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Nonlinear frequency conversion spanning from the ultraviolet to the mid-infrared (beyond 2.4 μm) is experimentally demonstrated in multicladding silicon nitride (𝑆𝑖𝑋𝑁𝑌) waveguides. By adjusting the waveguide cross-section the chromatic dispersion is flattened, which enhances both the efficiency and the bandwidth of the nonlinear conversion. How accurately the dispersion is tailored is assessed through chromatic dispersion measurements and an experiment/simulation comparison of the dispersive waves' wavelength locations. Undesirable fluctuations of both the refractive index and the dimensions of the waveguide during the fabrication process result in a dispersion unpredictability of at least 20 ps/nm/km. Finally, manipulation of the effective refractive index allows for multiple third harmonic generated tones spanning from 381 to 715 nm.

We describe the generation of powerful dispersive waves that are observed when pumping a dual concentric core microstructured fiber by means of a sub-nanosecond laser emitting at the wavelength of~1064 nm. The presence of three zeros in the dispersion curve, their spectral separation from the pump wavelength, and the complex dynamics of solitons originated by the pump pulse break-up, all contribute to boost the amplitude of the dispersive wave on the long-wavelength side of the pump. The measured conversion efficiency towards the dispersive wave at 1548 nm is as high as 50%. Our experimental analysis of the output spectra is completed by the acquisition of the time delays of the different spectral components. Numerical simulations and an analytical perturbative analysis identify the central wavelength of the red-shifted pump solitons and the dispersion profile of the fiber as the key parameters for determining the efficiency of the dispersive wave generation process., Comment: 11 pages, 12 figures

We review polarization stabilization techniques based on the polarization attraction effect in low-birefringence fibers. Polarization attraction or pulling may be based on cross-polarization modulation, on parametric amplification, and on Raman or Brillouin scattering. We also review methods for laser frequency conversion based on polarization modulation instabilities in low- and high-birefringence fibers, and photonic crystal fibers. Polarization instabilities in nonlinear fibers may also be exploited for sensing applications.

We study the performance of a nonlinear lossless polarizer (NLP), the device that transforms an input arbitrary state of polarization (SOP) of a signal beam into one and the same SOP toward the output and, unlike conventional passive polarizers, performs this transformation without polarization-dependent losses. The operation principle of this device is based on the nonlinear rotation of the SOP of the strong signal beam under the interaction with a copropagating strong pump beam in a Kerr medium, which in our case is a telecom fiber. We quantify the performance of this NLP by introducing the notion of instantaneous degree of polarization, which is a natural extension of the conventional notion of the degree of polarization appropriate for CW beams to the case of pulses whose SOP is not constant across the pulse. We pay particular attention to the regime when signal and pump beams experience a walk-off in the dispersive medium. In particular, we demonstrate that a signal pulse experiences much stronger repolarization when the walk-off effect is present as compared with the case of no walk-off. We also study the degradation of the efficiency of the NLP in the presence of polarization mode dispersion.

We study the dynamics of bound solitons of the complex cubic-quintic Ginzburg- Landau equation under the influence of external modulation. We consider periods of modulation being either smaller or larger than the soliton separation and the amplitude of modulation being either real or imaginary. For each case, we observe bifurcation and hysteresis phenomena in the parameters of the pair when changing the amplitude of modulation. Namely, soliton separation and phase difference between the solitons may take two or more values for the same modulation amplitude. In the case of gain-loss modulation, two solitons may split and be positioned in the two equilibrium states of the periodic potential. The complicated dynamics of this process is illustrated with numerical examples.

Four-photon mixing in a low-birefringence fiber is strongly influenced by the orientation of the pump and signal waves with respect to the fiber axes. We experimentally investigated the dependence of the modulational gain spectra on pump power and polarization by mixing orthogonal pump and probe light beams in a birefringent optical fiber. With a pump on the fast fiber axis, a cascade of sidebands was generated in the regime of normal fiber dispersion. These sidebands are shown to correspond to 0.2–0.3-THz trains of pulses with complex polarization profiles. The analysis reveals that, at particular values of the input pump and probe powers and signal frequency detuning, trains of dark-solitonlike pulses can be generated on the axis orthogonal to the pump.

