Broadband Directional Coupler Research Articles

Highly efficient and selective integrated directional couplers for multigas sensing applications

The design and fabrication of a compact, low-loss, broadband directional coupler (DC) based duplexer operating in the near-infrared (NIR) region are demonstrated. The duplexer exhibits high selectivity and coupling efficiency (CE), for target wavelengths of 1530 nm and 1653.7 nm, making it applicable in systems for the multi-gas detection of ammonia and methane. The measured CE for the duplexer is 73% and 76% at 1530 nm and 1653.7 nm respectively. These results demonstrate the effectiveness of the duplexer as a broadband and scalable power source for highly sensitive sensing techniques, like quartz-enhanced photoacoustic spectroscopy (QEPAS). Its compact size and low-loss characteristics make it highly portable and well-suited for drone-based multi-gas detection applications.

Integrate and scale: a source of spectrally separable photon pairs

Integrated photonics is a powerful contender in the race for a fault-tolerant quantum computer, claiming to be a platform capable of scaling to the necessary number of qubits. This necessitates the use of high-quality quantum states, which we create here using an all-around high-performing photon source on an integrated photonics platform. We use a photonic molecule architecture and broadband directional couplers to protect against fabrication tolerances and ensure reliable operation. As a result, we simultaneously measure a spectral purity of 99.1±0.1%, a pair generation rate of 4.4±0.1MHzmW−2, and an intrinsic source heralding efficiency of 94.0±2.9%. We also see a maximum coincidence-to-accidental ratio of 1644±263. We claim over an order of magnitude improvement in the trivariate trade-off among source heralding efficiency, purity, and brightness. Future implementations of the source could achieve in excess of 99% purity and heralding efficiency using the lowest reported propagation losses.

Broadband and Low-Loss Silicon Photonic Directional Coupler for Signal Power Tapping on the 3 μm SOI Waveguide Platform

Silicon photonics (SiPh) has emerged as a promising technology for photonic integrated circuits (PICs). One of the basic components in SiPh is the directional coupler (DC), which plays an important role in signal monitoring with the requirement of low wavelength dependence and low loss. This paper proposes a broadband and low-loss DC designed for signal power tapping on the 3 μm silicon-on-insulator (SOI) waveguide platform. By utilizing the advantages of multi-micron waveguides and replacing one of the straight waveguides with an optimized arc-shaped waveguide in the coupling region, the proposed DC enhances spectral stability and improves transmission with negligible loss. Experimental evidence indicates that the proposed DC showcases a minimal variation in the tapping ratio. From 1470 nm to 1630 nm, the largest deviation away from the tapping ratio at 1550 nm is 1.433%. Additionally, the device exhibits a low excess loss of −0.27 dB. These results suggest that the proposed device is well-suited to reliable signal power tapping and monitoring, particularly within PICs.

Broadband Directional Coupler with High Isolation

A novel wideband microstrip directional coupler operating at 1 to 5 GHz with high isolation is presented in this paper. Parallel coupling lines replace the traditional coupling transmission lines, providing broadband characteristics. The rectangular defected ground structure (DGS) on the RF ground on the back increases the isolation. The coupler was designed using 3D electromagnetic simulations. FR4-type dielectric substrate has been used due to its low cost. Simulation results prove that the proposed directional coupler provides an isolation of at least 20 dB on the full frequency band from 1 to 5 GHz, and reaches 66 dB at 2.89 GHz. The coupling degree is about 12 dB. Moreover, the proposed design can also achieve wide operating bandwidth (approximately 139% fractional bandwidth).

Compact and broadband directional coupler for high power applications

A compact, flat coupling, and broadband directional coupler are presented for high-power applications in the frequency band of 100 ∼ 1000 MHz. The coupler is mainly composed of two compact semi-flexible coaxial cables with the length of the λ 0/4 (λ 0 is the free-space wavelength at the frequency of 850 MHz), four chip ceramic capacitors, and two air wound inductors that can tune the coupling coefficient. The phase inverter of the coupler is well designed and experimentally validated. The directional coupler exhibits high performance in the whole operation frequency band with low insertion loss (less than 0.3 dB), high directivity (more than 18 dB), and high power (up to 300 W). The amplitude imbalance of the coupling coefficient is less than 0.3 dB. The measured results exhibit good consistency with the theoretical analysis. And the proposed design method can be readily extended to other low-cost, high-power, and broadband directional couplers.

