Wire Waveguides Research Articles

Topological converter for high-efficiency coupling between Si wire waveguide and topological waveguide.

Replacing part of a conventional optical circuit with a topological photonic system allows for various controls of optical vortices in the optical circuit. As an underlying technology for this, in this study, we have realized a topological converter that provides high coupling efficiency between a normal silicon wire waveguide and a topological edge waveguide. After expanding the waveguide width while maintaining single-mode transmission from the Si wire waveguide, the waveguides are gradually narrowed from both sides by using a structure in which nanoholes with C6 symmetry are arranged in a honeycomb lattice. On the basis of the analysis using the three-dimensional finite-difference time-domain method, we actually fabricated a device in which a Si wire waveguide and a topological edge waveguide were connected via the proposed topological converter and evaluated its transmission characteristics. The resulting coupling efficiency between the Si wire waveguide and the topological edge waveguide through the converter was -4.49 dB/taper, and the coupling efficiency was improved by 5.12 dB/taper compared to the case where the Si wire waveguide and the topological edge waveguide were connected directly.

Discrepancies in the free spectral range (FSR) of one-dimensional (1D) photonic crystal/photonic wire coupled-cavities

We present the simulation and experimental demonstration of a coupled-cavity 1D photonic-crystal/photonic-wire (PhC/PhW) structure that produces multiple resonance wavelengths. The combination of several cavities results in the assembly of a spectral response that exhibits multiple resonance wavelengths and potentially leads to the wavelength control required for wavelength division multiplexing (WDM) applications. By using a structure with three distinct in-line cavities, we have obtained three distinct resonance wavelengths—in conformity with the rule that the number of distinct resonance wavelengths is proportional to the number of cavities. The experimental photonic wire waveguide structure had cross-sectional dimensions of 600 nm (width) × 260 nm (height)—with an embedded photonic crystal (PhC) micro-cavity—all based on a silicon-on-insulator (SOI) platform. The embedded PhC structure was tailored to give resonance wavelengths in the C-band and L-band fiber telecommunication range. With the introduction of tapering in the multiple micro-cavity structure, it was possible to obtain three resonance wavelengths that correspond to WDM wavelengths of 1534.87, 1554.63 and 1594.86 nm—whereas, without tapering, the resonance wavelengths were 1645.60, 1670.76 and 1698.68 nm, respectively. We have observed an asymmetric free spectral range (FSR) situation with un-equal resonance wavelength spacing. The taper regions are also responsible for high optical transmission and lower Q-factor values at resonance. Transmission values of 0.17, 0.47 and 0.43 were obtained, together with Q-factor values of 1179.32, 930.05 and 970.35, respectively, without using tapered sections—while transmission values of 0.45, 0.74 and 0.43 were obtained, together with Q-factor values of 1083.24, 850.10 and 885.22, respectively, using tapered sections. (The normalisation values for the experiments were obtained with respect to an unstructured photonic wire). We have demonstrated that the taper structures used must be designed accurately, in order to maximize the transmission values at the desired resonance wavelengths. The demonstration of fabricated device structures that have measured properties that are in close agreement with predictions obtained using finite-difference time-domain (FDTD) computational software is an indication of the precision of the fabrication process. With the introduction of multiple cavities into the structures realised, the number of resonance wavelengths can be tailored for application as WDM components or other wavelength selective filters, such as arrayed-waveguide grating structures (AWGs) and Bragg gratings.

Influence of longitudinal mode components on second harmonic generation in III-V-on-insulator nanowires.

The large index contrast and the subwalength tranverse dimensions of nanowires induce strong longitudinal electric field components. We show that these components play an important role for second harmonic generation in III-V wire waveguides. To illustrate this behavior, an efficiency map of nonlinear conversion is determined based on full-vectorial calculations. It reveals that many different waveguide dimensions and directions are suitable for efficient conversion of a fundamental quasi-TE pump mode around the 1550 nm telecommunication wavelength to a higher-order second harmonic mode.

Second-harmonic generation enabled by longitudinal electric-field components in photonic wire waveguides

We investigate type I second harmonic generation in III-V semiconductor wire waveguides aligned with a crystallographic axis. In this direction, because of the single nonzero tensor element of III-V semiconductors, only frequency conversion by mixing with the longitudinal components of the optical fields is allowed. We experimentally study the impact of the propagation direction on the conversion efficiency and confirm the role played by the longitudinal components through the excitation of an antisymmetric second harmonic higher order mode.

