Broadband Coupling Research Articles

A Low-Cost Wideband 77-GHz Planar Butler Matrix in SIW Technology

This communication presents a low-cost 77-GHz switched-beam slot antenna array driven by a Butler matrix. In the proposed configuration, four broadband couplers are combined and crossovers are effectively avoided. In this case, the overall circuit and beamforming network losses are considerably reduced. A 4 × 4 planar SIW Butler matrix is designed and demonstrated for integrated beamforming network applications, which exhibits about 7 degrees phase error and ±0.75 dB coupling imbalance over 11% bandwidth. This experimentally prototyped matrix is integrated with a four-array slot antenna on the same substrate. An alternating displacement of the slots in subsequent radiating waveguides is proposed to save the arrangement of the input ports and to achieve broadband performances. Measured radiation patterns are in good agreement with simulated counterparts over the proposed 77-GHz frequency range.

Solar energy trapping with modulated silicon nanowire photonic crystals

We demonstrate the efficacy of nanostructured thin film silicon solar cells to trap and absorb approximately 75% of all sunlight incident (400 nm–1200 nm) with an equivalent bulk thickness of only 1 micron of silicon. This is achieved by sculpting the collection zone into a three-dimensional, simple-cubic-symmetry, photonic crystal consisting of modulated silicon nanowires embedded in SiO2 and sitting on a quartz substrate with no metallic mirrors. A specific modulation of the radius of nanowires provides antireflection, strong light trapping, and back-reflection mechanisms in targeted spectral regions. This modulation is linear at the top of the nano-rods leading to nanocones at the solar cell to air boundary. These silicon nanocones are very good absorbers at short wavelengths and act as broadband coupler to a light-trapping region below at longer wavelengths. In the light trapping region the modulation is periodic to form a simple cubic photonic crystal exhibiting a broad spectrum of strong parallel interface refraction resonances. Here, light incident from most angles is deflected into slow group velocity modes with energy flow nearly parallel to the interface, long dwell times, and strong light intensity enhancement (up to 150 times the incident intensity) in specific regions. Finally, a stronger and chirped modulation of the nanowire underneath provides back-reflection by means of a one-dimensional depth-dependent photonic stop-gap. The possibility of absorbing light at energies below the electronic band gap of silicon is illustrated using a graded index SixGe1−x alloy in the bottom section of each nanowire. Each nanowire is amenable to a radial P-N junction for proximal charge carrier separation and efficient collection of photo-generated current.

Photonic wire bonding: a novel concept for chip-scale interconnects

Photonic integration requires a versatile packaging technology that enables low-loss interconnects between photonic chips in three-dimensional configurations. In this paper we introduce the concept of photonic wire bonding, where polymer waveguides with three-dimensional freeform geometries are used to bridge the gap between nanophotonic circuits located on different chips. In a proof-of-principle experiment, we demonstrate the fabrication of single-mode photonic wire bonds (PWB) by direct-write two-photon lithography. First-generation prototypes allow for efficient broadband coupling with average insertion losses of only 1.6 dB in the C-band and can carry wavelength-division multiplexing signals with multi-Tbit/s data rates. Photonic wire bonding is well suited for automated mass production, and we expect the technology to enable optical multi-chip systems with enhanced performance and flexibility.

High Efficiency and Broadband Two-Dimensional Blazed Grating Coupler With Fully Etched Triangular Holes

A 2-D blazed grating coupler with fully etched triangular holes to realize high efficient and broadband coupling between silicon-on-insulator nanowaveguide and fiber is proposed. By changing the lateral width of a triangular hole along propagating direction in each single grating period gradually according to effective medium theory, a high coupling efficiency of -2.78 dB and an extremely large 1 dB bandwidth of 180 nm are achieved simultaneously. By optimizing the buried oxide thickness from 2 to 1.83 μm, the coupling efficiency can be further improved to -2.18 dB and 1 dB bandwidth is enlarged to 290 nm.

Imaging of Acoustic Waves in Piezoelectric Ceramics by Coulomb Coupling

The transport properties of bulk and guided acoustic waves travelling in a lead zirconate titanate (PZT) disc, originally manufactured to serve as ultrasonic transducer, have been monitored by scanned Coulomb coupling. The images are recorded by excitation and detection of ultrasound with local electric field probes via piezoelectric coupling. A narrow pulse has been used for excitation. Broadband coupling is achieved since neither mechanical nor electrical resonances are involved. The velocities of the traveling acoustic waves determined from the images are compared with characteristic velocities calculated from material properties listed by the manufacturer of the PZT plate.

