Low-loss Broadband Research Articles

Development and verification of an on-board integrated optics scheme for a 50G/10G combo PON system

We propose an on-board integrated optics (OBO) scheme within a 50G/10G combo passive optical network (PON) system, for the first time to our best knowledge, which can achieve high port density, low cost, and low power consumption required by the 50G PON system. We provide an OBO scheme that integrates record-high power distributed feedback (DFB) lasers, low insertion loss silicon modulators, a planar lightwave circuit wavelength-division multiplexer, and avalanche photodiodes on a single board, which can solve key issues faced by the demand of coexistence and smooth evolution from 10G to 50G PON for global access network operators. In this system, the slope efficiency of the lasers has been optimized at high temperatures, allowing the output power of the lasers to achieve a record-breaking 23.5 dBm at 1577 and 1342 nm, while reducing cooling power consumption to realize low carbon emission in access networks. We developed a low-loss broadband silicon modulator to meet the large power budget and high data rate requirements of 50G PON by optimizing doping parameters and constructing electrodes that match the driver. This silicon modulator has a 6.6 dB insertion loss and an electro-optic bandwidth of more than 38 GHz, making it the ideal silicon modulator for 50G PON systems. In addition, we verify the end-to-end performance, including receiver sensitivity in the upstream direction and the dispersion penalty in the downstream direction. This suggests that the OBO scheme can satisfy the criteria of the 50G PON system according to the ITU-T G.9804 standard series.

Novel Subterahertz High Isolation Stacked-SIW Power Divider

A new type of eight-way multi-layer power divider is presented in this paper. To form the power divider, eight layers of the same substrate integrated waveguides (SIWs) are stacked together and inserted into a WR10 rectangular waveguide (RWG). In order to achieve a low-loss broadband response when connecting two substrates with different dielectric constants, a wedge structure is designed and applied to the input port of the power divider. Auxiliary parts are designed for measurement, including SIW extension line, SIW absorbing load and SIW-to-RWG transition. The measurement results show that, from 90 to 110 GHz, the insertion loss is less than 1.35 dB, and the isolation is above 19 dB. The proposed power divider is small and simple, suitable for terahertz technology.

Low-loss, broadband MMI coupler based on thin film lithium niobate platform

A low-loss broadband power splitter based on X-cut lithium niobate is proposed by utilizing the multimode interference coupler with a shallow-etched slot, wedge-shaped structure and SiO2 cladding. The multimode waveguide with a shallow-etch slot could reduce the sensitivity of multi-mode region on the wavelength and reach to high-quality imaging requirements. Results show that there is an excess loss around 0.01 dB at the operation wavelength of 1550 nm, a broad operation bandwidth of ∼ 1314 nm and large tolerance in the shallow-etching. Additionally, our proposed device has been compared with the published and conventional multimode interference coupler for presenting good low-loss and broad bandwidth.

Ultra low loss broadband 1 × 2 optical power splitters with various splitting ratios

We designed Si-based all-dielectric 1 × 2 TE and TM power splitters with various splitting ratios by combining the use of the inverse design of adjoint and numerical 3D finite-difference time-domain methods. The structure of the designed Si-based power splitters contains two Si waveguide branches on a SiO2 substrate that is compatible with CMOS fabrication technology. The proposed devices exhibit ultra-high transmission efficiency above 98 and 99%, and excess losses below 0.1 and 0.035 dB, for TE and TM splitters, respectively. The merits of these devices include a minor footprint of 2.2 × 2.2 µm2 and a flat-broad operating bandwidth of 200 nm with a center wavelength of λ = 1.55 µm. Also, the other advantage of these optical power splitters is the very short optimization time of 2 h for each device. Because of the aforementioned merits, the optimized devices can be crucial candidates for optical integrated circuits.

Assembly of Guanine Crystals as a Low-Polarizing Broadband Multilayer Reflector in a Spider, Phoroncidia rubroargentea.

