Waveguide Circuits Research Articles

Design of an 845-nm GaAs Vertical-Cavity Silicon-Integrated Laser with an Intracavity Grating for Coupling to a SiN Waveguide Circuit

A short-wavelength hybrid GaAs vertical-cavity silicon-integrated laser (VCSIL) with in-plane waveguide coupling has been designed and optimized using numerical simulations. A shallow etched silicon nitride (SiN) grating is placed inside the cavity of the hybrid vertical-cavity silicon-integrated laser to both set the polarization state of the resonant optical field and to enable output coupling to a SiN waveguide with high efficiency. The numerical simulations predict that for apertures of 4 and 6-μm oxide-confined VCSILs operating at 845-nm wavelength, a slope efficiency for the light coupled to the waveguide of 0.18 and 0.22 mW/mA is achievable, respectively, while maintaining a low threshold gain of 583 and 589 cm−1, respectively, for the lasing.

Integrated Nanophotonic Excitation and Detection of Fluorescent Microparticles

Biological microentities, such as cells and bacteria, are often detected and identified by bulky optical setups such as fluorescent microscopes and flow cytometers. Integrated photonics provides the prospect of dramatically miniaturizing these setups, enabling highly parallelized on-chip detection. In this work we designed, fabricated, and characterized a SiN photonic circuit for fluorescence detection of microentities. A tailored diffraction grating excites fluorescent particles, whose emitted light is collected in a single-mode waveguide by a second neighboring diffraction grating. Both fluorescent polystyrene microparticles and labeled peripheral blood mononuclear cells were detected in a microfluidic channel integrated on top of the waveguide circuit. We quantified both numerically and experimentally the efficiency of this grating system and obtained a good agreement. The presented on-chip fluorescent detection structure and the obtained results will strongly contribute to the development of on-chip c...

Design and Fabrication of a 300 GHz Modified Sine Waveguide Traveling-Wave Tube Using a Nanocomputer Numerical Control Machine

The sine waveguide slow-wave structure is a promising interaction circuit for traveling-wave tubes in the terahertz region because it possesses advantageous properties such as high transmission, easy fabrication, and elimination of the electron beam tunnel. These waveguides could be fabricated by nanocomputer numerical control (CNC) machining, a fabrication method capable of fabricating microscale components. In our study, we evaluate the practical feasibility of manufacturing 300-GHz sine waveguides with the ideal design, using nano-CNC machining. It is found that the ideal sine waveguide circuit must be modified, because of the limitations imposed by the actual tool size of the nano-CNC machine. Simulations of cold- and hot-tests of the circuit—including the electron beam effect—were conducted for both the ideal and the modified sine waveguide circuits. A modified sine waveguide was successfully machined using a nano-CNC machine with a 0.12-mm diameter tool tip. The S-parameters of the fabricated circuit were measured and compared to simulation data. A detailed analysis of the measured transmission loss was performed, and this loss was found to be attributable to a gap left by the assembly process between the two copper plates.

Hybrid Plasmonics Slot THz Waveguide for Subwavelength Field Confinement and Crosstalk Between Two Waveguides

The slot waveguide has attracted considerable attention because of its ability to confine and guide electromagnetic energy at the subwavelength scale beyond the diffraction limit. We propose a novel terahertz slot waveguide structure to achieve a better tradeoff between propagation length and field confinement capacity, the novel waveguide consisting of a two slot structure. The performances of terahertz waveguides were investigated using the finite-element method. The results demonstrated that the hybrid slot waveguide (HSW) provides significantly enhanced field confinement in low index slot regions: more than five times that of traditional low index slot waveguides (LISWs). An optimized HSW structure was achieved by tuning the tradeoff between mode confinement and propagation length. We also showed that its integration in conventional planar waveguide circuits was greatly improved compared with the LISWs, by comparing their crosstalk. The proposed new HSW structure has great potential to enable THz production of compact integration and could lead to true semiconductor-basedTHz applications with high performance.

Optical emulation of photon-pair generation in nonlinear lossy waveguides

We establish theoretically and demonstrate experimentally that photon-pair generation through spontaneous parametric down-conversion in a nonlinear waveguide with scattering or material losses can be effectively emulated by classical laser light propagation through a specially designed linear waveguide circuit. This platform can represent arbitrary photon and pump losses, with a potential for the emulation of non-Markovian decay. We characterize the photon-pair correlation spectrum and observe its characteristic transformation from the well-known sinc-shape in lossless waveguides towards a Lorentzian shape in the presence of photon loss.

