Tunable Coupler Research Articles

Tunable Wavelength-Selective Coupler Based on Microtapered Long-Period Fiber Gratings

In this letter, the wavelength-selective couplers (WSCs) constructed with two- and three-parallel microtapered long-period fiber gratings (MLPFGs) are experimentally demonstrated, respectively. The MLPFGs are fabricated by periodically tapering the standard single mode fibers. In WSCs constructed with two-parallel MLPFGs, the maximum peak coupling efficiency is ~10.1%, while in WSCs constructed with three-parallel MLPFGs, the maximum peak coupling efficiencies are ~2.9% and ~3.2% for two tapping fibers, respectively. Moreover, we propose an effective and convenient method to tune the WSCs. By simply twisting the WSCs constructed with two-parallel MLPFGs, the wavelength tunability of −0.01985 nm/° with a maximum wavelength tuning range of 6.8 nm is achieved, together with a good linear relationship of the shift at resonance wavelength versus the twist angle.

Programmable True Time Delay Lines Using Integrated Waveguide Meshes

We analyze and explore the potential that waveguide-mesh-based architectures used in programmable photonic integrated circuits can be configured to enable true time optical delay lines, which can find applications in different microwave photonics functionalities, such as beamforming and optical filtering. We also propose and experimentally demonstrate an alternative standalone tunable basic unit (TBU) architecture where its internal coupling device is implemented by means of a dual-drive tunable directional coupler (DD-TDC) that performs independent amplitude beam splitting and phase shifting. Compared to the previous alternatives based on 3-dB balanced Mach–Zehnder interferometers, the DD-TDC reduces by more than two times the insertion losses of TBUs enabling the potential realization of larger meshes with a three-fold enhanced step-time resolution. Bandwidth and robustness analysis are also considered.

Tunable, Flexible, and Efficient Optimization of Control Pulses for Practical Qubits.

Quantum computation places very stringent demands on gate fidelities, and experimental implementations require both the controls and the resultant dynamics to conform to hardware-specific constraints. Superconducting qubits present the additional requirement that pulses must have simple parameterizations, so they can be further calibrated in the experiment, to compensate for uncertainties in system parameters. Other quantum technologies, such as sensing, require extremely high fidelities. We present a novel, conceptually simple and easy-to-implement gradient-based optimal control technique named gradient optimization of analytic controls (GOAT), which satisfies all the above requirements, unlike previous approaches. To demonstrate GOAT's capabilities, with emphasis on flexibility and ease of subsequent calibration, we optimize fast coherence-limited pulses for two leading superconducting qubits architectures-flux-tunable transmons and fixed-frequency transmons with tunable couplers.

Demultiplexing of photonic temporal modes by a linear system

Temporally and spatially overlapping but field-orthogonal photonic temporal modes (TMs) that intrinsically span a high-dimensional Hilbert space are recently suggested as a promising means of encoding information on photons. Presently, the realization of photonic TM technology, particularly to retrieve the information it carries, i.e., demultiplexing of photonic TMs, is mostly dependent on nonlinear medium and frequency conversion. Meanwhile, its miniaturization, simplification, and optimization remain the focus of research. In this paper, we propose a scheme of TM demultiplexing using linear systems consisting of resonators with linear couplings. Specifically, we examine a unidirectional array of identical resonators with short environment correlations. For both situations with and without tunable couplers, propagation formulas are derived to demonstrate photonic TM demultiplexing capabilities. The proposed scheme, being entirely feasible with current technologies, might find potential applications in quantum information processing.

Compact Design of Planar Quadrature Coupler With Improved Phase Responses and Wide Tunable Coupling Ratios

This paper presents a compact planar tunable quadrature coupler with improved phase responses. Wide coupling-tuning ratio is achieved by using two varactors loaded on the center of the transmission lines of the modified coupler. Closed-form equations are derived for design parameters. For verification, a 1-GHz tunable coupler is designed and measured. The measured results agree well with the simulated ones. The measured power-dividing ratio can be tuned in a range from 16.2 to −35 dB (from 42 to 3.2E-4) with better than 20-dB return loss and isolation, while the phase imbalance is smaller than 10°. The coupler size is $0.21\lambda \text{g}\,\,\times \,\,0.08\lambda \text{g}$ and reduced by 73.1% compared with the conventional branch-line coupler. The theoretical analysis shows that the phase imbalance and insertion loss are mostly caused by the loss from varactors. Then, a tunable coupler with improved phase and loss responses is proposed, where the additional phase difference and insertion loss resulting from the varactors are compensated for by introducing a negative resistance from the negative impedance converter. Measured results of a demonstrative 1-GHz coupler show a power-dividing ratio tuning range from 24.4 to −22.2 dB (from 275.4 to 6E-4) while maintaining 20-dB return loss and isolation. The phase imbalance is smaller than 1° and the insertion loss is improved by 1 dB and nearly close to theoretical values across the tuning range.

