Zero Group Velocity Dispersion Research Articles

Compression of Few-Microjoule Femtosecond Pulses in a Hollow-Core Revolver Fiber

Gas-filled hollow-core fibers are a convenient tool for laser pulse compression down to a few-cycle duration. The development of compact, efficient and high quality compression schemes for laser pulses of relatively low μJ-level energies is of particular interest. In this work, temporal pulse compression based on nonlinear spectral broadening in a xenon-filled revolver fiber followed by a chirped mirror system is investigated. A 250 fs pulse at a central wavelength of 1.03 μm is compressed to 13.3 fs when the xenon pressure was tuned to provide zero group velocity dispersion near 1.03 μm. The energies of input and compressed pulses are 3.8 and 2.7 μJ, respectively. The compression quality factor of 1.8 is achieved.

Zero GVD slow-light originating from a strong coupling of one-way modes in double-channel magneto-optical photonic crystal waveguides.

We have studied the coupling effect of topological photonic states in a double-channel magneto-optical photonic crystal waveguide by introducing a two-stranded ordinary Al2O3 photonic crystal as the coupling layer. There exist both M1 (odd) and M2 (even) one-way modes simultaneously in the bandgap. Interestingly, M1 mode is always a fast-light mode with large group velocity (vg) and large group velocity dispersion (GVD) regardless what the radius (RA) of Al2O3 rods is. However, when RA is appropriate, M2 mode becomes a very slow-light mode exhibiting near-zero vg and zero GVD simultaneously. The physical reason of such slow-light is attributed to the strong coupling effect between the one-way edge modes in both sub-waveguides. Furthermore, the simulation results show that the robustness of both the fast- and slow-light modes are extremely strong against perfect electric conductor defect and the one-way transmittance is close to 100%. Besides, the PEC defect can cause significant phase delay. These results hold promise for many fields such as signal processing, optical modulation, and the design of various topological devices.

Broadband quasi-phase-matched optical parametric generation in phase reversal optical superlattice devices

We have theoretically investigated the generation of ultra-flat broadband optical parametric generation (OPG) in the communication band using a lithium niobate based domain engineered phase reversal optical superlattice (PROS) structure by group velocity matching (GVM) technique. The simulation results show an ultra-flat broadband OPG of acceptance FWHM bandwidth Δλ = 210 nm centered at 1.55 μm in 10 mm long device with 19 μm period operating at 20 °C for 775 nm fixed pump wavelength. The selection of the period and pump wavelength relies on the group velocities of the signal and idler wavelengths close to zero-group velocity dispersion (GVD) point to achieve broadband OPG. The PROS quasi-phase matching (QPM) device generates broader and ultra-flat OPG compared to the conventional periodic QPM counterparts. Further, the variation in the flatness of the broadband QPG due to the change in the location and width of the phase reversal (PR) domain in the device is discussed.

Supercontinuum generation in the absence and in the presence of color centers in NaCl and KBr

We study supercontinuum generation with femtosecond mid-infrared laser pulses in NaCl and KBr crystals when pumped at the vicinity of their zero group velocity dispersion points. Almost three octave-spanning SC spectra (covering the 0.7–5.4 μm and 0.85–5.4 μm ranges in 5 mm-thick NaCl and KBr samples, respectively) were produced by filamentation of 70 fs, 3.6 μm input pulses in continuously translated samples. In the static setup, we show that rapid formation of persistent color centers (within a few seconds at 500 Hz repetition rate) inside these materials results in a marked decrease (by 40%) of the overall energy transmittance and in significant narrowing of SC spectra. We also demonstrate that the effect of color centers on the SC spectrum could be adequately simulated numerically using a simple phenomenological model which considers color centers as impurities with a certain energy bandgap.

