Silicon Nanowire Waveguides Research Articles

High-performance, adiabatically nanotapered fiber-chip couplers in silicon at 2 microns wavelength.

Fiber optic technology connects the world through the Internet, enables remote sensing, and connects disparate functional optical devices. Highly confined silicon photonics promises extreme scale and functional integration. However, the optical modes of silicon nanowire waveguides and optical fibers are very different, making efficient fiber-chip coupling a challenge. Vertical grating couplers, the dominant coupling method today, have limited optical bandwidth and are naturally out-of-plane. Here we demonstrate a new method that is low-loss, broadband, manufacturable, and naturally planar. We adiabatically couple a tapering silicon nanowire waveguide to a conic nanotapered optical fiber, measuring transmission between 2.0 µm and 2.2 µm wavelength. The silicon chip is fabricated at a commercial foundry and then post-processed to release the tapering nanowires. We estimate an optimal per-coupler transmission of -0.48dB (maximum; 95% confidence interval [+0.46, -1.68]dB) and a 1-dB bandwidth of at least 295 nm. With automated measurements, we quantify the device tolerance to lateral misalignment, measuring a flat response within ±0.968 µm. This new design can enable low-loss modular systems of integrated photonics irrespective of material and waveband.

Coupling characteristics between cylindrical hybrid plasmon waveguide and cylindrical silicon nanowire waveguide

Cylindrical waveguides have a wide range of applications in photonic integrated circuits because of lower transmission losses. This paper proposes a directional coupler composed of a cylindrical silicon nanowire waveguide and a metal-based cylindrical hybrid plasmon waveguide. The coupling characteristics of the coupler are analyzed theoretically and are also simulated using the finite-difference time-domain method. The results show that the coupler can operate effectively with a relatively short coupling length, displaying low insertion loss, and a high coupling efficiency and extinction ratio. This study also shows how the TM mode in the plasmon waveguide can be excited with the TE mode of the dielectric waveguide. This directional coupler has a potential application in signal exchanging and mode switching between the dielectric and plasmon waveguides in photonic integrated circuits.

Optimizing the Kerr Nonlinear Optical Performance of Silicon Waveguides Integrated With 2D Graphene Oxide Films

The Kerr nonlinear optical performance of silicon nanowire waveguides integrated with 2D layered graphene oxide (GO) films is theoretically studied and optimized based on experimentally measured linear and nonlinear optical parameters of the GO films. The strong mode overlap between the silicon nanowires and highly nonlinear GO films yields a significantly enhanced Kerr nonlinearity for the hybrid waveguides. A detailed analysis for the influence of waveguide geometry and GO film thickness on the propagation loss, nonlinear parameter, and nonlinear figure of merit (FOM) is performed. The results show that the effective nonlinear parameter and nonlinear FOM can be increased by up to 52 and 79 times relative to bare silicon nanowires, respectively. Self-phase modulation (SPM)-induced spectral broadening of optical pulses is used as a benchmark to evaluate the nonlinear performance, examining the tradeoff between enhancing Kerr nonlinearity and minimizing loss. By optimizing the device parameters to balance this, a high spectral broadening factor of 27.8 can be achieved, more than 6 times that achieved in previous experiments. Finally, the influence of pulse chirp, material anisotropy, and the interplay between saturable absorption and SPM is also discussed, together with the comparison between the spectral broadening after going through GO-coated and graphene-coated silicon waveguides. These results provide useful guidance for optimizing the Kerr nonlinear optical performance of silicon waveguides integrated with 2D layered GO films.

Polarization Diversified 16λ Demultiplexer Based on Silicon Wire Delayed Interferometers and Arrayed Waveguide Gratings

We proposed novel 16λ demultiplexer based on cascaded connection of silicon-nanowire waveguide type delayed interferometric filters and arrayed waveguide gratings, and experimentally verified 100-GHz-spaced bandpass filtering operation with nearly identical insertion loss (<; 6 dB) and low crosstalk (<; -19.5 dB to <; -34.6 dB for a single channel, <; -12.7 dB for entire output channels). We also demonstrated polarization diversified operation with 32 Gbps non-return-to-zero (NRZ) signal transmissions in C-band range. We also verified the operation of 140-GHz-spaced bandpass filtering in O-band spectral range.

Anisotropic metamaterial-assisted all-silicon polarizer with 415-nm bandwidth

Polarizers have been widely used in various optical systems to reduce polarization cross talk. The polarizers based on the silicon nanowire waveguide can provide chip-scale device size and a high polarization extinction ratio. However, the working bandwidth for the on-chip silicon polarizers is always limited ( 20 dB) over a >415 nm bandwidth from 1.26 to 1.675 μm, which is at least fourfold better than what has been demonstrated in all previous works. To the best of our knowledge, such a device is the first all-silicon polarizer that covers O-, E-, S-, C-, L-, and U-bands.

