Compact Directional Coupler Research Articles

Compact low loss and broadband hybrid plasmonic directional coupler

We propose a novel broadband coupler for silicon photonics using a hybrid plasmonic waveguide section. The hybrid plasmonic waveguide is used to create an asymmetric section in the middle of a silicon nanowire waveguide coupler to introduce a phase delay to allow for a 3-dB power coupling ratio over a 150 nm bandwidth around 1.55 µm. The device is very compact (<8.5 µm) and has a low insertion loss (<0.15 dB).

A Lumped-Element Unit Cell for Beam-Forming Networks and Its Application to a Miniaturized Butler Matrix

A lumped-element unit cell for designing compact beam-forming networks (BFNs) is proposed. It is a reciprocal four-port network, which is lossless, isolated, and matched at all ports. The unit cell is designed to behave as planar and compact directional couplers with both arbitrary output amplitude and phase distributions, as well as crossovers with arbitrary output phase distributions. For both cases, design equations are derived. The design procedure is validated numerically and experimentally through several examples. The versatility of the unit cell allows the design of BFNs using only the proposed network topology. As an example, a 4 $\,\times\,$ 4 Butler matrix is implemented. A strong size reduction is also achieved with respect to classical designs thanks to the lumped-element implementation. The prototype footprint is approximately 25 times smaller than typical transmission-line-based Butler matrices. The structure shows also a very small area compared to state-of-the-art miniaturized 4 $\,\times\,$ 4 Butler matrix designs. Measurement results confirm the behavior of the Butler matrix as a BFN. The achieved bandwidth is approximately 8%.

Tunable silicon photonics directional coupler driven by a transverse temperature gradient

A compact directional coupler fabricated on a silicon photonic platform is presented, with a power-splitting ratio that can be tuned through a transverse temperature gradient induced by a laterally shifted integrated heater. The tuning mechanism exploits the thermally induced phase velocity mismatch between the coupled modes of the silicon waveguides. The positions of the integrated heater and the waveguide design are optimized to maximize the tuning range and to reduce electric power consumption. Asynchronous devices with an intrinsic phase mismatch are demonstrated to be more efficient, providing a tunable coupled power from 0.7 to 0.01 with 36 mW maximum power dissipation.

Compact UWB directional coupler with one notch-band

An ultra-wideband (UWB) parallel coupled-line directional coupler (DC) with one notch-band based on folded stepped impedance resonator (SIR) is investigated in this paper. The proposed UWB DC is comprised of one conventional microstrip directional coupler, two folded shunt quarter-wavelength short-circuited stubs and one SIR with metalized via hole to ground. The SIR is studied and employed to generate one desired notched band. The predicted results are compared with measured data and good agreement is reported.

Ultra compact slow light photonic crystal directional coupler design with elliptical rods

We propose a novel design for a photonic crystal waveguide coupler, which uses elliptical shaped rods with refractive index equal to 3.4 arranged in square lattice. We investigate the performance of slow light in this photonic crystal coupler. By providing a slow-light region using elliptical cells, a coupling length of just 2.58μm is achieved at optical wavelength. This is extremely small coupling length then previously reported. The proposed structure would offer significant potential for novel compact high-speed optical signal processing devices.

25-terahertz-bandwidth all-optical temporal differentiator

An all-optical temporal differentiator with a record operation bandwidth of ~25 THz (~200 nm, at least one order of magnitude larger than any previously reported temporal differentiation technology) is experimentally demonstrated based on a simple and compact all-fiber wavelength-selective directional coupler. The fabricated directional coupler can be used to process optical signals with time features as short as a few tens of femtosecond. A Gaussian-like optical pulse with a time-width of 250-fs is experimentally differentiated with a processing error of 2.1%. As an application example, a chirp-free flat-top pulse with a time-width of 540-fs is also successfully generated.

All planar compact size microstrip CRLH arbitrary coupling directional coupler

A new compact size microstrip arbitrary coupling directional coupler (DC) based on the coupled line composite right/left handed (CL-CRLH) unit cell is proposed. The coupler is patterned on a single metal layer, does not use vias, and occupies 45 × 11 mm2 area. Its bandwidth is 400 MHz centered around 2.4 GHz and it provides −3 dB coupling coefficient. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:115–119, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27270

A Compact and Temperature-Sensitive Directional Coupler Based on Photonic Crystal Fiber Filled With Liquid Crystal 6CHBT

A directional coupler structure formed by a nematic liquid crystal (NLC)-filled photonic crystal fiber (PCF) represents a promising configuration in sensing applications. Because of large refractive index difference between the NLC and silica material, the mode coupling between the NLC waveguide and the silica core is more complicated than the situation of coupling between two fundamental modes of the waveguides. Therefore, it is necessary to perform a theoretical investigation of the mode properties associated with the experimental studies of the coupling characteristics. In this paper, we present a thorough analysis, both theoretically and experimentally, of the directional coupler structure, including the mode properties, coupling characteristics, and thermal sensing properties. The temperature response of the device is experimentally measured, showing a polynomial curve in nematic phase and a linear curve in isotropic phase. The nonlinearity of the temperature response of the device in nematic phase and the linearity in isotropic phase are attributed to the temperature dependence of the refractive index of the NLC. Specifically, the sensitivity is -3.86 nm/°C in isotropic phase of the 6CHBT with good linearity and shows good agreement with simulation results.

