Symmetric Coupler Research Articles

Small size dual-band coupled-line coupler for WiMAX applications

Abstract In this paper, a symmetric compact dual-band coupled-line coupler is proposed using microstrip transmission lines, circular resonators, and spiral lines for WiMAX applications. The proposed coupler provides two narrow operating bands, high coupling and isolation, and suitable return loss. The primary design consists of two parallel-coupled lines with a flat coupling over a wide frequency band. Then, the bandwidth, return loss, and isolation are optimized by adding circular resonators and spiral lines to achieve a compact size. In this design, the return loss is better than 30–35 dB in the bandwidth 3.4–5.79 GHz. Finally, this structure is fabricated using a Rogers-RT Duroid 5,880 substrate with a thickness of 20 mil, a loss tangent of 0.0009, a dielectric constant of 2.2, and an area of 18.8 × 14 mm2. The results of the measurements are in good agreement with those of the simulation.

Topological polarization beam splitter based on directional coupling between asymmetric valley photonic crystal waveguides.

Compared with symmetric directional couplers (DCs), asymmetric DCs constructed by two or over two different parallel waveguides offer a more flexible structure and allow for easier expansion of mode channels. In this Letter, we propose a kind of asymmetric topological DC based on two different valley photonic crystal waveguides (VPCWs). According to the coupled-mode theory, phase matching induces the complete coupling for the guide modes of two different VPCWs, whereas significant phase mismatching indicates no coupling occurs. Furthermore, the asymmetric topological DCs exhibit backscattering immunity and anti-disturbance robustness owing to the topological edge states (TESs), which greatly improve the performance of asymmetric DCs. We further design a new, to the best of our knowledge, kind of topological polarization beam splitter (TPBS) at the communication wavelength of 1550 nm by ensuring that the transverse electric mode satisfies the phase matching condition, while the transverse magnetic mode is phase-mismatched. The simulated results demonstrate that the proposed TPBS exhibits highly effective polarization separation and is robust against defects. This design holds significant potential for applications in optical communication systems.

A Symmetrical Fractal-based Balanced Branch-Line Coupler for Simultaneous Low- and Mid-band 5G Frequencies Applications

A Symmetrical Fractal-based Balanced Branch-Line Coupler for Simultaneous Low- and Mid-band 5G Frequencies Applications

Scalable Photonic Digital-to-Analog Converters

This work introduces a novel architecture for implementing a parallel coherent photonic digital-to-analog converter (PDAC), designed to transform parallel digital electrical signals into corresponding analog optical output, convertible to analog electrical signals using photodiodes. The proposed architecture incorporates microring resonator-based modulators (MRMs), phase shifters, and symmetric multimode interference couplers. Efficient modulation is achieved by MRMs utilizing carrier depletion-induced refractive index changes, while metal heaters facilitate tuning of the ring resonator resonance wavelength. The proposed architecture is scalable to higher bit resolutions and exhibits a dynamic range limited by MRM’s sensitivity to applied bias and noise levels. Experimental results of the fabricated chip in the silicon-on-insulator (SOI) platform showcase the successful realization of a 4 GSample/sec conversion rate in a 2-bit resolution operation, along with a stationary conversion of four parallel DC digital signals into 16 analog intensity levels in a 4-bit PDAC configuration. The study encompasses a proof-of-concept experimental demonstration of 8 Gbps data conversion, along with a 50 Gbps data conversion rate using the optimized design in the simulation, affirming the accuracy and quality of the PDAC architecture. These findings contribute to the advancement of PDAC technology, providing insights into performance characteristics, limitations, and potential applications.