The diminishing of the polarization effect is important in the applications of dielectric multilayer reflectors in many optical systems, such as low-loss broadband waveguides, optical fibers, and LEDs. Low-polarizing broadband reflections were identified from birefringent-guanine-crystal-based multilayer reflectors in the skins of some fish. Previous models for these intriguing natural optical phenomena suggested the combined action of two populations of guanine crystals with an orthogonal low-refractive-index optic axis. Here we report a novel realization of polarization-insensitive broadband reflectivity in a spider, Phoroncidia rubroargentea, based solely on the type of guanine crystals with the low-refractive-index optic axis normal to the crystal plates. We examined the three-dimensional structure of the guanine assembly in the spider and performed finite-difference time-domain (FDTD) optical modeling of the guanine-based multilayer reflector. Comparative modeling studies reveal that the biological selection of the guanine crystal type and specific spatial arrangement work synergistically to optimize the polarization-insensitive broadband reflection. This study demonstrates the importance of both crystallographic characteristics and 3D arrangement of guanine crystals in understanding relevant natural optical effects and also provides new insights into similar broadband, low-polarizing reflections in biological optical systems. Learning from relevant biofunctional assembly of guanine crystals could promote the bioinspired design of nonpolarizing dielectric multilayer reflectors.

Piezoelectric transducer design for simultaneous ultrasonic power transfer and backscatter communication

Ultrasonic waves can transfer power and data to sensors and devices deployed to traditionally inaccessible locations, such as inside the human body or deep in the ocean, eliminating the need for battery replacement. In ultrasonic power and data transfer systems, a piezoelectric transducer converts incident ultrasonic waves to useful electric power while transmitting data by modulating its reflected signal through backscatter communication. Existing approaches rely on reflecting a portion of the incident power to communicate, reducing the harvested power. This work realizes uninterrupted power harvesting with simultaneous backscatter communication through frequency multiplexing. A piezoelectric transducer is first designed and tested experimentally for high sensitivity and high bandwidth operation through low-loss broadband acoustic and electrical impedance matching. The transducer achieved 70% bandwidth at 1 MHz with a 10 dB difference between reflecting and absorbing incident ultrasonic waves. A frequency multiplexing technique is then developed to separate power and data into different frequency bands achieving simultaneous operation. The technique extends the range and bandwidth of ultrasonically powered devices such as biomedical implants and ocean monitoring sensors.

The anisotropic broadband surface plasmon polariton and hot carrier properties of borophene monolayer

Abstract Borophene monolayer with its intrinsic metallic and anisotropic band structures exhibits extraordinary electronic, optical, and transport properties. Especially, the high density of Dirac electrons enables promising applications for building low-loss broadband SPP devices. However, a systematic characterization of the surface plasmon polariton (SPP) properties and hot carriers generated from the inevitable SPP decay in borophene has not been reported so far. Most importantly, the mechanism for SPP losses remains obscurely quantified. In this work, from a fully first-principles perspective, we explicitly evaluate the main loss effects of SPP in borophene, including the Drude resistance, phonon-assisted intraband and direct interband electronic transitions. With this knowledge, we further calculate the frequency- and polarization-dependent SPP response of borophene, and evaluate some typical application-dependent figure of merits of SPP. On the other hand, we evaluate the generation and transport properties of plasmon-driven hot carriers in borophene, involving energy- and momentum-dependent carrier lifetimes and mean free paths, which provide deeper insight toward the transport of hot carriers at the nanoscale. These results indicate that borophene has promising applications in next-generation low-loss optoelectronic devices and photocatalytic reactors.

A Low-Loss Broadband Planar Transition From Ground Coplanar Waveguide to Substrate-Integrated Coaxial Line

In this letter, a three-dimensional ground coplanar waveguide (3D-GCPW) structure is proposed as a broadband planar transition between the conventional GCPW and substrate integrated coaxial line (SICL). The proposed 3D-GCPW contains a 3-D central conductor constructed by two copper layers which are connected together by metallized blind vias and two side ground planes. Its characteristic impedance is mainly determined by the width of the 3-D central conductor and the gap to the side ground plane; thus, the requirement for machining accuracy of metallized blind vias in 3D-GCPW can be reduced during the machining process. A back-to-back structure of the 3D-GCPW transition is fabricated by a multilayer printed circuit board (PCB) process and measured to validate the good performance. The experimental results show that the proposed 3D-GCPW transition exhibits a reflection coefficient less than −10 dB and an acceptable insertion loss within dc to 42 GHz, making it promising for high-performance multiple-channel transceiver or massive integrated applications.

