Optical phase conjugation and pseudolinear transmission

We describe the use of optical phase conjugation (OPC) to suppress intrachannel nonlinearities that limit pseudolinear transmission. We show that OPC combined with appropriate dispersion mapping is effective in suppressing intrachannel nonlinearities, even in the absence of signal-power evolution symmetry that is generally required by OPC to compensate fiber nonlinearity. An increase in signal power by 5.5 dB is observed when a single OPC is used for 40-Gbit/s pseudolinear transmission over 32 x 100 km of passive-fiber spans.     

Soliton-based ultra-high speed optical communications

Multi-terabit/s, ultra-high speed optical transmissions over several thousands kilometers on fibers are becoming a reality. Most use RZ (Return to Zero) format in dispersion-managed fibers. This format is the only stable waveform in the presence of fiber Kerr nonlinearity and dispersion in all optical transmission lines with loss compensated by periodic amplifications. The nonlinear Schrödinger equation assisted by the split step numerical solutions is commonly used as the master equation to describe the information transfer in optical fibers. All these facts are the outcome of research on optical solitons in fibers in spite of the fact that the commonly used RZ format is not always called a soliton format. The overview presented here attempts to incorporate the role of soliton-based communications research in present day ultra-high speed communications.     

Tailoring optical nonlinearities via the Purcell effect

We predict that the effective nonlinear optical susceptibility can be tailored using the Purcell effect. While this is a general physical principle that applies to a wide variety of nonlinearities, we specifically investigate the Kerr nonlinearity. We show theoretically that using the Purcell effect for frequencies close to an atomic resonance can substantially influence the resultant Kerr nonlinearity for light of all (even highly detuned) frequencies. For example, in realistic physical systems, enhancement of the Kerr coefficient by one to two orders of magnitude could be achieved.     

Stable helical solitons in optical media

We present a review of new results which suggest the existence of fully stable spinning solitons (self-supporting localised objects with an internal vorticity) in optical fibres with self-focusing Kerr (cubic) nonlinearity, and in bulk media featuring a combination of the cubic self-defocusing and quadratic nonlinearities. Their distinctive difference from other optical solitons with an internal vorticity, which were recently studied in various optical media, theoretically and also experimentally, is that all the spinning solitons considered thus far have been found to be unstable against azimuthal perturbations. In the first part of the paper, we consider solitons in a nonlinear optical fibre in a region of parameters where the fibre carries exactly two distinct modes, viz., the fundamental one and the first-order helical mode. From the viewpoint of application to communication systems, this opens the way to doubling the number of channels carried by a fibre. Besides that, these solitons are objects of fundamental interest. To fully examine their stability, it is crucially important to consider collisions between them, and their collisions with fundamental solitons, in (ordinary or hollow) optical fibres. We introduce a system of coupled nonlinear Schrödinger equations for the fundamental and helical modes with nonstandard values of the cross-phase-modulation coupling constants, and show, in analytical and numerical forms, results of collisions between solitons carried by the two modes. In the second part of the paper, we demonstrate that the interaction of the fundamental beam with its second harmonic in bulk media, in the presence of self-defocusing Kerr nonlinearity, gives rise to the first ever example of completely stable spatial ring-shaped solitons with intrinsic vorticity. The stability is demonstrated both by direct simulations and by analysis of linearized equations.     

Quasi-stationary stage of a nonequilibrium optical discharge in the field of a Bessel wave beam

The self-consistent stationary distributions of the field and plasma produced by Bessel wave beams in a gas with the ionization and Kerr nonlinearities are studied analytically. Using a stationary model based on the condition of a constant field amplitude in the ionized region, the structures are considered to be formed by Bessel beams of two types: with an azimuthal electric field equal to zero at the symmetry axis and a quasitrans-verse field having a maximum at the axis. A specific feature of the plasma channel formed in the first case is the presence of a nonionized region in its central part (tubular discharge), whose radius is independent of the incident power. In the second case, the channel is continuous. The relation is found between the incident radiation power and the external radius of the discharge. It is shown that the Kerr nonlinearity, which is especially important at small divergence angles of the beam, enhances the maximum plasma density and reduces the discharge radius. The parameters of plasma structures produced upon focusing a Gaussian beam by a conical lens are estimated using the model proposed.     