A broadband directional coupler based on ferrite cores

A broadband directional coupler based on ferrite cores is proposed for high frequency (HF)/Very high frequency (VHF)/Ultra High Frequency (UHF) communication systems. The transformer-type coupler is composed of two double-aperture ferrite cores and 1-mm-diameter strands. The ferrite cores with different relative permeabilities are screened to achieve an ultra-wide bandwidth (5 –1000 MHz). The coupler demonstrates excellent electric performances. The voltage standing wave ratio of each port is less than 1.2, a stable −20 dB coupling level is maintained over the operating bandwidth and the insertion loss is less than 0.8 dB. Such couplers have compact sizes and are of very low cost.

Compact broadband suspended silicon photonic directional coupler.

Directional couplers are extensively used in photonic integrated circuits as basic components for efficient on-chip photonic signal routing. Conventionally, directional couplers are fully encapsulated in the technology's waveguide cladding material. In this Letter, we demonstrate a compact broadband directional coupler, fully suspended in air and exhibiting efficient power coupling in the cross state. The coupler is designed and built based on IMEC's iSiPP50G standard platform, and hydrofluoric (HF) vapor-etching-based post-processing allows to release the freestanding component. A low insertion loss of 0.5 dB at λ=1560nm and a 1 dB bandwidth of 35 nm at λ=1550nm have been confirmed experimentally. With a small footprint of 20µm×30µm and high mechanical stability, this directional coupler can serve as a basic building block for large-scale silicon photonic microelectromechanical systems (MEMS) circuits.

A Compact and Broadband Directional Coupler for High-Power Radio Frequency Applications

In this letter, a compact and broadband directional coupler is proposed for high-power radio frequency applications in the frequency band of 5–150 MHz. The coupler is composed of a suspended semiflexible coaxial cable and a series of equalizers composed of lumped components. Such design exhibits excellent performance in high power handling capacity (~kW), high directivity (~35 dB), low return loss (>20 dB), and flat coupling coefficient (45 ± 1.3 dB).

Robust and broadband integrated terahertz coupler conducted with adiabatic following

As the key concept in fabricating integrated device, surface plasmon-polaritons (SPPs) have been widely employed to artificially manipulate the electromagnetic waves in metallic surfaces. However, due to the highly structure-dependent resonance of SPPs, it is challengeable to develop a fixed device which can function at wide band. Here, we propose a novel broadband and robust SPPs directional coupler based on the tri-layered curved waveguides working at terahertz (THz) frequencies, where the coupling of SPPs between the adjacent waveguides can be modeled with coupled mode theory. By introducing the stimulated raman adiabatic passage quantum control technique, we achieve the complete transfer of SPPs from the input to the output waveguides in the range of 0.9–1.3 THz, and even considering the propagation loss, the transfer rate is still above 70%. Furthermore, the performance of our device is eminently robust because of its insensitivity to the geometry of structure and the wavelength of SPPs. As a result, our device can tolerate defect induced by fabrication processing and manipulate THz waves at broadband. This finding provides a new theoretical guideline in promoting THz on-demand applications, which is of significance in developing integrated THz devices.

Simplified Approach for Optimizing Optical Asymmetric Curved Waveguides of Broadband Directional Couplers