Cyclic olefin copolymer‐based copper clad laminate and the application for low loss broadband baluns

In this article, a novel copper clad laminate based on cyclic olefin copolymer (COC) is proposed and manufactured by using lamination process. The loss characteristic of manufactured laminate is investigated by comparing the measured results of two microstrips on proposed COC-based laminate and traditional Rogers RT/duroid 5880, respectively. Moreover, the SIW/HMSIW baluns operated at X-band are designed on the proposed laminate, and the measured results verify the inherent out-of-phase performance and good amplitude characteristics in broadband frequency range. This work shows a novel type of high-frequency circuit boards on COC material, which is significantly to be one of the advanced candidates for microwave/millimeter wave circuit applications.

Low-loss broadband bi-layer edge couplers for visible light.

Low-loss broadband fiber-to-chip coupling is currently challenging for visible-light photonic-integrated circuits (PICs) that need both high confinement waveguides for high-density integration and a minimum feature size above foundry lithographical limit. Here, we demonstrate bi-layer silicon nitride (SiN) edge couplers that have ≤ 4 dB/facet coupling loss with the Nufern S405-XP fiber over a broad optical wavelength range from 445 to 640 nm. The design uses a thin layer of SiN to expand the mode at the facet and adiabatically transfers the input light into a high-confinement single-mode waveguide (150-nm thick) for routing, while keeping the minimum nominal lithographic feature size at 150 nm. The achieved fiber-to-chip coupling loss is about 3 to 5 dB lower than that of single-layer designs with the same waveguide confinement and minimum feature size limitation.

Monolithic integration of laser onto multilayer silicon nitride photonic integrated circuits with high efficiency at telecom wavelength.

We propose a multilayer silicon nitride (SiN) -on-silicon photonic integrated circuit (PIC) platform with a monolithic laser at the C-band. A tapered edge coupler and a meta-structure-based interlayer directional coupler in the platform were designed to realize low-loss broadband laser-to-chip 3D coupling with small footprint. The coupling length of the interlayer directional coupler and the gap between different SiN layers were optimized as 12.7 µm and 1.4 µm. We measured the 1-dB-drop optical operation bandwidth of greater than 76 nm and the coupling loss of 6.1 ± 0.1 dB at 1550 nm for the interlayer directional coupler. The hybrid integration was demonstrated as a proof of concept for monolithic integration of light sources. The butt-coupling loss of 3.7 ± 0.1 dB between an on-chip DFB laser and a SiN edge coupler at 1549.48 nm was achieved. This approach opens the possibility of employing monolithic laser in the silicon photonics platform.

Hyperbolic plasmon modes in tilted Dirac cone phases of borophene

Hyperbolic materials are receiving significant attention due to their ability to support electromagnetic fields with arbitrarily high momenta and, hence, to achieve very strong light confinement. Here, based on first-principles calculations and many-body perturbation theory, we explore the characteristic of two-dimensional plasmon modes and its hyperbolic properties for two phases of single-layer boron hosting tilted Dirac cone, namely, the $hr\text{\ensuremath{-}}sB$ and 8Pmmn borophene. In-plane anisotropy in borophene is manifested in the structural, electronic, vibrational, and optical properties. We find two hyperbolic regimes for both phases of borophene, where the high-energy one is located in the visible range. The $hr\text{\ensuremath{-}}sB$ borophene is characterized with an intrinsic high carrier density and it supports strong hyperbolic plasmon modes in the visible part of the spectrum. The 8Pmmn borophene, on the other hand, resembles the prototypical Dirac material graphene and on carrier doping acquires anisotropic Dirac plasmons in the mid-infrared. We have also investigated the impact of the electron-phonon coupling and Landau damping on these hyperbolic plasmon modes. Our results show that borophene, having high anisotropy, intrinsic high carrier concentration, low-loss hyperbolic Dirac plasmon modes, and high confinement can represent a promising candidate for low-loss broadband surface plasmon polariton devices.

