Tunable Directional Coupler Research Articles

A fixed phase tunable directional coupler based on coupling tuning

The field of photonic integrated circuits has witnessed significant progress in recent years, with a growing demand for devices that offer high-performance reconfigurability. Due to the inability of conventional tunable directional couplers (TDCs) to maintain a fixed phase while tuning the reflectivity, Mach-Zehnder interferometers (MZIs) are employed as the primary building blocks for reflectivity tuning in constructing large-scale circuits. However, MZIs are prone to fabrication errors due to the need for perfect balanced directional couplers to achieve 0-1 reflectivity, which hinders their scalability. In this study, we introduce a design of a TDC based on coupling constant tuning in the thin film Lithium Niobate platform and present an optimized design. Our optimized TDC design enables arbitrary reflectivity tuning while ensuring a consistent phase across a wide range of operating wavelengths. Furthermore, it exhibits fewer bending sections than MZIs and is inherently resilient to fabrication errors in waveguide geometry and coupling length compared to both MZIs and conventional TDCs. Our work contributes to developing high-performance photonic integrated circuits with implications for various fields, including optical communication systems and quantum information processing.

Fully reconfigurable MEMS-based second-order coupled-resonator optical waveguide (CROW) with ultra-low tuning energy.

Integrated microring resonators are well suited for wavelength-filtering applications in optical signal processing, and cascaded microring resonators allow flexible filter design in coupled-resonator optical waveguide (CROW) configurations. However, the implementation of high-order cascaded microring resonators with high extinction ratios (ERs) remains challenging owing to stringent fabrication requirements and the need for precise resonator tunability. We present a fully integrated on-chip second-order CROW filter using silicon photonic microelectromechanical systems (MEMS) to adjust tunable directional couplers and a phase shifter using nanoscale mechanical out-of-plane waveguide displacement. The filter can be fully reconfigured with regard to both the ER and center wavelength. We experimentally demonstrated an ER exceeding 25 dB and continuous wavelength tuning across the full free spectral range of 0.123 nm for single microring resonator, and showed reconfigurability in second-order CROW by tuning the ER and resonant wavelength. The tuning energy for an individual silicon photonic MEMS phase shifter or tunable coupler is less than 22 pJ with sub-microwatt static power consumption, which is far better than conventional integrated phase shifters based on other physical modulation mechanisms.

Harnessing self-heating effect for ultralow-crosstalk electro-optic Mach–Zehnder switches

This paper presents a novel approach to counterbalance free-carrier-absorption (FCA) in electro-optic (E-O) Mach–Zehnder interferometer (MZI) cells by harnessing the self-heating effect. We show insights on crosstalk limitations in MZIs with direct carrier-injection and provide a detailed design methodology on a differential phase shifter pair. Leveraging both free-carrier dispersion (FCD) and self-heating effects, our design enables arbitrary phase tuning with balanced FCA loss in the pair of arms, eliminating the need for additional phase corrections and creating ultralow crosstalk MZI elements. This neat design disengages from the commonly used nested structure, thus providing an opportunity of embedding tunable couplers for correcting imperfect splitting ratios given that only two are needed. We show that with the use of tunable directional couplers, a standard ±10 nm process variation is tolerated, while achieving a crosstalk ratio below −40 dB. By direct carrier injection in both arms, the proposed device operates at nanosecond scales and can bring about a breakthrough in the scalability of E-O switch fabrics, as well as other silicon integrated circuits that have stringent requirements for crosstalk leakage.

