Low Propagation Loss Research Articles

Optical fibres with embedded two-dimensional materials for ultrahigh nonlinearity

Nonlinear optical fibres have been employed for a vast number of applications, including optical frequency conversion, ultrafast laser and optical communication1-4. In current manufacturing technologies, nonlinearity is realized by the injection of nonlinear materials into fibres5-7 or the fabrication of microstructured fibres8-10. Both strategies, however, suffer from either low optical nonlinearity or poor design flexibility. Here, we report the direct growth of MoS2, a highly nonlinear two-dimensional material11, onto the internal walls of a SiO2 optical fibre. This growth is realized via a two-step chemical vapour deposition method, where a solid precursor is pre-deposited to guarantee a homogeneous feedstock before achieving uniform two-dimensional material growth along the entire fibre walls. By using the as-fabricated 25-cm-long fibre, both second- and third-harmonic generation could be enhanced by ~300 times compared with monolayer MoS2/silica. Propagation losses remain at ~0.1 dB cm-1 for a wide frequency range. In addition, we demonstrate an all-fibre mode-locked laser (~6 mW output, ~500 fs pulse width and ~41 MHz repetition rate) by integrating the two-dimensional-material-embedded optical fibre as a saturable absorber. Initial tests show that our fabrication strategy is amenable to other transition metal dichalcogenides, making these embedded fibres versatile for several all-fibre nonlinear optics and optoelectronics applications.

Design of terahertz fibers using polymer tube bundles and their guided mode and propagation loss analyses

Flexible terahertz fibers were designed for terahertz wave applications. In this paper, new terahertz fiber structures using polymer tube bundles were proposed. A triangle structure fiber with three-tube core and a hexagonal structure fiber with mono-tube core were considered. Mode field distributions in the proposed terahertz fibers and their propagation losses were numerically calculated using the finite element method. It was found that lower propagation loss was obtained for longer operating wavelength and smaller thickness of polymer tube due to the fraction increment of electromagnetic field propagating in the air. It was also clarified that lower propagation loss was brought by introducing the core tube material with lower absorption coefficient, where the propagation loss increased for longer wavelength due to the electromagnetic field expansion into the cladding region with larger absorption coefficient compared with the core material.

Free‐Space Communications Enabled by Quantum Cascade Lasers

Future generations of wireless communication systems are expected to support orders of magnitude faster data transfer with much lower latency than the currently deployed solutions. Development of wireless transceivers of higher bandwidth, low energy consumption, and small footprint becomes challenging with radio frequency (RF) electronic technologies. Photonics‐assisted technologies show many advantages in generating signals of ultrabroad bandwidth at high carrier frequencies in the millimeter‐wave, terahertz, and IR bands. Among these frequency options, the mid‐IR band has recently attracted great interest for future wireless communication due to its intrinsic merits of low propagation loss and high tolerance of atmospheric perturbations. A promising source for mid‐IR free‐space communications is the semiconductor quantum cascade laser (QCL), which can be directly modulated at a high speed and facilitates monolithic integration for compact transceivers. Herein, the research and development of QCL‐based free‐space communications are reviewed and a recent experimental study of multi‐gigabit transmission with a directly modulated mid‐IR QCL and a commercial off‐the‐shelf IR photodetector is reported on. Up to 4 Gb s−1 transmission of two advanced modulation formats, namely, four‐level pulse amplitude modulation (PAM‐4) and discrete multitone (DMT) modulation, is demonstrated.

Channel waveguide lasers in bulk Tm:LiYF4 produced by deep diamond-saw dicing.

We report on a novel approach to fabricate channel (ridge) waveguides (WGs) in bulk crystals using precision diamond saw dicing. The channels feature a high depth-to-width aspect ratio (deep dicing). The proof-of-the-concept is shown for a Tm3+:LiYF4 fluoride crystal. Channels with a depth of 200 µm and widths of 10-50 µm are diced and characterized by confocal laser microscopy revealing a r.m.s. roughness of the walls well below 100 nm. The channels obtained possess waveguiding properties at ∼815 nm with almost no leakage of the guided mode having a vertical stripe intensity profile into the bulk crystal volume and relatively low propagation losses (0.20-0.43 dB/cm). Laser operation is achieved in quasi-CW regime by pumping at 780 nm. The maximum peak output power reaches 0.68 W at ∼1.91 µm with a slope efficiency of 53.3% (in σ-polarization). The proposed concept is applicable to a variety of laser crystals with different rare-earth dopants.

