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.     

Application of variational calculus to propagation of coupled pulses in optical fibers

Two nonlinear Schrödinger equations, linked by cross-modulation terms, are used to study the nature of coupled pulse propagation in an optical fiber. The problem is formulated within the framework of variational calculus. Using Gaussian trial functions for the propagating pulses, an expression is constructed for the effective Lagrangian of the system. It is shown that this Lagrangian, via the Ritz optimization procedure, provides a basis to construct approximate solutions of the problem. Some judicious approximations are invoked to investigate how the cross modulation affects the behavior of pulse propagation. Conditions are derived under which both pulses can propagate without distortion.     

Spatiotemporal propagation dynamics of intense optical pulses in loosely confined gas-filled hollow-core fibers

We numerically study the propagation dynamics of intense optical pulses in gas-filled hollow-core fibers(HCFs). The spatiotemporal dynamics of the pulses show a transition from tightly confined to loosely confined characteristics as the fiber core is increased, which manifests as a deterioration in the spatiotemporal uniformity of the beam. It is found that using the gas pressure gradient does not enhance the beam quality in large-core HCFs, while inducing a positive chirp in the pulse to lower the peak power can improve the beam quality. This indicates that the self-focusing effect in the HCFs is the main driving force for the propagation dynamics. It also suggests that pulses at longer wavelengths are more suitable for HCFs with large cores because of the lower critical power of self-focusing, which is justified by the numerical simulations. These results will benefit the generation of energetic few-cycle pulses in large-core HCFs.     

Modulation instability at propagation of narrowband 100-ns pulses in optical fibers of various types

The instability of the propagation of narrowband 100-ns laser pulses in the transparency window of a single-mode fiber in the presence of continuous broadband noise is demonstrated. The modulation instability developed due to the nonlinearity and anomalous dispersion leads to a rapid decay of the narrowband component after passing through the fiber. The modulation-instability threshold is shown to be sensitive to the noise level in a spectral interval of about 100 GHz near a central wavelength of about 1.55 μm and is inversely proportional to the fiber length (varied from 1.6 to 21.0 km in the experiments). The effect is completely eliminated owing to the application of a dispersion-shifted fiber with normal dispersion.     

Suppression of magnetic state decoherence using ultrafast optical pulses

It is shown that magnetic state decoherence produced by collisions in a thermal vapor can be suppressed by the application of a train of ultrafast optical pulses.     

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.     

大芯径石英光纤中皮秒脉冲激光的自聚焦

本文报道了对大芯径石英光纤中皮秒脉冲激光自聚焦现象的实验研究和理论分析.文中测量了自聚焦先场的近场光强轮廓,研究了自聚焦光场的时域和频域性质,并对环形自聚焦光场的成因进行了解释.     

Propagation of optical pulses in periodic nonlinear fibers

We study the conditions of formation and propagation of soliton pulses in a periodic two-mode nonlinear fiber with allowance for dispersion of the fiber material and intermode dispersion of interacting modes under the condition of their phase synchronism. State University, Ul’yanovsk, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 8, pp. 1032–1042, August, 1998.     

Dynamics of optical pulses in periodic nonlinear fibers

Dynamics of a two-mode wave packet with strong linear and nonlinear (cross-modulation) coupling is investigated. The effect of the initial conditions of the pulse excitation on its further transformation is considered. The possibility to control the dispersion parameters of the wave packet by varying the initial conditions of its input is pointed out. The effective parameters of the dispersion and nonlinearity that govern the dynamics of an optical pulse in a periodic nonlinear fiber light guide are obtained.     

Shaping of picosecond pulses for pumping optical parametric amplification

The use of temporally shaped pump pulses for optical parametric chirped-pulse amplification can increase the efficiency and suppress possible spectral distortions in this process. With this goal in mind a novel method for shaping narrowband picosecond pulses has been developed. The method is based on the pulse stacking principle, where replicas of the incoming pulse are created in a specially designed four-beam interferometer. The replicas are recombined with appropriate relative delays. The interferometer design allows for a unique flexibility in varying the pulse shape, since all relevant degrees of freedom, such as relative intensities and delays between the pulse replicas are independently adjustable. Here we describe the design of our device in detail and report on the experimental demonstration of its pulse-shaping capabilities. PACS 42.65.Yj; 42.65.Re; 42.81.Gs     

Twisted optical fibers sustaining propagation of optical vortices

The mode structure (the orbital number l = 1) of a few-mode weakly guiding optical fiber with high linear birefringence and a regular twist of its anisotropy axes is investigated. It is shown that, for certain values of twist pitch, the modes with l = 1 are almost pure linearly polarized optical vortices in the local coordinate system associated with the anisotropy axes. The range of values of twist pitch in which twisted fibers sustain propagation of linearly polarized optical vortices is determined numerically.     

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.     

Intraband effects on ultrafast pulse propagation in semiconductor optical amplifier

High bit-rate (>10 Gb/s) signals are composed of very short pulses and propagation of such pulses through a semiconductor optical amplifier (SOA) requires consideration of intraband phenomena. Due to the intraband effects, the propagating pulse sees a fast recovering nonlinear gain which introduces less distortion in the pulse shape and spectrum of the output pulse but introduces a positive chirping at the trailing edge of the pulse.     

Controllable propagation of light pulses in Er-doped fibers with saturable absorption

We report controllable (slow or fast) propagation of low-intensity probe-light pulses through erbium-doped fiber periodically saturated by the synchronized master-pulse sequence. These two pulse sequences could have significantly different carrier wavelengths within the fundamental absorption spectrum 1470-1570 nm of Er(+3) ions. The effect of fractional delay or advancement grew with the fiber optical absorption at the probe wavelength and could be significantly stronger than that at the saturating wavelength. The probe-pulse advancement was observed in the case when the saturating and probe waves were modulated approximately in antiphase. The observed effects are explained in the framework of a simple model of a periodically saturated homogeneously broadened absorption line.     

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.     

Self-similar propagation of high-power parabolic pulses in optical fiber amplifiers

Self-similarity techniques are used to study pulse propagation in a normal-dispersion optical fiber amplifier with an arbitrary longitudinal gain profile. Analysis of the nonlinear Schr?dinger equation that describes such an amplifier leads to an exact solution in the high-power limit that corresponds to a linearly chirped parabolic pulse. The self-similar scaling of the propagating pulse in the amplifier is found to be determined by the functional form of the gain profile, and the solution is confirmed by numerical simulations. The implications for achieving chirp-free pulses after compression of the amplifier output are discussed.