Electro-optic tunable grating-assisted optical waveguide directional coupler in lithium niobate

In this paper, a high-performance electro-optical tunable grating-assisted directional coupler (GADC) is proposed and demonstrated experimentally. Our proposed GADC consists of a two-mode waveguide (TMW) and a single-mode waveguide (SMW) formed with lithium niobate (LN). By introducing a long-period waveguide grating into the side-wall of the TMW to compensate for the phase mismatch of the fundamental modes of the SMW and TMW, the fundamental modes of the two waveguides can be coupled efficiently each other at a specific wavelength. Furthermore, push–pull electrode structure is introduced to achieve the electro-optic (EO) tuning function featuring high speed, low driving voltage, and large tuning range. The best LN GADC we fabricated on x-cut LN substrate using annealing proton exchange process shows a high isolation of ~ 34 dB at 1532.9 nm wavelength, quite large EO tuning efficiency of 1.195 nm/V (1526.4–1549.1 nm) or 1.736 nm/V (1576.1–1602.2 nm), and a thermo-optic tuning efficiency of 0.128 nm/°C (22–60 °C). Our proposed LN GADC can find applications in the fields of high-speed tunable wavelength filtering, mode filtering, and EO modulation.

MEMS enabled suspended silicon waveguide platform for long-wave infrared modulation applications

Long-wave infrared (LWIR, 6–14 µm) is an important wavelength range as it covers abundant vibrational molecular fingerprints that can be used for gas/liquid sensing and absorption spectroscopy. Silicon waveguide-based passive photonic devices that offer advantages toward chip-scale miniaturization of photonic integrated circuits (PIC) and photonic sensors in LWIR have been explored. However, the modulation of Si-based guided-wave propagation in LWIR remains less reported. Here, we demonstrate a MEMS tunable directional coupler operating at a long-wave infrared wavelength range (6.4–7 µm) using suspended Si waveguides with subwavelength gratings. Leveraging the membrane transfer technology, the whole photonic waveguide device membrane is transferred onto a receiver substrate with a cavity that allows sufficient displacement for MEMS electrostatic actuation. The proposed device experimentally achieved an optical attenuation of −14.25 dB with 90 V DC bias voltage, while having a response time of 177 µs. In addition, the demonstration of using such a device for computational spectroscopy has been validated.

Quantum key distribution transmitter chip based on hybrid-integration of silica and lithium niobates

A quantum key distribution transmitter chip based on hybrid-integration of silica planar light-wave circuit (PLC) and lithium niobates (LN) modulator PLC is presented. The silica part consists of a tunable directional coupler and 400-ps delay line, and the LN part is made up of a Y-branch, with electro-optic modulators on both arms. The two parts are facet-coupled to form an asymmetric Mach–Zehnder interferometer. We successfully encode and decode four BB84 states at 156.25-MHz repetition rate. Fast phase-encoding of 0 or π is achieved, with interference fringe visibilities 78.53% and 82.68% for states |+〉 and |–〉, respectively. With the aid of an extra off-chip LN intensity modulator, two time-bin states are prepared and the extinction ratios are 18.65 dB and 15.46 dB for states |0〉 and |1〉, respectively.

Principles, fundamentals, and applications of programmable integrated photonics

Programmable integrated photonics is an emerging new paradigm that aims at designing common integrated optical hardware resource configurations, capable of implementing an unconstrained variety of functionalities by suitable programming, following a parallel but not identical path to that of integrated electronics in the past two decades of the last century. Programmable integrated photonics is raising considerable interest, as it is driven by the surge of a considerable number of new applications in the fields of telecommunications, quantum information processing, sensing, and neurophotonics, calling for flexible, reconfigurable, low-cost, compact, and low-power-consuming devices that can cooperate with integrated electronic devices to overcome the limitation expected by the demise of Moore’s Law. Integrated photonic devices exploiting full programmability are expected to scale from application-specific photonic chips (featuring a relatively low number of functionalities) up to very complex application-agnostic complex subsystems much in the same way as field programmable gate arrays and microprocessors operate in electronics. Two main differences need to be considered. First, as opposed to integrated electronics, programmable integrated photonics will carry analog operations over the signals to be processed. Second, the scale of integration density will be several orders of magnitude smaller due to the physical limitations imposed by the wavelength ratio of electrons and light wave photons. The success of programmable integrated photonics will depend on leveraging the properties of integrated photonic devices and, in particular, on 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 multiport interferometers and waveguide meshes provide regular and periodic geometries, formed by replicating unit elements and cells, respectively. In the case of waveguide meshes, the cells 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 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 provide the basic foundations and principles behind the construction of these complex programmable circuits. We also review some practical aspects that limit the programming and scalability of programmable integrated photonics and provide an overview of some of the most salient applications demonstrated so far.

