Variable Coupler Research Articles

High-Power-Density Wireless Power Transfer System for Autonomous Underwater Vehicle Based on a Variable Ring-Shaped Magnetic Coupler

High-Power-Density Wireless Power Transfer System for Autonomous Underwater Vehicle Based on a Variable Ring-Shaped Magnetic Coupler

Design of a high sensitivity and wide range angular rate sensor based on exceptional surface

Abstract It is found that when the parity–time symmetry phenomenon is introduced into the resonant optical gyro system and it works near the exceptional point, the sensitivity can in theory be significantly amplified at low angular rate. However, in fact, the exceptional point is easily disturbed by external environmental variables, which means that it depends on harsh experimental environment and strong control ability, so it is difficult to move towards practical application. Here, we propose a new angular rate sensor structure based on exceptional surface, which has the advantages of high sensitivity and high robustness. The system consists of two fiber-optic ring resonators and two optical loop mirrors, and one of the resonators contains a variable ratio coupler and a variable optical attenuator. We theoretically analyze the system response, and the effects of phase and coupling ratio on the system response. Finally, compared with the conventional resonant gyro, the sensitivity of this exceptional surface angular rate sensor can be improved by about 300 times at low speed. In addition, by changing the loss coefficient in the ring resonator, we can achieve a wide range of 600 rad/s. This scheme provides a new approach for the development of ultra-high sensitivity and wide range angular rate sensors in the future.

Magneto-thermal limitations in superconducting cavities at high radio-frequency fields

The performance of superconducting radio-frequency Nb cavities at high radio-frequency (rf) fields in the absence of field emission can be limited by either a sharp decrease of the quality factor Q0(Bp) above peak surface magnetic fields Bp ∼100 mT or by a quench. We have measured Q0(Bp) at 2 K of several 1.3 GHz single-cell Nb cavities with different grain sizes, and with different ambient magnetic fields and cooldown rates below the critical temperature. Temperature mapping and a novel magnetic field mapping systems were used to find the location of “hot-spots” and regions of trapped magnetic flux. The use of a variable input coupler allowed further exploring the dissipative state. The results showed a remarkable thermal stability in some cavities with up to 200 W of rf power dissipation at 2 K, whereas other cavities quenched at much lower rf power. We observed a narrow distributions of the onset fields of hot-spots which were not affected by thermal cycling or by conditions which favor the formation of Nb hydrides. Furthermore, a poor correlation was found between the location of hot-spots and trapped vortices. We suggest that the totality of our experimental data can be explained by a sharp increase of the residual surface resistance above 120–140 mT due to the field-induced breakdown of a proximity-coupled metallic suboxide layer at the surface.

Broadband multi-wavelength Brillouin-Raman fiber laser with frequency switching

We purpose and demonstrate the switchable multi-wavelength Brillouin–Raman fiber laser (MBRFL) through a bi-directional Raman pumping scheme. The laser structure is arranged in a linear cavity by including a physical mirror at one side of the cavity. The switching operation for MBRFL with single- and double-wavelength spacing is implemented by optimizing the Raman power distribution through a variable optical coupler. This effect on feedback power of the physical mirror provides the difference between odd- and even-order Stoke lines’ maximum power on different sides of the cavity with 10 GHz and 20 GHz spacing. A 90/10 coupler is found to be the optimal. Up to 460 flat-amplitude lines within only a 0.5-dB flatness range, average −5 dBm Stokes peak power (SPP), 10 GHz frequency spacing, and an average optical signal-to-noise ratio (OSNR) of 26 dB are observed. All the counted laser lines are spread across a 37 nm bandwidth. Simultaneously, 170 Stoke lines with overall −2 dBm SPP, 28 dB OSNR, and 20 GHz frequency spacing are attained on other side of the cavity. These are achieved when the Raman pump power is set only at 900 mW. To date, this is the simplest cavity design with the flattest spectrum and highest output power for both wavelength spacing and excellent OSNR achieved in multi-wavelength fiber lasers that incorporate a single low-power Raman pump unit.

Novel Lumped-Distributed Variable Coupler

Novel Lumped-Distributed Variable Coupler

An Integrated Pump-Controlled Variable Coupler Fabricated by Ultrafast Laser Writing.

The design and fabrication of a integrated symmetric directional coupler dependent o the pumping power and operating at a 1534 nm wavelength is reported. The twin-core waveguide was inscribed into Er3+/Yb3+ co-doped phosphate glass by a femtosecond laser direct writing technique. By optical pumping, the coupling ratio can be modulated due to the changes induced in the refractive index of the material. The experimental results demonstrated that the coupling ratio can be tuned continuously from 100/0 to 50/50 by increasing the pump's power from 0 to 350 mW. The developed twin-core coupler has promising applications for on-chip all-optical signal processing and communication systems.

