Femtosecond pulse delivery using a chirped long-period grating of multi-mode fiber for mode conversion

A period-chirped long-period fiber grating (CLPFG), using ordinary multi-mode fiber (MMF), for broadband mode conversion (60 nm bandwidth of coupling efficiency >90%) has been proposed. Through using such a MMF-CLPFG, a broad-spectrum pulse can be converted into LP ₀₂ for femtosecond pulse distortion-free delivery in MMF. The simulation shows that the pulse suffers less nonlinear distortion than that in a single-mode fiber. In addition, the negative pre-chirp introduced by MMF-CLPFG can offset the linear and positive SPM-induced frequency chirp and thus compress the pulse. Using this scheme, an initial 18 kW peak power 75 fs pulse distortion-free delivery in MMF has been numerically demonstrated.     

Multiwavelength picosecond and single wavelength femtosecond pulses emission in a passively mode-locked fiber laser using a semiconductor saturable absorber mirror and a contrast ratio tunable comb filter

We propose and demonstrate a highly flexible fiber laser capable of generating stable multiwavelength picosecond and single wavelength femtosecond pulses by using a semiconductor saturable absorber mirror and a contrast ratio tunable comb filter. In the multiwavelength lasing regime, up to 11-wavelength stable mode-locked pulses in 3 dB bandwidth with a channel spacing of 0.8 nm were obtained. While in the single wavelength with broadband spectrum lasing regime, the fiber laser emitted 576 fs soliton pulse. Through changing the contrast ratio of the comb filter, the conversion between the multiwavelength picosecond and single wavelength femtosecond pulsed operations could be efficiently achieved.     

Propagation of solitary waves in inhomogeneous erbium-doped fibers with third-order dispersion,self-steepening and gain/loss

We propose a generalized inhomogeneous Hirota–Maxwell–Bloch system which explains ultrashort optical pulse propagation in an inhomogeneous nonlinear, dispersive fiber doped with two-level resonant atoms. For this system, higher order effects like third-order dispersion and self-steepening are considered, which are assumed to be inhomogeneous together with group velocity dispersion and self-phase modulation. We have obtained a general solitary wave solution by using Lax pair and Bäcklund transformation techniques. We have analyzed various solitary wave forms like snaking solitons, classic solitons and compressed pulses by controlling the physical parameters and inhomogeneous functions.     

Dispersion effects on the interaction of Airy beams and dark solitons

We simulate and analyse an Airy pulse coupled with a dark soliton in a single-mode fibre by solving the non-linear Schrödinger equation (NLSE). Simulations show that the group velocity parameter β₂₂ has a huge impact on the interaction between the Airy pulse and the dark soliton. At some values of β₂₂, a bright soliton arises from the Airy pulse. The intensity of the bright soliton reaches a maximum when β₂₂ takes a certain value. Meanwhile, the transmission distance and the primary energy of the Airy pulse are affected by the different values of β₂₂. However, when the Airy pulse propagates in a linear medium, varying the value of β₂₂ has only a slight impact on the interaction, and no bright soliton detaches from the Airy pulse. In brief, the dispersion effect has a large impact on the interaction of Airy pulses and dark solitons in a non-linear medium; however, dispersion has a lesser impact on interactions in a linear medium.     

Seeded femtosecond optical parametric amplification in the mid-infrared spectral region above 3 mum

Femtosecond optical parametric amplification that results in microjoule mid-infrared pulses at wavelengths exceeding 3 mum is demonstrated. Narrow-band quasi-cw seeding at the signal wavelength is applied to ensure the generation of nearly transform-limited femtosecond pulses at the idler wavelength. The broad bandwidth of the parametric amplification provided by pumping with femtosecond pulses from a Ti:sapphire regenerative amplifier at high intensity results in idler pulse durations shorter than the pump pulse length. The potentials of three nonlinear optical crystals that belong to the potassium titanyl phosphate family are comparatively studied. At 1-kHz repetition rate our all-solid-state system produces highly synchronized ~100-fs pulses in the spectral range between 3 and 4 mum.     

Continuously tunable femtosecond pulses covering 2·1–2·5 μm from an optical parametric oscillator based on RbTiOAsO4

Abstract

The operation of a synchronously pumped femtosecond optical parametric oscillator based on the nonlinear material RbTiOAsO₄ is described. The idler output was continuously tuned from 2·1 to 2·5 μm, the pulses having powers as high as 140 mW and durations as short as 140 fs. Characterization of the pulses was achieved by use of a novel autocorrelator based on an InGaAs photodiode and a LiNbO₃ frequency-doubling crystal, capable of producing high quality interferometric and intensity autocorrelations.     

Hidden possibilities in soliton switching through tunneling in erbium doped birefringence fiber with higher order effects

In this paper, coupled higher order nonlinear Schrödinger–Maxwell–Bloch equations are studied for the ultrashort pulse propagation in erbium-doped birefringent fiber system. Lax pair is constructed for the CHNLS–MB system. Through the Darboux method and symbolic computation, analytic two-soliton solution is obtained. To analyze the specific problem, we design an optimal system by properly choosing the special form of variable coefficients. Here, using obtained two-soliton solution, tunneling of femtosecond soliton through barrier and well is investigated. While solitons are passing through barrier and well, some unexpected behavior is observed and hidden possibilities in switching action through tunneling is investigated.     