Terahertz current oscillations in a gated two-dimensional electron gas with antenna integrated at the channel ends

We studied terahertz current oscillations induced by a frequency-tunable radiation source in a AlGaAs/InGaAs/AlGaAs heterostructure field effect transistor channel. A planar antenna was integrated on-chip, and a substrate lens was used for broadband coupling of free-space radiation at 0.18–0.72 THz to the channel ends. Through spectral analysis of the detection signal, we identified two different mixing mechanisms: one related to channel current oscillations and the other to modulation of the gate-to-channel potential. Depending on gate bias and radiation frequency, the two mechanisms either compete or cooperate, leading to responsivity up to 300 V/W and noise equivalent power of 1 nW/Hz0.5

Substrate Integrated Composite Right-/Left-Handed Leaky-Wave Structure for Polarization-Flexible Antenna Application

An effective development of a composite right-/left-handed (CRLH) leaky-wave (LW) structure for polarization-flexible antenna applications is presented. The proposed leaky transmission line (TL) is a planar passive circuit built using the substrate integrated waveguide technology. It consists of two symmetrical waveguide lines loaded with series interdigital capacitors which radiate orthogonal 45° linearly polarized waves. Its dispersion, Bloch impedance and radiation characteristics are extracted by applying a comprehensive analysis on the unit cell. Its backfire-to-endfire beam-steering capability through frequency scanning due to the CRLH nature is demonstrated and discussed. It is able to generate arbitrary different polarization states by changing the way of excitation, including linear polarization (LP) and circular polarization (CP). This leaky TL is fabricated by the standard printed-circuit board process. Two broadband couplers are also designed and fabricated for the specified excitation purpose. Six different polarization states, including four LP cases and two CP ones, are experimentally verified. The propagation and radiation parameters, including the S-parameters, radiation patterns, gain, and axial ratio (for CP states) are presented for these modes. Measured results are consistent with the simulation. The proposed LW structure shows some desirable merits, such as the simplicity in design, low-cost fabrication, and beam-steering and polarization-flexible capabilities, providing a high degree of flexibility for the real application.

Electromagnetically induced transparency for Λ-like systems with a structured continuum and broad-band coupling laser

In this paper, electric susceptibility of a laser-dressed atomic medium is calculated for a model Λ-like system which includes two lower states and a structured continuum. It is assumed that the laser control field applied in the experiment of electromagnetically induced transparency can be decomposed into two parts: a deterministic (coherent) part and a randomly fluctuating chaotic component (white noise). The exact formulae determining the dispersion and absorption spectra of the medium susceptibility are derived by solving a set of coupled stochastic integro-differential equations. The results show that the position and width of the transparency window change dramatically in comparison with the case when the noise of the control laser is absent.

A Novel Balanced Amplifier Based on HMSIW Coupler

In this letter, a broadband coupler is presented that makes use of a half mode substrate integrated waveguide (HMSIW) technique using a printed circuit board process. The coupler is realized by a parallel HMSIW line which couples energy by magnetic field. Compared with microstrip coupler and conventional HMSIW coupler it has lower loss and better Electromagnetic Compatibility owning to the closed field structure. Compared with SIW coupler, it has smaller size and lower cost owing to the half TE10 model. Based on coupler, a two-way balanced power combining amplifier is designed and measured. The measured results show a good agreement with simulation and a combining efficiency of 88% is achieved at 9.7 GHz.

Design and optimization of a novel broadband and polarization-insensitive dual-core photonic crystal fiber coupler over the whole optical communication band

In this paper, a novel broadband polarization-insensitive dual-core photonic crystal fiber with elliptical central hole is proposed and the influence of its structural parameters on coupling characteristic is investigated in detail by the full-vector finite element method. Through optimizing the fiber structural parameters, broadband and polarization-insensitive characteristics are achieved over the whole optical communication band from 1.225 m to 1.675 m. The variation of coupling ratio is stabilized at 50% 1%, and the coupling ratio difference between x polarization and y polarization is less than 0.5% overall the wavelength range. Due to its relatively independent of cores and elliptical central hole and suitable structural parameters, this fiber meets the application requirements, that is, easy to fabricate, easy to splice and low splice loss. This research is freed from the current coupler limit: narrow bandwidth, wavelength dependence, polarization-sensitive, difficulty of fabricating, thereby provides the theoretical basis for the study of the large capacity high speed all-optical networks and multi-wavelength tunable fiber laser.