A broadband asymmetric microwave metamaterial based on LC and standing-wave resonances

Three metamaterial samples were simulated, numerically analyzed, and fabricated with different resonator lengths on similar unit cells. According to the results in addition to conventional LC resonance of the resonator, standing wave resonance was also observed within the resonator-dielectric-grounded back wire waveguide. We observed that the orientation of the sides of this waveguide relative to the polarization of the incident electromagnetic wave has a direct effect on the resonant frequency. The best asymmetric shape of the resonator with the minimum reflection coefficient in a wide frequency range of about 5.5 GHz with only 3% of reflection was introduced. According to the results, metamaterial polarizers and optical filters can be produced based on the standing wave resonances.

High-efficiency edge-coupling based on lithium niobate on an insulator wire waveguide.

Edge-coupling on single-crystal thin-film lithium niobate on insulator (LNOI) was systematically studied in this paper. An inverse taper-shaped spot-size converter (SSC) to convert the mode field from the laser chip to a nanoscale LNOI waveguide was adopted to improve the coupling efficiency. Structure of the edge coupler was fully investigated and optimized by using the eigenmode expansion method. The single-mode conditions of the LNOI waveguide for three common communication bands were taken into consideration. Further, the length and tip width of the inverse taper, the cross-section dimensions of SiON waveguide, and the sidewall angle were investigated with respect to coupling efficiency. As a result, the maximum coupling efficiency from an edge coupler to laser chip can reach 54%, 48%, and 58% at 1550, 1310, and 850 nm in Z-cut LNOI for quasi-TM mode, respectively. This proposed work gives a better understanding of the function of the edge coupler based on LNOI material and provides an appropriate method for the design of an edge coupler with high efficiency, which could benefit the further application of high-density monolithic integrated optical components.

A wire waveguide channel for terabit-per-second links

The rise in consumer data usage has increased the demand for higher data rates in telecommunication in both wireless and wired systems. In order to meet the demands for increased data rates for wired services, one possibility is to switch to higher frequencies, beyond the MHz-range frequencies typically used in digital subscriber line (DSL) services. In this work, we investigate the channel properties of a 200 GHz signal transmitted through a waveguide structure that is designed to approximately emulate the type of paired phone cable typically used for DSL transmissions. We report the attenuation characteristics of such a channel and explore the achievable data rates of a realistic vectored scenario that exploits the modal diversity of this multi-mode channel. We find that aggregate data rates on the order of terabits per second are feasible over short distances.

A High Finesse Serial Coupled Microresonator Filter

AbstractA serially coupled double-ring resonator using the low loss multilayer silicon wire waveguide is presented that has featured high out-of-band rejection and high finesse. The round trip loss and Q factor has also been calculated and found moderate. However, extremely low loss waveguide has shown reduced coupling between the ring and the waveguide.

Advances in Interfacing Optical Fibers to Nanophotonic Waveguides Via Mechanically Compliant Polymer Waveguides

Interfacing standard optical fibers with high index-contrast photonic waveguides remains a substantial challenge restricting the cost-efficiency and scalability of silicon photonic devices. To address this, we have proposed a mechanically compliant polymer interface, formed of a standard fiber connection and a flexible ribbon with lithographically defined polymer waveguides. Self-aligned mechanical assembly and optical mode evolution designs are employed for low cost and large spectral bandwidth. In this paper, we review our progress leading to the full demonstration of this concept with −1.8 dB peak transmission from a plugged-in standard fiber connector to Si wire waveguides. This with 0.7 dB roll-off on a 100 nm bandwidth and all polarization. We present a comprehensive analysis of loss sources and discuss intermediate results, self-alignment accuracy, and reliability.

Low-cost and high-efficiency single-mode-fiber interfaces to silicon photonic circuits

A high-efficiency and large-alignment-tolerance fiber interface to silicon-on-insulator wire waveguides is introduced. A spot-size converter is built through a full three-dimensional polymer waveguide and a silicon inverse taper is employed to expand the optical mode field from a silicon sub-micro waveguide to a normal cleaved single-mode fiber with a mode size of about 10μm. The fabrication for the three-dimensional structure relies on simple spin-coating processes. The coupling loss from the fiber to the silicon waveguide through the proposed structure reaches -1.0dB for both transverse electrical and transverse magnetic modes, respectively. The polarization-dependent loss for the best coupling case is kept within -0.6dB in the whole measured wavelength range. The coupling facet of the proposed fiber interface structure is polishable and facilitates an easy package procedure with standard fiber arrays.