Role of external focusing geometry in supercontinuum generation in bulk solid-state media

We present a detailed experimental and numerical study of supercontinuum generation in sapphire crystal at the vicinity of its zero group velocity dispersion point (1.3 μm) when pumped by relatively long (210 fs) femtosecond pulses. We uncover very different evolutions of the spectral broadening versus the input pulse energy when the incident beam is focused either onto the input face or inside the 4-mm-thick sapphire sample. In particular, when the input beam was focused inside the crystal, we captured a surprising variation of the spectral width as a function of the input pulse energy, demonstrating supercontinuum generation, its suppression, and eventually, its recovery. The experimental findings were nicely reproduced by the numerical simulations, revealing a specific supercontinuum generation scenario, which relies on strong reshaping of the wave packet due to defocusing and absorption of free electron plasma, subsequent replenishment of the pulse on the propagation axis, and its splitting. We also demonstrate that this particular supercontinuum generation scenario takes place regardless of the external focusing geometry.

Period-Timing Bifurcations in a Dispersion-Managed Fiber Laser With Zero Group Velocity Dispersion

We report on the numerical results of period timing bifurcations in a dispersion-managed (DM) fiber laser with zero group velocity dispersion (GVD).The fiber laser is passively mode-locked by using the nonlinear polarization rotation technique. Period-tripling bifurcation route to chaos is numerically observed apart from the conventional period-doubling bifurcation route to chaos. The small signal gain is the only varying parameter, while all the other cavity parameters are fixed. Different from the conventional period-doubling bifurcation route to chaos where the route begins from order, the period-tripling bifurcation route to chaos is led by a direct route from period one to chaos and then to a period-tripling state. The appearance of nonlinear window of period-tripling suggests that the ultrashort pulses generated from the DM fiber laser with zero GVD have rich nonlinear dynamics.

Experimental GVD engineering in slow light slot photonic crystal waveguides.

The use in silicon photonics of the new optical materials developed in soft matter science (e.g. polymers, liquids) is delicate because their low refractive index weakens the confinement of light and prevents an efficient control of the dispersion properties through the geometry. We experimentally demonstrate that such materials can be incorporated in 700 μm long slot photonic crystal waveguides, and hence can benefit from both slow-light field enhancement effect and slot-induced ultra-small effective areas. Additionally, we show that their dispersion can be engineered from anomalous to normal regions, along with the presence of multiple zero group velocity dispersion (ZGVD) points exhibiting Normalized Delay Bandwidth Product as high as 0.156. The reported results provide experimental evidence for an accurate control of the dispersion properties of fillable periodical slotted structures in silicon photonics, which is of direct interest for on-chip all-optical data treatment using nonlinear optical effects in hybrid-on-silicon technologies.

High-energy, shock-front-assisted resonant radiation in the normal dispersion regime

We present a simple yet effective theory that predicts the existence of resonant radiation bands in the deep normal group velocity dispersion region of a medium, even in absence of a zero-group velocity dispersion point. This radiation is evident when the medium is pumped with high-energy ultrashort pulses, and it is driven by the interplay between the Kerr and the shock terms in the NLSE. Accurate experiments performed in bulk silica fully support the theoretical phase-matching condition found by our theory.

A genetic algorithm based approach to fiber design for high coherence and large bandwidth supercontinuum generation

We present a new approach to the design of optical microstructured fibers that have group velocity dispersion (GVD) and effective nonlinear coefficient (gamma ) tailored for supercontinuum (SC) generation. This hybrid approach combines a genetic algorithm (GA) with pulse propagation modeling, but without include it into the GA loop, to allow the efficient design of fibers that are capable of generating highly coherent and large bandwidth SC in the mid-infrared (Mid-IR) spectrum. To the best of our knowledge, this is the first use of a GA to design fiber for SC generation. We investigate the robustness of these fiber designs to variation in the fiber's structural parameters. The optimized fiber structure based on a type of tellurite glass (70TeO(2) - 10 Na(2)O - 20 ZnF(2)) is predicted to have near-zero group velocity dispersion (< +/-2 ps/nm/km) from 2 to 3 microm, and a effective nonlinear coefficient of gamma approximately 174 W(-1)km(-1) at 2 microm. The SC output of this fiber shows a significant bandwidth and coherence increase compare to a fiber with a single zero group velocity dispersion wavelength at 2 microm.

Soliton effects in photonic crystal fibres at 850 nm

Soliton effects are observed at 850 nm in a pure silica photonic crystal fibre with group velocity dispersion (GVD) characteristics unattainable in conventional fibre. Zero GVD is obtained at 740 nm.