Symmetric Directional Coupler Research Articles

All-optical logic gates based on XPM effect under the PAM-ASK modulation in a symmetric dual NLDC

We investigate the possibility of achieving all-optical logical gates based in a symmetric double non-linear directional coupler (NLDC) working with pulses of 2 ps coded for the pulse amplitude modulation-amplitude shift keying (PAM-ASK), in a digital keying. The effects of group velocity dispersion, nonlinear self phase modulation and cross phase modulation (XPM) are taken into account in a lossless theoretical setup scheme. We analyze the amplitude and shape of the output pulses as a function of the modulation factor \((|\varepsilon |)\) to show that an OR gate can be readily realizable. Further, we also implement an optical sign phase shift in one of the NLDC input channels after the PAM-ASK modulator. We identify parameter regions on which the logical operations NAND, OR and XOR can be performed with the respective re-configurable scheme set by PAM-ASK modulation technique under the XPM effect.

Wideband Three-Section Symmetrical Coupled-Line Directional Coupler Operating in Differential Mode

In this letter, a concept of wideband multisection symmetrical coupled-line directional couplers operating in a differential mode has been presented for the first time. The required different coupling coefficients for each of the coupled-line sections have been achieved by introducing a multilayer dielectric structure and by placing sections on different metallization layers. The theoretical investigation has been supported by measurements of an exemplary three-section symmetrical 3-dB coupled-line directional coupler operating in a differential mode at the center frequency of $f_{0} = 3$ GHz.

Symmetric polarization-insensitive directional couplers fabricated by femtosecond laser writing.

We study analytically the polarization behaviour of directional couplers composed of birefringent waveguides, showing that they can induce polarization transformations that depend on the specific input-output path considered. On the basis of this study, we propose and demonstrate experimentally, by femtosecond laser writing, directional couplers that yield a polarization-independent power splitting and, at the same time, preserve the polarization state of the propagating light. More in detail, we devise two different approaches to realize such devices: the first one is based on local birefringence engineering by additional refractive index modification tracks, while the second one exploits ultra-low birefringence waveguides (b = 1.2 × 10-6), obtained by thermal annealing.

Mode Coupling and Scattering Parameters in a Parity-Time Symmetric Resonant Integrated Coupler

We investigate theoretically, in the frame of coupled-mode theory, a parity-time symmetric directional coupler inserted into a symmetric resonant cavity, all ports being terminated by four identical lossless reflectors. The scattering parameters are expressed under analytic closed forms in several equivalent bases, underlining the fact that PT Symmetry can be thought of as mode coupling between the old Eigenmodes of the passive structure. We also derive the oscillation threshold of such a complex cavity, and comment upon this apparent paradox: as a matter of fact, a laser effect is not necessarily expected from a device with more losses than gain.

A Compact Silicon-Based TE-Pass Polarizer Using Three-Guide Directional Couplers

A compact TE-pass polarizer for silicon wires (SWs) based on a symmetrical three-guide directional coupler (DC) is proposed, where two identical hybrid plasmonic wave-guides (HPWs), including bends are symmetrically located at both sides of the central SW. By properly choosing the structural parameters, the phase-matching condition is only satisfied for TM mode. Therefore, the input TE-mode can directly pass through while the injected TM-mode will be evenly coupled to the adjacent HPWs and attenuated via the HPWs. Consequently, an efficient TE-pass polarizer is realized. With the symmetrical three-guide DC configuration, device length is significantly reduced, and more importantly, an ultralow insertion loss (IL) can be obtained. From the results, the proposed polarizer has an extinction ratio (ER) of 40.8 dB and an IL of 0.09 dB at $1.55~\mu \text{m}$ , and its bandwidth is ~160 nm for an ER > 30 dB dB. In addition, fabrication tolerances to the key structural parameters are analyzed and field evolution through the polarizer is also presented.

Compact broadband polarization beam splitter using a symmetric directional coupler with sinusoidal bends.

We design and demonstrate a compact broadband polarization beam splitter (PBS) using a symmetric directional coupler with sinusoidal bends on a silicon-on-insulator platform. The sinusoidal bends in our PBS suppress the power exchange between two parallel symmetric strip waveguides for the transverse-electric (TE) mode, while allowing for the maximum power transfer to the adjacent waveguide for the transverse-magnetic (TM) mode. Our PBS has a nominal coupler length of 8.55 μm, and it has an average extinction ratio (ER) of 12.0 dB for the TE mode, an average ER of 20.1 dB for the TM mode, an average polarization isolation (PI) of 20.6 dB for the through port, and an average PI of 11.5 dB for the cross port, all over a bandwidth of 100 nm.