Linewidth Narrowing of a Dual Wavelength-Selectable, Ring Cavity Erbium-Doped Fiber Laser Using a Saturable Absorber

The narrow linewidth fiber laser is useful in applications such as fiber sensing, optical communications, and spectroscopy. This paper presents an investigation of the model and an experiment of a stable, wavelength-selective, narrow linewidth, ring cavity erbium-doped fiber laser incorporating two fiber Bragg gratings (FBG) at 1530.18 nm and 1550.08 nm, respectively. An F-P tunable filter was used to select a specific wavelength after optimizing the spectral output from the two FBGs to measure their respective linewidths. The erbium-doped ring fiber laser was optimized by adjusting the optical cavity loss using a variable optical coupler at a coupling ratio of 95%. The variable coupler was set to an optimal coupling ratio of 95%, where the spectral output powers of 3.4 mW at 1530.18 nm and 3.1 mW at 1550.08 nm were achieved as the optimal fiber laser output powers. The balanced output power had an optical signal-to-noise ratio of (OSNR) of 61 dB for each wavelength. The linewidth was measured for both wavelengths without saturable absorbers, and 27.7 kHz and 28.3 kHz for 1530.18 nm and 1550.08 nm were obtained. Using the saturable absorber, the linewidths were narrowed to 25.3 KHz and 21.1 kHz for 1530.18 nm and 1550.08 nm, respectively.

Superconducting quantum circuit of NOR in quantum annealing

The applicability of quantum annealing to various problems can be improved by expressing the Hamiltonian using a circuit satisfiability problem. We investigate the detailed characteristics of the NOR/NAND functions of a superconducting quantum circuit, which are the basic building blocks to implementing various types of problem Hamiltonians. The circuit is composed of superconducting flux qubits with all-to-all connectivity, where direct magnetic couplers are utilized instead of the variable couplers in the conventional superconducting quantum circuit. This configuration provides efficient scalability because the problem Hamiltonian is implemented using fewer qubits. We present an experiment with a complete logic operation of NOR/NAND, in which the circuit produces results with a high probability of success for arbitrary combinations of inputs. The features of the quantum circuit agree qualitatively with the theory, especially the mechanism for an operation under external flux modulation. Moreover, by calibrating the bias conditions to compensate for the offset flux from the surrounding circuit, the quantum circuit quantitatively agrees with the theory. To achieve true quantum annealing, we discuss the effects of the reduction in electric noise in quantum annealing.

Cavity-dumped nanosecond Cr:LiSAF laser in the 985\u20131030\xa0nm region for versatile seeding of Yb-based amplifiers

We report nanosecond (ns) cavity-dumped operation of a low-cost diode-pumped Cr:LiSAF laser around 1000 nm. The system is pumped with one 1-W single-emitter multimode diode at 665 nm. A Pockell cell (PC) and thin-film-polarizer (TFP) combination placed inside the cavity chops up an adjustable portion of the intracavity power and creates a variable time-dependent output coupler. Via adjusting the length and magnitude of the electrical signal going into the PC, output pulses with pulsewidths in the 2.5–500 ns range and with peak power levels above 10 W are generated at repetition rates up to 100 kHz. The central wavelength of the pulses could be smoothly tuned in the 985–1030 nm region, and is only limited by the anti-reflection coating bandwidth of the current PC and TFP. This versatile nanosecond source with 100 nJ level energies could serve as an attractive low-cost seed source for Yb-based amplifiers, including the cryogenic Yb:YLF systems.

Realization of high interference visibility based on silica-on-silicon planar lightwave circuit

Quantum key distribution can realize unconditional security of communication. Here, a quantum decoding chip based on silica-on-silicon planar lightwave circuit (PLC) platform, which is compatible for multiprotocol has been demonstrated. PLC technology provides stability, high integration, and practicality for the chip, whereas silica-on-silicon material makes it low loss and low cost. The chip is integrated with three variable optical splitters, two asymmetric Mach–Zehnder interferometers, and four variable directional couplers. The interference visibility is investigated by self-interfering method, and the results showed it is high to 98.2% under temperature control. The extinction ratio for the phase states is kept between 23 and 25 dB for 6 h without active phase correction.

Quantum communication with itinerant surface acoustic wave phonons

Surface acoustic waves are commonly used in classical electronics applications, and their use in quantum systems is beginning to be explored, as evidenced by recent experiments using acoustic Fabry–Pérot resonators. Here we explore their use for quantum communication, where we demonstrate a single-phonon surface acoustic wave transmission line, which links two physically separated qubit nodes. Each node comprises a microwave phonon transducer, an externally controlled superconducting variable coupler, and a superconducting qubit. Using this system, precisely shaped individual itinerant phonons are used to coherently transfer quantum information between the two physically distinct quantum nodes, enabling the high-fidelity node-to-node transfer of quantum states as well as the generation of a two-node Bell state. We further explore the dispersive interactions between an itinerant phonon emitted from one node and interacting with the superconducting qubit in the remote node. The observed interactions between the phonon and the remote qubit promise future quantum-optics-style experiments with itinerant phonons.