Reconfigurable and tunable compact comb filter and (de)interleaver on silicon platform

We propose and demonstrate a reconfigurable and tunable chip-scale comb filter and (de)interleaver on a silicon platform. The silicon-based photonic integrated device is formed by Sagnac loop mirrors (SLMs) with directional couplers replaced by multi-mode interference (MMI) assisted tunable Mach-Zehnder interferometer (MZI) couplers. The device can be regarded as a large SLM incorporating two small SLMs which form a Fabry-Perot (FP) cavity. By appropriately adjusting the micro-heaters in tunable MZI couplers and cavity, switchable operation between comb filter and (de)interleaver and extinction ratio and wavelength tunable operations of comb filter and (de)interleaver are achievable by thermo-optic tuning. Reconfigurable comb filter and (de)interleaver is demonstrated in the experiment. The central wavelength shifts of comb filter and (de)interleaver are demonstrated with wavelength tuning efficiencies of ~0.0224 nm/mW and ~0.0193 nm/mW, respectively. The 3-dB bandwidth of the comb filter is ~0.032 nm. The 3-dB and 20-dB bandwidths of the (de)interleaver passband are ~0.225 nm and ~0.326 nm. The obtained results indicate that the designed and fabricated device provides switchable comb filtering and interleaving functions together with extinction ratio and wavelength tunabilities. Reconfigurable and tunable silicon-based comb filter and (de)interleaver may find potential applications in robust wavelength-division multiplexing (WDM) optical communication systems.

A Compact Tunable Directional Coupler with Continuously Tuned Differential Phase

A tunable directional coupler with outputs that have continuously tuned phase difference and constant magnitude is presented. The initial design is based on a 3-dB branch-line coupler with two arms having variable electrical lengths. To realize the variable-length lines, a novel concept of tunable phase shifting unit, which includes a pair of inductor-varactor loaded coupled lines, is proposed. By controlling the shifting phase of the two arms, the differential phase (i.e., the phase difference between the two output ports) can be tuned continuously. Explicit relation between the objective differential phase of the device and the required shifting phase of those units is analyzed and explained. To validate the design, a prototype is built, simulated, and tested. The experimental and predicted results agree well and show that the device can realize arbitrary and continuously tunable differential phase from 45° to 135°. The overall size of the design is only $0.18 \lambda _{g} \times 0.24 \lambda _{g}$ , which is extremely compact compared with using a cascaded coupler-phase shifters and is thus suitable for miniaturized wireless systems.

Design and Simulation of a Tunable Stripline Wideband Coupler

The basic theory of coupler is introduced, and a tunable strip-line wideband coupler is designed by using radio-frequency/microwave software which is named ADS on the basis of theoretical calculations and analysis. The broadband coupler can be adjusted appropriately in different frequency regions by using a tunable band line. It has a wide bandwidth of 2G~4G, excellent return loss of (>30.5) dB, stable coupling coefficient of (9.85~10.81) dB, and high isolation of (>20.08) dB, the straight-through pass-band transmission coefficient of (0.41~0.62) dB. The coupler has high power tolerance and small size. It can reduce design costs by used in communication systems.

Analysis of a parametrically driven exchange-type gate and a two-photon excitation gate between superconducting qubits

A current bottleneck for quantum computation is the realization of high-fidelity two-qubit quantum operations between two or more quantum bits in arrays of coupled qubits. Gates based on parametrically driven tunable couplers offer a convenient method to entangle multiple qubits by selectively activating different interaction terms in the effective Hamiltonian. Here, we theoretically and experimentally study a superconducting qubit setup with two transmon qubits connected via a capacitively coupled tunable bus. We develop a time-dependent Schrieffer-Wolff transformation and derive analytic expressions for exchange-interaction gates swapping excitations between the qubits (iswap) and for two-photon gates creating and annihilating simultaneous two-qubit excitations (bswap). We find that the bswap gate is generally slower than the more commonly used iswap gate, but features favorable scalability properties with less severe frequency-crowding effects, which typically degrade the fidelity in multiqubit setups. Our theoretical results are backed by experimental measurements as well as exact numerical simulations including the effects of higher transmon levels and dissipation.