Tantalum pentoxide nanophotonic circuits for integrated quantum technology.

Nanophotonics holds great promise for integrated quantum technologies, but realizing all functionalities for processing quantum states of light in optical waveguides poses an outstanding challenge. Here we show that tantalum pentoxide-on-insulator offers significant advantages for such purpose and experimentally demonstrate crucial photonic integrated circuit components. Exploiting advanced nanophotonic design and state-of-the-art nanofabrication processes, we realize low-loss waveguiding with 1 dB/cm propagation loss, efficient optical fiber-chip interfaces with more than 100 nm bandwidth, micro-ring resonators with quality factors of 357,200 and tunable directional couplers. We further achieve active functionality with nano-electromechanical phase-shifters. Our work enables reconfigurable photonic circuit configurations in the Ta2O5 material system with highly favorable optical properties for integrated quantum photonics.

Comprehensive modeling and design of Raman lasers on SOI for mid-infrared application

In this article, we present a comprehensive model for a CW Raman laser in a silicon-on-insulator (SOI) platform. We validate this model by comparing its output with experimental results from the literature. We then design a 2.232 μm Raman laser with an on-chip ring resonator cavity. Numerical analysis of laser characteristics, such as threshold power and output power, are conducted. The optimized values for the cavity length and power coupling ratios are determined taking into account fabrication variations. Finally, by designing a tunable directional coupler (DC) for the laser cavity, a robust design of Raman laser is presented. We show that the reduction of propagation loss down to 0.6 dB/cm for the silicon waveguides and free carriers removal, by the use of a p-i-n junction, will enable a Raman laser with a threshold power of 16 mW and a slope efficiency of 62%.

Tunable Directional Coupler With Very Wide Tuning Range of Power Division Ratio

A new tunable directional coupler with a very wide tuning range of power division ratio (PDR) is proposed and demonstrated. The key uniqueness is the introduction of phase reconfigurable synthesized transmission lines (PRSTLs) as the tuning elements whose electrical length and input matching can be controlled simultaneously. By inserting PRSTLs in between two branch-line couplers (BLCs), the PDR at the two outputs of the tunable coupler can be theoretically tuned from 0 to $\infty $ (or equivalently from $-\infty $ to $\infty $ dB) with low reflection coefficient and good port isolation. As an exemplum, the proposed tunable directional coupler was fabricated and tested using microstrip technology at 2.4 GHz. The experimental demonstration features the widest PDR tuning range (−39 ~ 29 dB) among the literature, with a power loss of 1.2 dB and good port isolation higher than 22 dB.

Magnon Straintronics to Control Spin-Wave Computation: Strain Reconfigurable Magnonic-Crystal Directional Coupler

The effect of electrical field control of the spin-wave spectrum in adjacent magnonic crystals stripes was investigated by micromagnetic simulation, microwave spectroscopy, and Brillouin light scattering (BLS). The electric-field-induced dipolar coupling between magnetic stripes leads to the formation of symmetric and antisymmetric collective modes in the spectra of propagating spin waves. In particular, the strain-induced variation of the spin-wave coupling length in the stripe, predicted by micromagnetic simulation and measured by BLS, show the possibility of application of a lateral array of magnonic crystals as tunable microwave directional couplers, power dividers, and spin-wave demultiplexers, which can be used simultaneously as an interconnection unit in reconfigurable magnonic networks.

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.

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.

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.