Compact cantilever couplers for low-loss fiber coupling to silicon photonic integrated circuits

We demonstrate coupling from tapered optical fibers to 450 nm by 250 nm silicon strip waveguides using compact cantilever couplers. The couplers consist of silicon inverse width tapers embedded within silicon dioxide cantilevers. Finite difference time domain simulations are used to design the length of the silicon inverse width taper to as short as 6.5 μm for a cantilever width of 2 μm. Modeling of various strip waveguide taper profiles shows reduced coupling losses for a quadratic taper profile. Infrared measurements of fabricated devices demonstrate average coupling losses of 0.62 dB per connection for the quasi-TE mode and 0.50 dB per connection for the quasi-TM mode across the optical telecommunications C band. In the wavelength range from 1477 nm to 1580 nm, coupling losses for both polarizations are less than 1 dB per connection. The compact, broadband, and low-loss coupling scheme enables direct access to photonic integrated circuits on an entire chip surface without the need for dicing or cleaving the chip.

A broadband and compact asymmetrical backward coupled-line coupler with high coupling level

A new wideband asymmetric microstrip coupled-line coupler with 3dB coupling value and quadrature phase difference is presented. Compared with the conventional edge-coupled couplers, this structure, consisting of two different transmission lines (interdigital and conventional microstrip transmission lines) as coupled lines, achieves wider operating bandwidth and larger coupling level. The coupled-line length of the proposed structure is approximately λg/4. To characterize the structure, an equivalent circuit model has been established. A 3dB designed and fabricated coupler with 0.2mm spacing between coupled lines exhibits an amplitude balance of 2dB from 2.2GHz to 4.2GHz. Good agreements between the full-wave simulation and equivalent circuit model results has been achieved and verified the effectiveness of the proposed circuit model. Also, measurement results have been presented.

Efficient Strip to Strip-Loaded Slot Mode Converter in Silicon-on-Insulator

We present a mode converter in the silicon-on-insulator platform, which enables highly efficient, broadband coupling between a strip waveguide and a strip-loaded slot waveguide near 1.55 μm. The proposed converter has the benefit of electrically isolating the two arms of the slot waveguide. We fabricated and tested our mode converters, and achieved 0.81 ±0.49 dB per converter insertion loss in the best set of devices, in agreement with simulation.

A terahertz broadband 3dB directional coupler based on bridged PPDW

In this paper, a novel broadband 3 dB directional coupler with very flat coupling based on bridged parallel plate dielectric waveguide (PPDW) is proposed and demonstrated. In the uniform coupling section, a bridge structure between the two PPDWs is employed to obtain accurate coupling value and achieve a broadband coupling. It is found that this new type of coupling structure exhibits excellent performance at terahertz frequencies. In order to achieve strong isolation between the adjacent ports and reduce the power reflection in all ports, two quarter-circle bend arms are introduced as the curved transition sections to connect the uniform coupling section. For this bridged coupler, it only needs the value of the uniform coupling length as short as 400 μm to achieve a broadband 3 dB coupling. In this case, the coupler's average return loss is greater than 28 dB, average isolation is better than 27 dB and average coupler loss is only 0.9 dB, over a percentage bandwidth of 12.5% at 1 THz. Compared to the conventional PPDW coupler, the bridged PPDW coupler shows significantly greater bandwidth (about 4.2 times), compact and mechanically stable with a much shorter uniform coupling length (reduced about 61%), which may have potential applications for terahertz integrated circuits and systems.

Polymer Hybrid Plasmonic Waveguides and Microring Resonators

We report the design and fabrication of hybrid plasmonic waveguides based on the Au/SiO2/SU-8 material system. Monolithic integration of these devices with dielectric SU-8 waveguides on the same chip enabled efficient and broadband coupling between photonic and plasmonic devices to be achieved. Compact hybrid plasmonic microring resonators were also demonstrated with high intrinsic Q-factors close to 300.