A Highly Reconfigurable Low-Power CMOS Directional Coupler

This paper presents a highly reconfigurable, low-power, and compact directional coupler. The coupler uses varactors and novel active inductors to achieve wide tuning ranges of operating frequencies and coupling coefficients. The use of a low-pass circuit architecture with only two inductors minimizes chip area, power consumption, and noise. The coupler is implemented in a 0.13-μm CMOS process. It occupies an area of 350 μm× 340 μm and consumes 40 mW or less power. The obtained 1-dB compression point is -3.2 dBm, and the measured noise figure is ~ 23 dB. These parameters compare favorably with previously published reconfigurable couplers. The measured coupling coefficient can be tuned from 1.3 to 9.0 dB at 4 GHz with 32 dB or better isolation and 15 dB or better return loss. The operating center frequency can be tuned from 2.0 to 6.0 GHz for a nominal 3-dB operation. These results agree with theoretical predictions and simulations reasonably well.

Compact microstrip line directional coupler with high directivity

A compact directional coupler with high directivity is presented. The coupling passbands characteristics of the multisections coupler which cascades two coupled lines via transmission line are analyzed. In addition, asymmetric coupled line and stepped impedance matching method are used to generate the isolation zero. A 2.45 GHz coupler is designed and implemented. The 3 dB bandwidth of the coupling is from 1.2 to 3.55 GHz with the corresponding fractional bandwidth (FBW) of 98%, and the directivity is better than 20 dB from 0.5 to 3.5 GHz. The proposed coupler presents small size and excellent directivity compared with the traditional parallel coupled line coupler.

Low cost and compact directional coupler for ultrawideband applications

AbstractA design of a compact size (35 × 15 mm2) 3‐dB coupler with ultrawideband (UWB) characteristic is demonstrated in this article. The proposed design is accomplished by forming two diamond‐shaped microstrip lines that are broadside coupled by a slot at the middle ground plane. The dimension of the design was initialized using a simple mathematical formula and optimized using the CST microwave studio simulator. The design coupler is fabricated and measured. It shows comparable results of the simulation and measurement for the phase difference between two output ports, return loss, isolation, and 3‐dB coupling over an UWB frequency range between 4 and 8.34 GHz. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:670–674, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26616

A CMOS Power Amplifier With Integrated-Passive-Device Spiral-Shaped Directional Coupler for Mobile UHF RFID Reader

A CMOS power amplifier (PA) with a compact spiral-shaped directional coupler for a mobile UHF RF identification (RFID) reader is proposed here, and its output power combiner and the directional coupler are implemented using an integrated passive device process. The two-chip solution not only enables a CMOS PA to be highly efficient, but also allows the directional coupler and the power combiner to be mounted in a compact standard package. A polar transmitter is implemented using the CMOS PA with the directional coupler to verify the operation of the proposed configuration for a UHF RFID reader. Measurements indicate that the CMOS PA with the directional coupler transmits 27.3 dBm of output with 44.6% of power-added efficiency and that the implemented polar transmitter satisfies the required UHF RFID reader specifications.

Experimental demonstration of tunable directional excitation of surface plasmon polaritons with a subwavelength metallic double slit

We demonstrate experimentally the directional excitation of surface plasmon polaritons (SPPs) on a metal film by a subwavelength double slit under backside illumination, based on the interference of SPPs generated by the two slits. By varying the incident angle, the SPPs can be tunably directed into two opposite propagating directions with a predetermined splitting ratio. Under certain incident angle, unidirectional SPP excitation can be achieved. This compact directional SPP coupler is potentially useful for many on-chip applications. As an example, we show the integration of the double-slit couplers with SPP Bragg mirrors, which can effectively realize selective coupling of SPPs into different ports in an integrated plasmonic chip.

A Novel Compact Forward-Wave Directional Coupler Design Using Periodical Patterned Ground Structure

A novel forward-wave directional coupler with periodical patterned ground structures is proposed. The input impedance of the even and odd mode of the proposed coupler can be matched, and the phase difference between the even and odd mode is also strongly enhanced. The equivalent models are constructed to predict the propagation properties based on the Bloch-Floquet theorem. A completed design procedure is also proposed based on the equivalent models. This two-layer structure is fabricated on Duroid substrate. 0.5-dB coupling level can be measured at 1.5 GHz, and the coupler length is 0.5 λg. Compared to previous studies, the proposed coupler can be designed at a lower frequency range with a smaller size.

Compact capacitive compensated directional coupler using planar artificial transmission lines

A novel compact directional coupler based on planar artificial transmission lines is proposed. Interdigital capacitors are introduced to compensate for the odd-mode phase velocity in order to achieve a high directivity. The size of this proposed coupler is about 25% that of a conventional one. A 20 dB directional coupler at 900 MHz has been designed and fabricated. The directivity reaches 36 dB at the centre frequency. The design, simulation and measurements are presented.