InGaAs/InAlAs avalanche photodetectors integrated on silicon-on-insulator waveguide circuits

We simulated an evanescently coupled InGaAs/InAlAs avalanche photodetector integrated on silicon-on-insulator waveguide circuits using benzocyclobutene as the bonding layer. A silicon fiber-grating coupler is adopted to couple light from the fiber to the Si waveguide, and light is ultimately absorbed in the absorption region. Simulations aimed at improving the optical coupling efficiency using different device dimensions such as different benzocyclobutene bonding thicknesses, different InP layer parameters and different silicon waveguide widths were carried out. The simulation result shows a detection efficiency of 96.7% for a 2 pm silicon waveguide device with a photodetector length of 40 pm, which can obtain a gain-bandwidth product of 135 GHz at 1550 nm.

Special Issue on Substrate integrated Waveguide Circuits and Systems for MHz-Through-THz Wireless and Photonics Applications

Special Issue on Substrate integrated Waveguide Circuits and Systems for MHz-Through-THz Wireless and Photonics Applications

(Invited) Mechanically Flexible Integrated Photonic Systems for Sensing and Communications

Conventional integrated photonic devices are fabricated on rigid semiconductor or dielectric substrates and are therefore inherently incompatible with soft biological tissues. Over the past few years, we have developed a suite of active and passive photonic devices and systems integrated on plastic substrates which can be bent, twisted, and stretched without compromising their optical performance. Here we review the latest progress in this emerging field, and discuss the rational material and mechanical engineering principles underlying the extraordinary flexibility of these photonic structures. Leveraging these design strategies, we demonstrated bendable glass waveguide circuits, flexible waveguide-integrated nanomembrane photodetectors, and stretchable photonics.

Design Study of GW-THz Wave Transmission Without Mode Competition in an Oversized Relativistic Backward Wave Oscillator

Researchers have investigated plasma-related phenomena and internal breakdowns in the gigawatt–terahertz relativistic backward wave oscillator (RBWO) caused by field emissions from the surfaces of the waveguide circuits in these oscillators. In order to increase the power-handling capability of relativistic electron beam devices, an RBWO is proposed in the form of an oversized structure in which the diameter ( $D$ ) of the electrodynamic structure is increased to several times the free-space wavelength ( $\lambda )$ . An optimal periodic rectangular corrugation structure of which the axial mode is matched to the $\pi $ mode is designed in order to determine only the TM01 mode without mode competition in a slow-wave structure (SWS). The oversized RBWO has a fairly large diameter and uses a resonant reflector to achieve optimum efficiency while also preventing mode competition in the SWS. Absorption of a decelerated electron beam is achieved by the collector, which protects against breakdown and damage. The results of 0.5–0.1 THz from 500 kV–5 kA in the relativistic region are observed by means of a particle-in-cell simulation, and the dispersion relationship is determined by a finite-difference time-domain simulation. The outcomes show that a high-power oversized RBWO can control the selection of the mode and the interaction efficiency.

A tunable waveguide-coupled cavity design for scalable interfaces to solid-state quantum emitters

Photonic nanocavities in diamond have emerged as useful structures for interfacing photons and embedded atomic color centers, such as the nitrogen vacancy center. Here, we present a hybrid nanocavity design that enables (i) a loaded quality factor exceeding 50 000 (unloaded Q>106) with 75% of the enhanced emission collected into an underlying waveguide circuit, (ii) MEMS-based cavity spectral tuning without straining the diamond, and (iii) the use of a diamond waveguide with straight sidewalls to minimize surface defects and charge traps. This system addresses the need for scalable on-chip photonic interfaces to solid-state quantum emitters.

Design and fabrication of a sub-millimeter multi-beam folded waveguide structure

A novel multi-beam folded waveguide (MBFW) circuit, which can enhance the output power and interaction efficiency of sub-terahertz (THz) traveling wave tube (TWT), is presented in the paper. Operating with fundamental mode and multiple electron beams means that a larger beam current can be used for a higher output power. The characteristics of the MBFW structure are analyzed and optimized. Compared with the single-beam folded waveguide (SBFW) TWT, the output power of the MBFW TWT increases from 3.64 W to 25.45 W at 140 GHz and its electronic efficiency increases from 1.06% to 7.4% under the conditions of an input peak power of 10 mW, a beam voltage of 9.55 kV and a current of 12 mA. The optimized MBFW structure can be successfully fabricated by micro milling, with dimension errors below expectation, and the measured transmission characteristics are in good agreement with the design.

