Journal of Nonlinear Optical Physics & Materials

An all-normal dispersion photonic crystal fiber with nearly zero flattened dispersion at 1550 nm is designed for generating high coherent broadband supercontinuum. It is found that an all normal dispersion photonic crystal fiber with nearly zero flattened dispersion at 1550 nm can be obtained by appropriately designing geometrical parameters and optimizing the index of the first ring of air-holes with filling different index liquid of the photonic crystal fiber. The results show that the optimized design photonic crystal fiber for pumping at 1550 nm is suitable for flat broadband and high coherent supercontinuum generation with only 4 kW input peak power in a 40 cm long of the photonic crystal fiber. It is also found the weaker the dispersion effect is, the more advantage to the high coherent broadband supercontinuum generation due to the self-phase modulation.

In this paper, the channel drop filter based on two dimensional photonic crystal is proposed. The structure is made of silicon rods with the refractive index n₁ = 3.4641 which are perforated in air with refractive index n₂ = 1. The simulation results are obtained using 2D finite difference time domain (FDTD) method. The photonic band gap is calculated by plane wave expansion solver method. Resonant mode of the ring resonator and the filter transmission spectrum is calculated using 2D FDTD method. Full width half maximum (FWHM) bandwidth of the filter at the output transmission spectrum from 1.508 μm to 1.512 μm is 4 nm. The quality factor of the filter is 377.5 and the proposed filter design is around 21 × 15 μm which is suitable for photonic integrated circuits.

Three different structures of chiral side-chain azobenzene polyurethane (CAPU-1), chiral graft azobenzene polyurethane (CAPU-2) and chiral bisazo polyurethane (CAPU-3) were prepared by diazo coupling reaction. The structures, thermal, mechanical properties, refractive index (n) and transmission loss of CAPU-1~3 were investigated. The Y-branch and Mach–Zehnder polymeric thermo-optic switches were simulated based on the synthesized three chiral azo polyurethanes as waveguide materials. The response time differences are owing to the different substituents and structure of the azo compound. The conclusion had potential significance to improve and develop the new digital optical and thermo-optic switch (TOS) with short response time and low driving power.

We introduced a coupled waveguide arrays with intrinsic Parity-Time (PT)-symmetric nonlinearity. The system is described by a set of coupled Schrödinger equations with linear couplings and a complex nonlinearity. Two-component soliton modes are found to exist in this system. The stability of the solitons are investigated and the stable map is plotted numerically in the parameter plane of linear coupling and the strength of PT nonlinearity. The effects of the PT nonlinearity on the amplitude and the effective area of the soliton modes are studied. It is found that the PT nonlinearity can manipulate the amplitude and the propagation constant of the soliton modes. The mobility of these soliton modes are studied numerically. The soliton modes can be kicked to move and increase the PT nonlinearity will enhance the mobility. Collision of two soliton modes are investigated and the results indicates that we can control the types and properties of the collision by adjusting the linear coupling and the PT nonlinearity.

The D4σ-Z-scan technique is used to evaluate the sign and the magnitude of the nonlinear (NL) refractive index and the NL absorption (NLA) coefficient with a laser delivering single pulses in the picosecond regime at 355 nm, 532 nm and 1064 nm. The NL optical response of the zinc porphyrins (Pph) has been found to be significantly enhanced. The study of the NLA and refraction is performed, taking into account the time dependence and the spectral width of the incident laser. The variations of the NL coefficients as a function of the intensity and the concentration are provided and are found to be linear.

We experimentally study the pressure dependent high-order harmonic generation in a semi-infinite gas cell with different absorbing gaseous media in order to reveal the influence of absorption in the high harmonic generation (HHG) process. Theoretical fits to the experimental results allow us to measure the absorption cross-section of the harmonics and to investigate the pressure independent phase mismatches. Our findings provide a better understanding about HHG in absorbing media and show that a sharp and intense harmonic spectrum can be generated as phase-matched and absorption-limited in a long interaction length geometry.

Frequency-resolved thermal lensing (FRTL) is an approach used to measure thermal diffusivity coefficients of liquid samples. The theoretical model of this approach is based on thermal waves and the Fresnel diffraction approximation. This model predicts that only two experimental parameters are needed to estimate the thermal diffusivity of the sample. Experiments performed on pure solvents validate the theoretical predictions and confirm the feasibility of using this approach to thermally characterize liquid samples. The results show that the method is a simple and sensitive approach to determine thermal diffusivity coefficients in liquid samples.