Polarization mode hybridization and conversion in phononic wire waveguides

Phononic wire waveguides of subwavelength cross sections support two orthogonal polarization modes: the out-of-plane motion dominated Rayleigh-like and the in-plane motion dominated Love-like modes, analogous to transverse-electric and transverse-magnetic modes in photonic waveguides. Due to the anisotropic elasticity of the substrate material, the polarization states of phonons propagating along certain crystallographic orientations can strongly hybridize. Here, we experimentally investigate the orientation-dependent mode hybridization in phononic wire waveguides patterned from GaN-on-sapphire thin films. Such mode hybridization allows efficient actuation of piezoelectrically inactive Love-like modes using common interdigital electrodes designed for Rayleigh-like modes and further enables on-chip polarization conversion between guided transverse modes. Both are important for on-chip implementation of complex phononic circuits.

An Ultracompact GRIN‐Lens‐Based Spot Size Converter using Subwavelength Grating Metamaterials

AbstractGraded‐index materials offer virtually complete control over light propagation in integrated photonic chips but can be challenging to implement. Here, an anisotropic graded‐index metamaterial, synthesized with fully etched silicon subwavelength structures, is proposed. Based on this material, a spot size converter that expands the transverse electric (TE) mode field profile from a 0.5 µm wide silicon wire waveguide to a 15 µm wide waveguide within a length of only 14 µm is designed. Measured insertion losses are below 1 dB in an unprecedented 130 nm bandwidth, limited by the measurement setup, with full 3D finite‐difference time‐domain (FDTD) simulations predicting a bandwidth in excess of 300 nm. Furthermore, the device is well suited to feed fiber‐to‐chip grating couplers, while requiring a footprint ten times smaller than conventional adiabatic tapers.

Alignment-tolerant broadband compact taper for low-loss coupling to a silicon-on-insulator photonic wire waveguide.

We experimentally demonstrate a broadband, fabrication-tolerant compact silicon waveguide taper (34.2 μm) in a silicon-on-insulator wire waveguide. The taper works on multimode interference along the length of the taper. A single taper design has broadband operation with coupling efficiency >70% over 700 nm that can be used in O-, C-, and L-bands. The compact taper is highly tolerant to fabrication variations; ±100 nm change in the taper and end waveguide width varies the taper transmission by <5%. The footprint of the device, i.e., the taper along with linear gratings, is ≈442 μm2, 11.5× smaller than the adiabatic taper. The taper with linear gratings provides coupling efficiency comparable to standard focusing gratings. We have also experimentally compared the translational and rotational alignment tolerance of the focusing grating with linear grating couplers.

Spin-dependent directional emission from a quantum dot ensemble embedded in an asymmetric waveguide.

In this study, we examine a photonic wire waveguide embedded with an ensemble of quantum dots (QDs) that directionally emits into the waveguide depending on the spin state of the ensemble. The directional emission is facilitated by the spin-orbit interaction of light. The waveguide has a two-step stair-like cross section and QDs are embedded only in the upper step, such that the circular polarization of emission from the spin-polarized QDs controls the direction of the radiation. We numerically verify that more than 70% of the radiation from the ensemble emitter is toward a specific direction in the waveguide. We also examine a microdisk resonator with a stair-like edge, which supports selective coupling of the QD ensemble radiation into a whispering gallery mode that rotates unidirectionally. Our study provides a foundation for spin-dependent optoelectronic devices.

Silicon-Nitride-Assisted Edge Coupler Interfacing With High Numerical Aperture Fiber

Reducing the mode size mismatch between high-index-contrast silicon waveguide and single-mode fiber takes an important role in photonic integration. A low-loss and broadband silicon-nitride-assisted edge coupler for the coupling of light between silicon wire waveguides and high numerical aperture fiber is proposed and optimized for TE polarization. The mode size converter is composed of silicon inverse taper waveguides in the core and two silicon nitride interlayers spaced by silica in cladding layers. The edge coupler is characterized by cleaved high numerical aperture fiber (mode field diameter is about $6.5~\mu \text{m}$ ). With the help of refractive index matching oil ( $n =1.46$ ), the coupling loss for TE polarized light input is 0.35 dB at 1570 nm. The 1-dB bandwidth is 95 nm. The 1-dB alignment tolerances are higher than ± 2.5 and $\pm ~1~\mu \text{m}$ in the horizontal and vertical directions, respectively, and higher than $25~\mu \text{m}$ in the axial direction.