Recent progress of the application of surface plasmon polariton in quantum information processing

Surface plasmon polariton has attracted more and more attention and has been studied extensively in the recent decades, owing to its ability to confine the electro-magnetic field to a sub-wavelength scale near the metal-dielectric interface. On one hand, the tightly confined surface plasmonic modes can reduce the size of integrated optical device beyond the diffraction limit; on the other hand, it provides an approach to enhancing the interaction between light and matter. With the development of experimental and numerical simulation techniques, its investigation at a quantum level has become possible. In the recent experiments, scientists have realized quantum interference between single plasmons in a nanoscale waveguide circuit and achieved the strong coupling between photons and single molecules by using plasmonic structure, which demonstrates its superiority over the traditional optics. Here, we review the theoretical and experimental researches of surface plasmon polariton in the field of quantum information processing. First, we introduce the experiments on the basic quantum properties of surface plasmons, including the preservation of photonic entanglement, wave-particle duality and quantum statistical property. Second, we review the research work relating to the generation, manipulation and detection of surface plasmons in a quantum plasmonic integrated circuit. Then, we present the research of the interaction between surface plasmons and single quantum emitters and its potential applications. Finally, we make a discussion on how the intrinsic loss affects the quantum interference of single plasmons and the coupling between quantum emitters. The collision and combination of quantum optical and plasmonic fields open up possibilities for investigating the fundamental quantum physical properties of surface plasmons. It can be used to make ultra-compact quantum photonic integrated circuits and enhance the interaction strength between photons and quantum emitters.

Increase of Recognizable Label Number with Optical Passive Waveguide Circuits for Recognition of Encoded 4- and 8-Bit BPSK Labels

Optical label processing is expected to reduce power consumption in label switching network nodes. Previously, we proposed passive waveguide circuits for the recognition of BPSK labels with a theoretically infinite contrast ratio. The recognizable label number was limited to four and eight for 4-bit and 8-bit BPSK labels, respectively. In this paper, we propose methods to increase the recognizable label number. The proposed circuits can recognize eight and sixteen labels of 4-bit BPSK codes with a contrast ratio of 4.00 and 2.78, respectively. As 8-bit BSPK codes, 64, 128, and 256 labels can be recognized with a contrast ratio of 4.00, 2.78, and 1.65, respectively. In recognition of all encoded labels, that is, 16 and 256 labels for 4-bit and 8-bit BPSK labels, a reference signal is employed to identify the sign of the optical output signals. The effect of phase deviation and loss along the optical waveguides of the devices is also discussed.

A new twist on glass: A brittle material enabling flexible integrated photonics

AbstractGlass is in general brittle and therefore usually cannot sustain large deformation. Recent advances in glass material development as well as micro‐mechanical designs, however, are set to defy the conventional wisdom through the demonstration of flexible integrated photonics that can be bent, twisted, and even stretched without compromising its structural integrity and optical performance. In this paper, we review the latest progress in this emerging field, and discuss the rational material and mechanical engineering principles underlying the extraordinary flexibility of these photonic structures. Leveraging these design strategies, we demonstrated bendable chalcogenide glass waveguide circuits, flexible glass waveguide‐integrated nanomembrane photodetectors, and stretchable glass photonics.

Noise tolerance in optical waveguide circuits for recognition of optical 16 quadrature amplitude modulation codes

In photonic label routers, various optical signal processing functions are required; these include optical label extraction, recognition of the label, optical switching and buffering controlled by signals based on the label information and network routing tables, and label rewriting. Among these functions, we focus on photonic label recognition. We have proposed two kinds of optical waveguide circuits to recognize 16 quadrature amplitude modulation codes, i.e., recognition from the minimum output port and from the maximum output port. The recognition function was theoretically analyzed and numerically simulated by finite-difference beam-propagation method. We discuss noise tolerance in the circuit and show numerically simulated results to evaluate bit-error-rate (BER) characteristics against optical signal-to-noise ratio (OSNR). The OSNR required to obtain a BER less than 1.0×10−3 for the symbol rate of 2.5 GBaud was 14.5 and 27.0 dB for recognition from the minimum and maximum output, respectively.

