Simulation of integrated coupled nonlinear microring resonators all-optical pulse restorer

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We propose a simple way to design a fully integrated and passive all-optical pulse reshaper in the aim of reamplifying and reshaping (2R) regeneration of optical digital signals. The device could be achieved from a coupled microresonator structure with Kerr nonlinearity. By using time-domain simulation, we show that we can obtain a clear nonlinear power transfer that is capable of improving signal-to-noise ratio and reducing bit error rate for digital signals. The reshaping performances are estimated by calculating <formula formulatype="inline"><tex>$Q$</tex> </formula>-factor enhancement. Since we take benefit from field enhancement at resonance, this integrated reshaper could be much smaller than other gates based on nonlinear fibers or waveguides. </para>

A microring resonator with a narrow gap between a waveguide and a microring resonator has been fabricated by using nanoimprint lithography. An organic spin-on-glass has been used as the material for the microring resonator. The outer diameter of the microring resonator is 20 µm and the coupling gap between the straight waveguide and the microring resonator is 40 nm. The filtering characteristics of the microring resonator have been evaluated using a metal halide lamp. The free spectral range is 3.9 nm and the Q-factor is 310. The structure of the microring resonator with a narrow gap can be used as a fundamental component of optical devices.

Recently there has been much interest in designing optical tapers for nonlinear pulse manipulation in both standard[1] and semiconductor filled fibres[2]. Such tapers can be used to overcome the large linear losses in semiconductor filled fibres while allowing parabolic pulse generation or soliton propagation for example. However once the taper profile has been chosen to overcome the linear loss problem in semiconductor filled fibres the problem of two photon absorption (TPA) and associated free carrier effects remain. In this work we show that it is possible to use a genetic algorithm to design tapers that alleviate both linear and nonlinear losses and hence should allow high quality pulse propagation over several centimetres of semiconductor filled fibres.

Layered 2D graphene oxide (GO) films are integrated with micro-ring resonators (MRRs) to experimentally demonstrate enhanced nonlinear optics. Both uniformly coated (1-5 layers) and patterned (10-50 layers) GO films are integrated on complementary-metal-oxide-semiconductor (CMOS)-compatible doped silica MRRs using a large-area, transfer-free, layer-by-layer GO coating method with precise control of the film thickness. The patterned devices further employ photolithography and lift-off processes to enable precise control of the film placement and coating length. Four-wave-mixing (FWM) measurements for different pump powers and resonant wavelengths show a significant improvement in efficiency of ≈7.6 dB for a uniformly coated device with 1 GO layer and ≈10.3 dB for a patterned device with 50 GO layers. The measurements agree well with theory, with the enhancement in FWM efficiency resulting from the high Kerr nonlinearity and low loss of the GO films combined with the strong light-matter interaction within the MRRs. The dependence of GO's third-order nonlinearity on layer number and pump power is also extracted from the FWM measurements, revealing interesting physical insights about the evolution of the GO films from 2D monolayers to quasi bulk-like behavior. These results confirm the high nonlinear optical performance of integrated photonic resonators incorporated with 2D layered GO films.

This paper addresses the problem of Bayesian wavelet estimating the mean vector of multivariate normal distribution under a multivariate normal prior distribution based on linear and nonlinear exponential balanced loss functions. The covariance matrix of multivariate normal distribution is considered known. Bayes estimators of mean vector parameter of multivariate normal distribution are achieved. Then two soft shrinkage wavelet threshold estimators based on Stein’s unbiased risk estimate (SURE) and Bayes estimators are provided. Finally, the performance of the soft shrinkage wavelet estimators was checked through simulation study and Electrical Grid Stability Simulated data set. Simulation and real data results showed the better performance of SURE thresholds based on linear and nonlinear exponential balanced loss functions compared to other classical wavelet methods. Also, they showed better performance for SURE threshold based on nonlinear exponential balanced loss function in multivariate normal distribution with small dimensions.

Investigating the effects of structural parameters on the optical characteristics of add-drop filters

A simple all-optical regeneration technique is described. The regenerator suppresses the noise in zeros and the amplitude fluctuations in ones of return-to-zero optical data streams. Numerical simulations and experimental results are presented.

