Modulational instability in optical fibers with arbitrary higher-order dispersion and delayed Raman response

We analyse modulational instability (MI) of electromagnetic waves in a large variety of optical fibers having different refractive-index profiles. For the normal-, anomalous-, and zero-dispersion regimes of the wave propagation, we show that whenever the second-order dispersion competes with higher-order dispersion (HOD), propagation of plane waves leads to a rich variety of dynamical behaviors. Most of the richness comes from the existence of critical behaviors, which include situations in which the HOD suppresses MI in the anomalous dispersion regime, and other situations in which the HOD acts in the opposite way by inducing non-conventional MI processes in the normal- and anomalous-dispersion regimes. We show that non-conventional MI sidebands are more prone to Raman-induced degradations than ordinary MI sidebands can be.     

Modulational instability and space-time dynamics in nonlinear parabolic-index optical fibers

Beam propagation in multimode graded-index parabolic optical fibers in the presence of group-velocity dispersion and Kerr nonlinearity is theoretically investigated. It is shown that a modulational instability arising from the periodic spatial focusing of the beam takes place regardless of the sign of fiber dispersion, leading to a highly nonlinear space-time dynamics and the generation of ultrashort optical pulses.     

Modulational instability of electromagnetic waves in birefringent fibers with periodic and random dispersion

Modulational instability (MI) of electromagnetic waves in a birefringent fiber with a periodic dispersion (two-step dispersion management scheme) is investigated. The properties of new sidebands are studied. The strong variation of dispersion leads to the decreasing of the main MI region and the suppression of additional resonance. In the random dispersion case the MI of all frequencies of modulation in the normal dispersion region is predicted. In the anomalous dispersion case the decreasing of the main MI peak is calculated and changes in the spectral bandwidth of MI gain are found. The analytical predictions are confirmed by the numerical simulations of the full coupled nonlinear Schr?dinger equations with periodic coefficients.     

Optical amplitude multiplexing through parametric amplification in optical fibers

We propose a new technique that uses parametric amplification in optical fibers to combine two binary signals into a single quaternary amplitude-shift keying (4-ASK) signal. We develop a theoretical model to predict the power level distribution of the 4-ASK signals as a function of the extinction ratios of the input binary signals. Computer simulation results agree with the predictions of this theoretical model within a 0.9 dB margin. We also analyze the application of this technique to the generation of quaternary-amplitude optical packets, which are attractive for allowing the label to be transmitted in the same bandwidth and simultaneously to the payload. Our results indicate that such optical packets may be propagated through lengths comparable to those involved in metropolitan area networks.     

Influence of Raman susceptibility on optical parametric amplification in optical fibers

The real part of the Raman susceptibility is shown to have a strong influence on the peak parametric gain of single-pump parametric amplifiers. This results in a 35% variation in the peak parametric gain over the frequency range 0-30 THz. We are able to experimentally demonstrate this effect in a photonic crystal fiber and obtain good agreement between the experimentally measured and theoretically predicted gains.     

Modulational instability effects in PSK modulated coherent fiber systems and their reduction by optical loss

We show using the propagating beam technique that if optical loss is neglected, the efficiency of PSK modulation in a coherent fiber system is severely compromised by modulational instability effects at sufficiently high signal powers. We note, however, that the intrinsic optical loss of a single mode fiber will to a large degree counteract the modulational instability and preserve the initial PSK pulse shape.     

Visible pulse compression to 4 fs by optical parametric amplification and programmable dispersion control

Angular dispersion of pump frequencies is shown to be an efficient mechanism for bandwidth enhancement in a noncollinear optical parametric amplifier. We demonstrate the generation of a continuous, simultaneously phase-matched 250-THz parametrically amplified spectrum. The resultant visible-near-IR signal-wave pulses were compressed to a 4-fs duration by a micromachined flexible mirror. Feedback for an iterative computer-controlled dispersion compensation algorithm is based on pulse characterization by second-harmonic generation frequency-resolved optical gating.     

Polarization mode dispersion induced pulse broadening in optical fibers

A simple and exact analytical expression is derived for the amount of broadening that a pulse suffers when it is subjected to the combined actions of polarization mode dispersion, chromatic dispersion, and chirping. The theory is then applied to various manifestations of second-order polarization mode dispersions.     

Modulational instability of optically induced nematicon propagation

We report the modulational instability （MI） analysis for the modulation equations governing the propagation of coherent polarized light through a nematic liquid crystal （NLC） slab, in the limit of low light intensity and local material response. The linear stability analysis of the nonlinear plane wave solutions is performed by considering both the wave vectors （k,l） of the basic states and wave vectors （K,L） of the perturbations as free parameters. We compute the MI gain, and the MI gain peak is reduced and the stable bandwidth is widened with the increase of the strength of the applied electric field. Further, we invoke the extended homogeneous balance method and Exp-function method aided with symbolic computation and obtain a series of periodic solitonic humps of nematicon profiles admitting the propagation of laser light in the NLC medium.     

