Symmetric Directional Coupler Research Articles

Analysis of an Ultrashort PCF-Based Polarization Splitter

We propose a new structure of polarization splitter based on a dual-elliptical-core photonic crystal fiber. The polarization splitter has a simple and symmetric directional coupler configuration. The vector boundary element method is used to investigate the propagation characteristics of the device under various conditions. Results of the numerical calculation relevant to the design conditions are presented. A 0.3-mm-long splitter with an extinction ratio of 23 dB is obtained.

A performance study of an all-optical logic gate based in PAM-ASK

In this paper a numerical study of an all-optical logical gate based in a symmetric nonlinear directional coupler (NLDC) operating with two ultrashort fundamental soliton pulses of 2ps, modulated by PAM-ASK with binary amplitude modulation to represent the logical level 1 and 0, is presented. The performance of a symmetric dual core NLDC performing two-input logical functions is examined. Initially, we evaluate the effect resulting from an increment in the PAM coding parameter offset (ϵ), considering the anomalous Group Velocity Dispersion (GVD), nonlinear Self Phase Modulation (SPM) in a lossless configuration. We can conclude that is possible to get logical operations for the cores 1 or 2 since a phase control exists which is applied to the input pulse in fiber 1.

Broadband 4 $\times$ 4 Butler Matrices as a Connection of Symmetrical Multisection Coupled-Line 3-dB Directional Couplers and Phase Correction Networks

An innovative approach for realization of broadband 4 times 4 Butler matrices, in which broadband symmetrical multisection 3-dB/90deg directional couplers are used as basic elements, has been presented. A new technique for realization of center crossover together with 45deg phase shifters is proposed where phase correction networks, consisting of a number of coupled-line sections, are applied. The use of the proposed phase correction networks allows to achieve equal-ripple 45deg differential phase characteristics in a wide frequency range. Numerical results for the design of broadband 4 times 4 Butler matrices consisting of multisection symmetrical 3-dB/90deg directional couplers having three, five, seven, and nine coupled-line sections in conjunction with phase correction networks having an appropriate number of coupled-line sections are given in a tabular form. A method of performance improvement for broadband Butler matrices by the combinatory use of directional couplers consisting of different number of coupled-line sections is also proposed. To verify the presented analysis a broadband 4 times 4 Butler matrix consisting of six 3-dB/90deg three-section symmetrical coupled-line directional couplers and two single-section coupled-line phase correction networks has been designed, manufactured, and measured. Results of measurements of the designed Butler matrix fully confirm the validity of the proposed technique of broadband 4 times 4 Butler matrix realization.

The Complete Design of Microstrip Directional Couplers Using the Synthesis Technique

A symmetrical microstrip directional coupler design using the synthesis technique without prior knowledge of the physical geometry of the directional coupler is analytically given. The introduced design method requires only the information of the port impedances, the coupling level, and the operational frequency. The analytical results are first validated by using a planar electromagnetic simulation tool and then experimentally verified. The error between the experimental and analytical results is found to be within 3% for the worst case. The design charts that give all the physical dimensions, including the length of the directional coupler versus frequency and different coupling levels, are given for alumina, Teflon, RO4003, FR4, and RF-60, which are widely used in microwave applications. The complete design of symmetrical two-line microstrip directional couplers can be obtained for the first time using our results in this paper.

Analysis of an optical logic gate using a symmetric coupler operating with pulse position modulation (PPM)

In this paper we propose the operation of an all-optical logical gate based in a symmetric nonlinear directional coupler (NLDC) operating with a pulse position modulation (PPM). The performance of a symmetric NLDC realizing two-input AND/OR logical functions, which can be applied in transmission and processing of signals in all-optical form in TDM systems, is examined. This integrated symmetric NLDC logical gate operates with two ultrashort soliton light pulses (2ps), which are modulated in agreement with the technique of pulse position modulation (PPM). Initially, we evaluate the effect resulting of an increment in the PPM coding parameter offset (ε), for the temporal position of the output pulse, considering the anomalous group-velocity dispersion (GVD), nonlinear self phase modulation (SPM) and without loss propagation regime of input pulses, in the cores 1 and 2 of the NLDC. In this situation, we analyze the four possible situations for the two-input logical gate, modulating the 1 and 2 input pulses through temporal displacement and allowing a variation in the coding parameter offset. We can conclude that is possible to get AND/OR logical operations for the cores 1 or 2, without to insert PPM error, since a phase control (Δϕ) exists applied in agreement with the logical level of the input pulse in the core 1. Finally we define the truth table, considering the adequate phase difference and coding parameter offset for the stable operation of the AND/OR logical gate based in the symmetric NLDC.