ABSTRACTOptical broadband directional couplers (BDCs) are indispensable components for providing wavelength-insensitive and flexible optical splitting in the construction of functional photonic integrated circuits (PICs). The existing BDC device structures are usually required to determine specific design parameters for different waveguide structures and operating wavelength bands. To circumvent this dilemma, here we present a novel optimization procedure to realize a compact BDC by using the asymmetric curved waveguide structure. The versatile particle swarm optimization (PSO) technique is adopted to determine the optimal device parameters of the compact and broadband asymmetric curved directional couplers (ACDCs) for different coupling ratios. In order to reduce the computational complexity in the optimization, the 3D ACDC is first converted to an equivalent 2D structure by using the modified effective index method (MEIM). The device parameters of the equivalent 2D ACDC are optimized by the PSO with the objective function of a wavelength flattened coupling ratio. Afterward, the optimized 2D structure is converted to the 3D one by including the waveguide thickness. To cope with the approximation error by the MEIM, the 3D ACDC is further fine-tuned by sweeping one of the device parameters with the full 3D simulation but keeping all of the other optimal parameters obtained from the PSO intact. As a result, a DC with broad bandwidth of 100 nm is obtained over the wavelength range from 1.50 µm to 1.60 µm with a very small coupling length of 6 µm. The semi-optimized ACDC is used to construct an unbalanced Mach-Zehnder interferometer (MZI) and a Sagnac loop mirror (SLM), both of which show high extinction ratios of >25 dB over a broad wavelength range with low excess loss.

Broadband feeding networks based on directional filters for two-beam antenna arrays

ABSTRACT An alternative approach for the design of feeding networks designated for two-beam antenna arrays that ensure similar radiation patterns in a very broad frequency range has been proposed. The presented concept is based on directional filters that are utilized for smooth change of excitation signals of two broadband radiating elements and a broadband directional coupler used for ensuring quadrature excitations between two identical parts of the antenna arrays. The proposed concept has been confirmed by measurements of two-beams antenna arrays operating in frequency ratio fH /fL ≈ 2:1 and 3:1.

Ultrabroadband lattice filters for integrated photonic spectroscopy and sensing

We report the design, fabrication, and measurement of waveguide lattice filters for use in integrated Raman- or fluorescence-based spectroscopy and sensing systems. The filters consist of a series of broadband directional couplers and optical delay sections that create an n-stage unbalanced Mach–Zehnder interferometer specifically designed to segregate pump light and redshifted signal light in the two output ports. We first report the design criteria for optimal filter performance. Then, we use these criteria with numerical beam propagation methods to design specific broadband couplers. The filters were fabricated by a photonic integrated circuit foundry and measured using white-light spectroscopy. We report both four-stage and eight-stage filters, with the eight-stage filter demonstrating a 190-nm-wide signal passband (1100 cm − 1) on the “through” port with <1.5 dB of ripple and a 17-nm-wide, 20-dB extinction band at the filter resonance.

Silicon-on-Insulator Substrates as a Micromachining Platform for Advanced Terahertz Circuits

This paper presents a comprehensive overview of the development and utilization of a micromachined silicon-on-insulator (SOI) fabrication process that has enabled the development of terahertz (THz) frequency superconducting-insulator-superconducting (SIS) and hot-electron bolometer (HEB) mixers, broadband directional couplers, on-wafer probes, as well as several multipliers. Through the detailed presentation of these circuits, it is demonstrated that ultrathin silicon is able to provide the required characteristics to enable the heterogeneous integration of multiple device technologies that is likely to be required for future THz system-on-chip (T-SoC) development.

Polarization-Insensitive and Broadband Optical Power Splitter With a Tunable Power Splitting Ratio

In this paper, we propose a novel tunable power splitter based on symmetric Mach–Zehnder (MZ) structure to achieve the polarization-insensitive and broadband performance. This good performance is realized by utilizing two 3 dB broadband directional couplers and square waveguides in a conventional symmetric MZ structure. Based on thermo-optic effect, the power splitting ratio of our power splitter for the TE and TM modes can be equally controlled in a wide operating bandwidth of 63 nm. Our results show a low polarization-dependent loss of 0.001–0.089 dB over a wide wavelength range of 1541–1604 nm.

Ultra-wideband high-speed Mach-Zehnder switch based on hybrid plasmonic waveguides.

In this paper, the distinctive dispersion characteristic of hybrid plasmonic waveguides is exploited for designing ultra-wideband directional couplers. It is shown that by using optimized geometrical dimensions for hybrid plasmonic waveguides, nearly wavelength-independent directional couplers can be achieved. These broadband directional couplers are then used to design Mach-Zehnder-interferometer-based switches. Our simulation results show the ultra-wide bandwidth of ∼260 nm for the proposed hybrid plasmonic-waveguide-based switch. Further investigation of the proposed Mach-Zehnder switch confirms that because of the strong light confinement in the hybrid plasmonic waveguide structure, the switching time, power consumption, and overall footprint of the device can be significantly improved compared to silicon-ridge-waveguide-based Mach-Zehnder switches. For the Mach-Zehnder switch designed by using the optimized directional coupler, the switching time is found to be less than one picosecond, while the power consumption, VπLπ figure of merit, and active length of the device are ∼61 fJ/bit, 85 V×μm, and 30μm, respectively.