Third-order nonlinear optical properties of 2-adamantylamino-5-nitropyridine caused by cascaded second-order nonlinearity

2-Adamantylamino-5-nitropyridine (AANP) was reported to have large nonlinear optical properties d₃₁ of 80 pm/V and have an ability of type II phase-matching by using d₁₅. The χ⁽²⁾:χ⁽²⁾ cascading effect via second-order optical nonlinearity produces the same effect of the third-order optical nonlinearity in phase-matching condition. We investigated the third-order nonlinear optical properties via cascaded second-order optical nonlinearity of AANP crystal at telecommunication wavelength range.     

Dispersion-managed soliton propagation in optic fibers with random dispersion

Propagation of dispersion-managed solitons in optic fibers with randomly distributed dispersion is studied. It is shown that the effect of the dispersion fluctuations can be described within the framework of a modified nonlinear Schrödinger equation with a frequency-dependent damping term (～ω⁴). The presence of randomly modulated dispersion leads to the damping of optical pulses. The condition for stable pulse propagation is determined based on the corresponding variational equations.     

Superradiance and lasing properties of new two-photon absorption dye DEASPS

Pumped by picosecond pulses from a Nd:YAG laser, a new lasing dye, trans-4-[4′-(N,N-diethylamino)styryl]-N-methyl pyridinium methyl sulfate (abbreviated to DEASPS), shows both intense superradiance and strong lasing properties in benzyl alcohol solution. By using streak camera systems, the superradiance and lasing can be distinguished both spectrally and temporally. It has been found that the peak wavelength of lasing is at 620 nm with a red-shift of about 12 nm to the superradiance wavelength. The lasing pulse shows an oscillatory effect that it is not found in the superradiance pulse. The fluorescence lifetime is 529 ± 40 ps and the effective molecular two-photon absorption is (1.25 ± 0.1) × 10^?48 cm⁴ ·s·photon ^?1, measured using a nonlinear transmittance method. This dye shows effective optical limiting of the pumping wavelength.     

Picosecond measurement of the nonlinear refractive index of new salts of carboxylate anions with chiral ammonium cations

The third order nonlinear optical properties of novel carboxylate anions with chiral amines and ammonium cations have been studied by a new alternative of the classical z-scan technique with picosecond laser pulses (pulse duration of 30 ps at 1064 nm wavelength). The materials do not present linear and nonlinear (two-photon) absorptions at the wavelength and the intensities used for the experiments. The non-resonant molecular second order hyperpolarizability γ obtained can reach 4.4 × 10⁻⁴⁸ SI. These values are two and three orders of magnitude larger than those CS₂ and of C₆H₆ respectively.     

Investigation of gas-dynamic processes in optical discharges with an ablating polymer wall in air and vacuum conditions

Experimental research results are presented on the formation dynamics and macrostructure of optical discharges of condensed matter of a polymer series ((C₂F₄) _n , (CH₂O) _n ) under the action of a femtosecond laser (τ _0.5 ∼ 45–70 fs) pulses (I ₀ ∼ 10¹³−10¹⁵ W/cm²) in the UV — NIR spectral region (λ ∼ 266, 400, 800 nm) under air and vacuum conditions. Electron density distributions in the near-surface area of the optical discharge, vapor expansion, and velocities of shock-wave front propagation are determined for the first time by precise laser pulse micro-interferometry with high spatial and time resolution. The correspondence is shown of the values of the laser ablation spectral-energy threshold, as determined by interference microscopy and the interferometry of a gas-plasma flow. An estimation technique for the total momentum of light-erosion gas-plasma flow in the sub-nanonewton range is proposed and implemented for the first time. The results of comparative analysis are presented on the laser radiation conversion efficiency at different stages of femtosecond optical discharges.     

Dynamics of solitons in optical fibres

Abstract

A multiple-scale perturbation method is developed to study the optical solitons described by a perturbed nonlinear Schrödinger equation. We show that, by properly defining the phase of the soliton pulse, we can obtain corrections to the pulse where a standard soliton perturbation approach fails. A comparison is made with results obtained by other methods as well as with numerical simulations.     

Influence of spectral broadening on femtosecond wavelength conversion based on four-wave mixing in silicon waveguides

Femtosecond wavelength conversion in the telecommunication bands via four-wave mixing in a 1.5 mm long silicon rib waveguide is theoretically investigated. Compared with picosecond pulses, the spectra are greatly broadened for the femtosecond pulses due to self-phase modulation and cross-phase modulation in the four-wave mixing process, and it is difficult to achieve a wavelength converter when the pump and signal pulse widths are close to or less than 100 fs in the telecommunication bands because of the spectral overlap. The influence of the spectral broadening on the conversion efficiency is also investigated. The conversion bandwidth of 220 nm and peak conversion efficiency of -8 dB are demonstrated by using 500 fs pulses with higher efficiency than the picosecond pulse-pumped efficiency when the repetition rate is 100 GHz.     

