Tunable Coupler Research Articles

Design of Thermally Poled Twin-Core Fiber Coupler With Electrical Tunability

Tunable coupler based on thermally poled twin-core fiber (TCF) with a hole-core-core-hole configuration was proposed in this letter. The thermal poling process therein was investigated based on the 2-D charge dynamics model. Effective χ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> in the twin-cores pointing toward opposite directions were introduced. By subjecting the poled TCF to modulating voltage, the tunability of coupling ratio between the twin-cores was investigated. The results indicate that the coupling characteristics of thermally poled twin-cores could be controlled flexibly by modulating voltage, and consequently achieving a stable and highly compacted electrically tunable TCF coupler.

Sitewise manipulations and Mott insulator-superfluid transition of interacting photons using superconducting circuit simulators

The Bose-Hubbard model (BHM) of interacting bosons in a lattice has been a paradigm in many-body physics, and it exhibits a Mott insulator-superfluid (MI-SF) transition at integer filling. Here a quantum simulator of the BHM using a superconducting circuit is proposed. Specifically, a superconducting transmission line resonator supporting microwave photons is coupled to a charge qubit to form one site of the BHM, and adjacent sites are connected by a tunable coupler. To obtain a mapping from the superconducting circuit to the BHM, we focus on the dispersive regime where the excitations remain photonlike. Standard perturbation theory is implemented to locate the parameter range where the MI-SF transition may be simulated. This simulator allows single-site manipulations, and we illustrate this feature by considering two scenarios where a single-site manipulation can drive a MI-SF transition. The transition can be analyzed by mean-field analyses, and the exact diagonalization was implemented to provide accurate results. The variance of the photon density and the fidelity metric clearly show signatures of the transition. Experimental realizations and other possible applications of this simulator are also discussed.

Tunable dual-parallel Mach–Zehnder modulator with ultra linearity and high tolerance

An electro-optic modulator with excellent linearity and high tolerance can be crucial for optimizing the performance of optical communication systems by improving modulation efficiency. In this paper, we propose a novel dual-parallel Mach–Zehnder modulator with three Multimode Interference couplers (MMI-DPMZM) consisting of a reconfigurable type and two 3 dB ones. The structure can maintain a high dynamic range over a wide operating range and is highly tolerant of manufacturing errors because the MMI couplers split the light precisely and flexibly. When the value of the radio frequency voltage splitting ratio deviates from its optimum value within , the Spurious-Free Dynamic Range can be maintained above 104.00 dB Hz through the adjustment of the tunable MMI coupler. Further analysis of the effects of modern MMI couplers’ fabrication errors demonstrates the dynamic range can still be 104.10 dB Hz.

Highly linear dual parallel Mach-Zehnder modulator incorporating MMI couplers

The strict requirements on splitting ratios of optical power and radio-frequency (RF) voltage affect the performance of the Dual Parallel Mach-Zehnder modulator (DPMZM). In this paper, a modified DPMZM with three Multimode Interference (MMI) couplers (MMI-DPMZM) is proposed. The three MMI couplers, comprised of a reconfigurable one and two 3 dB ones, function as optical splitters. The theoretical analysis shows that the structure can prohibit the third-order intermodulation distortion (IMD3) from the third order term and fifth order term of the transfer function’s Taylor series, leading to the peak Spurious-Free Dynamic Range (SFDR) as high as 104.12 dB. The adjustment of the tunable MMI coupler ensures the SFDR is kept above 104.00 dB even the radio frequency (RF) signal’s voltage splitting ratio deviates from its optimum value. It is also demonstrated that the SFDR can reach 104.10 dB with the precision and uniformity of MMI couplers considered.

Compact tunable silicon photonic differential-equation solver for general linear time-invariant systems.