Self-accelerating self-trapped optical beams

We present self-accelerating self-trapped beams in nonlinear optical media, exhibiting self-focusing and self-defocusing Kerr and saturable nonlinearities, as well as a quadratic response. In Kerr and saturable media such beams are stable under self-defocusing and weak self-focusing, whereas for strong self-focusing the beams off-shoot solitons while their main lobe continues to accelerate. Self-accelerating self-trapped wave packets are universal, and can also be found in matter waves, plasma, etc.     

四波混频在长距离相干光纤传输中的影响

本文研究光纤非线性现象之一的四波混频对长距离相干多路光纤传输系统的影响.通过对一条长度为550km,采用9个掺铒光纤放大器的频分复用系统的实验,清楚地观察到了四波混频这一光纤非线性.实验的结果符合对四波混频的理论分析与计算.结果表明:对信息容量在每秒10~9bit数量级,采用常规的不归零码作信号方式的多路传输,光纤中的四波混频现象将严重地影响其传输性能.     

Giant Kerr nonlinearities and slow optical solitons in coupled double quantum-well nanostructure

We investigate the enhancement of Kerr nonlinearities in an asymmetric coupled double quantum-well (CQW) by analyzing the nonlinear optical response. With the consideration of real parameters in AlGaAs-based CQWs, we indicate the possibility to obtain the giant Kerr nonlinearities with the cancellation of linear absorption simultaneously. We also reveal both analytically and numerically that under appropriate conditions a probe field with very low intensity can evolve into a stable shape-preserving waveform that propagates with a slow group velocity. The stability of such slow optical solitons is also demonstrated numerically.     

Optical soliton perturbation in a non-Kerr law media

This paper studies the optical soliton perturbation by the aid of soliton perturbation theory. The various perturbation terms, that arise in the study of optical solitons, are exhaustively studied in this paper. The adiabatic parameter dynamics of optical solitons are obtained in presence of these perturbation terms. The types of nonlinearities that are considered are Kerr law, power law, parabolic law as well as the dual-power law.     

Cavity-enhanced optical parametric chirped-pulse amplification

A novel method for the generation of high-energy ultrashort optical pulses is described and studied theoretically and numerically. Through the combination of parametric amplification and enhancement cavities, this method opens a route to generate few-cycle pulses at unprecedented average power levels through the use of a low-energy, high average-power pump source and energy storage in the enhancement cavity. Dispersion in the enhancement cavity ceases to be a concern with the use of long pump pulses. Limitations set by the Kerr nonlinearity of the amplifier crystal are analyzed, and ways to overcome them using self-defocusing nonlinearities are discussed.     

Saturable absorbers for low power optical phase conjugation

Properties of a few saturable absorbers as candidates for low power optical phase conjugation (OPC) are examined. The specific systems studied are the dyes eosin, erythrosin B and Rose Bengal, doped in gelatin, polyvinyl alcohol and boric acid glass films. A method for quickly checking saturated absorption is suggested and demonstrated. Examples for OPC with these systems are cited and limitations are briefly discussed. An erratum to this article is available at .     

Pilot-based cross-phase modulation compensation for coherent optical orthogonal frequency division multiplexing long-haul optical communications systems

A pilot-based nonlinearity compensator (PB-NLC) is shown in this Letter to be an effective method for compensating cross-phase modulation (XPM) in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. An unmodulated pilot tone is transmitted at the center of each OFDM channel to detect phase errors caused by the Kerr effect, which converts intensity fluctuations from all channels to phase errors. The pilots are then used to cancel the XPM phase errors for each OFDM channel at the receiver after each channel's self-phase modulation (SPM) has been compensated, using its intensity waveform to determine its SPM. Numerical simulations of a 58 Gb/s single polarization 2375 km system with inline dispersion compensation show that the signal quality, Q, at the optimal launch power is increased by 2.4 dB if SPM compensation is used before the PB-NLC. This contrasts with only a 0.9 dB improvement if the PB-NLC is used without an SPM compensator for the same link. This shows the PB-NLC can effectively mitigate XPM but not SPM.     

Bright and dark solitons in a cascaded system

This paper studies coupled nonlinear Schrödinger's equation (NLSE) that appears in a cascaded system. Both Kerr law and power law nonlinearities are considered. Bright and dark soliton solutions are retrieved for these nonlinearities. The corresponding constraint conditions naturally fall out that from the mathematical expressions that must remain valid for solitons to exist.     