Electrical activity at the AlN/Si Interface: identifying the main origin of propagation losses in GaN-on-Si devices at microwave frequencies

AlN nucleation layers are the basement of GaN-on-Si structures grown for light-emitting diodes, high frequency telecommunication and power switching systems. In this context, our work aims to understand the origin of propagation losses in GaN-on-Si High Electron Mobility Transistors at microwaves frequencies, which are critical for efficient devices and circuits. AlN/Si structures are grown by Metalorganic Vapor Phase Epitaxy. Acceptor dopant in-diffusion (Al and Ga) into the Si substrate is studied by Secondary Ion Mass Spectroscopy and is mainly located in the first 200 nm beneath the interface. In this region, an acceptor concentration of a few 1018 cm-3 is estimated from Capacitance–Voltage (C–V) measurements while the volume hole concentration of several 1017 cm-3 is deduced from sheet resistance. Furthermore, the combination of scanning capacitance microscopy and scanning spreading resistance microscopy enables the 2D profiling of both the p-type conductive channel and the space charge region beneath the AlN/Si interface. We demonstrate that samples grown at lower temperature exhibit a p-doped conductive channel over a shallower depth which explains lower propagation losses in comparison with those synthesized at higher temperature. Our work highlights that this p-type channel can increase the propagation losses in the high-frequency devices but also that a memory effect associated with the previous sample growths with GaN can noticeably affect the physical properties in absence of proper reactor preparation. Hence, monitoring the acceptor dopant in-diffusion beneath the AlN/Si interface is crucial for achieving efficient GaN-on-Si microwave power devices.

High-temperature-resistant silicon-polymer hybrid modulator operating at up to 200\u2009Gbit\u2009s\u22121 for energy-efficient datacentres and harsh-environment applications

To reduce the ever-increasing energy consumption in datacenters, one of the effective approaches is to increase the ambient temperature, thus lowering the energy consumed in the cooling systems. However, this entails more stringent requirements for the reliability and durability of the optoelectronic components. Herein, we fabricate and demonstrate silicon-polymer hybrid modulators which support ultra-fast single-lane data rates up to 200 gigabits per second, and meanwhile feature excellent reliability with an exceptional signal fidelity retained at extremely-high ambient temperatures up to 110 °C and even after long-term exposure to high temperatures. This is achieved by taking advantage of the high electro-optic (EO) activities (in-device n3r33 = 1021 pm V−1), low dielectric constant, low propagation loss (α, 0.22 dB mm−1), and ultra-high glass transition temperature (Tg, 172 °C) of the developed side-chain EO polymers. The presented modulator simultaneously fulfils the requirements of bandwidth, EO efficiency, and thermal stability for EO modulators. It could provide ultra-fast and reliable interconnects for energy-hungry and harsh-environment applications such as datacentres, 5G/B5G, autonomous driving, and aviation systems, effectively addressing the energy consumption issue for the next-generation optical communication.

Chalcogenide glass photonic integration for improved 2 μm optical interconnection

In this work, on-chip chalcogenide glass photonic integrations with several fundamental photonic building blocks are designed and fabricated based on the As 2 S 3 platform for improved 2 μm optical interconnection, achieving a broadened wavelength bandwidth and improved fabrication tolerance. A 600 nm thick As 2 S 3 strip waveguide has low propagation loss of 1.447 dB/cm at 2 μm. Broadband vertical coupling is realized by a grating coupler with 4.3 dB coupling loss. A Bragg grating filter, power splitter, Mach–Zander interferometer, and mode converter for on-chip mode division multiplexing (MDM) are also reported at 2 μm with reliable performances. Finally, a record high MDM optical interconnection capacity of 3 × 80 Gbps at 2 μm is experimentally demonstrated based on the proposed As 2 S 3 chip, drawing promising prospects for future photonic integration and high-speed interconnection at the 2 μm waveband.