Coupling interaction of electromagnetic wave in a groove doublet configuration

Based on the waveguide mode (WGM) method, coupling interaction of electromagnetic wave in a groove doublet configuration is studied. The formulation obtained by WGM method for a single groove [Prog. Electromagn. Res. 18, 1-17 (1998)] is extended to two grooves. By exploring the total scattered field of the configuration, coupling interaction ratios are defined to describe the interaction between grooves quantitatively. Since each groove in this groove doublet configuration is regarded as the basic unit, the effects of coupling interaction on the scattered fields of each groove can be investigated respectively. Numerical results show that an oscillatory behavior of coupling interaction is damped with increasing groove spacing. The incident and scattering angle dependence of coupling interaction is symmetrical when the two grooves are the same. For the case of two subwavelength grooves, the coupling interaction is not sensitive to the incident angle and scattering angle. Although the case of two grooves is discussed for simplicity, the formulation developed in this article can be generalized to arbitrary number of grooves. Moreover, our study offers a simple alternative to investigate and design metallic gratings, compact directional antennas, couplers, and other devices especially in low frequency regime such as THz and microwave domain.

Electro-optically switched compact coupled photonic crystal waveguide directional coupler

In this paper, we present a compact photonic crystal directional coupler in a silicon on insulator platform electro-optically switched at 150 kHz with a switching time of 620 ns under a low voltage operation of 2.9 V. The switch design utilizes a coupled photonic crystal structure designed to operate in the slow light regime. Switching is attained by modulating the coupling coefficient of the coupled photonic crystal waveguide system by using a p-i-n diode to modulate the carrier concentration with a density of ∼104 A/cm2 across the plane of the photonic crystal.

Artificial-Synthesized Edge-Coupled Transmission Lines for Compact CMOS Directional Coupler Designs

This paper presents artificial-synthesized edge- coupled transmission lines (TLs) based on complementary metal- oxide semiconductor (CMOS) technology. The artificial coupled- line (CL), the so-called the complementary-conducting-strip coupled line (CCS CL), consists of the unit cell and is meandered in the 2-D plane. The unit cell, whose dimensions are much smaller than the guiding wavelength at the operating frequency, provides five structural parameters for CL syntheses. The synthesized CLs, which are fully characterized by using theoretical and experimental procedures, reveal the following unique guiding properties. First, the coupling coefficient (K) can be 7.7% higher than that of the meandered conventional edged-coupled TLs. Second, the even- and odd-mode propagation constants can be synthesized as identical at the same time. Two backward-wave directional couplers for achieving the tight coupling and high directivity are designed by incorporating artificial CCS CL. Two prototypes are fabricated by the standard 0.18-?m CMOS technology, occupying the chip areas of 120.0 ?m × 240.0 ?m. The measured results show that the isolation is higher than 25.0 dB from 10.0 to 40.0 GHz with a coupling of 14.4 dB. On the tight coupler design, the through and coupling are 4.65 ± 0.15 dB from 30.0 to 40.0 GHz with an amplitude imbalance of less than 0.3 dB.

Miniaturized Dual-Band Directional Couplers Using Composite Right/Left-Handed Transmission Structures and Their Applications in Beam Pattern Diversity Systems

This paper demonstrates the feasibility of dual-band compact and bandwidth-enhanced directional couplers, including both 90deg and 180deg hybrids, using composite right/left-handed (CRLH) transmission structures. Novel and systematic design methodology leading to size miniaturization is proposed. By considering all possible dual-band phase transitions of the constituent transmission lines of the directional couplers, this design approach ensures the compactness of the hybrids and reduces footprint size significantly compared to the previously exhibited standard transmission-line- and CRLH-based hybrids. The proposed dual-band 90deg and 180deg hybrids achieve 10% and 43.7% size reduction, respectively, compared to conventional microstrip couplers in the lower band of the dual-frequency operation. In addition, the fabricated 180deg hybrid is shown to have enhanced bandwidth performance in both isolation and 3-dB magnitude balance. The combination of either of the proposed couplers with the proposed dual-band CRLH-based delay line enables antenna pattern diversity with distinct beam patterns in dual bands. This serves as an effective dual-band beam pattern diversity system when connected to the antenna array. Moreover, the isolation improvement by cascading the antenna array with this assembly was measured experimentally at two operational bands: 22.7 dB at 1 GHz and 10 dB at 2 GHz.

Compact silicon microring-assisted directional couplers for optical signal processing applications

We propose and demonstrate a compact microring-assisted directional coupler in the silicon-on-insulator material system. An analytical model based on the strongly coupled mode theory shows that the device can be designed to have a wide range of spectral characteristics, including sharp asymmetric Fano line shapes, which can be used for WDM, switching, and sensing applications. The measured transmission spectra of the fabricated device with a 5-mum-radius microring resonator exhibited an asymmetric spectral response characteristic of the Fano resonance as predicted by the analytical model.