Micro-resonators (MR) have become a key element for integrated optical sensors due to their integration capability and their easy fabrication with low cost polymer materials. Nowadays, there is a growing need on MRs as highly sensitive and selective functions especially in the areas of food and health. The context of this work is to implement and study integrated micro-ring resonators devoted to sensing applications. They are fabricated by processing SU8 polymer as core layer and PMATRIFE polymer as lower cladding layer. The refractive index of the polymers and of the waveguide structure as a function of the wavelength is presented. Using these results, a theoretical study of the coupling between ring and straight waveguides has been undertaken in order to define the MR design. Sub-micronic gaps of 0.5 μm to 1 μm between the ring and the straight waveguides have been successfully achieved with UV (i-lines) photolithography. Different superstrates such as air, water, and aqueous solutions with glucose at different concentrations have been studied. First results show a good normalized transmission contrast of 0.98, a resonator quality factor around 1.5 × 104 corresponding to a coupling ratio of 14.7%, and ring propagation losses around 5 dB/cm. Preliminary sensing experiments have been performed for different concentrations of glucose; a sensitivity of 115 ± 8 nm/RIU at 1550 nm has been obtained with this couple of polymers.

A photonic approach for generating low frequency drifting noise, arbitrary and large frequency chirping rate (FCR) optical pulses based on the Kerr effect in the nonlinear optical fiber is theoretically analyzed and experimentally demonstrated. Due to the Kerr effect-induced sinusoidal phase modulation in the nonlinear fiber, high order Kerr pulse with a large chirping rate is generated. In the concept-proof experiments, the FCR of the mth Kerr pulse has been significantly improved by a factor of 2m+1. In addition, dynamic strain measurement along with a random fiber grating array (RFGA) sensor by using different order Kerr pulse is carried out for demonstrating a large strain measurement range with lower uncertainty sensing capability. Benefiting from the use of a single laser source and large FCR Kerr pulse, the system exhibits a 3.9 µɛ static strain measurable range, 0.24 µɛ measurement uncertainty by using -4th order Kerr pulse that has an FCR up to 0.8 GHz/ns. Note that the FCR of the chirped pulse could be further enhanced by using larger FCR chirped pulse seed or choosing higher order Kerr pulses.

A refractive index (RI) sensor based on straight silicon strip waveguide and a hybrid plasmonic microring resonator is presented. The hybrid waveguide is made up of a Metal-Air-Silicon structure, where the electric field is significantly enhanced in the narrow air gap. The transmission spectrum and electric field distribution of this sensor structure are simulated using Finite Element Method (FEM). Due to a large overlap of the optical field in the hybrid microring resonator and the sensing medium, this sensor can be used for both gas and bio sensing applications. The best sensitivity of 690 nm/RIU and 401 nm/RIU is obtained for gas and bio sensing, respectively. The obtained values are relatively high as compared to conventional dielectric microring resonator sensors. Our proposed sensor design has a potential to find further applications in biomedical science and nanophotonic circuits.

Photonic crystal and split ring resonator (SRR) metamaterial waveguides with Kerr nonlinear dielectric impurities are studied. The transmission coefficients for two guided modes of different frequencies scattering from the Kerr impurities are computed. The systems are shown to exhibit multiple transmission coefficient solutions arising from the Kerr nonlinearity. Multiple transmission coefficients occur when different input intensities into a waveguide result in the same transmitted output intensities past its nonlinear impurities. (In the case of a single incident guided mode the multiplicity of transmission coefficients is known as optical bistability.) The analytical conditions under which the transmission coefficients are single and multiple valued are determined, and specific examples of both single and multiple valued transmission coefficient scattering are presented. Both photonic crystal and split ring resonator systems are studied as the Kerr nonlinearity enters the photonic crystal and SRR systems in different ways. This allows for an interesting comparison of the differences in behaviors of these two types of system which are described by distinctly different mathematical structures. Both the photonic crystal and SRR models used in the calculations are based on a difference equation approach to the system dynamics. The difference equation approach has been extensively employed in previous papers to model the basic properties of these systems. The paper is a continuation of work on the optical bistability of single guided modes interacting with Kerr impurities in photonic crystals originally considered by McGurn [Chaos 13, 754 (2003)] and work on the resonant scattering from Kerr impurities in photonic crystal waveguides considered by McGurn [J. Phys.: Condens. Matter 16, S5243 (2004)]. It generalizes this work making the extension to the more complex interaction of two guided modes at different frequencies. It extends the two guided mode treatment by McGurn [Organ. Electron. 8, 227 (2007)] which was limited to a special case of one of the photonic crystal systems considered here.