Physical origin of higher-order soliton fission in nanophotonic semiconductor waveguides

Supercontinuum generation in Kerr media has become a staple of nonlinear optics. It has been celebrated for advancing the understanding of soliton propagation as well as its many applications in a broad range of fields. Coherent spectral broadening of laser light is now commonly performed in laboratories and used in commercial “white light” sources. The prospect of miniaturizing the technology is currently driving experiments in different integrated platforms such as semiconductor on insulator waveguides. Central to the spectral broadening is the concept of higher-order soliton fission. While widely accepted in silica fibers, the dynamics of soliton decay in semiconductor waveguides is yet poorly understood. In particular, the role of nonlinear loss and free carriers, absent in silica, remains an open question. Here, through experiments and simulations, we show that nonlinear loss is the dominant perturbation in wire waveguides, while free-carrier dispersion is dominant in photonic crystal waveguides.

Athermal echelle grating filter in silicon-on-insulator using a temperature-synchronized input.

Athermal design of integrated photonic devices can reduce the need for active temperature stabilization and consequently the energy required to operate photonic integrated circuits. For silicon photonic filters such as AWGs which employ wire or ridge waveguides, temperature insensitivity can be achieved using cladding materials with negative thermo-optic coefficients. On the other hand, in echelle grating filters the inteference takes place in the slab free-propagation region, and therefore the modal overlap with the cladding is small, rendering this method ineffective. In this work we present an approach to design an athermal echelle grating filter exploiting a temperature-synchronized Mach-Zehnder interferometer as input. This reduces the spectral shift over a temperature range of 20 K to less than ±45 pm compared to the 1.6 nm shift for the same echelle grating with a conventional waveguide input. Furthermore, the proposed design relies exclusively on a standard fabrication process for silicon-on-insulator photonic devices and exhibits a good tolerance to fabrication uncertainties.

Carbon-nanotube-on-waveguide thermo-optic tuners.

We demonstrate on-waveguide thermo-optic tuners based on solution-processed metallic carbon nanotubes (CNTs) on silicon-on-insulator (SOI) and silicon nitride (SiN) microring resonators operating around 1550nm. On SOI microring resonators using planarized wire waveguides, a thermo-optic power efficiency of 29mW/FSR and a thermal transient of 1.3μs are achieved. The heater is shown to be more power-efficient than conventional metal heaters and has lower thermal transient than both metal heaters and graphene-based heaters. On SiN microring resonators using rib waveguides, improvement in power efficiency with an increase in coverage of CNTs is demonstrated, indicating localized heating using the CNTs; this is favorable for low thermal cross-talk. An optimal power efficiency of 142mW/FSR and a thermal transient of 5.8μs are achieved.

Numerical Modeling of Abnormal Blocking Effect for the Design of Novel Optical Sensor Element Constructed by Periodic Grating Strips Over Si/SiO2 Wire Waveguide

In this paper the numerical modeling of periodic structures for implementation of a novel type of photonic sensors by using the 3D finite difference time domain (FDTD) method is presented. The sensing is based on the optical phenomena occurring in the segmented grating which is placed on the thin silica buffer over the silicon wire waveguide in the silicon‐on‐insulator (SOI) structure. This design provides the effective resonance interaction of the guided wave with the virtual leaky wave supported by the segmented grating evanescently coupled with the silicon wire. The dropping wavelength of this interaction is strongly dependent on the grating environment and it provides a strong refractometricsensitivity (Sn &gt; 420 nm RIU−1) and surface sensitivity Sh &gt; 0.17, evaluated in the case of water. The modeling proves that the effect of abnormal blocking due to virtual leaky wave could be the base for the design of optical sensors with extremely high sensitivity.

Highly Nondegenerate Two-Photon Absorption in Silicon Wire Waveguides

Non-degenerate two-photon absorption (TPA) is investigated in a nanophotonic silicon waveguide in a configuration such that the dispersion of the nonlinear absorption and refraction cannot be neglected. It is shown that a signal wave can strongly be absorbed by cross-TPA by interaction with a low energy pump pulse (1.2\,pJ), close to the half-bandgap and experiencing low nonlinear absorption. The experiments are very well reproduced by numerical simulations of two-coupled generalized nonlinear Schr\"{o}dinger equations (GNLSE), validating the usual approximation made to compute the cross nonlinear coefficients in indirect-gap semiconductors. We show that the nonlinear dynamics can be well described by a single GNLSE despite the wavelength separation between the pump and the signal waves. We also demonstrate that in silicon wire waveguides and contrary to optical fibers, the dispersion of the nonlinear absorption is much larger than the dispersion of the Kerr effect. This could have an impact in the design of all-optical functions based on cross-TPA, as well as on the study of supercontinuum and frequency comb generation in integrated semiconductor-on-insulator platforms.