Control of adiabatic light transfer in coupled waveguides with longitudinally varying detuning

We study adiabatic light transfer in systems of two coupled waveguides with spatially varying detuning of the propagation constants, providing an analogy to the quantum phenomena of rapid adiabatic passage (RAP) and two-state stimulated Raman adiabatic passage (two-state STIRAP). Experimental demonstration using a photoinduction technique confirms the robust and broadband character of the structures that act as broadband directional couplers and broadband beam splitters, respectively.

Compact Broadband Directional Couplers Using Subwavelength Gratings

We experimentally demonstrate compact broadband directional couplers using subwavelength gratings for silicon-on-insulator wafers with silicon layers of 220 nm. The dispersion properties of the optical modes are engineered using subwavelength gratings, which allow broadband operation. Finite-difference time-domain (FDTD)-based band structure calculations, with significantly reduced simulation time, were used to analyze the design, which included both the structure and material dispersions. Compact broadband direction couplers, with device lengths shorter than 14 $\mu\text{m}$ , which cover a bandwidth of 100 nm for power splitting ratios of 50/50, 40/60, 30/70, and 20/80, are designed and fabricated for the fundamental transverse electric mode with a central operating wavelength of 1550 nm.

Broadband efficient directional coupling to short-range plasmons: towards hybrid fiber nanotips.

We present a broadband and efficient short-range plasmonic directional coupler design, for the delivery and collection of deeply sub-wavelength radiation to tapered plasmonic nanowires. We show a proof-of-concept design using a planar geometry operating at wavelengths between 1.2 -2.4 μm, showing that the propagation characteristics predicted by an Eigenmode analysis are in excellent agreement with finite element simulations. This analytical formulation is straightforward to implement and immediately provides the power-exchange properties of hybrid plasmonic waveguides. An investigation of both waveguide delivery and collection performance to and from a plasmonic nano-tip is performed. We show that this design strategy can be straightforwardly adapted to a realistic hybrid fiber geometry, containing wire diameters more than one order of magnitude larger than the planar geometries, with important applications in all-fiber plasmonic superfocussing, and nonlinear plasmonics.

Broadband silicon photonic directional coupler using asymmetric-waveguide based phase control.

We design and demonstrate broadband directional couplers that use asymmetric-waveguide based phase control sections, on the silicon-on-insulator platform. Broadband directional couplers with various power splitting ratios, including 10%/90%, 20%/80%, 30%/70%, 40%/60% and 50%/50%, were realized for both transverse electric (TE) and transverse magnetic (TM) modes. Some of the devices exhitbit bandwidths in excess of 100 nm, and all in excess of 75 nm. The footprints of the TE mode couplers are 32 μm ×1.3 μm, or less, and those of the TM mode couplers are 13 μm ×1.3 μm, or less.

Multisection Couplers With Coupled-Line Sections Having Unequal Lengths

A new type of broadband multisection coupled-line directional couplers is proposed. The investigated directional couplers are composed of coupled-line sections having unequal electrical lengths and a transmission-line section. The presented couplers feature similar bandwidth to the standard multisection directional couplers with significantly reduced maximum coupling coefficient, and the overall size smaller than in the case of tandem couplers. Moreover, it is shown that in the presented couplers an ideal 90 ° differential phase characteristic can be achieved when such couplers are solely composed of coupled-line sections having appropriate electrical lengths. Additionally, it is shown that the proposed couplers' bandwidth can be increased by increasing the number of quarter-wave-long coupled-line sections. Theoretical analysis has been validated by three designs of broadband directional couplers operating at a frequency f 0 =1.8 GHz. Measurements of the manufactured directional couplers have been shown to experimentally confirm their properties.