Nonlinear Pulse Distortion Research Articles

Nonlinear Hysteretic Torsional Waves.

We theoretically study and experimentally report the propagation of nonlinear hysteretic torsional pulses in a vertical granular chain made of cm-scale, self-hanged magnetic beads. As predicted by contact mechanics, the torsional coupling between two beads is found to be nonlinear hysteretic. This results in a nonlinear pulse distortion essentially different from the distortion predicted by classical nonlinearities and in a complex dynamic response depending on the history of the wave particle angular velocity. Both are consistent with the predictions of purely hysteretic nonlinear elasticity and the Preisach-Mayergoyz hysteresis model, providing the opportunity to study the phenomenon of nonlinear dynamic hysteresis in the absence of other types of material nonlinearities. The proposed configuration reveals a plethora of interesting phenomena including giant amplitude-dependent attenuation, short-term memory, as well as dispersive properties. Thus, it could find interesting applications in nonlinear wave control devices such as strong amplitude-dependent filters.

Nonlinear light-matter interaction with femtosecond high-angle Bessel beams

We show that high-angle Bessel beams may significantly reduce nonlinear pulse distortions due, for example, to nonlinear Kerr effects (self-phase-modulation and self-focusing) yet enhance ionization and plasma generation. Holographic reconstruction of Bessel beams in water show intensity clamping at increased intensities and evidence of nontrivial plasma dynamics as the input energy is increased. The solvated electron density increases significantly and the cavitation-induced bubbles are ejected from the focal region indicating a significant excess plasma heating in the Bessel-pulse wake.

2.03] Development of a two-photon endoscope

Objective An endoscopic probe is proposed for in-vivo high-resolution, two-photon imaging. This probe, based on a flexible fiber bundle combined with a miniaturized high numerical aperture objective, allows 3D-imaging of superficial tissue and therefore may be an important tool for optical biopsy. The optical sectioning strength was characterized by the axial resolution. The two-photon microendoscope may offer a favorable imaging depth within scattering tissue compared to the confocal approach. However, a disadvantage of the two-photon endoscopic application is the lower fluorescence signal due to linear and non-linear pulse distortion. Simulations and experiments were carried out to study these effects. Material and methods Two femtosecond laser sources, a Ti:sapphire laser system (Coherent, 80 MHz, 800 nm, 120 fs) and a fiber-based system (Toptica, 90 MHz, 1090 nm, 20 fs) were used in combination with various fiber bundles (Sumitomo Electrics Industries). Two miniaturized objectives were examined, a GRIN-lens system (GRINTECH) and a custom made system (Storz), with a numerical aperture of 0.75 and 0.6, respectively. The fluorescent dyes, fluorescein and cresyl violet acetate, were used on sample materials and excised bladder wall tissue. Simulations of short pulse propagation in the fiber materials were done with LAB2-A. Results First measurements with our two-photon microendoscope showed that it is possible to detect fluorescence signals without compensating for linear and non-linear pulse broadening. The outer diameter is already compatible with the working channel of a conventional cystoscope. Furthermore, the optical performance showed a good image quality at high axial resolution. Consequently, intracellular details could be resolved from bulky tissue with high-speed performance. For future optimizations, the simulations indicate an effective decrease of linear and nonlinear pulse distortions by mainly quadratic pre-chirping, which may be implemented for example, using a simple grating compressor. Conclusion The current microendoscope for two-photon imaging shows the potential to resolve intracellular details and can be integrated into a commercial endoscope. Two-photon endoscopy may be the possible next step to effectively overcome the limitations of confocal microendoscopy. In particular, the higher penetration depth may contribute to a broader acceptance for in-vivo tissue diagnostics.

Picosecond pulse amplification in a core-pumped large-mode-area erbium fiber

Amplification in a single-clad, large-mode-area erbium fiber as an alternative to double-clad Er-Yb amplifiers is presented. Both signal and pump are coupled through a mode-matched splice into the fundamental mode, which ensures preferential gain in the fundamental mode while minimizing the amplified spontaneous emission (ASE). The 875 microm(2) effective area of the Er fiber enables amplification of 6 ps pulses at 1.55 microm wavelength by approximately 33 dB in a single stage to >25 kW peak power with low nonlinear pulse distortion and a diffraction-limited output beam with M(2)<1.1.

Confined propagation and near single-mode laser oscillation in a gain-guided, index antiguided optical fiber

The authors report laser oscillation in what appears to be a single transverse mode with very large mode area in optical fibers having heavily Nd-doped 100μm diameter cores with refractive index significantly lower in the core than in the surrounding cladding. Since fibers of this type cannot support conventional index-guided modes, their results appear to confirm a recent analysis which predicts gain-guided single-mode propagation in index antiguided fibers, provided the gain coefficient in the core exceeds a threshold value. Fibers of this type may be of significant interest for amplifiers and oscillators having large power outputs and/or small nonlinear pulse distortion.