Zero-ϕeff non-Bragg gap solitons in 1D Kerr polaritonic/metamaterial heterostructures

A theoretical study of one-dimensional heterostructures composed of alternate layers of a Kerr polaritonic material and a linear dispersive metamaterial is performed. For frequency values at the edges of the non-Bragg zero-ϕ_eff gap of the heterostructure in the linear regime, a switching from very low to high transmission states is obtained and localized gap solitons of various orders are found, depending on the particular value of the incident power. Soliton solutions are shown to be robust with respect to absorption effects and a study is presented for gap soliton phases at the top and bottom of the zero-ϕ_eff gap in the case of defocusing and focusing nonlinearities.     

Performance investigation of suppression of four wave mixing using optical phase conjugation with different modulation format in DWDM soliton communication system

The performance of dense wavelength division multiplexed (DWDM) soliton transmission system for returnto-zero (RZ) and non-return-to-zero (NRZ) modulation formats have been investigated. The main aim of this paper is to estimate and mitigate the four wave mixing (FWM) power by using in-line optical phase conjugator (OPC). The effect of FWM has been estimated using real fiber link having nonlinear and attenuation losses. The FWM power is strongly suppressed by introducing destructive interference between the first and second halves of in-line OPC. It has been indicated that RZ with OPC yields the better performance with FWM power suppression (more than 20 dBm in certain cases) with reasonable bit error rate and Q-factor.     

Optical solitons for non-Kerr law nonlinear Schrödinger equation with third and fourth order dispersions

In this paper, we obtain optical soliton solutions for non-Kerr law nonlinear Schrödinger equation (NLSE) with third order (3OD) and fourth order dispersions (4OD). We will use two integration schemes, namely sin-cosine method and Bernoulli’s equation approach with five laws of nonlinearities. Sine-cosine method is applicable to Kerr, power and anti-cubic laws, this method provides bright soliton solutions. The second method is applicable to parabolic and cubic quintic laws, this method generates dark soliton. The results may be used in discussing the propagation of optical solitons in highly dispersive media with Kerr, power, anti-cubic, parabolic and cubic quintic law nonlinearities.     

Enhancement of Node-Induced Crosstalk by Nonlinear Effects in Nonzero Dispersion-Shifted Fiber Rings with Optical Add/Drop Multiplexers

Wavelength division multiplexed metropolitan networks have been attracting increasing attention in the networking research community. We discuss an issue that is pertinent to node design and transmission systems in these networks. The problem of node-induced crosstalk and its interaction with fiber nonlinearities is investigated. In this respect, we consider transmission systems that are based on nonzero dispersion-shifted fiber rings with simple optical add/drop multiplexing nodes. The system power penalty due to the crosstalk/nonlinear interaction phenomenon is calculated for several cases. Dependence of this penalty on the frequency difference between signal and crosstalk carriers, input-signal power, and extinction ratio is investigated. It is shown that the design of regional and metropolitan networks or links with physical spans in excess of 200 km has to take this crosstalk/nonlinear interaction phenomenon into consideration.     

Excitation of stable transverse wavepackets with quadratic and cubic susceptibilities

We have identified a family of (2+1)D spatial solitary waves which can stably propagate in bulk media in the presence of coexisting diffraction, self-focusing Kerr and quadratic nonlinearities. In a conspicuous range of excitation conditions close to the stationary solutions, the emerging wavepackets are immune to the detrimental occurrence of filamentation and collapse, typical of pure Kerr media. The presence of a second-order contribution to the cubic nonlinear response is, therefore, able to prevent optical damage in applications relying on self-guidance. We show that the cross-phase modulation plays an important effect on stability. Our estimate shows that the effects of the cubic susceptibility cannot be neglected below a certain beam size in realistic crystals (e.g. KTP or similar).     

Femtosecond pulse shaping using counter-propagating pulses in a semiconductor optical amplifier

An efficient pulse shaping method using counter-propagating pulses in the femtosecond regime is proposed and investigated. The effects of pump pulse power and pulsewidth on probe pulse are studied in counter-propagation scheme. It is shown that, with the proposed method, output probe pulse temporal and spectral peak shift due to femtosecond nonlinearities can be compensated, while the output pulse is amplified sufficiently. Furthermore, in relatively high power regime, the probe pulsewidth and time-bandwidth product are improved using counter-propagating pump pulse.