A Low propagation loss planar waveguide structure fabricated on lithium niobate via low fluence argon-ion irradiation

A low propagation loss waveguide structure was fabricated on lithium niobate crystal using 70 MeV argon-ion irradiation with a low fluence of 1 × 1012 ions/cm2. Guided modes were detected in visible and near-infrared regions; the propagation loss at 633 nm is ∼0.8 dB/cm. meaning this waveguide can be used for visible and near-infrared communications The Raman spectra of the waveguide layer and substrate indicate the lithium niobate lattice structure has been preserved after the low dose argon-ion irradiation. The results indicated that the low fluence Ar-ion can be used for optical waveguide fabrication on lithium niobite crystal.

Fluorescent diamond microparticle doped glass fiber for magnetic field sensing

Diamond containing the nitrogen-vacancy (NV) center is emerging as a significant sensing platform. However, most NV sensors require microscopes to collect the fluorescence signals and therefore are limited to laboratory settings. By embedding micron-scale diamond particles at an annular interface within the cross section of a silicate glass fiber, we demonstrate a robust fiber material capable of sensing magnetic fields. Luminescence spectroscopy and electron spin resonance characterization reveal that the optical properties of NV centers in the diamond microcrystals are well preserved throughout the fiber drawing process. The hybrid fiber presents a low propagation loss of ∼4.0 dB/m in the NV emission spectral window, permitting remote monitoring of the optically detected magnetic resonance signals. We demonstrate NV-spin magnetic resonance readout through 50 cm of fiber. This study paves a way for the scalable fabrication of fiber-based diamond sensors for field-deployable quantum metrology applications.

Integration of low loss vertical slot waveguides on SOI photonic platforms for high efficiency carrier accumulation modulators.

Silicon accumulation type modulators offer prospects of high power efficiency, large bandwidth and high voltage phase linearity making them promising candidates for a number of advanced electro-optic applications. A significant challenge in the realisation of such a modulator is the fabrication of the passive waveguide structure which requires a thin dielectric layer to be positioned within the waveguide, i.e. slotted waveguides. Simultaneously, the fabricated slotted waveguide should be integrated with conventional rib waveguides with negligible optical transition losses. Here, successful integration of polysilicon and silicon slot waveguides enabling a low propagation loss 0.4-1.2 dB/mm together with an ultra-small optical mode conversion loss 0.04 dB between rib and slot waveguides is demonstrated. These fabricated slot waveguide with dielectric thermal SiO2 layer thicknesses around 6 nm, 8 nm and 10 nm have been characterized under transmission electron microscopy allowing for strong carrier accumulation effects for MOS-capacitor electro-optic modulators.

Weak One-Dimensional Field Confinement Silica Waveguides

A novel weak one-dimensional field confinement silica waveguide (WCSW) with a large horizontal and vertical aspect ratio and low refractive index difference is proposed and fabricated via sol-gel technique. This new type of waveguide exhibits properties of high lithography tolerance, low polarization dependence, low propagation loss and low coupling loss with a single-mode fiber. More importantly, WCSW can provide extremely large evanescent field in vertical direction. The measured propagation loss and coupling loss are as low as 0.81 dB/cm and 0.71 dB/facet, respectively. In addition, high efficiency vertical coupling with a 1.9 μm gap between a WCSW bus waveguide and a WCSW based high Q (5.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> ) ring resonator is achieved, which demonstrates that vertical integration of WCSW is potentially applicable.

Machine Learning to Improve Multi-Hop Searching and Extended Wireless Reachability in V2X

Multi-hop relay selection is a critical issue in vehicle-to-everything networks. In previous works, the optimal hopping strategy is assumed to be based on the shortest distance. This study proposes a hopping strategy based on the lowest propagation loss, considering the effect of the environment. We use a two-step machine learning routine: improved deep encoder-decoder architecture to generate environmental maps and ${Q}$ -learning to search for the multi-hopping path with the lowest propagation loss. Simulation results show that our proposed method can improve environmental recognition and extend the reachability of multi-hop communications by up to 66.7%, compared with a shortest-distance selection.