Numerical design and scattering losses of a one-dimensional metallo–dielectric multilayer with broadband coupling of propagating waves to plasmon modes in the visible range

We report on the design, fabrication, and optical characterization of a one-dimensional (1D) metallo–dielectric metamaterial tuned to enhance evanescent wave transmission with kx≤4k0. Taking reflection and transmission measurements from a 1D subwavelength diffraction grating placed on the metamaterial, we show broadband-propagating wave to surface plasmon coupling in the visible. However, the fabricated device falls short of the design expectations based on coupled-wave numerical simulations. The dips in the reflection spectrum associated with surface plasmon coupling are 40% smaller than predicted, and the transmission exhibits strong depolarization. Overall, the numerical results support that intrinsic metallic losses do not preclude the development of these devices in the visible, but there are considerable plasmon scattering losses from the metamaterial’s imperfections. This extrinsic limiting factor needs to be overcome to develop metallo–dielectric metamaterials for practical components for use in superlenses.

Ultrabroadband efficient intracavity XUV output coupler

We report an efficient intracavity XUV output coupler based on an anti-reflection-coated grazing incidence plate (GIP). Conceptually, GIP is an extension of a Brewster plate, affording low loss of the circulating fundamental light and serving as a highly efficient, extremely broadband output coupler for XUV. Due to the grazing incidence geometry, the short wavelength reflectivity can be extended to the keV range. The first GIP realized shows parameters close to the design. We discuss both the limitations of the GIP in comparison with other XUV output couplers and the applicability of the GIP extension at longer wavelengths, down to the MIR.

Design and optimization of broadband and polarization-insensitive dual-core photonic crystal fiber coupler

In this paper, we propose a novel (to our knowledge) broadband and polarization-insensitive dual-core photonic crystal fiber (PCF) coupler through the introduction of an elliptical-shaped central air hole to offset the slight birefringence arising from the dual core. With a full vectorial finite element method and anisotropic perfectly matched layers as the external boundaries, the impact of several fiber parameters on the coupling characteristics of dual-core PCF is investigated in detail. Through optimizing the main fiber parameters, including core diameter, size and ellipticity of the central air hole, and refractive index difference, broadband and polarization-insensitive characteristics are achieved in the wavelength range from 0.8 to 1.7 μm. The variation of the coupling ratio is stabilized at 50±1%, and the coupling ratio difference between x polarization and y polarization is less than 2% over the wavelength range. This dual-core PCF makes it easier to develop a 3 dB coupler over a wide wavelength for passive optical networks and large optical systems.

Compact and broadband tunable hybrid ring coupler using synthetic left- and right-handed transmission lines

In this letter, we present a frequency tunable hybrid ring coupler based on the concepts of a synthetic left- and right-handed transmission line.The proposed ring coupler makes use of lumped inductor-capacitor (LC) elements to greatly reduce the circuit size; meanwhile, the frequency responses of the coupler can be tuned by direct current (DC) bias voltages. The fabricated hybrid ring coupler shows a wideband performance with an insertion loss of less than 2.5 dB from 1.7 to 3.3 GHz. For this frequency band, the isolation is maintained at more than 24 dB. When the circuit is excited to get a pair of differential signal, less than ±5° phase error is observed for the whole frequency range. This hybrid has a 78% size reduction compared to the conventional ones and achieved a 64% of fractional bandwidth at 2.5 GHz. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1379–1382, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25959

High Frequency and Broadband Coupling Characteristics of Filter Circuit Based on Low Voltage Power Lines

The low voltage power line has some advantages, for example, wide application, connecting fastness and economy. Power line communication technology is becoming a hot research area. Coupling filter circuit is the key link of power line communication. In this paper, a 1MHz-30MHz high frequency and broadband coupling circuit which consists of filter, coupling capacitor, isolation transformer was designed. This circuit can be used to test the input impedance, noise characteristics, and attenuation characteristics of channel. Designed the Butterworth band-pass filter, through simulation, got the amplitude frequency characteristics, and proved the feasibility of the filter. According to the working frequency, selecting the suitable coupling capacitor with band-pass filter and isolation transformer constituted the coupling circuit. In this paper, chose the 50Hz and 15MHz AC signal as contrast signals to simulate and got the two signals’ simulating results. The results show that the coupling circuit has isolation effect of the 50Hz industrial frequency signal, instead the 1MHz-30MHz signals can have no distortion to couple to the electric network.