Silica-silicon based planar lightwave circuit quantum key distribution decoding chip for multi-protocol

Silica-silicon planar lightwave circuit (PLC) has the advantages of miniaturization, low cost, and low loss, and it provides a stable, highly integrated, and practical platform for quantum key distribution (QKD). A quantum decoding chip based on this platform which have potential to be compatible for multi-protocols has been designed and measured. The chip consists of three variable optical splitters (VOSs), two Mach-Zehnder interferometers (AMZIs) and four variable directional couplers (VDCs). The results of experiment show that the chip can achieve up to 98.6% interference visibility with measured delay times of 405 ps and 402 ps for two AMZI respectively under temperature control. The extinction ratio for the phase states is kept between 23 and 25 dB for 6 h without active phase correction. The chip has advantageous to achieve high interference visibilities and low quantum bits error rate (QBER).

Realization of simultaneous balanced multi-outputs for multi-protocols QKD decoding based on silica-based planar lightwave circuit* *Project supported by the National Key Research and Development Program of China (Grant No. 2018YFA0306403) and the National Nature Science Foundation of China (Grant No. 61805232).

Silica-based planar lightwave circuit (PLC) devices can reduce transmission loss and cost in a quantum key distribution (QKD) system, and have potential applications in integration and production. A PLC-based quantum decoding integrated chip for multi-protocols is designed and fabricated, which is composed of variable optical splitters (VOSs), asymmetric Mach–Zehnder interferometers (AMZIs), and variable directional couplers (VDCs). Balanced pulse-pairs of four outputs are obtained simultaneously with measured delay times of 405 ps and 402 ps, respectively. The chip has advantages in achieving high interference visibility and low quantum bit error rate (QBER).

A fast and large bandwidth superconducting variable coupler

Variable microwave-frequency couplers are highly useful components in classical communication systems and likely will play an important role in quantum communication applications. Conventional semiconductor-based microwave couplers have been used with superconducting quantum circuits, enabling, for example, the in situ measurements of multiple devices via a common readout chain. However, the semiconducting elements are lossy and furthermore dissipate energy when switched, making them unsuitable for cryogenic applications requiring rapid, repeated switching. Superconducting Josephson junction-based couplers can be designed for dissipation-free operation with fast switching and are easily integrated with superconducting quantum circuits. These enable on-chip, quantum-coherent routing of microwave photons, providing an appealing alternative to semiconductor switches. Here, we present and characterize a chip-based broadband microwave variable coupler, tunable over 4–8 GHz with over 1.5 GHz instantaneous bandwidth, based on the superconducting quantum interference device with two parallel Josephson junctions. The coupler is dissipation-free and features large on-off ratios in excess of 40 dB, and the coupling can be changed in about 10 ns. The simple design presented here can be readily integrated with superconducting qubit circuits and can be easily generalized to realize a four- or more port device.

Improved 2D continuously variable output couplers for an exit pupil expander fabricated through ion beam etching

An important goal in designing a head-mounted display device is to improve its exit pupils' output light areal uniformity. The goal cannot be reached by using an output grating coupler of uniform surface profile depth. To solve this problem, we propose a method to fabricate 2D and continuously variable depth gratings by using a specially modified reactive ion-beam etching system, which features variable speed scanning etching and ion beam cross-sectional shaping in two orthogonal dimensions. We experimentally verified that the uniformity of the light outcoupled from the coupler fabricated by using the proposed method was 52% better than that of a uniform-depth output grating coupler.

Triggering of different pulsed regimes in fiber cavity laser by a waveguide electro-optic switch.

A novel practical method for electronic triggering of essentially different pulsed regimes in fiber cavity lasers is introduced. The method relies on electronic control of complementary transmission characteristics of a fiber-coupled LiNbO3 waveguide electro-optic switch (WEOS) which plays the role of the variable output coupler in a fiber cavity. The method was studied using a testbed laser configuration comprised of a semiconductor optical amplifier (SOA) and an all-fiber cavity. Modulation of the WEOS-based output coupling in the fast gain recovery configuration allowed not only high-quality mode locking and harmonic mode-locking at certain pulse repetition rates determined by the cavity round trip time, but it also allowed nanosecond pulsed output of the same quality to be yielded by cavity dumping at widely and continuously tunable repetition rate (ranging from kHz to MHz). Thus, WEOS-based electronically variable output coupling allows uniquely high flexibility for lasing regimes and characteristics within a single all-fiber cavity configuration.