We investigate exact traveling wave solutions of higher order nonlinear Schrödinger equation (NLSE) in the absence of third-order dispersion, which exhibit nontrivial self-phase modulation. It is shown that the corresponding dynamical equation, governing the evolution of intensity in the femtosecond regime, is that of NLSE with a source. The exact localized solutions to this system can have both super and subluminal propagation belonging to two distinct classes. A number of these solitons exhibit chirality, thereby showing preferential propagation behavior determined by group velocity dispersion. Both localized bright and dark solitons are found in complementary velocity and experimental parameter domains, which can exist for anomalous and normal dispersion regimes. It is found that dark solitons in this system propagate with nonzero velocity, unlike their counterpart in nanosecond regime. Interestingly, subluminal propagation is observed for solitons having a nontrivial Padé type intensity profile.

We study the modulation instability (MI) for nonlinear Schrödinger equation phase locked to an external source. We analytically compute the growth rate and analyze the possibility of existence of MI for self-focusing and self-defocusing nonlinearities for positive and negative values of the homogeneous source term coefficient. We further investigate the impact of variation of nonlinear coefficient and variation of source term coefficient on the existence of MI. Phase-locking is the key characteristic that enables us to conduct analytical investigations of MI in this dynamical system, that appertains to pulse propagation through asymmetric twin-core fibers.

We present a numerical study for the design of efficient MgO:PPLN based pulsed nanosecond optical parametric oscillator (OPO), with input pump pulses of top-hat transverse intensity profile, and a double-pass pump cavity configuration. In particular, we investigate the effect of domain shape on the conversion efficiency. We considered two types of domain structures: "perfect vertical-wall" and "wedge-shaped" domains and our numerical results show that at higher pump pulse energies, the conversion efficiency saturates, and the saturated efficiency is almost independent of the domain shapes. Based on our study, we also propose a new scheme for development of compact high-power OPO sources using MgO:PPLN chips of relatively short lengths and large cross-sectional area.

In this work, we present a numerical study to investigate the transmission, extinction ratio, crosstalk and compression factor outputs of a Mach–Zehnder interferometer in photonic crystal fibers by varying the nonlinearity parameter of the fiber from β = 1 to β = 2 for three different nonlinearity profiles (constant, increasing and decreasing) and in three different scenarios (P₀ = Pc, P₀ < Pc and P₀ > Pc). We observed that the highest transmission (T = 67%), the highest extinction ratio (XR = 3.57 dB) and the lowest crosstalk (Xtalk = -5.25 dB) occurred in the third scenario (P₀ = 110 kW > Pc) considering the decreasing profile for β = 1.35. Such condition enables applications such as TDMA and logic circuits.

Optical bistability (OB) for a homogeneously broadened two-level atomic system in a ring cavity is investigated within and without the rotating wave approximation (RWA) using nonautonomous Maxwell–Bloch equations subject to a time delay. It is shown that the dynamics both within and without the RWA are susceptible to the introduction of time delays in the differential equations. A range of instability scenarios are found as certain parameters are ramped up and down and these can affect the bistable operation of the physical devices. However, the introduction of a time delay can also result in an important positive application; for the first time, as far as the authors are aware, it is shown that a type of butterfly hysteresis can occur in the fundamental component, which is a relatively strong output signal, and may be desired in optical signal processing.

Two-pump fiber optical parametric amplifiers (2-P FOPAs) are investigated to obtain a broadband and flat gain simultaneously with a low pump-to-signal noise transfer. The analysis is performed based on the six-wave model to find the optimum pump wavelength separation and also the fiber length to simultaneously maximize the gain and minimize the noise transfer from the pump to the signal. The results of this paper show the potential of 2-P FOPAs for application in fiber optical communication systems; for example, wavelength division multiplexing (WDM).

An experimental work has been done to measure the nonlinear refractive index of poly(2,5-dimethylaniline) (PDMA). In this work, a continuous wave (cw) Ar-ion laser has been used to calculate n₂ for a novel polymer; poly(2,5-dimethylaniline): synthesized from 2,5 dimethylaniline. The single beam Z-scan technique was used with laser beams of 514 and 488 nm wavelengths, taking PDMA in methanol solvent at different concentrations. The order of n₂ was found to be of the order of ~ 10^-14 esu.