A Compact Low-Loss Broadband Polarization Independent Silicon 50/50 Splitter

We present a low-loss, compact, polarization insensitive 3-dB optical power-splitter for submicron silicon waveguides. The splitter is optimized over a broad wavelength span ranging from 1300 to 1700 nm through finite difference time domain simulations. The splitter junction footprint is only 1.8 × 1.3 μm, which is smaller than previously reported broadband multimode junction splitters. Furthermore, the demonstrated device overcomes the fabrication limitations of previous splitters while maintaining a compact size without any sharp angles. The device was fabricated using electron beam lithography. The experimental results show a small insertion loss of less than 0.3 dB for both the TE and TM polarizations.

Low-Loss Broadband Transverse Electric Pass Hybrid Plasmonic Fiber Polarizers Using Metallic Nanomaterials

Metals were for decades perceived as devoid of interesting optical properties that could be harnessed for optical components and devices. However, with the development of accurate nanofabrication techniques and precise control over architectural parameters, metals can be structured and characterized on the nanoscale. Metallic plasmonic nanomaterials exhibit a number of unique structural and optical properties, which offer the potential for developing new types of plasmonic devices. Here, we demonstrate a low-loss broadband polarizer based on a hybrid plasmonic fiber structure using metals as polarization-selective absorption materials. The polarization mechanism, design, fabrication, and characteristics of the plasmonic polarizers are investigated theoretically, numerically, and experimentally. The theoretical analysis predicts that the polarization-selective absorption with insensitivity to wavelength enables hybrid plasmonic fibers to function as broadband polarizers. Numerical simulations give the comparison of the polarization-selective absorption of various metallic nanomaterials (Ag, Au, In, Al, Cr) and show that aluminum is regarded as the optimum absorption material for the plasmonic polarizer. Experimental results show that through precise control over geometrical parameters, this device is capable of offering a high polarization extinction ratio (PER) of over 40 dB and a low insertion loss (IL) of less than 1.3 dB in the wavelength region of 810.1-870.0 nm. Compared with commercial birefringent-crystal-fiber polarizers, the plasmonic fiber polarizer has a better PER and IL bandwidth. These merits, combined with a compact and robust configuration, enable the plasmonic polarizer to have great potential in a broad range of applications.

Low-Loss Broadband Silicon TM-Pass Polarizer Based on Periodically Structured Waveguides

A compact and high performance integrated silicon TM-pass polarizer is proposed and experimentally demonstrated. The device is formed by periodically structuring a waveguide with a slot section within each period, and is implemented on a siliconon-insulator platform. The fabricated device has low insertion loss for the TM mode (average 0.7dB) and high extinction for the TE mode (average 41.25dB), over a 1.5 μm to 1.6 μm wavelength range. Numerical simulations indicate that a large fraction of the blocked TE mode radiates out of the periodic structure, resulting in weak back reflections. The compact footprint of the device (21 μm × 0.5 μm) makes it suitable for dense integration in photonic integrated circuits.

Analysis and design of novel wideband and high efficiency millimeter-wave antenna arrays for 60-GHz applications

A type of millimeter-wave antenna array with flexible design is proposed for a variety of applications at 60 GHz. The antenna array can be adjusted to be linearly or circularly polarized by simply changing the radiation part of the antenna array. High gain, wideband, and high radiation efficiency characteristics can be achieved by adopting a low insertion loss feeding network and broadband antenna elements. For the linearly polarized antenna array, simulation results show that the impedance bandwidth of the 2×2 antenna subarray reaches 21.6%, while the maximum gain achieves 15.1 dBi and has a fluctuation of less than 0.4 dBi within the working bandwidth. Simulation results of the 8×8 linearly polarized antenna array show a bandwidth of 21.6% and a gain of (26.1±1) dBi with an antenna efficiency of more than 80%. For the 8×8 circularly polarized antenna array, simulation results show that an impedance bandwidth of 18.2% and an axial ratio (AR) bandwidth of 13.3% are obtained. Gain and efficiency of up to 27.6 dBi and 80% are achieved, respectively. A prototype of antenna array is fabricated, and results are compared and analyzed.