We propose and experimentally demonstrate an all-optical temporal differential-equation solver that can be used to solve ordinary differential equations (ODEs) characterizing general linear time-invariant (LTI) systems. The photonic device implemented by an add-drop microring resonator (MRR) with two tunable interferometric couplers is monolithically integrated on a silicon-on-insulator (SOI) wafer with a compact footprint of ~60 μm × 120 μm. By thermally tuning the phase shifts along the bus arms of the two interferometric couplers, the proposed device is capable of solving first-order ODEs with two variable coefficients. The operation principle is theoretically analyzed, and system testing of solving ODE with tunable coefficients is carried out for 10-Gb/s optical Gaussian-like pulses. The experimental results verify the effectiveness of the fabricated device as a tunable photonic ODE solver.

UV-soft Imprinted Tunable Polymer Waveguide Ring Resonator for Microwave Photonic Filtering

A tunable polymer waveguide ring resonator is proposed and demonstrated for microwave photonic filtering. The ring resonator consists of a Mach–Zehnder interferometer (MZI) as the tunable coupler and two micro-heaters as the thermo-optic phase shifters. It was fabricated with inorganic–organic hybrid optical material polysiloxane-liquid series by a novel and simple UV-soft selective imprint technique. The coupling conditions, over-, critical- and under-coupling, were obtained with the micro-heater on the arm of MZI and the maximum notch depth of about 18 dB was achieved. The resonant wavelength was also tuned with the micro-heater on the ring waveguide. The dispersion induced power fading of radio over fiber link was overcome by the tunable ring resonator with its notch filtering function.

Controllable preparation of two-mode entangled coherent states in circuit QED

Although the multi-level structure of superconducting qubits may result in calculation errors, it can be rationally used to effectively improve the speed of gate operations. Utilizing a current-biased Josephson junction (λ-type rf-SQUID) as a tunable coupler for superconducting transmission line resonators (TLRs), under the large detuning condition, we demonstrate the controllable generation of entangled coherent states in circuit quantum electrodynamics (circuit QED). The coupling between the TLRs and the qubit can be effectively regulated by an external bias current or coupling capacitor. Further investigations indicate that the maximum entangled state can be obtained through measuring the excited state of the superconducting qubits. Then, the influence of the TLR decay on the prepared entangled states is analyzed.

Graphene-based terahertz tunable plasmonic directional coupler

We propose and numerically analyze a terahertz tunable plasmonic directional coupler which is composed of a thin metal film with a nanoscale slit, dielectric grating, a graphene sheet, and a dielectric substrate. The slit is employed to generate surface plasmon polaritons (SPPs), and the metal-dielectric grating-graphene-dielectric constructs a Bragg reflector, whose bandgap can be tuned over a wide frequency range by a small change in the Fermi energy level of graphene. As a graphene-based Bragg reflector is formed on one side of the slit, the structure enables SPP waves to be unidirectionally excited on the other side of the slit due to SPP interference, and the SPP waves in the Bragg reflector can be efficiently switched on and off by tuning the graphene's Fermi energy level. By introducing two optimized graphene-based Bragg reflectors into opposite sides of the slit, SPP waves can be guided to different Bragg reflectors at different Fermi energy levels, thus achieving a tunable bidirectional coupler.

Tunable acoustic couplers for two fluids with large impedance mismatch

The tunneling effects of acoustics have been found when the proposed solid–fluid superlattices (SLs) are used to match two fluids with a large impedance mismatch. One of the examples shows that, by embedding a rubber/water SL between air and crude oil, the transmission of acoustic energy reaches 0.97 at 5° at 8.73 kHz. According to the results of theoretical analyses and full-wave simulations, this energy tunneling is attributed to the acoustically resonant states induced by the SL at the interface. The tunneling frequencies can be tuned in real time by varying the interspacing between the solid layers of the SL, which may have wide potential applications in matching two fluids, such as acoustic coupling between air and crude oil during petroleum exploration.

CRIT-Alternative Narrow-Passband Waveguide Filter for Microwave Photonic Signal Processors

This letter presents a novel use of a waveguide circuit consisting of a two-ring resonator-assisted asymmetrical Mach-Zehnder interferometer, which realizes a narrow-passband filter in the context of microwave photonics (MWP). The filter principle is an alternative to the coupled resonator induced transparency and features easy implementation and robust performance. In the experimental demonstration, such a circuit fabricated in SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> waveguide technology exhibits a narrow transmission window surrounded by a region of significant suppression. The transmission window features a -3-dB bandwidth of 1 GHz and a -20-dB bandwidth of 3.5 GHz, equivalently a 20-dB roll-off enhancement of three times as compared with a regular add-drop ring resonator. In addition, the investigated waveguide circuit features full reconfigurability based on tunable phase shifters and power couplers. This allows the proposed filter functionality to be combined with other functionalities in a common device, which is of high interest for the realization of flexible on-chip MWP signal processors.