A 60-GHz SiGe BiCMOS Monostatic Transceiver for FMCW Radar Applications

In this paper, a high-range 60-GHz monostatic transceiver system suitable for frequency-modulated continuous-wave (FMCW) applications is presented. The RF integrated circuit is fabricated using a 0.13- $\mu \text{m}$ SiGe BiCMOS technology with $f_{T}$ / $f_{\max }$ of 250/340 GHz and occupies a very compact area of $1.42 \times 0.72$ mm2. All of the internal blocks are designed fully differential with an in-phase/quadrature receiver (RX) conversion gain of 14.8 dB and −18.2 dBm of input-referred 1-dB compression point and a transmitter (TX) with 6.4 dBm of output power. The 60-GHz voltage-controlled oscillator is of a push-push type Colpitts oscillator integrated into a frequency divider with an output frequency between 910 MHz and 1 GHz with the help of 3-bit frequency tuning mechanism for external phase-locked loop operations. Between the TX and RX channels, a tunable coupler is placed to guarantee a high isolation between channels which could withstand any fabrication failures and provide a single differential antenna output. On the TX side, two power detectors are placed in order to monitor the transmitted and reflected powers on the TX channel by passing through a branch-line coupler for built-in-self-test purposes. The total current consumption of this transceiver is 156 mA at 3.3 V of single supply. Considering the successful real-time radar measurements, which the radar is able to detect the objects in more than 90-m range, it proves the suitability of this monostatic chip in high-range FMCW radar systems.

Universal Stabilization of a Parametrically Coupled Qubit.

We autonomously stabilize arbitrary states of a qubit through parametric modulation of the coupling between a fixed frequency qubit and resonator. The coupling modulation is achieved with a tunable coupling design, in which the qubit and the resonator are connected in parallel to a superconducting quantum interference device. This allows for quasistatic tuning of the qubit-cavity coupling strength from 12MHz to more than 300MHz. Additionally, the coupling can be dynamically modulated, allowing for single-photon exchange in 6ns. Qubit coherence times exceeding 20 μs are maintained over the majority of the range of tuning, limited primarily by the Purcell effect. The parametric stabilization technique realized using the tunable coupler involves engineering the qubit bath through a combination of photon nonconserving sideband interactions realized by flux modulation, and direct qubit Rabi driving. We demonstrate that the qubit can be stabilized to arbitrary states on the Bloch sphere with a worst-case fidelity exceeding 80%.

A Compact Trans-Directional Coupler With Wide Frequency Tuning Range and Superior Performance

In this paper, a compact coupled-line trans-directional (TRD) coupler with wide continuous frequency tuning range and superior performance is presented. The proposed tunable TRD coupler is composed of three coupled lines, two shunt stubs, and three types of varactors. By providing suitable reverse bias voltages on these varactors, the operating frequency of the proposed TRD coupler can flexibly be adjusted as desired. Moreover, better impedance match and port isolation are maintained at all tuning center frequencies. A theoretical analysis using the even–odd mode theory is presented to clarify the influence of the design parameters on the minimum and maximum operating frequencies. It is exhibited that the proposed TRD coupler has approximately 150% frequency tuning range in ideal conditions. Due to the manufacturing limitation, a microstrip TRD coupler with a tunable frequency of 1.0–2.5 GHz (85.7%) is designed and fabricated. The measured results show that the proposed TRD coupler can be tuned from 1.0 to 2.5 GHz with more than 27 dB return loss and more than 35 dB isolation in the entire tuning range, agreeing well with the simulation results.

Coupling coefficient range extension technique for broadband branch-line coupler

In a conventional branch-line coupler (BLC), high coupling coefficients require very narrow lines, which results in high fabrication complexity and reduction in power handling capacity. In addition, changes in multiple design parameters are required to realize arbitrary coupling coefficients. In this paper, we present the design, fabrication, and characterization of a novel broadband branch-line coupler that extends the range of arbitrary coupling coefficients. The proposed structure is also able to achieve different coupling through tuning only one design parameter, and therefore can be used as a tunable coupler. Closed-form design equations are presented for the synthesis of broadband BLCs based on the proposed structure. To validate the proposed structure and the corresponding design methodology, a variety of broadband BLCs with different coupling coefficients have been designed, fabricated, and measured. Measurements show that couplers with 6, 9, and 12 dB coupling coefficients provide wide fractional bandwidths of 49, 49, and 52%, respectively.

Giant THz surface plasmon polariton induced by high-index dielectric metasurface

We use computational approaches to explore the role of a high-refractive-index dielectric TiO2 grating with deep subwavelength thickness on InSb as a tunable coupler for THz surface plasmons. We find a series of resonances as the grating couples a normally-incident THz wave to standing surface plasmon waves on both thin and thick InSb layers. In a marked contrast with previously-explored metallic gratings, we observe the emergence of a much stronger additional resonance. The mechanism of this giant plasmonic resonance is well interpreted by the dispersion of surface plasmon excited in the air\\TiO2\\InSb trilayer system. We demonstrate that both the frequency and the intensity of the giant resonance can be tuned by varying dielectric grating parameters, providing more flexible tunability than metallic gratings. The phase and amplitude of the normally-incident THz wave are spatially modulated by the dielectric grating to optimize the surface plasmon excitation. The giant surface plasmon resonance gives rise to strong enhancement of the electric field above the grating structure, which can be useful in sensing and spectroscopy applications.