Analysis of Bragg reflection waveguides with finite cladding: An accurate matrix method formulation

Ideal Bragg reflection waveguides (BRWs) are assumed to have an infinitely extended periodic cladding whereas in practice, the cladding of BRWs is of a finite extent. Bloch theorem is widely used to analyze the propagation characteristics of the BRWs. Since Bloch theorem is ideally valid only for an infinitely extended periodic medium, its application to study such BRWs is an approximation. We present a matrix method for a more accurate analysis of finite-clad BRWs and estimate the extent of errors involved in the values of the propagation constant obtained by the Bloch wave formalism. The proposed method can be used to obtain the mode effective indices as well as the radiation loss of a finite-clad BRW without resorting to solving any complex transcendental equation. In addition, since the method does not inherently assume a periodic cladding, it can also be used to analyze symmetric multi-channel BRWs, chirped structures and directional couplers.

Optical switching in a symmetric three-waveguide nonlinear directional coupler

In this paper we study optical switching in a system of a three-waveguide nonlinear directional coupler. We consider a waveguide structure with three parallel waveguides in a linear, symmetric coupling configuration. We work in the case that the (absolute value of the) difference of the propagation constants of the outer and centre waveguides is much larger than the linear coupling constants. When one of the outer waveguides is initially excited, we show that for small values of the initial power the system behaves as a two-waveguide nonlinear directional coupler where only the two outer waveguides are involved in the switching. For larger values of the initial power, transfer of light between the initially excited waveguide and the centre waveguide is observed.

Three-dimensional photonic devices fabricated in glass by use of a femtosecond laser oscillator

Three-dimensional photonic waveguide devices are fabricated in glass by use of femtosecond pulses from an extended-cavity laser oscillator. Three-dimensional devices, including a symmetric three-waveguide directional coupler and a three-dimensional microring resonator, are fabricated and tested. Waveguides can be fabricated at depths of approximately 1 mm inside a glass substrate, thus demonstrating the capability of achieving dramatic increases in device density. These results demonstrate the potential to fabricate new classes of devices that are not possible in two dimensions.

The Calculation of Switching Power of Symmetric and Asymmetric Nonlinear Directional Couplers With Variable Coupling Coefficient

It is proposed that the switching power of symmetric one-coupling-length nonlinear directional couplers (NLDC) with variable coupling coefficient (VCC) can be calculated as the weighted average of the local critical power of the coupler. Further, it is shown that the switching characteristics of a one-coupling-length VCC NLDC are determined mainly by the first half of the coupler and the switching power of an asymmetric VCC NLDC can be calculated as the weighted average of the local critical power over the first half of the coupler. Both symmetric and asymmetric examples are presented.

Even - odd-mode analysis and experimental studies on tap-offs for CATV networks

A new application method was proposed, where the even-odd-mode theory for a symmetrical coupled-line directional coupler was applied to a transformer-type directional coupler. In addition, the tap-offs were analyzed by the proposed theory, which are widely used in CATV (cable television), DBS (direct broadcasting satellite), CS (communication satellite) systems. By comparing the calculation results to the measured ones using the even-odd-mode reflection coefficients only, the validity of the proposed analysis method was confirmed. Then, the tap-off has been broadened from 5 MHz over 4,000 MHz by adopting the proposed theory.

Polarization splitter based on photonic crystal fibers

We report a new kind of polarization splitter based on dual-core photonic crystal fibers. The polarization splitter has a symmetric directional coupler configuration. Each core exhibits high birefringence, which gives rise to an adequate difference in the coupling lengths for the two orthogonal polarizations. A 1.7-mm-long splitter is obtained with the splitting ratio better than -11 dB and a bandwidth of 40 nm. The relationship between the length of the polarization splitter and the diameter of the air hole in the middle of the two cores is discussed.

Switching in symmetric second-order nonlinear directional couplers by weak optical control waves

Coupled nonlinear waveguides are basic components for all-optical signal processing. The existence and stability of states with no power exchange between the coupled waves (stationary solutions, nonlinear modes) is important for the switching properties of such structures. In this contribution, nonlinear modes of second-harmonic generation in symmetric directional couplers are analyzed with respect to their use for all-optical switching controlled by weak optical signals. Two configurations with pump and control at the fundamental frequency only and low output powers at the second harmonic are proposed. The first employs longitudinally varying profile of the wave-vector mismatch, and the second is based on induced upconversion/downconversion.

Add-drop multiplexer with UV-written Bragg gratings and directional coupler in SiO/sub 2/-Si integrated waveguides

The needs of wavelength multiplexers is growing and therefore optical add-drop multiplexers (OADMs) become key components for the advanced high bit rate telecommunication networks. Currently, one of the solutions to make OADMs use fiber Bragg gratings and circulators. In order to take advantage of integrated optics and to avoid the use of expensive circulators, we perform this kind of OADM in ridge silica waveguide on silicon. This technology makes possible short based directional coupler (DC) device. In previous work, we firstly realized an OADM which involves a symmetrical directional coupler combined with two Bragg gratings inside the coupling zone. The results meet difficult tradeoff between a high extinction ratio and a narrow spectral width of the rejection band. This paper reports some fabrication improvements for DC-OADM that lead to more efficient components, progress results from a lot of works concerning both the photo-writing process of Bragg gratings and the lowering of the waveguide internal stresses. Thus, we show that apodization of gratings and low birefringence in the waveguides enables us to achieve DC-OADM with an extinction ratio of 30 dB and a spectral bandwidth less than 1.5 nm.