Kerr nonlinearity induced four-wave mixing of CMOS-compatible PECVD deposited ultra-Si-rich-nitride

Benefitting from low propagation loss and complementary metal–oxide–semiconductor compatibility, Si3N4 is heavily explored for the applications of nonlinear optical signal processing. However, the Si3N4 waveguide is limited by its low optical Kerr nonlinearity. In this work, we introduce highly nonlinear ultra-Si-rich-nitride (USRN) deposited by the plasma enhanced chemical vapor deposition system. The measured linear refractive index of USRN is 3.09 at a wavelength of 1550 nm, while the Kerr nonlinearity of the USRN waveguide is experimentally extracted with a value of 2.25×10−17 m2/W. Moreover, a broadband wavelength conversion ranging from S-band to L-band by a four-wave-mixing experiment is realized via designed USRN waveguide with a relatively short length of 3 mm. The measured bandwidth is 190 nm with a continuous-wave pump laser located at 1530 nm. The conversion efficiency is measured approximately −48 dB under a relatively low pump power of 7 dBm.

Electro-Optically Switchable Optical True Delay Lines of Meter-Scale Lengths Fabricated on Lithium Niobate on Insulator Using Photolithography Assisted Chemo-Mechanical EtchingSupported by the National Key R&D Program of China (Grant No. 2019YFA0705000), the National Natural Science Foundation of China (Grant Nos. 11734009, 61590934, and 11874375), the Strategic Priority Research Program of CAS (Grant No. XDB16030300), and the Key Project of the

Optical true delay lines (OTDLs) of low propagation losses, small footprints and high tuning speeds and efficiencies are of critical importance for various photonic applications. Here, we report fabrication of electro-optically switchable OTDLs on lithium niobate on insulator using photolithography assisted chemo-mechanical etching. Our device consists of several low-loss optical waveguides of different lengths which are consecutively connected by electro-optical switches to generate different amounts of time delay. The fabricated OTLDs show an ultra-low propagation loss of ∼ 0.03 dB/cm for waveguide lengths well above 100 cm.

Hybrid integrated photonics using bulk acoustic resonators

Integrated photonic devices based on Si3N4 waveguides allow for the exploitation of nonlinear frequency conversion, exhibit low propagation loss, and have led to advances in compact atomic clocks, ultrafast ranging, and spectroscopy. Yet, the lack of Pockels effect presents a major challenge to achieve high-speed modulation of Si3N4. Here, microwave-frequency acousto-optic modulation is realized by exciting high-overtone bulk acoustic wave resonances (HBAR) in the photonic stack. Although HBAR is ubiquitously used in modern communication and superconducting circuits, this is the first time it has been incorporated on a photonic integrated chip. The tight vertical acoustic confinement releases the lateral design of freedom, and enables negligible cross-talk and preserving low optical loss. This hybrid HBAR nanophotonic platform can find immediate applications in topological photonics with synthetic dimensions, compact opto-electronic oscillators, and microwave-to-optical converters. As an application, a Si3N4-based optical isolator is demonstrated by spatiotemporal modulation, with over 17 dB isolation achieved.

High-dimensional communication on etchless lithium niobate platform with photonic bound states in the continuum

Photonic bound states in the continuum (BICs) have been exploited in various systems and found numerous applications. Here, we investigate high-order BICs and apply BICs on an integrated photonic platform to high-dimensional optical communication. A four-channel TM mode (de)multiplexer using different orders of BICs on an etchless lithium niobate (LiNbO3) platform where waveguides are constructed by a low-refractive-index material on a high-refractive-index substrate is demonstrated. Low propagation loss of the TM modes in different orders and phase-matching conditions for efficient excitation of the high-order TM modes are simultaneously achieved. A chip consisting of four-channel mode (de)multiplexers was fabricated and measured with data transmission at 40 Gbps/channel. All the channels have insertion loss <4.0 dB and crosstalk <−9.5 dB in a 70-nm wavelength band. Therefore, the demonstrated mode (de)multiplexing and high-dimensional communication on LiNbO3 platform can meet the increasing demand for high capacity in on-chip optical communication.