Lossless microwave photonic delay line using a ring resonator with an integrated semiconductor optical amplifier

Optical delay lines implemented in photonic integrated circuits (PICs) are essential for creating robust and low-cost optical signal processors on miniaturized chips. In particular, tunable delay lines enable a key feature of programmability for the on-chip processing functions. However, the previously investigated tunable delay lines are plagued by a severe drawback of delay-dependent loss due to the propagation loss in the constituent waveguides. In principle, a serial-connected amplifier can be used to compensate such losses or perform additional amplitude manipulation. However, this solution is generally unpractical as it introduces additional burden on chip area and power consumption, particularly for large-scale integrated PICs. Here, we report an integrated tunable delay line that overcomes the delay-dependent loss, and simultaneously allows for independent manipulation of group delay and amplitude responses. It uses a ring resonator with a tunable coupler and a semiconductor optical amplifier in the feedback path. A proof-of-concept device with a free spectral range of 11.5 GHz and a delay bandwidth in the order of 200 MHz is discussed in the context of microwave photonics and is experimentally demonstrated to be able to provide a lossless delay up to 1.1 to a 5 ns Gaussian pulse. The proposed device can be designed for different frequency scales with potential for applications across many other areas such as telecommunications, LIDAR, and spectroscopy, serving as a novel building block for creating chip-scale programmable optical signal processors.

Silicon-Based Integrated Tunable Fractional Order Photonic Temporal Differentiators

Two integrated fractional-order photonic temporal differentiators based on two Mach–Zehnder interferometer (MZI) structures implemented on a silicon-on-insulator (SOI) platform are designed, fabricated, and experimentally evaluated. The first photonic temporal differentiator employs a multimode interference (MMI) coupler as one of the two 3-dB couplers of the MZI. By changing the polarization state of the input optical signal, the coupling coefficient of the MMI is changed, which leads to the change of the phase shift in the destructive interference wavelength, and a photonic temporal differentiator with a tunable fractional order is implemented. The second photonic temporal differentiator is designed to have two cascaded MZIs, a balanced MZI, and an unbalanced MZI. A phase modulator (PM) is incorporated in one of the two arms of each of the MZIs. The balanced MZI with a PM forms an active tunable coupler, which is used to actively tune the fractional order of the temporal differentiator. The PM in the unbalanced MZI is used to tune the operating wavelength. The two photonic temporal differentiators are designed and fabricated in a CMOS compatible SOI platform, and their performance is evaluated experimentally. The experimental results show that both temporal differentiators can have a tunable fractional order from 0 to 1. In addition, the use of the active temporal differentiator to perform high-speed coding with a data rate of 16 Gb/s is experimentally evaluated.

A tunable transformer-based CMOS directional coupler for UHF RFID readers

A tunable transformer-based CMOS directional coupler for UHF RFID readers

基于LiNbO3的长周期波导光栅可调谐耦合器的设计

基于LiNbO₃的长周期波导光栅可调谐耦合器的设计

Coupled‐line directional coupler with tunable power division ratio and operating frequency

This study presents a novel planar coupled-line directional coupler with tunable both power division ratio and operating frequency, which is achieved using microstrip technology for the first time. Its tunability of the operating frequency and the power division ratio, or coupling ratio, can be independently controlled by the bias voltages. Moreover, it can simultaneously maintain perfect port match and isolation. On the basis of the even–odd mode analysis method, both the closed-form analytical equations and parameter discussions are provided to facilitate the practical design. An experimental prototype of the tunable coupler is simulated, fabricated, and measured to demonstrate how it works. The measured results indicate that power division ratio can vary from 0.35 (9 dB) to 0.71 (3 dB) when operating frequency ranges from 0.55 to 1.48 GHz (91.63%) while power division ratio varies from 0.35 (9 dB) to 1 (0 dB) when operating frequency is fixed at 1 GHz, which validated the reliability of the proposed model.

Fully reconfigurable terahertz devices enabled by T-shaped graphene two-parallel-sheet

In this letter, we investigate fully reconfigurable terahertz (THz) devices enabled by a T-shaped graphene two-parallel-sheet (GTPS). The proposed T-shaped GTPS is constructed by two series-connected GTPS sections and one shunt-connected open-ended GTPS section. By applying electrical gating to the T-shaped GTPS, both electrical length and characteristic impedance of the terahertz waveguide (i.e. T-shaped GTPS) can be fully tuned. To demonstrate the tuning feature of the T-shaped GTPS, a variety of tunable THz devices are designed based on it, including reconfigurable tunable THz transmission lines and a tunable THz coupler. The simulation results verify the large tuning range and multifunction of proposed devices.