Nonlinear mode coupling in graphene-buried optical waveguides

The photothermal effect of graphene, which refers to the effect of converting light absorbed by graphene into heat, offers an effective physical mechanism for the realization of all-optical control devices. In this paper, we explore this physical mechanism for the study of nonlinear mode-coupling effects with three graphene-buried waveguide structures: a graphene-buried long-period waveguide grating, a symmetric directional coupler with graphene buried in two cores, and a symmetric directional coupler with graphene buried in one core. We establish physical models for these graphene-buried waveguide structures based on the coupled-mode theory and experimentally implement these structures with polymer waveguides. Our experimental results agree well with the theoretical analyses. The nonlinear mode-coupling effects generated in the graphene-buried waveguide structures show similar characteristics as those achieved with Kerr nonlinearity, but the input powers required in our experiments are much lower (only several tens of milliwatts), which can be delivered by common continuous-wave lasers. The graphene-buried waveguide platform makes feasible the generation of strong nonlinear mode-coupling effects at low powers and offers much flexibility for nonlinearity engineering, which can greatly facilitate the investigation of nonlinear mode-coupling effects in different waveguide structures for practical applications.

Multi-wavelength fiber laser with high-purity LP11 output mode based on a symmetrical fiber mode selective coupler and Sagnac interferometer

In this work, a multi-wavelength fiber laser with a high-purity LP11 output mode was proposed and experimentally demonstrated. More specifically, a novel symmetrical fiber mode selective coupler (MSC) was utilized for mode conversion and a Sagnac interferometer is fabricated as a multi-channel filter for wavelength selection. From the experimental results, it was shown that the laser was able to emit stable single-, dual-, and triple-wavelength outputs both around 1530 nm and 1550 nm, and the purity of the LP11 mode was above 97 % and 98 % at the two bands respectively. In addition, the laser was capable of simultaneous dual-wavelength emission around 1530 nm and 1550 nm, resulting in an overall four-wavelength output with an LP11 mode purity of above 98 %. The proposed high-order mode multi-wavelength fiber laser has potential applications in various fields including optical communications, optical sensing, and materials processing.

A Balanced Symmetrical Branch-Line Microstrip Coupler for 5G Applications

Symmetry in designing a microstrip coupler is crucial because it ensures balanced power division and minimizes unwanted coupling between the coupled lines. In this paper, a filtering branch-line coupler (BLC) with a simple symmetrical microstrip structure was designed, analyzed and fabricated. Based on a mathematical design procedure, the operating frequency was set at 5.2 GHz for WLAN and 5G applications. Moreover, an optimization method was used to improve the performance of the proposed design. It occupied an area of 83.2 mm2. Its harmonics were suppressed up to 15.5 GHz with a maximum level of −15 dB. Meanwhile, the isolation was better than −28 dB. Another advantage of this design was its high phase balance, where the phase difference between its output ports was 270° ± 0.1°. To verify the design method and simulation results, the proposed coupler was fabricated and measured. The results show that all the simulation, design methods, and experimental results are in good agreement. Therefore, the proposed design can be easily used in designing high-performance microstrip-based communication systems.

An Integrated Pump-Controlled Variable Coupler Fabricated by Ultrafast Laser Writing.

The design and fabrication of a integrated symmetric directional coupler dependent o the pumping power and operating at a 1534 nm wavelength is reported. The twin-core waveguide was inscribed into Er3+/Yb3+ co-doped phosphate glass by a femtosecond laser direct writing technique. By optical pumping, the coupling ratio can be modulated due to the changes induced in the refractive index of the material. The experimental results demonstrated that the coupling ratio can be tuned continuously from 100/0 to 50/50 by increasing the pump's power from 0 to 350 mW. The developed twin-core coupler has promising applications for on-chip all-optical signal processing and communication systems.