Conformable Nanowire-in-Nanofiber Hybrids for Low-Threshold Optical Gain in the Ultraviolet.

The miniaturization of diagnostic devices that exploit optical detection schemes requires the design of light sources combining small size, high performance for effective excitation of chromophores, and mechanical flexibility for easy coupling to components with complex and nonplanar shapes. Here, ZnO nanowire-in-fiber hybrids with internal architectural order are introduced, exhibiting a combination of polarized stimulated emission, low propagation losses of light modes, and structural flexibility. Ultrafast transient absorption experiments on the electrospun material show optical gain which gives rise to amplified spontaneous emission with a threshold lower than the value found in films. These systems are highly flexible and can conveniently conform to curved surfaces, which makes them appealing active elements for various device platforms, such as bendable lasers, optical networks, and sensors, as well as for application in bioimaging, photo-cross-linking, and optogenetics.

Nonlocal nonlinear optical X waves and their active control in a Rydberg atomic gas

$X$ waves are a special type of wave packet that can maintain their transverse profile of $X$ shape during propagation, and they are of much interest for the study of fundamental physics and practical applications. Here we present a scheme to generate nonlinear $X$ waves and realize their active control by using a cold gas of Rydberg atoms via electromagnetically induced transparency (EIT). We show that, due to the EIT effect contributed by a control laser field and the strong, nonlocal Kerr nonlinearity contributed by Rydberg-Rydberg interaction between atoms, the system supports high-dimensional, nonlocal, and nonlinear optical $X$ waves, which have low propagation loss, ultraslow propagation velocity, and ultralow generation power. We also show that the stability domain of such $X$ waves can be greatly enlarged by increasing the nonlocality degree of the Kerr nonlinearity, and their motion trajectory can be manipulated by using an external magnetic field. Our study opens a route for generating and controlling nonlocal ultraslow nonlinear optical $X$ waves, which may have promising applications in optical information processing and transmission.

Interleaved Subwavelength Gratings Strip Waveguide Based TM Pass Polarizer on SOI Platform

We report on an ultra-compact, low loss and broadband TM-pass polarizer, which is constructed by two interleaved subwavelength-gratings (SWGs) waveguides inserted in the center of the strip waveguide on a silicon-on-insulator (SOI) platform. The device structure is optimally engineered to support Bloch mode for fundamental TM polarization only. Therefore, the TM mode transmits along the strip waveguide with rather low propagation loss while the TE mode is completely reflected by the embedded SWGs waveguides. Simulation results show that an efficient and ultra-compact TM-pass polarizer of 5.2 μm in length is realized with extinction ratio (ER) of 31.63 dB and insertion loss (IL) of 0.18 dB at the operating wavelength of 1550 nm. The operation bandwidth for ER over 20 dB and IL below 0.32 dB is 155 nm (from 1465 to 1620 nm). By increasing the period number, the ER performance and bandwidth can be further improved. Furthermore, the fabrication of the present polarizer only requires mono-lithography and single etch process, which will greatly reduce the fabrication difficulty.

Silicon-embedded photonic crystal fiber for high birefringence and nonlinearity

In this study, we propose a hexagonal photonic crystal fiber (PCF) including an elliptical Si core for high nonlinearity and birefringence, simultaneously. The propagation properties are rigorously investigated by numerical full-vector analysis. The designed fiber provides high nonlinearity and birefringence for a wide range of wavelengths, from 1.25 to 2 μm, maintaining low propagation loss by virtue of a high nonlinear coefficient and large Si refractive index. The optimal values of nonlinearity and birefringence are 2 × 105 W−1 km−1 and 0.92, respectively. In addition, the proposed hexagonal PCF can be easily fabricated with a conventional draw technique compared with other PCFs composed of complex structures to achieve high nonlinearity. It also has a high fabrication tolerance for the rotational angle of the elliptical Si core. We believe that this type of the fiber could be applied to various efficient nonlinear applications.