Asymmetric directional couplers based on silicon nanophotonic waveguides and applications

Directional couplers (DCs) have been playing an important role as a basic element for realizing power exchange. Previously most work was focused on symmetric DCs and little work was reported for asymmetric directional couplers (ADCs). In recently years, silicon nanophotonic waveguides with ultra-high index contrast and ultra-small cross section have been developed very well and it has been shown that ADCs based on silicon-oninsulator (SOI) nanophotonic waveguides have some unique ability for polarization-selective coupling as well as mode-selective coupling, which are respectively very important for polarization-related systems and modedivision-mulitplexing systems. In this paper, a review is given for the recent progresses on silicon-based ADCs and the applications for power splitting, polarization beam splitting, as well as mode conversion/(de)multiplexing.

Исследование способов увеличения направленности симметричных направленных ответвителей диапазона 2–20 ГГц

Исследование способов увеличения направленности симметричных направленных ответвителей диапазона 2–20 ГГц

Non-linear switching based on dual-core non-linear optical fiber couplers with XPM and Raman intrapulse applied to femtosecond pulse propagation

In this work, we investigated the optical switching process for three shapes of femtosecond pulses (soliton, Gaussian and super-Gaussian) propagating inside a symmetrical dual-core non-linear directional coupler by simulating their propagation via the coupled non-linear Schrödinger equations. In all simulations, we considered the dispersive effects of second and third order, besides the self-phase modulation and self-steepening non-linear effects. We studied three scenarios for each of the three pulse shapes under investigation. In the first scenario, we added only cross-phase modulation (XPM); in the second approach, we added only Raman scattering; in the third one, we combined both. The study was performed for distinct polarization modes and for different values of the Raman factor, with power range varying from 1 to 300 W. We noted that the XPM non-linear effect results in a decrease in the critical power threshold, whereas the Raman scattering causes an increase. For the first scenario (only XPM effect), the critical power threshold reduced from 113.72 to 104.69 W for the soliton pulse, from 111.49 to 100.77 W for the Gaussian and from 92.79 to 80.47 W for the Super-Gaussian pulse shape. For the second scenario (only Raman scattering), the critical power increased for a Raman factor varying from 1 to 10 fs, and the three pulse shapes reached thresholds above 150 W from a 5 fs factor, reaching more than 200 W for the super-Gaussian pulse as the Raman factor increased. For the third scenario (with both effects combined), we highlight that for a fixed XPM factor of 2, the critical power remained unchanged with the variation of the Raman factor. Hence, we observed that the Super-Gaussian pulse reached lower values for critical power when compared to the other pulse shapes.

Analysis of optical fiber-based LP(01) ↔ LP(02) mode converters for the O-, S-, and C-Band.

This paper proposes a double fiber taper-based mode converter that converts LP(01) to LP(02) and vice versa. The first taper is used to convert LP(01) to some higher order LP(0m) (m>1) modes while the second taper suppresses the undesired higher order modes (m>2) and results in LP(02) overwhelmingly dominated. A simulation shows that conversion efficiency of almost 100% at the central wavelength of O-, S-, and C-Band, and above 98% over the S- and C-Band was achieved. Moreover, suppression of undesired higher order modes is more than 10 dB over the whole O-, S-, and C-Band. In particular, the suppression is more than 19 dB for the whole C-Band. The analysis also shows that the performance of the mode converter is not sensitive to slight variations of the converter's parameters. The same converter also can be used to convert LP(02) back to LP(01). It is shown that conversion efficiency higher than 99% and suppression of undesired higher order modes higher than 30 dB can be obtained. Further, a (de)multiplexer for an LP(02) and an LP(01) mode was designed using the mode converter combined with a symmetric directional coupler. The multiplexer is broadband and has an insertion loss less than 0.5 dB in the C-Band.

Quasi -symmetry in passive photonic lattices

The concept of quasi- symmetry in an optical wave-guiding system is elaborated by comparing the evolution dynamics of a -symmetric directional coupler and a passive directional coupler. In particular, we show that in the low-loss regime, apart from an overall exponentially damping factor that can be compensated via a dynamical renormalization of the power flow in the system along the propagation direction, the dynamics of the passive coupler fully reproduce the one in the -symmetric system.