High-Mode Purity 1 μm Cylindrical Vector Beam All-Fiber Laser Based on a Symmetric Two-Mode Coupler

Cylindrical vector beams (CVBs) are the product of polarization modulation of optical fields, and possess both unique focusing characteristics and excellent properties applicable to machining, imaging, communication and other fields. Mode selection couplers comprise a promising new method to realize the long-term stable output of cylindrical vector beam all-fiber lasers. Mode selection couplers have the advantages of a simple structure, high mode conversion efficiency and high mode purity. However, the production process of conventional asymmetric mode selection couplers is more complicated. Therefore, in this paper, a symmetric two-mode coupler with a 1 μm band is designed and fabricated using the finite element method, beam propagation method and fused pull-cone method, and a set of all-fiber passive mode-locked lasers based on symmetric dual-mode couplers are constructed. Finally, we obtain cylindrical vector beam outputs with central wavelengths of 1038.97 nm/1067.72 nm, a repetition rate of 8.78 MHz, pulse durations of 660 ps/656 ps, maximum average powers of 5.25 mW/5.2 mW, and the high mode purity of TM (transverse magnetic)01 mode and TE (transverse electric)01 mode is 97.18% and 97.07%, respectively.

Finding straight line generators through the approximate synthesis of symmetric four-bar coupler curves

The approximate path synthesis of four-bar linkages with symmetric coupler curves is presented. This includes the formulation of a polynomial optimization problem, a characterization of the maximum number of critical points, a complete numerical solution by homotopy continuation, and application to the design of straight line generators. Our approach specifies a desired curve and finds several optimal four-bar linkages with a coupler trace that approximates it. The objective posed simultaneously enforces kinematic accuracy, loop closure, and leads to polynomial first order necessary conditions with a structure that remains the same for any desired trace leading to a generalized result. Ground pivot locations are set as chosen parameters, and it is shown that the objective has a maximum of 73 critical points. The theoretical work is applied to the design of straight line paths. Parameter homotopy runs are executed for 1440 different choices of ground pivots for a thorough exploration. These computations found the expected linkages, namely, Watt, Evans, Roberts, Chebyshev, and other previously unreported linkages which are organized into a 2D atlas using the UMAP algorithm.

A Fully Symmetrical Uni-Planar Microstrip Line Comparator Network for Monopulse Antenna

In this letter, a fully symmetrical uni-planar comparator network is proposed for monopulse antenna feeding. To break through the fundamental limitation of a conventional 180 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> coupler (crossing between input and output ports and amplitude imbalance), a novel symmetric ports coupler is designed by cascading two identical 90 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> couplers with two-phase delay lines, where the total phase delay of two lines is 180 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> . To realize in planar structure, a zero-phase delay planar crossover is designed to resolve signal crossing between two stage couplers in a comparator network. To verify our design concept, a prototype monopulse array operating at 5.7 GHz is designed, fabricated, and measured, and results are aligned well with simulation and theory.

Graded-index core-based polarization beam splitters realized with symmetric polymer directional couplers

In this paper, we design and demonstrate polymer-based polarization beam splitters realized with parabolic graded-index (GI) directional couplers at 1550 nm. We design and investigate these beam splitters by using the beam propagation method (BPM) and the RSoft CAD BeamPROP solver is used for model design and characterization. We study the splitting ratio of 50:50 and 100:0 of these beam splitters for both transverse electric (TE) and transverse magnetic (TM) modes of the polarized light. In particular, the 50:50 beam splitters exhibit an output efficiency of 95% due to the absence of polarization. However, in the presence of the polarization effect, the polarization dependency is analyzed and manipulated successfully, and the output efficiency increases by approximately 98% at an incident angle of 31∘ due to the polarized launch field. Moreover, the 100:0 beam splitters act as the switching devices realized with parabolic GI polymer directional couplers. The insertion losses of the GI beam splitters are 0.162 dB and 0.186 dB, while the excess losses are 0.148 dB and 0.149 dB for TE and TM modes, respectively. Also, the polarization extinction ratio (PER) of the 100:0 beam splitters is &gt;25 dB and &gt;20 dB for TE and TM modes, respectively.

