a doped optical fiber, were obtained.     

Adaptive control of femtosecond pulse propagation in optical fibers

Nonlinear effects present fundamental obstacles to the propagation of femtosecond pulses of detectable energy in single-mode optical fibers, inducing severe distortion even after a very short (a few meters) propagation distance. We show here that adaptive pulse shaping can overcome these limitations by synthesizing pulses that are self-correcting for higher-order nonlinear effects when they are launched in the fiber. This approach would not only affect optical communications but also yield benefits in various disciplines requiring optimized fiber-based femtosecond pulse delivery, for example, nonlinear imaging techniques such as multiphoton microscopy, material processing, and medical diagnostics.     

Light propagation with ultralarge modal areas in optical fibers

We demonstrate robust single-transverse-mode light propagation in higher-order modes of a fiber, with effective area A(eff) ranging from 2,100 to 3,200 microm(2). These modes are accessed using long-period fiber gratings that enable higher-order-mode excitation over a bandwidth of 94 mm with greater than 99% of the light in the desired mode. The fiber is designed such that the effective index separation between modes is always large, hence minimizing in-fiber mode mixing and enabling light propagation over lengths as large as 12 m, with bends down to 4.5 cm radii. The modal stability increases with mode order, suggesting that A(eff) of this platform is substantially scalable.     

Nonlinear optical propagation and self-limiting effect in liquid-crystalline fibers

The nonlinear optical properties of the isotropic phase of liquid crystals induced by nanosecond laser pulses are analyzed in the context of nonlinear multi-mode propagation in a liquid-crystal-cored fiber. The negative thermal density nonlinearity of the core gives rise to an intensity-dependent loss in the core-guided transmission and optical action. Experiments conducted with such liquid-crystal-cored fibers show that the optical limiting threshold for nanosecond laser pulses can be as low as 0.09 J/cm², which is one of the lowest among known nonlinear optical materials and structures, including bulk liquid-crystal films.     

Nonlinear propagation and continuum generation in microstructured optical fibers

A theoretical investigation of the propagation of femtosecond pulses under conditions similar to those of recent experiments in which a white-light continuum was generated in a microstructured fiber is presented. It is found that higher-order dispersion primarily determines the shape and width of the generated spectrum and that the fine spectral substructure exhibits extreme sensitivity to the initial pulse energy.     

Three-dimensional FDTD method for optical pulse propagation analysis in microstructured optical fibers

We propose a three-dimensional (3D) finite-difference time-domain (FDTD) method to analyze the pulse propagation characteristics in microstructured optical fibers (MOFs). The computation domain size is greatly reduced by adopting the technique of moving problem space. The propagating pulse is virtually held in the buffer cell of the problem space as simulation continues. This method is capable to investigate the temporal evolution of the propagating pulse. Spectral information can be obtained by Fourier analysis. As an example, the influence of the kerr nonlinearity on the optical pulse propagation in a Lorentz dispersive MOF is demonstrated. The model is also used to simulate the nonlinear interactions between the pump spectral broadening and third harmonic generations in a highly nonlinear fused silica nanowire with good agreement with the generalized nonlinear envelop equation (GNEE) model.     

Modulated optical vortices

Single-beam optical gradient force traps created by focusing helical modes of light are known as optical vortices. Modulating the helical pitch of such a mode's wave front yields a new class of optical traps whose dynamically reconfigurable intensity distributions provide new opportunities for controlling motion in mesoscopic systems. An implementation of modulated optical vortices based on the dynamic holographic optical tweezer technique is described.