Projecting light beams with 3D waveguide arrays

Free-space light beams with complex intensity patterns, or non-trivial phase structure, are demanded in diverse fields, ranging from classical and quantum optical communications, to manipulation and imaging of microparticles and cells. Static or dynamic spatial light modulators, acting on the phase or intensity of an incoming light wave, are the conventional choices to produce beams with such non-trivial characteristics. However, interfacing these devices with optical fibers or integrated optical circuits often requires difficult alignment or cumbersome optical setups. Here we explore theoretically and with numerical simulations the potentialities of directly using the output of engineered three-dimensional waveguide arrays, illuminated with linearly polarized light, to project light beams with peculiar structures. We investigate through a collection of illustrative configurations the far field distribution, showing the possibility to achieve orbital angular momentum, or to produce elaborate intensity or phase patterns with several singularity points. We also simulate the propagation of the projected beam, showing the possibility to concentrate light. We note that these devices should be at reach of current technology, thus perspectives are open for the generation of complex free-space optical beams from integrated waveguide circuits.

Optically Integrated InP–Si$_3$ N$_4$ Hybrid Laser

We describe the first demonstration and characterization of an optically integrated InP–Si $_3$ N $_4$ hybrid laser. The laser is formed by integration of an InP-based reflective semiconductor optical amplifier with a Si $_3$ N $_4$ based feedback waveguide circuit. The circuit comprises a frequency selective and tunable Vernier mirror composed of two microring resonators with slightly different radii. A wide tuning range of more than 43 nm is achieved via the thermo-optic effect. The typical side mode suppression ratio is 35 dB. The narrowest linewidth achieved is about 90 kHz, and the relative intensity noise is less than $-$ 135 dBc/Hz.

Parametric down-conversion photon-pair source on a nanophotonic chip

Quantum-photonic chips, which integrate quantum light sources alongside active and passive optical elements, as well as single-photon detectors, show great potential for photonic quantum information processing and quantum technology. Mature semiconductor nanofabrication processes allow for scaling such photonic integrated circuits to on-chip networks of increasing complexity. Second-order nonlinear materials are the method of choice for generating photonic quantum states in the overwhelming majority of linear optic experiments using bulk components, but integration with waveguide circuitry on a nanophotonic chip proved to be challenging. Here, we demonstrate such an on-chip parametric down-conversion source of photon pairs based on second-order nonlinearity in an aluminum-nitride microring resonator. We show the potential of our source for quantum information processing by measuring the high visibility anti-bunching of heralded single photons with nearly ideal state purity. Our down-conversion source yields measured coincidence rates of 80 Hz, which implies MHz generation rates of correlated photon pairs. Low noise performance is demonstrated by measuring high coincidence-to-accidental ratios. The generated photon pairs are spectrally far separated from the pump field, providing great potential for realizing sufficient on-chip filtering and monolithic integration of quantum light sources, waveguide circuits and single-photon detectors.

Design and Analysis of 2-μm InGaSb/GaSb Quantum Well Lasers Integrated Onto Silicon-on-Insulator (SOI) Waveguide Circuits Through an Al2O3 Bonding Layer

GaSb-based quantum well (QW) laser diode, with emission wavelength ∼2 μm, integrated onto a silicon-on-insulator (SOI) waveguide circuit through a high-thermal-conductivity Al2O3 bonding layer has been designed and analyzed. Prior to bonding, the fabricated Fabry–Perot GaSb QW laser worked under continuous wave operation at room temperature, with a low threshold current of 37 mA at the emission wavelength of 2019 nm, demonstrating high material quality. A tapered structure has been used for evanescent coupling of light from the GaSb laser to the underlying Si waveguide. Instead of using SiO2 for direct bonding or Benzocyclobutene for adhesive bonding, the use of Al2O3 to directly bond GaSb lasers onto SOI wafers is proposed. The optical mode distribution simulations by a beam propagation method software show that light can be coupled efficiently to the underlying Si waveguide through the tapered structure without compromise in optical coupling efficiency. Furthermore, there is a significant reduction (∼70%) in the total thermal resistance compared with the same structure using a SiO2 bonding layer. Our results suggest that the Al2O 3 bonding layer could be a promising candidate for III–V lasers integrated on SOI circuits, where thermal dissipation is very critical.

Developing an integrated photonic system with a simple beamforming architecture for phased-array antennas.

We have designed a simplified true-time-delay beamforming architecture using integrated photonics for phased-array antennas. This architecture can independently control multiple RF beams simultaneously with only a single tuning parameter to steer the beam in each direction for each beam. We have made a proof-of-the-principle demonstration of an X-band, 30×4-elements, fiber-optics-based beamformer for one-dimensional steering in transmission mode. The goal is to develop a semiconductor-based integrated photonic circuit so that a 2D beamforming array for both transmit and receive operations can be made on a single chip. For that, we have designed a Si-based integrated waveguide circuit using two types of "slow-light" waveguide for tunable time delays for two-dimensional steering.