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.     

Modulational instability on triangular dynamical lattices with long-range interactions and dispersion

The influence of long-range interactions on the stability of stationary solutions of triangular lattices described by the continuum-discrete nonlinear Schrödinger equation is analyzed. By virtue of the linear stability analysis and a variational approach we demonstrate that both soliton array and continuous-wave solutions are modulationally unstable. Analytical expressions for instability thresholds and growth rate spectra are presented and compared with the corresponding results in the approximation of a nearest neighbor interaction.     

Compensating losses in negative-index metamaterials by optical parametric amplification

Optical parametric amplification controlled by the auxiliary electromagnetic field enables transparency, amplification, and oscillation with no cavity in strongly absorbing negative-index metamaterials. The opposite directions of the wave vector and the Poynting vector in such materials result in extraordinary optical properties, including "backward" phase matching and the generation of entangled pairs of left- and right-handed counterpropagating photons.     

Modulational instability in optical fiber with stochastic parameters and noninstantaneous response

We investigate analytically and numerically the modulational instability (MI) in optical fiber, where the effect of noninstantaneous nonlinear response as well as stochastic coefficients are taken into account. Applying the linear stability analysis and stochastic calculus, we show that the MI gain spectrum reads as the maximal eigenvalue of a constant matrix. In the limiting cases of small fluctuations, we give explicit expressions for the MI gain spectra. In the general configurations, we derive an explicit form of the effective matrix and compute numerically the maximal eigenvalue. The moment MI peak is enhanced and the delayed Raman response reduces the maximum MI gain caused by stochasticity both in anomalous and normal dispersion regimes. Numerical simulations of the full stochastic nonlinear Schródinger equation show that, the phenomenon of MI gives rise to periodic pulse arrays of waves train, as well as to a chain of peaks with continuously growing amplitudes.     

Diffusion-induced parametric dispersion and amplification in doped semiconductor plasmas

Using the hydrodynamic model of semiconductor plasma, the diffusion-induced nonlinear current density and the consequent second-order effective susceptibility are obtained under off-resonant laser irradiation. The analysis deals with the qualitative behaviour of the anomalous parametric dispersion and the gain profile with respect to the excess doping concentration and pump electric field. The analysis suggests that a proper selection of doping level and pump field may lead to either positive or negative enhanced parametric dispersion, which can be of great use in the generation of sequeezed states. It is found that gain maximizes at moderate doping concentration level, which may drastically reduce the fabrication cost of parametric amplifier based on this interaction.      

Pump-noise transfer in optical parametric chirped-pulse amplification

We report on direct observation of temporal contrast degradation of short pulses amplified by optical parametric chirped-pulse amplification. We show that, despite injection seeding, quantum-noise-induced fast modulations (< 50 ps) of the temporal profile of the pump pulse are imprinted on the spectrum of the amplified chirped pulse and give rise to a large picosecond pedestal in the time domain.     

Stacking chirped pulse optical parametric amplification

Stacking chirped pulse optical parametric amplification based on a home-built Yb³⁺-doped mode-locked fiber laser and an all-fiber pulse stacker has been demonstrated. Energic 11 mJ shaped pulses with pulse duration of 2.3 ns and a net total gain of higher than 1.1 × 10⁷ at fluctuation less than 2% rms are achieved by optical parametric amplification pumped by a Q-switched Nd:YAG frequency-doubled laser, which provides a simple and efficient amplification scheme for temporally shaped pulses by stacking chirped pulse.     

Bending-induced optical loss in random-hole optical fibers

We present the experimentally determined optical bend loss for random hole optical fibers in the spectral range 1520 to 1570 nm induced by wrapping the optical fiber around a fixed diameter mandrel. The optical losses are compared to those obtained for a single-mode fiber and a multimode fiber using the same bending procedures. The bending induced optical losses in the random hole optical fibers were several orders of magnitude lower than for the single-mode fiber and were about 1 order of magnitude lower than for the multimode fiber.     

Modulational instability in wind-forced waves

We consider the wind-forced nonlinear Schrödinger (NLS) equation obtained in the potential flow framework when the Miles growth rate is of the order of the wave steepness. In this case, the form of the wind-forcing terms gives rise to the enhancement of the modulational instability and to a band of positive gain with infinite width. This regime is characterised by the fact that the ratio between wave momentum and norm is not a constant of motion, in contrast to what happens in the standard case where the Miles growth rate is of the order of the steepness squared.     

Optical parametric amplification in all-silica triangular-core photonic crystal fibers

The present analysis demonstrates that photonic crystal fibers can be designed to enhance their properties for optical parametric amplification. In particular, it is possible to achieve high nonlinear coefficient values and flattened dispersion curves around 1550 nm to fully exploit the four-wave-mixing effect. These results have been obtained for triangular-core photonic crystal fibers without the need of modifying the refractive-index profile by adding dopants in the fiber cross section.