Controllable quantum information network with a superconducting system

We propose a controllable and scalable architecture for quantum information processing using a superconducting system network, which is composed of current-biased Josephson junctions (CBJJs) as tunable couplers between the two superconducting transmission line resonators (TLRs), each coupling to multiple superconducting qubits (SQs). We explicitly demonstrate that the entangled state, the phase gate, and the information transfer between any two selected SQs can be implemented, respectively. Lastly, numerical simulation shows that our scheme is robust against the decoherence of the system.

A carbon nanotube-embedded fiber-optic tunable coupler for flexible repetition rate control of a passively Q -switched fiber laser

We propose a carbon nanotube (CNT)-embedded fiber-optic tunable coupler as a passive Q-switch that can provide flexible control of the output pulse repetition rate in passively Q-switched fiber lasers, and experimentally demonstrate its feasibility. The device is implemented by sandwiching a thin CNT layer between two side-polished fibers. Using the coupler, it was experimentally shown that repetition-rate-tunable Q-switched pulses can readily be produced from an erbium-doped fiber ring cavity by adjusting the cross-coupling ratio through control of the core alignment mismatch angle between the two side-polished fibers. Flexible tuning of the pulse repetition rate is shown to be achievable from 20.96 to 12.98 kHz at a fixed pump power by adjusting the core alignment mismatch angle while the pulse width is varied from 4.98 to 5.44 μs.

Photonic Integrated Filter With Widely Tunable Bandwidth

We present a comprehensive design, fabrication, and characterization analysis of compact silicon-on-insulator bandpass filters with widely tunable bandwidth. The filter architecture is based on an unbalanced Mach-Zehnder interferometer loaded with a pair of ring resonators. A wide bandwidth tunability (from 10% to 90% FSR) can be achieved by controlling the resonant frequency of the rings while preserving a good filter off-band rejection. Design rules are provided that take into account fabrication tolerances as well as losses. Furthermore, the use of tunable couplers allows a more flexible shaping of the spectral response of the filter. The sensitivity with respect to nonlinear effects is carefully investigated. Operation over a wavelength spectrum of 20 nm is demonstrated, making the device suitable for channel subset selection in WDM systems, reconfigurable filters for gridless networking and adaptive filtering of signals.

Tunable Optofluidic Couplers for Dynamic Card-to-Backplane Optical Interconnect

Polymer waveguides with tunable optofluidic couplers are fabricated by the vacuum assisted microfluidic technique for dynamic card-to-backplane optical interconnect applications. The backplane optofluidic coupler consists of polymer waveguides with 45° integrated waveguide mirrors and perpendicular microfluidic channel structures for translational movement of index matching liquid and air bubbles. Controlled positioning of the air bubble or index matching liquid in contact with the integrated mirror can effectively turn on or off each optofluidic coupler for surface normal coupling in card-to-backplane optical interconnection. Experimental results demonstrated about 23 dB on/off surface normal interconnect signal change for both 1/3 and half size waveguide mirrors of the optofluidic couplers. 10 Gbps card-to-backplane optical interconnect eye diagrams have been demonstrated showing the effectiveness of the interconnect structure.

Flexible-Format Modulator With a Lattice Configuration

We discuss the general design concept of lattice optical modulators and detailed characteristics of a flexible-format modulator with a lattice configuration. Compared to the conventional modulators, the lattice modulator provides smaller intrinsic signal-synthesis losses because it utilizes both of the two complementary quadrature-phase-shift-keying (QPSK) signals to synthesize higher-order signals, although in practice the decrease in the intrinsic loss may be canceled out with the increase in the propagation loss. The flexible-format modulator, which we fabricated with a hybrid configuration of silica planar lightwave circuits and a LiNbO3 chip, enables us to select the format of the output signal from QPSK, 8-level PSK (8PSK), 8-level quadrature amplitude modulation (8QAM) and 16-level QAM (16QAM) just by using static tunable couplers and a phase shifter, without electronic digital-to-analog converters.