A travelling wave array system as solution for the ion cyclotron resonance frequencies heating of DEMO

Travelling wave array (TWA) antennas distributed along the periphery of the tokamak are presently considered as an ion cyclotron resonance frequencies (ICRF) heating solution for the DEMO reactor. Compared to the conventional ICRF antenna systems currently in use or designed for future machines like ITER, the TWA consists of antenna sections integrated in the breeding blanket scattered around the machine, each one fed through a variable coupler in a resonant ring configuration. Previous modelling of an antenna system for DEMO with 16 quadruple TWA sections of eight straps shows that a power capability exceeding 50 MW can be obtained in the frequency band of interest using the reference low coupling plasma profile of ITER. The described system optimizes the coupling to the plasma by providing a large number of radiating elements, which results in enhanced antenna directivity, hereby decreasing the antenna power density. This results in a maximum strap voltage amplitude of only 15 kV and maximum inter-strap voltage amplitude of 18 kV. The generators remain matched for all loading conditions: the system is totally load resilient. Following the recommendation of the work package heating and current drive Review Panel, a TWA ion cyclotron resonant heating (ICRH) system consisting of fewer sections concentrated in front of the equatorial ports is analysed in this paper and compared to the previous design. Reducing the number of sections increases the power density and its associated voltages. To couple 50 MW on the ITER density profile, voltages up to 30 kV are now required. Some aspects like the coupling between sections and its repercussion on the feeding network are briefly discussed. To assess the feasibility of the TWA fed by a resonant ring as an ICRH system for a DEMO reactor, a test on an existing medium size tokamak is under study.

Ultracompact Optical Comparator for 4-bit QPSK-Modulated Signal Based on Silicon Photonic Waveguide

Optical comparators have the potential to improve the operational speed and power consumption of optical communication systems, as they directly perform the required operations in the optical domain to avoid any optic-electro conversions. Various comparators are optically implemented; however, most studies have been confined to an on-off keying modulation. Although there is a comparator available for the phase-shift keying (PSK) modulation, the device has a limitation that requires integration into an actual system. In this paper, the author provides an optical comparator based on a silicon waveguide for a quadrature PSK (QPSK) signal. The proposed comparator is a simple 2.0 × 0.1 mm 2 rectangle, which is approximately 1/7500 the size of comparators from previous studies, and the device mainly comprises a delay line interferometer, a variable splitting ratio optical coupler, and a phase shifter. The performance of the comparator designed for 00 00 and 11 00 codewords was experimentally evaluated. The comparator generates the coupled symbol with the comparison result between the target code and the input signal. The paper demonstrates the feasibility of the ultracompact optical comparator for a 4-bit QPSK-modulated signal at 10 Gbaud.

Flexible Filtering Element on SOI With Wide Bandwidth Tunability and Full FSR tuning

We demonstrate a silicon-on-insulator flexible filtering element, relying on two serially coupled micro-ring resonators equipped with three thermo-optically tuned Mach–Zehnder interferometers, acting as variable optical couplers. A single electrode configuration is adopted, enabling the low-complexity thermo-optical control of the filtering element for bandwidth and wavelength tunability. The module's performance has been studied in detail via simulation analysis, and the obtained results were verified through experimental evaluation. Wide bandwidth tunability from 9 GHz to 103 GHz is achieved with a power efficiency of 0.284 mW/GHz, while full free spectral range wavelength tuning is achieved with an efficiency of 0.297 mW/GHz. Module's insertion loss lays below 3 dB.

Programmable multifunctional integrated nanophotonics

Abstract Programmable multifunctional integrated nanophotonics (PMIN) is a new paradigm that aims at designing common integrated optical hardware configurations, which by suitable programming can implement a variety of functionalities that can be elaborated for basic or more complex operations in many application fields. The interest in PMIN is driven by the surge of a considerable number of emerging applications in the fields of telecommunications, quantum information processing, sensing and neurophotonics that will be calling for flexible, reconfigurable, low-cost, compact and low-power-consuming devices, much in the same way as how field programmable gate array (FPGA) devices operate in electronics. The success of PMIN relies on the research into suitable interconnection hardware architectures that can offer a very high spatial regularity as well as the possibility of independently setting (with a very low power consumption) the interconnection state of each connecting element. Integrated waveguide meshes provide regular and periodic geometries, formed by replicating a unit cell, which can take the form of a square, hexagon or triangle, among other configurations. Each side of the cell is formed by two integrated waveguides connected by means of a Mach-Zehnder interferometer (MZI) or a tunable directional coupler that can be operated by means of an output control signal as a crossbar switch or as a variable coupler with independent power division ratio and phase shift. In this paper, we review the recent advances reported in the field of PMIN and, especially, in those based on integrated photonic waveguide meshes, both from the theoretical as well as from the experimental point of view. We pay special attention to outlining the design principles, material platforms, synthesis algorithms and practical constraints of these structures and discuss their applicability to different fields.