Analysis of SiO 2/TiO 2–SiO 2/SiO 2 coupled parallel waveguide structures using computer aided design techniques

The analysis of two coupled parallel rib waveguides of SiO 2/TiO 2–SiO 2/SiO 2 on silicon substrate is presented. The indigenously developed beam propagation method (BPM) simulation tools are applied to observe the power coupling between two identical parallel rib waveguides and to study the dependence of the coupling length on various parameters of the rib waveguides including the refractive indices, rib height, guide thickness, clad thickness and separation between them. The designs for the 3 dB coupler and demultiplexer for DWDM, based on the symmetric directional coupler, are optimized for minimum dimensions using the techniques, for the first time. The optimum device length for 3 dB coupler was calculated to be 650 μm with 0.016 dB losses. The 1.3 μm and 1.304 μm demultiplexer has been designed with the optimized device length of 1.02 mm and the losses less than 0.40 dB at both the wavelengths.

A new analysis method and broad-band design of tap-offs for CATV/DBS systems

A new analysis method of tap-off for CATV (cable television) and DBS (direct broadcasting satellite) system was proposed and investigated by using 4-port and 3-port equivalent circuits. As a new analysis method the even-odd-mode theory of a symmetrical coupled-line directional coupler was introduced, and then it was applied to transformer type directional coupler and analyzed. By comparing the results of simulation and measurement the validity of proposed analysis method was confirmed. Furthermore, the tap-off has been broadened from 5 MHz to 2450 MHz by adopting the proposed theory.

Self-switching with a nonlinear birefringent loop mirror

We investigate switching characteristics of the nonlinear birefringent loop mirror which consists of a symmetric directional coupler, a birefringent fiber, and a symmetry-breaking element that converts linearly polarized light to circularly polarized light, and vice versa. The condition for total reflection in the linear regime is obtained using the Jones matrix formalism, The switching curves have been obtained through numerical computations both in the nondispersive pulse regime and in the soliton-like pulse regime. Parameter values suitable for practical applications of our device as an intensity filter are obtained.

Comparison of an in-line asymmetric directional coupler modulator with distributed optical loss to other linearized electrooptic modulators

The transfer function of an external modulator is the critical factor that determines the spurious free dynamic range, signal power gain, and noise figure of a wide-band analog RF-photonic link. We present an in-line asymmetric directional coupler modulator with distributed optical loss capable of providing a transfer function linearized to the fourth order for multioctave bandwidths. This modulator compares favorably with multiple modulator suboctave linearization techniques consisting of series symmetric directional coupler and series Mach-Zehnder modulators and superoctave parallel Mach-Zehnder, and cascaded sections containing series Mach-Zehndersymmetric directional coupler modulators. The optimum bias point for the asymmetric directional coupler modulator is determined by device dimensions and material parameters with relative insensitivity to errors introduced during fabrication. Optical insertion loss imposes significant limitations on modulator performance. An implementation of an in-line asymmetric directional coupler modulator is discussed that eliminates excess loss due to fiber coupling in order to achieve a large RF power gain and competitive noise figure compared to the linearization configurations.

Beam-propagation analysis of tapered nonlinear directional couplers

We analyze symmetric and parallel nonlinear directional couplers with the finite-difference beam-propagation method. The results exhibit a cutoff phenomenon of an antisymmetric mode. We also investigate tapered nonlinear couplers for their possible applications to all-optical logic gates. The numerical results show that tapered nonlinear couplers can provide an XOR (eXclusive OR) logic function. © 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 15: 43–46, 1997.

Nonclassical light in symmetric nonlinear directional coupler

We examine the quantum-statistical properties of light beams in a directional symmetric nonlinear coupler composed of one linear waveguide and two nonlinear waveguides operating by the second-harmonic generation. Linear and nonlinear mismatches are taken into account. Non-classical behaviour of single and compound modes in such a device is investigated.

Experimental characterization of coupled waveguides by normal-mode excitation

We present a novel method for measuring the coupling coefficient of a symmetric directional coupler, based on the direct excitation of the normal modes of the entire structure. The output-power distribution at different coupling lengths was related to the experimentally measured normal modes in order to find the coupling coefficient. The device was alternatively characterized by means of the individual-waveguide modes according to coupled-mode theory in the strong-coupling regime. Both methods were implemented on single-mode directional couplers in Ti:LiNbO/sub 3/. The validity and accuracy of both approaches are discussed.