The Gordon–Haus time jitter and the soliton–soliton interactions in a soliton fiber loop memory or in a transmission link may be suppressed by the periodic injection of a weak cw control beam. In the case of a fiber ring cavity cw injection may also permit clock regeneration from a randomly modulated soliton train.

We characterize the soliton-train emission from an Er–Yb-doped fiber loop laser. We discuss the self-starting dynamics and pulse-repetition-rate control in this sliding-frequency soliton laser. We show that the laser truly self-starts after only one cavity round trip. In the steady state the laser emits a closely spaced train of solitons. We also show that the output pulse width may be controlled by the interplay of continuous frequency shifting, bandwidth-limited amplification, and nonlinear polarization rotation of the circulating solitons. The repetition rate is fixed by means of a weak intracavity feedback. The laser is tunable by shifting of the filter wavelength through the whole spectral band of the active fiber.

We study the role of polarization in modulational instabilities in a synchronously pumped ring resonator that is filled with an isotropic nonlinear dispersive medium. To describe nonlinear propagation of the polarized field through the ring, we introduce two coupled driven and damped nonlinear Schrodinger equations. These equations, which result from averaging propagation and boundary conditions over each circulation through the ring, permit a simple stability analysis. This analysis predicts polarization multistability in the steady state as well as the emergence of stable pulse trains whose polarization state is either parallel or orthogonal to a linearly polarized synchronous pump beam. The analytical predictions are confirmed and extended by numerical simulations of polarized wave propagation in the cavity.

Whenever a bright soliton is superimposed onto a much longer pulse or a continuous wave at the input of an optical fiber, its nonlinear propagation in the anomalous dispersion regime of the fiber critically depends on the relative phase at input of the two pulses. Whenever the relative phase between the pulses is in the range [−π/2, π/2], a stable solitonlike waveform is formed. Otherwise the soliton pulse rapidly decays into dispersive waves. We describe the potential of this effect for coherent-phase detection of solitons, optical-pulse compression, and all-optical switching. We also discuss the influence on the propagation of solitons plus a background of perturbations such as modulational instability and the Raman self-scattering effect.

We predict the formation and the resonant coupling of a new type of optical solitary wave that swings between orthogonal polarizations in a periodically twisted birefringent fiber filter. We point out the potential of these solitons for all-optical switching applications. In particular, power-dependent tuning of the Bragg resonance condition may lead to optically controlled wavelength demultiplexing of whole soliton pulses. The Raman effect may inhibit the polarization coupling for femtosecond pulses. However, we find that the twisting solitons may be still be formed for input-pulse widths as short as 20 fsec.

The operation of a new all-optical switch potentially capable of ultrafast exchange of information between two orthogonally polarized light beams with identical or slightly different wavelength and comparable intensity are described in terms of two interacting nonlinear couplers. We consider the four lowest-order modes (including polarization) of a dual-core optical-fiber nonlinear coupler. We show how the output power ratio between two modes with identical linear polarization propagating in parallel cores can be controlled in an incoherent manner by the presence of another orthogonally polarized light beam by virtue of the optical Kerr effect. An interesting new feature is that, owing to the spatial instability of the coupled-mode equations, when the fiber is sufficiently long (e.g., two linear coupling lengths), a relatively weak pulse is capable of completely switching out a stronger (ideally square) orthogonally polarized pulse. We find that nonlinear coupling may lead to spatial stochasticity and chaotic power-dependent switching whenever the input conditions are given in the vicinity of the normal (or array) modes of the coupler. Conversely, when the modes of the individual cores are excited, the solutions turn out to be regular for almost any input power level.

We derive an exact solution for both amplitude and phase of a signal that is back-reflected by collinear nonlinear degenerate mixing of four circularly polarized waves of arbitrary intensity in a Kerr-like medium. Our analysis suggests that taking pump depletion into account may spoil the ideal phase-conjugation condition, even in the limit of weak nonlinear coupling, and indicates that substantial deterioration of the probe phase-reversal property may occur for strongly interacting waves in a manner that is highly sensitive to the anisotropy of the nonlinear susceptibility.