High Contrast Optical “OR” Logic Gates Using a Photonic Crystal Fiber Modulated by PAM-ASK

In this work, we present a numerical study of an all-optical logical gate based in a symmetric nonlinear directional coupler (NLDC) in Photonic Fiber Crystals (PCFs) PCFs operating with two ultrashort fundamental soliton pulses of 100 fs, modulated by PAM-ASK with binary amplitude modulation to represent the logical level 1 and 0. The performance of a symmetric dual core NLDCPCF to execute two-input logical functions is examined. We evaluate the effect resulting from an increment in the PAM coding parameter offset (ε), considering the anomalous Group Velocity Dispersion (GVD), third-order dispersion (β3) and nonlinear effects Self Phase Modulation (SPM), Self-Steepening (SS), lntrapulse Raman Scattering (IRS) in a lossless configuration. Our results indicate that is possible to get logical operations for the cores 1 or 2 using a phase control to the input pulse in one of the fibers.

Analysis of tem three‐section symmetrical directional couplers with tandem‐connected center section

ABSTRACTThe analysis of TEM three‐section symmetrical directional couplers with tightly coupled section realized as a tandem connection of two coupled‐line sections has been shown. The design equations allowing for calculating the required even‐mode characteristic impedances of coupled‐line sections for arbitrarily chosen coupling coefficient and coupling ripple have been derived. The proposed method has been utilized for analysis of three‐section 3‐dB symmetrical couplers with different coupling ripple and the results are given in a tabular form. The theoretical analysis has been confirmed by measurements of a 3‐dB three‐section symmetrical coupler, in which coupled‐line sections constituting a tightly coupled section have been miniaturized to for better fitting to the coupler's structure. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2578–2582, 2013

Novel injection methods for substitution of irradiation under different coupling routes of electromagnetic interference

At present, the maximum electromagnetic radiated field strength which can be simulated is not able to satisfy the requirements of electromagnetic environment effects tests on electronic systems. To broaden the effective frequency range of the current injection technique, new current injection methods were put forward for substitution of the radiation. A wide-band current injection device was designed based on the symmetric directional coupler principle. With a typical antenna receiving system under test, the validity of the methods were analyzed by theory and verified by experiments when the Equipment Under Test (EUT) had different coupling routes of electromagnetic interference (EMI). The results indicate that: firstly, the relationship between radiated fields and injected voltage is linear, even if the responses of the system are non-linear; secondly, the method is valid even if the test frequency is up to gigahertz; thirdly, the novel methods of current injection are able to substitute for irradiation effectively when the antenna and the interconnected cable are the main coupling routes of EMI respectively.

Broadband three-section symmetrical directional couplers with reduced coupling coefficient requirements

A new class of broadband symmetrical directional couplers has been investigated. The proposed couplers consist of three coupled-line sections with the middle coupled-line section connected between two identical coupled-line sections in such a way that the coupled and transmission ports of the resulting three-section directional coupler are exchanged in comparison to the well-known three-section symmetrical directional coupler. By choosing coupling coefficients and electrical lengths it is possible to achieve an equal-ripple coupling characteristic. Theoretical analysis has been performed and the properties of the proposed directional couplers are compared to the well-known multisection couplers. The presented theoretical analysis has been verified by the design. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:639–645, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27401

Generalized Methods for the Design of Quasi-Ideal Symmetric and Asymmetric Coupled-Line Sections and Directional Couplers

Generalized methods for the design of quasi-ideal symmetric and asymmetric coupled-line sections have been proposed. Two cases of coupled-line sections, in which the inductive coupling coefficient is either greater or smaller than the capacitive coupling coefficient, have been separately considered. To compensate for the coupling coefficients' inequality, a combination of lumped capacitors and short coupled-line sections has been introduced into the coupled-line section. The proposed methods allow for designing quasi-ideal coupled-line sections in nearly arbitrarily chosen dielectric media and for arbitrarily chosen coupling. The theoretical analyses have been verified by the design and measurement of three compensated coupled-line directional couplers illustrating application of the proposed methods in different dielectric structures.

Study of the Performance of an All-Optical Half-Adder Based on Three-Core Non-Linear Directional Fiber Coupler Under Delayed and Instantaneous Non-Linear Kerr Responses