Wavelength-switchable cylindrical vector fiber laser based on symmetrical two-mode coupler

Wavelength-switchable cylindrical vector fiber laser based on symmetrical two-mode coupler

Analysis of splitting ratio of a symmetric directional coupler fabricated with triangular-index fibers in nonlinear condition-a design perspective

We report here one of the significant performance parameters of a 2 × 2 symmetric directional coupler, the splitting ratio, in the nonlinear conditions considering the coupler fabricated with two similar mono-mode triangular index fibers. Different sets of physical aspects like V-number, core-radius of the fibers used, core-to-core distance of separation between the fibers in coupler structure, operating wavelength, index of refraction of the core have been considered for evaluating the splitting ratio at different lengths of fused fiber coupler. This analytical exercise is an effort for extending an indication to the optical device design engineers. This has been worked out involving the reliable mathematical instrument, the Chebyshev technique. The change of behavior of the splitting ratio in different linear and nonlinear conditions and with different physical parameters has been studied rigorously. Our theoretical analysis goes superbly with that derived using the complicated and lengthy finite element technique. This exercise needs modest computation. Hence, this report on splitting ratio analysis of 2 × 2 symmetric directional coupler composed of two mono-mode triangular- index fibers will prove its feasibility as a ready reference in the area of nonlinear photonic design engineering.

Dual-Mode 2 × 2 Thermo-Optic Switch Based on Polymer Waveguide Mach-Zehnder Interferometer

A dual-mode <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> thermo-optic switch based on Mach-Zehnder interferometer with symmetric directional couplers is proposed and experimentally demonstrated by utilizing polymer waveguides. The present switch consists of two <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2\,\,3$ </tex-math></inline-formula> -dB directional couplers, which are designed to be mode-insensitive by choosing the core separation and coupling length optimally. Meanwhile, a mode-insensitive phase shifter is employed to alter the phase of the two different modes simultaneously. With the switching power of ~9.0 mW, the measured extinction ratios for the E11 mode and E21 mode can reach to 17.5 dB and 16.4 dB, respectively, at 1550 nm, and the switching time is ~1.30 ms. The performance of the switch is also independent to the polarization state of light. Our proposed dual-mode switch with the advantages of low power-consumption can accommodate MDM system and would be applied in future large capacity communication systems.

Few-mode 3-dB power splitters based on polymer waveguide directional couplers

Two designs of few-mode 3-dB power splitter based on symmetric directional couplers are proposed and experimentally demonstrated by utilizing polymer waveguides. The present devices are designed to be mode-insensitive by choosing the coupling distance and coupling length optimally. The power splitting ratio for the dual-mode 3-dB power splitter which supports E11 and E21 modes are measured to be about 0.5:0.5 with the insertion losses of ∼ 5.1 dB and ∼ 5.5 dB, respectively, at the wavelength of 1550 nm. The power splitting ratio for the three-mode 3-dB power splitter which supports E11, E21 and E12 modes are also measured to be about 0.5:0.5 with the insertion losses of ∼ 6.1 dB, ∼6.4 dB and ∼ 6.7 dB at 1550 nm, respectively. Our presented few-mode 3-dB power splitters could be extended to support more modes and find important applications in mode (de)multiplexing systems.

A Symmetrical Broadband Tight-Coupled Directional Coupler With High Directivity Using Three-Folded-Coupled Lines

In this brief, a novel symmetrical broadband tight-coupled directional coupler with high directivity using three-folded-coupled lines is proposed and implemented. The directional coupler is consisted of two simple coupled lines and two three-folded-coupled (TFC) lines, which realizes advantages of broadband, tight coupling, and high directivity. The TFC line is simplified as two-level coupled lines. Electrical lengths of coupled lines and precise mathematical equations for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}$ </tex-math></inline-formula> -parameters of the proposed TFC-structure coupler are calculated based on the method of even- and odd-mode analysis. The gap between the simplified two-level coupled lines is also considered and discussed. For demonstrative purposes, a broadband coupler using TFC lines is designed and fabricated at about 2 GHz with a power-dividing coefficient of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k} = 0.6$ </tex-math></inline-formula> . The measured fractional bandwidth (FBW) of the proposed coupler is about 54.0% as the return loss <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(|S_{11}|)$ </tex-math></inline-formula> and the isolation <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(|S_{41}|)$ </tex-math></inline-formula> are both higher than 20 dB. The fabricated directional coupler also shows advantages of tight coupling (about 3 dB) and high directivity (the highest directivity is about 33.1 dB).