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

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

CMOS integrated low TX noise digital transmitter with tunable directional coupler for high performance UHF RFID reader

A fully CMOS integrated low TX noise UHF RFID digital transmitter with tunable directional coupler is proposed. The transmitter decreases the AM noise density to −144.6 dBc/Hz at 10 kHz offset by eliminating noisy conventional analog components and optimizing LO chain. The tunable transformer-based directional coupler which is feasible for CMOS integration exhibits high isolation performance of −55 dB over the entire band. The transmitter with quadrature DPA structure supports DSB/SSB/PR-ASK modulation, and with the directional coupler, it achieves 22.8 dBm peak power with drain efficiency of 37.8%. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1750–1756, 2016

Silicon electro-optic modulator based on an ITO-integrated tunable directional coupler

Indium-tin-oxide (ITO) has attracted great attention because of its electrically-induced epsilon-near-zero (ENZ) characteristics, which has allowed us to develop electro-absorption optical modulators. As an extended application of ITO in a silicon optical modulator, we propose a silicon electro-optic modulator based on an ITO-integrated tunable directional coupler. An ITO block placed at the center of the two-core silicon directional coupler plays a key role in attenuating the optical power of the guided modes. Strong confinement of the optical field at the ENZ layer of ITO with the nonzero imaginary part of ITO’s permittivity let the guiding light experience high absorption as it propagates along the directional coupler. Numerical simulations reveal that the highest modulation depth of 5 dB is achievable at the ENZ region of ITO at a 1.55 μm wavelength. We can modulate the optical signals on an entire C-band ranging from a 1.530 to 1.565 μm wavelength with an on/off extinction ratio of larger than 4.6 dB.

Orbital Angular Momentum-Controlled Tunable Directional Plasmonic Coupler

In order to realize unidirectional surface plasmon polariton coupling, here, we design an orbital angular momentum-controlled tunable directional plasmonic coupler which can distinguish optical vortex with a topological charge of ±1 precisely, as well as act as surface plasmon polariton propagation controller to determine the surface plasmon polariton coupling directions. The method of design for the directional plasmonic coupler is explained in detail by both the theoretical analysis and the numerical simulation showing a directionality of $\sim 5:1$ . It is believed the directional plasmonic coupler can potentially be applied in the plasmonic control system.

Tunable directional coupler for RF front‐end applications

A tunable directional coupler for radio frequency (RF) front-ends of handheld mobile devices is presented. The coupler is implemented in a 0.13 μm bulk RF-CMOS technology with all elements monolithically integrated on a single die. A digitally-tuned termination network at the isolated port of the coupler is realised solely with RF-switch transistors using the channel resistance of a metal-oxide-semiconductor (MOS) device. The fine-tuned termination network allows the coupler to achieve a directivity of over 25 dB in the whole cellular frequency range from 699 MHz up to 2.7 GHz. The worst-case insertion loss in the transmitted path is 0.15 dB at 2.7 GHz.

Silicon Optical Modulators Based on Tunable Plasmonic Directional Couplers

For the development of Si-based next generation electronic-photonic integrated circuits, a silicon optical modulator is designed based on tunable plasmonic directional couplers. The characteristics are investigated using numerical simulations with a full-vectorial beam propagation method. The center waveguide in the three-core coupler is a MOS-type hybrid plasmonic waveguide that consists of a Si-ITO-SiO2-Si structure. By electrically tuning the ITO's refractive index, the coupling efficiency of the directional coupler is switchable and, hence, we can obtain modulated optical signals at the outer waveguide, which is configured with a vertical offset. The extinction ratios are 6.7 and 2.3 dB at 1.31- and 1.55-μm wavelength, respectively. The optical modulator can be transformed into a plasmonic absorption modulator based on a two-core directional coupler at the cost of a long coupling length. The proposed silicon optical modulator has the potential to play a key role in complementary metal-oxide-semiconductor-compatible 3-D silicon photonic integrated circuits.