A Frequency-Compensated Rat-Race Coupler With Wide Bandwidth and Tunable Power Dividing Ratio

This paper presents, for the first time, the theoretical and experimental study of broadband rat-race coupler design with a tunable power dividing ratio. Wide fractional bandwidth (around 40%) is accomplished by the adoption of frequency compensating networks with specific derivatives of even- and odd-mode admittances. General formulation is analytically carried out to lay down the design principles of these networks. The proposed design also features wide tuning range (almost 20 dB), low insertion loss, high port isolation (over 25 dB), good return loss, simple structure, compact size, zero dc power consumption, and single voltage operation with a minimal number of tuning diodes. For demonstration, both simulated and measured results of a 1-GHz tunable rat-race coupler implemented on microstrip are shown.

Tunable broadband light coupler based on two parallel all-fiber acousto-optic tunable filters

Based on the evanescent-field coupling between the cladding modes of two adjacent and parallel all-fiber acousto-optic tunable filters, tunable broadband light coupling with relatively uniform insertion loss of trapping spectrum was achieved. In the experiments, a wide spectral tuning range from 1490 nm to 1610 nm, covering the whole C- and L-band and parts of S-bands, was demonstrated with a wavelength tunability slope of -0.72 nm/kHz. The insertion loss of the trapping spectrum was uniform (around -5.0 dB, which can be improved with a longer evanescent-field coupling length) within the whole tuning spectral range. Such a light coupling structure would be useful in tunable broadband light coupler and broadband optical fiber add/drop multiplexer for applications in coarse wavelength division multiplexing systems.

Catch-Disperse-Release Readout for Superconducting Qubits

We analyze a single-shot readout for superconducting qubits via the controlled catch, dispersion, and release of a microwave field. A tunable coupler is used to decouple the microwave resonator from the transmission line during the dispersive qubit-resonator interaction, thus circumventing damping from the Purcell effect. We show that, if the qubit frequency tuning is sufficiently adiabatic, a fast high-fidelity qubit readout is possible, even in the strongly nonlinear dispersive regime. Interestingly, the Jaynes-Cummings nonlinearity leads to the quadrature squeezing of the resonator field below the standard quantum limit, resulting in a significant decrease of the measurement error.

Liquid-Crystal Tunable Long-Range Surface Plasmon Polariton Directional Coupler

A tunable plasmonic directional coupler is proposed based on the electro-optical control of a nematic liquid-crystal layer placed above two side-coupled gold stripe waveguides. It is shown that by proper adjustment of the structural parameters and the control voltage, the coupling length of the structure and the effective modal index of the individual waveguides can be tuned, allowing for the design of electro-optical liquid-crystal plasmonic directional coupler switches.

Embedded RF Test Circuits: RF Power Detectors, RF Power Control Circuits, Directional Couplers, and 77-GHz Six-Port Reflectometer

Modern integrated circuits (ICs) are becoming an integrated parts of analog, digital, and radio frequency (RF) circuits. Testing these RF circuits on a chip is an important task, not only for fabrication quality control but also for tuning RF circuit elements to fit multi-standard wireless systems. In this paper, RF test circuits suitable for embedded testing are introduced: RF power detectors, power control circuits, directional couplers, and six-port reflectometers. Various types of embedded RF power detectors are reviewed. The conventional approach and our approach for the RF power control circuits are compared. Also, embedded tunable active directional couplers are presented. Then, six-port reflectometers for embedded RF testing are introduced including a 77-GHz six-port reflectometer circuit in a 130 nm process. This circuit demonstrates successful calibrated reflection coefficient simulation results for 37 well distributed samples in a Smith chart. The details including the theory, calibration, circuit design techniques, and simulations of the 77-GHz six-port reflectometer are presented in this paper.