SIGNAL PROCESSING IN PHOTONIC CRYSTALS AND NANOSTRUCTURES

Optical devices employing photonic crystals and novel nanostructure materials may exhibit useful properties for applications to all-optical signal processing. In this work we analyze as a first example four-wave mixing of polarized beams in photonic crystal fibers. We show that by properly tuning the pump wavelength and the linear dispersion properties of the fiber one may obtain broadband parametric amplification and frequency conversion. Next we consider the in-line periodic amplification of short optical pulses by means of quantum-dot semiconductor optical amplifiers. We show by numerical simulations that pattern-free amplification of a 40 Gbit/s soliton signal at 1300 nm is possible without any inter-symbol interference or nonlinear pulse distortion caused by the fast gain dynamics.

Measurement Interpretation and Uncertainty Resulting from Nonlinear Ultrasonic Wave Propagation

Nominally identical ceramic-element ultrasonic transducers, of the type used in the aerospace and metals industries for nondestructive evaluation (NDE), often have variability in the peak pressure of the generated ultrasonic pulses. The generated pressures are also high enough for nonlinear propagation to be present in the water medium, often used as coupling in these types of inspections. In this study, a measurement system for monitoring ultrasonic pulse pressure level and nonlinear propagation in water is presented. Several different industrial NDE measurement applications are examined to quantify the impact of variable pressure and nonlinear propagation with respect to test interpretation and measurement uncertainty. In particular, pulse frequency content, velocity measurements, reference block calibrations, and beam-width measurements are examined. In addition, an experimental demonstration of why nonlinear pulse distortion is not normally observed in practice is presented.

Measurement Interpretation and Uncertainty Resulting from Nonlinear Ultrasonic Wave Propagation

Nominally identical ceramic-element ultrasonic transducers, of the type used in the aerospace and metals industries for nondestructive evaluation (NDE), often have variability in the peak pressure of the generated ultrasonic pulses. The generated pressures are also high enough for nonlinear propagation to be present in the water medium, often used as coupling in these types of inspections. In this study, a measurement system for monitoring ultrasonic pulse pressure level and nonlinear propagation in water is presented. Several different industrial NDE measurement applications are examined to quantify the impact of variable pressure and nonlinear propagation with respect to test interpretation and measurement uncertainty. In particular, pulse frequency content, velocity measurements, reference block calibrations, and beam-width measurements are examined. In addition, an experimental demonstration of why nonlinear pulse distortion is not normally observed in practice is presented.

Dynamic range and switching speed limitations of an N×N optical packet switch based on low-gain semiconductor optical amplifiers

Two important system performance limitations-dynamic range and switching speed-of an integrated packet switch fabric based on low-gain semiconductor optical amplifiers (SOA's) have been examined by using cascaded blocks of an SOA model, which includes transient effect, nonlinear pulse distortion effect, and amplified spontaneous emission (ASE) noise. Low-gain SOA's were used to minimize ASE noise considering that no optical filters can be integrated in an SOA-based switch fabric. The system performance with and without a narrowband optical filter at the receiver were both studied. By assuming fixed-wavelength transmitters and no optical filter can be used at the receiving end owing to the unpredictability of arriving packet wavelengths, our simulation results indicate that the dynamic ranges of 4/spl times/4 and 8/spl times/8 SOA-based packet switches at 2.5 Gb/s can only be about 3.2 and 0.8 dB, respectively. However, at 155 Mb/s, even without a receiving-end optical filter, the dynamic range of each switch size can be increased by more than 17 dB as compared to the cases of 2.5 Gb/s. Note that the dynamic ranges were estimated under the conditions of a bit error rate (BER) /spl les/10/sup -9/ and a pulse distortion ratio /spl les/30%. We have also shown that, when an optical filter with a 1 nm bandwidth was used at the receiving end to simulate (1) a circuit-switched condition where the center wavelength of the filter can be adjusted according to the established circuit, or (2) a packet-switched condition where each receiver has a wavelength demultiplexer and a detector array, the dynamic range of 4/spl times/4 and 8/spl times/8 switches can be increased to 16.3 and 14 dB, respectively, at 2.5 Gb/s.

Nonlinear pulse distortion at the zero dispersion wavelength of an optical fibre

Pulse propagation at the zero dispersion wavelength of a standard optical fibre is investigated experimentally and theoretically. The respective results are in remarkably good agreement and demonstrate that the combined influence of third order dispersion and fibre nonlinearity leads to severe distortions of the pulse temporal and spectral profiles. Using a straightforward physical model an analytical expression is derived which allows to calculate the critical experimental parameters for which significant nonlinear pulse distortion occurs.

Nonlinear pulse distortion in single-mode optical fibers at the zero-dispersion wavelength.

The propagation of optical pulses is considered at the zero-dispersion wavelength of nonlinear dispersive fibers. Even in the absence of group-velocity (first-order) dispersion, higher-order dispersive effects in single-mode silica fibers are found to be strong enough to cause significant broadening and distortion of picosecond optical pulses for fiber lengths of 10--100 km. Using the parameters appropriate for a 1.55-\ensuremath{\mu}m dispersion-shifted single-mode fiber, we have studied the evolution of pulse shapes and pulse spectra along the fiber length for a wide range of initial pulse widths. For peak powers \ensuremath{\sim}10 mW, the dispersive and nonlinear effects are comparable for pulse widths \ensuremath{\sim}1 ps and their mutual interplay leads to new qualitative features in the pulse shape and spectrum that are largely independent of the input profile. The theoretical results are useful for an understanding of the higher-order dispersion and, at the same time, have implications for high-capacity, long-haul, optical communication systems.