We report experiments on high-amplitude sound wave propagation in an acoustic metamaterial composed of an air-filled waveguide periodically side-loaded by holes. In addition to the linear viscothermal and radiation losses, high amplitude sound waves at the locations of the side holes introduce nonlinear losses. The latter result in an amplitude-dependent reflection, transmission, and absorption, which we experimentally characterize. First, we evidence that nonlinear losses change the nature of the device from a reflective to an absorbing one, showing the possibility to use the system as a nonlinear absorber. Second, we study the second-harmonic generation and its beating phenomenon both experimentally and analytically. We find that when considering the propagation of both the fundamental and the second harmonic, nonlinear losses cannot be neglected. Our results reveal the role of nonlinear losses in the proposed device and also provide a quite accurate analytical model to capture the effect of such losses.

The idea of nonlinear ‘transformation optics-inspired’ [–] electromagnetic cylindrical field concentrators has been taken up in a preliminary manner in a number of conference reports [–]. Such a concentrator includes both external linear region with a dielectric constant increased towards the centre and internal region with nonlinearity characterized by constant coefficients. Then, in the process of farther investigations we realized the following factors considered neither in [–] nor in the recent paper []: saturation of nonlinearity, nonlinear losses, linear gain, numerical convergence, when nonlinear effect becomes very strong and formation of ‘hotspots’ starts. It is clearly demonstrated here that such a strongly nonlinear process starts when the nonlinear amplitude of any incident beam(s) exceeds some ‘threshold’ value. Moreover, it is shown that the formation of hotspots may start as the result of any of the following processes: an increase of the input amplitude, increasing the linear amplification in the central nonlinear region, decreasing the nonlinear losses, a decrease in the saturation of the nonlinearity. Therefore, a tendency to a formation of ‘hotspots’ is a rather universal feature of the strongly nonlinear behaviour of the ‘nonlinear resonator’ system, while at the same time the system is not sensitive to the ‘prehistory’ of approaching nonlinear threshold intensity (amplitude). The new proposed method includes a full-wave nonlinear solution analysis (in the nonlinear region), a new form of complex geometric optics (in the linear inhomogeneous external cylinder), and new boundary conditions, matching both solutions. The observed nonlinear phenomena will have a positive impact upon socially and environmentally important devices of the future. Although a graded-index concentrator is used here, it is a direct outcome of transformation optics. Numerical evaluations show that for known materials these nonlinear effects could be readily achieved.

Optical response of materials to intense terahertz electric fields has become a new frontier in optics in the recent decade. We focus on the nonlinear optical responses that are quadratic in terahertz electric field, which can arise in second and third orders of nonlinearity. The second order nonlinear polarizability can lead to terahertz second harmonic generation, a phenomenon that has not been experimentally observed yet. The difficulty in detecting terahertz second harmonic generation stems from vanishingly small conversion efficiencies in this region of spectrum. However, an additional experimental difficulty results from a significant overlap of the fundamental and second-harmonic terahertz pulses both in the time and frequency domain. This makes it hard to distinguish the second harmonic from the fundamental terahertz wave. The third order nonlinear polarizability results in terahertz Kerr effect, which has been observed as an induced gating-beam birefringence that is quadratic in terahertz electric field. In noncentrosymmetric materials, terahertz Kerr effect may coexist with terahertz Pockels effect that is commonly used for time-domain terahertz detection via electro-optic sampling. Distinguishing the terahertz Kerr effect from the Pockels effect can also be difficult if the latter is significantly stronger. In this paper, we will present a method for measuring quadratic terahertz nonlinearities based on the second-harmonic lock-in detection in a time-domain electro-optic sampling experiment. We illustrate our method using a measurement of terahertz Kerr effect in a zinc blende semiconductor in geometry where both terahertz Kerr and Pockels effects are present. We will also discuss the possibility of measuring terahertz second harmonic generation in metamaterials.

We demonstrate and compare the on-chip Hong-Ou-Mandel interference be-tween two micro-ring resonators and two straight waveguides used as indistinguishable twin photons sources for quantum photonic applications.