Nonlinear quantum spectroscopy with parity–time-symmetric integrated circuits

We propose a novel quantum nonlinear interferometer design that incorporates a passive parity–time (PT)-symmetric coupler sandwiched between two nonlinear sections where signal–idler photon pairs are generated. The PT symmetry enables efficient coupling of the longer-wavelength idler photons and facilitates the sensing of losses in the second waveguide exposed to analyte under investigation, whose absorption can be inferred by measuring only the signal intensity at a shorter wavelength where efficient detectors are readily available. Remarkably, we identify a new phenomenon of sharp signal intensity fringe shift at critical idler loss values, which is distinct from the previously studied PT symmetry breaking. We discuss how such unconventional properties arising from quantum interference can provide a route to enhancing the sensing of analytes and facilitate broadband spectroscopy applications in integrated photonic platforms.

Bistable soliton switching dynamics in a PT -symmetric coupler with saturable nonlinearity

We investigate the switching dynamics in a $\mathcal{PT}$-symmetric fiber coupler composed of a saturable nonlinear material as the core. In such a saturable nonlinear medium, bistable solitons may evolve due to the balance between dispersion and saturable nonlinearity, which we extend in the context of the $\mathcal{PT}$-symmetric coupler. Our investigations of power-controlled and phase-sensitive switching show richer soliton switching dynamics than the currently existing conventional counterparts, which may lead to ultrafast and efficient all-optical switching dynamics at very low power owing to the combined effects of $\mathcal{PT}$ symmetry and saturable nonlinearity. In addition to the input power, the relative phase of the input solitons and saturable coefficient are additional controlling parameters that efficiently tailor the switching dynamics. Also, we provide a suitable range of system and pulse parameters that would be helpful for the practical realization of the coupler to use in all-optical switching devices and photonic circuits. Finally, we develop a variational approach to analytically investigate the switching dynamics in such $\mathcal{PT}$-symmetric couplers that excellently predicts the numerical findings.

Tensorial‐Analysis‐of‐Networks Applied to Bandpass Negative‐Group‐Delay Analysis of Resistorless LC‐Coupler‐Network

AbstractThis paper develops a bandpass (BP) negative‐group‐delay (NGD) analysis of multi‐port topology. The circuit under study consists of resistorless symmetric coupler constituted by passive LC‐networks. By using the tensorial‐analysis‐of‐networks (TAN) formalism, an innovative study enables to verify if the four‐port circuit is susceptible to exhibit the unfamiliar BP‐NGD function. Using the TAN formalism, the different steps of the method is described from the branch and mesh spaces via the tensor connectivity to determine the equivalent S‐parameters. The general methodology of the S‐matrix modeling is expressed in the function of the coupler L and C parameters. To validate the BP‐NGD behavior, an arbitrary parametrized lumped four‐port coupler is considered as a proof‐of‐concept. The feasibilities of the TAN S‐parameter modeling added by the BP‐NGD function is validated comparison results between TAN mathematical calculations and simulations from a commercial tool. Excellent correlations between the calculated and simulated results are confirmed with BP‐NGD behavior of the connections between port 1 and port 2 with −30 ns NGD value at the center frequency, 124 MHz, and between port 1 and port 4 with −27 ns NGD value at the center frequency, 92 MHz. The present results confirm a scientific result never being found before, the first time, which demonstrates the possibility to generate the BP‐NGD function with a four‐port resistorless circuit. The TAN method flexibility advantages are the flexibility to lumped and distributed circuits. However, the sensitivity to the component inaccuracies constitute the main drawbacks.