Mass Producible Low-Loss Broadband Optical Waveguides in Eagle2000 by Femtosecond Laser Writing

Optical waveguides were fabricated in alkaline earth boro-aluminosilicate glass, by femtosecond laser direct writing, with varying pulse energy and scan velocity. A spectral characterization, from 500 nm to 1700 nm, was made in order to determine their losses and understand its dependence on the processing parameters. Three major loss mechanisms were identified. At longer wavelengths, loss is mainly due to weak coupling. On the other hand, the behavior at shorter wavelengths is governed by propagation loss due to Rayleigh scattering, which was shown to be practically eliminated ( $\cdot$ cm $^{-1} {\cdot }\,\,\mu \text{m}^{4}$ ) at higher scan velocities. Bulk absorption was also found to have an influence in the propagation losses at higher wavelengths. The combination of intermediate pulse energies (between 125–250 nJ) and high scan velocities (above 6 cm/s) allowed the fabrication of optical waveguides offering low losses across the entire range of wavelengths tested, facilitating applications that require larger wavelength working bands. Furthermore, since optimal fabrication conditions are achieved at higher scanning velocities, mass production with reduced fabrication times can be achieved.

Hybrid plasmonic graphene modulator with buried silicon waveguide

We propose a low-loss broadband modulator by combining plasmonic effect with graphene to realize highly efficient modulation on the SOI (Silicon-On-Insulator) platform. Due to a hybrid plasmonic waveguide including buried silicon waveguide and silver waveguides, the fundamental mode is mainly confined in two single-layer graphene sheets, and the electric field vector is parallel to the graphene sheets ensuring the high efficient light absorption of graphene. The specific hybrid structure makes it easy for graphene transfer. Also, non-coplanar structure of silicon waveguide and silver waveguides provides simplified fabrication process and fabrication tolerance. The simulation results demonstrated that modulation bandwidth of 346 GHz is obtained, and the 3 dB modulation length is only 9.493μm. In addition, competitively low propagation loss of 0.85 dB was realized of the hybrid modulator.

Design and Simulation of Broadband Beam Splitter on a Silicon Nitride Platform for Optical Coherence Tomography

ABSTRACTIntegrated photonics enables the miniaturization of bulk optical components for biosensing applications such as optical coherence tomography (OCT) and is therefore promising for future lab-on-chip solutions. Here, we report the design and simulation of a compact low loss broadband beam splitter with arbitrary coupling ratios on silicon nitride platform for OCT systems. The reported coupler uses asymmetric waveguide-based phase control section for 10:90, 20:80, 30:70, 40:60, and 50:50 splitting ratios and is broadband over 100 nm with the central wavelength of 850 nm. The couplers are realized for transverse electric, transverse magnetic, and fully vectorial modes, and maximum excess loss for all mode types is reported to be less than 0.19 dB. The design tolerance of waveguide width and thickness of the designed coupler is further calculated and is within fabrication limit.

Multiple hollow-core anti-resonant fiber as a supermodal fiber interferometer

Hollow-core anti-resonant fiber technology has made a rapid progress in low loss broadband transmission, enabled by its much reduced light-material overlap. This unique characteristic has driven emerging of new applications spanning from extreme wavelength generation to beam delivery. The successful demonstrations appear to suggest progression of the technology toward device level development and all-fiberized systems. We investigate this opportunity and report an in-fiber interferometer built in a dual hollow-core anti-resonant fiber. By placing multiple air cores in a single fiber, coherently interacting transverse modes are excited, which becomes a basis of an interferometer. We use this hollow core based inherent supermodal interaction to demonstrate highly sensitive in-fiber interferometer. Unique combination of the air guidance and the supermodal interaction offers robust, simple yet highly sensitive interferometer with suppressed temperature cross-talk that has been an enduring problem in fiber strain sensing applications. The in-fiber interferometer is further investigated as a sensing element for pressure measurement based on an interferometric phase change upon external strain. The interferometer features 39.3 nm/MPa of ultrahigh sensitivity with 0.14 KPa/°C of negligible gas pressure temperature crosstalk. The performance, which is much improved from prior fiber sensors, testifies advances of hollow core fiber technology toward a device level.