Glass formation and third-order optical nonlinear characteristics of bismuthate glasses within Bi2O3–GeO2–TiO2 pseudo-ternary system

Glass-forming region of Bi₂O₃–GeO₂–TiO₂ (BGT) pseudo-ternary system was determined by using melt-quench method. A series of high transparent glass samples were selected and their structural characteristics were investigated by FT-IR and Raman spectra. By employing Z-scan and optical Kerr shutter techniques with femtosecond laser pulses as excitation source, third-order optical nonlinearities (TON) of the BGT glasses as well as the TON response time were investigated at wavelength of 800 nm. The ultrafast nonlinear response and high figure of merit suggest great potentials of BGT glasses in applications of all-optical switching or related optical devices.     

Periodic and solitary wave solutions for ultrashort pulses in negative-index materials

Abstract

We present a detailed analysis for the existence of dark and bright solitary waves as also fractional-transform solutions in a nonlinear Schrödinger equation model for competing cubic–quintic and higher-order nonlinearities with dispersive permittivity and permeability. Parameter domains are delineated in which these ultrashort optical pulses exist in negative-index materials (NIMs). For example, dark solitons exist for the case of normal second-order dispersion, anomalous third-order dispersion, self-focusing Kerr nonlinearity, and non-Kerr nonlinearities, while the bright solitons exist for the case of anomalous second-order dispersion, normal third-order dispersion, self-focusing Kerr nonlinearity, and non-Kerr nonlinearities. This is contrary to the situation in ordinary materials.     

Temporal solitons in second-harmonic generation with a noncollinear phase-mismatching scheme

A noncollinear second-harmonic-generation scheme that includes two gratings and a nonlinear optical crystal generates temporal solitons with a noncollinear phase mismatch and frequency-chirped laser pulses. At 180-fs pulse duration, 25-GW/cm2 fundamental intensity, -7647.3-m(-1) wave-vector mismatch, 66-fs delay time, and +/-3.07163 x 10(25) s(-2) frequency-chirp rates, temporal solitons with durations from 139 to 155 fs and Gaussian shapes can be obtained. The corresponding conversion efficiency is greater than 40%.     

Parallel transmission of solitons without interactions

Soliton interactions affect the pulse quality and capacity of communication systems. In this paper, parallel transmission of solitons without interactions are investigated to improve the pulse quality and capacity. Analytic two-soliton solutions for the variable coefficient nonlinear Schrödinger equation are obtained.The exponential profile of optical fibers is suggested to derive the parallel transmission of solitons, and influences of corresponding parameters are analyzed.     

Ultrafast laser processing: New options for three-dimensional photonic structures

Abstract

Using tightly focused ultrashort laser pulses allows the direct writing of three-dimensional photonic structures in different glasses and also crystalline media. One of the main drawbacks of this technology is, however, the limited writing speed achieved so far. In this paper we shall review our recent advances in the direct writing of three-dimensional integrated-optical devices and discuss a new approach using a fibre-based femtosecond laser system producing 300 fs pulses with pulse energies of 0.6 μJ at 2 MHz repetition rate. Using this laser system we fabricated low-loss waveguides (less than 0.5 dBcm^?1) at writing speeds of 100 mms^?1 for the first time. The influence of the writing speed on the produced structures as well as their optical properties will be discussed in detail.     

Cr-doped II–VI materials for lasers and nonlinear optics

The paper reviews material, spectroscopic, laser and nonlinear optical properties of wide-band Cr²⁺-doped II–VI materials. The strong revival of research interest in these materials is explained by the announcement of the extremely efficient (up to >70% slope efficiency) laser operation at room-temperature. With up to 11 W of average output power, one can achieve super broad tunability (up to 1100 nm between 2 and 3.1 μm) in narrow-line continuous-wave operation and 4 ps pulses at 400 mW in mode-locked regime. Directly diode-pumping and lasers based on ceramic active media have been demonstrated, allowing development of cost-efficient compact tunable and mode-locked lasers, with a possibility to generate few-optical cycle pulses. In this wavelength range the Cr²⁺-doped lasers proved to be viable competitors to the conventional semiconductor lasers or more complex laser systems, based on frequency conversion techniques in such applications as medicine, trace gas monitoring, remote sensing, spectroscopy, metrology, optical radars, optical communications, and all-optical switching. In contrast to conventional dielectric laser materials the Cr²⁺-doped II–VI compounds combine properties of semiconductors with that of the traditionally used dielectric active media. The semiconductor nature determines strong nonlinear optical response of these materials, giving rise to charge transport and photorefractive-like phenomena, harmonic generation and parametric processes, and self-focusing effects of various origins. This calls for a considerable modification of the mode-locking techniques and reconsideration of the existing theories, which should finally enable generation of few-optical cycle pulses directly from the laser oscillator in the mid-infrared. In this connection a number of important new aspects are being discussed, such as contribution of cascaded second-order nonlinearity and Raman processes to the third-order nonlinearity, its dispersion and anisotropy, and others.