Recently, much attention has been given to the influence of the relaxation process of the non-linear response, because the usual assumption of instantaneous non-linear response fails for ultra-short pulses, and additional contributions coming from non-linear dispersion and delayed non-linearity have to be taken into account. This article presents a numerical analysis of the symmetric planar and asymmetric planar three-core non-linear directional fiber couplers operating with a soliton pulse, where effects of both delayed and instantaneous non-linear Kerr responses are analyzed for implementation of an all-optical half-adder. To implement this all-optical half-adder, eight configurations were analyzed for the non-linear directional fiber coupler, with two symmetric and six asymmetric configurations. The half-adder is the key building block for many digital processing functions, such as shift register, binary counter, and serial parallel data converters. The optical coupler is an important component for applications in optical-fiber telecommunication systems and all integrated optical circuit because of its very high switching speeds. In this numerical simulation, the symmetric/asymmetric planar presents a structure with three cores in a parallel equidistant arrangement, three logical inputs, and two output energy. To prove the effectiveness of the theoretical model for generation of the all-optical half-adder, the best phase to be applied to the control pulse was sought, and a study was done of the extinction ratio level as a function of the Δ > parameter, the normalized time duration, and the Sum and Carry outputs of the (symmetric planar/asymmetric planar) non-linear directional fiber coupler. In this article, the interest is in transmission characteristics, extinction ratio level, normalized time duration, and pulse evolution along the non-linear directional fiber coupler. To compare the performance of the all-optical half-adders, the figure of merit of the logic gates was used. All results were obtained numerically, considering a simple model for generation of an all-optical half-adder.

950 MHz, -60dB TX-Cancellation Active Directional Couplers for UHF RFID Application

We propose two types of active directional couplers to assure high TX cancellation: an asymmetric type and a symmetric type. For attaining low receiving through loss, coupling capacitors used in conventional couplers are replaced by amplifiers in the proposed active directional couplers. The asymmetric active directional coupler is composed of a small number of components and simple structure. The symmetric active directional coupler has wide-bandwidth TX cancellation. Measurement results show that receiving through loss of -5.3dB and the TX cancellation of -67.6dB are obtained in the asymmetric active directional coupler, and receiving through loss of -6.7dB and the TX cancellation of -66.4dB are obtained in the symmetric active directional coupler. Compared to the asymmetric active directional coupler, the symmetric active directional coupler has advantage of wider bandwidth of 1.25MHz to reduce TX leakage of less than -55dB. Both the proposed active directional couplers achieve high TX cancellation, and the symmetric active directional coupler can be applied in a UHF RFID system with 10-m communication range.

The properties and design concepts of photonic directional couplers made of photonic crystal slabs

The properties and design concepts of air-defect symmetric and asymmetric directional couplers (DCs) made of photonic crystal slabs (PCSs) are investigated by the tight-binding theory. We give criteria in both cases for determining the specific frequency, named as the decoupling point, at which the even (-like) and odd (-like) parities of eigenmodes should switch. Two dispersion curves will cross in symmetric DCs but not in asymmetric DCs because eigenfrequencies of isolated photonic waveguides (PCWs) are different in asymmetric DCs. In the dielectric-rod PCS, the decoupling point of the DC is located almost at the same wave vector as a two-dimensional (2D) DC with only a blue shift in its frequency. Therefore, 2D simulation can give a primary result in designing a dielectric-rod DC. On reducing the radius of the defect rods, the increase in coupling coefficients leads to a faster group velocity and a longer coupling length. On the other hand, the dispersion curves of the air-hole PCS DC are no longer parallel to those of the 2D DC so that performing a 3D simulation is necessary in designing an air-hole PCS DC with enlarged air holes. Moreover, the dispersion curve of the single PCW is no longer located at the centre of the curves of the symmetric DC and the group velocity may become negative which is not observed in the dielectric-rod PCS. The simulation results indicate that the coupling length of an air-defect DC can be achieved as short as 5 lattice constants, which is much smaller than that made of dielectric defects in air-hole PCS DCs.

All-Optical Half-Adder Using All-Optical XOR and AND Gates for Optical Generation of “Sum” and “Carry”

In this article, a numerical simulation study using the symmetric planar three-core non-linear directional coupler, operating with a short light pulse (2 ps), for the implementation of an all-optical half-adder is presented. The half-adder is the key building block for many digital processing functions such as shift register, binary counter, and serial parallel data converters. Optical couplers are an important component for application in optical fiber telecommunication systems and all integrated optical circuits because of very high switching speeds (as high as the femto-second range). In this numerical simulation, the symmetric planar three-core non-linear directional coupler presents a planar symmetrical structure with three cores in a parallel equidistant arrangement, three logical inputs (CP, A, and B), and two output logic functions (C and S). The CP(ΔΦ) input is a control pulse with a phase difference ΔΦ = Δθπ (0 ≤ Δθ ≤ 2) between inputs A and B (logical inputs of the half-adder) and one amplitude discriminator circuit. The half-adder uses two output logic functions of Sum(S) and Carry(C), which can be demonstrated by using XOR and AND gates, respectively. For the half-adder, the phase [ΔΦMIN, ΔΦMAX] intervals are studied, allowing the operation of the device as a half-adder. For the selected range of CP(ΔΦBETTER), the extinction ratio was studied, the compression factors for both Sum(S) and Carry(C) outputs of the symmetric planar three-core non-linear directional coupler.