Ridge Resonance in Silicon Photonics Harnessing Bound States in the Continuum

AbstractThe theoretical analysis and experimental demonstration of a waveguide resonator are presented based on bound states in the continuum in an integrated photonic chip platform. The continuum has the form of a collimated beam of light which is confined vertically in a transverse electric (TE) mode of a silicon slab. The bound state is a discrete transverse‐magnetic (TM)‐like mode of a ridge on the silicon slab. The coupling between the slab and ridge modes results in a single sharp resonance at the wavelength where they phase match. This phenomenon is experimentally demonstrated on a silicon photonic chip using foundry‐compatible parameters and it is interfaced on‐chip to standard single‐mode silicon nanowire waveguides. The fabricated chip exhibits a single sharp resonance near 1550 nm with a line width of a few nanometers, an extinction ratio of 25 dB, and a thermal stability of 19.5 pm °C−1. It is believed that the demonstration of a resonance operating near a bound state in the continuum realized using guided wave components will form the basis of a new approach to on‐chip wavelength filtering and sensing applications.

Design of compact mode splitters using identical coupled waveguides with slots

In the past, multimode transmission through fibers has not been considered a preferred means of signal propagation due to the intermodal dispersion, which limits the data rate. However, recently, there has been an interest in using different modes as individual channels to increase the overall data rate. However, the challenging task in implementing such a system is that each individual mode has to be excited separately without mixing, and these should remain separated even at the bents and splices, and finally, they need to be collected separately. In this paper, design and optimization of a compact optical mode splitter by introducing a small slot in a silicon nanowire waveguide is demonstrated by employing a full-vectorial finite element method. The authors report here that by creating a slot within a waveguide, the desired coupling length ratio of 1:2 between the fundamental and the second modes can be obtained. The waveguide junctions have also been analyzed by using a rigorous least squares boundary residual method to study power transfer efficiency and effect of fabrication tolerances.

Numerical analysis of film‐loaded silicon nanowire optical rectangular waveguide: an effective optical sensing

Subwavelength waveguide using silicon nanowires is investigated for its compatibility with silicon-on-insulator platform. A layer of silicon film over SiO2 substrate is employed to construct silicon nanowire waveguides that complies with standard 220 nm silicon height technology. The proposed structure is further investigated for its utility as an optical sensing element in photonic integrated circuits. Film-loaded silicon nanowire optical rectangular waveguide (SNORW) allows light to be guided inside low refractive indexed gapping region with optimised dimension. Fully numerical analysis is performed using finite-element method as evidence to support the guiding phenomena of this waveguide. Eventually, confinement factor and waveguide sensitivity is found to be linear with measurand refractive index. As a result, the purpose of this SNORW leads to an effective sensing in the new era of photonics.

On-chip generation of time-and wavelength-division multiplexed multiple time-bin entanglement.

Optical quantum states based on entangled photons are the key resource in quantum-information science. The realization of multiplexed multiple entanglement are necessary for developing high-capacity quantum information process. Silicon-on-insulator (SOI) has recently become a leading platform for generating and processing of non-classical optical states. In this work, by combining the wavelength- and time-division multiplexing technologies, we demonstrate a multiplexing time-bin entangled photon pair source based on a silicon nanowire waveguide and distribute entangled photons into 3(time) × 14(wavelength) channels independently. The indistinguishability of photon pairs in each time channel is confirmed by a fourfold Hong-Ou-Mandal quantum interference. Our work paves a new and promising way to achieve a high capacity quantum communication and to generate a multiple-photon non-classical state.

$2-\mu$ m Wavelength Grating Coupler, Bent Waveguide, and Tunable Microring on Silicon Photonic MPW

The $2-\mu \text{m}$ wave band is emerging as a promising spectral window for optical communications and gas sensing. With the increasing demand for components at this wavelength range, developing $2-\mu \text{m}$ devices is thus required. Silicon photonics is an ideal device technology as it offers several unique advantages of potential low cost and large scale integration. Besides, the silicon nanowire waveguide is transparent and suffers from negligible two photon absorption at $2-\mu \text{m}$ wavelengths. Here, we report the demonstration of grating coupler, waveguide bend, and thermally tunable microring filter on a multi-project wafer which is fabricated using CMOS compatible process. The peak coupling efficiency of the grating coupler is measured to be −8.4 dB at 1952 nm. The bending losses for the rib waveguide and the strip loaded waveguide are measured with different radii. The ring resonator is fabricated and tested with an underestimated $Q$ factor of 1520 (through port) which is due to the limitation of the laser linewidth and wavelength tuning resolution. The extinction ratio exceeds 20 dB and the ring resonance can be thermally tuned by a Titanium Nitride heater. The tuning efficiency is 0.17 nm/mW.

Silicon-nanowire-based optical hybrid with insensitive operation for TE/TM states of polarization

An optical 90° hybrid consisting of a polarization splitter–rotator (PSR) and a multimode interferometer (MMI) is proposed to realize insensitive optical frequency mixing for TE/TM states of polarization. The PSR is based on the mode hybridization and mode coupling between waveguides; input light at a fundamental transverse electric (TE) mode and a transverse magnetic (TM) mode are output at separate ports, but both output signals are in the fundamental TE mode. These two output fields are then guided to the MMI, where the optical frequency mixing with the light of a local oscillator takes place. This mixing is then independent of the state of polarization of the hybrid input light. We designed such an optical hybrid based on silicon-nanowire waveguides. At a wavelength of 1540nm, simulation results for the hybrid show a transmission efficiency of 98.6% or 95.8% for TE- or TM-polarized input light, respectively, while the transmission efficiencies for both polarizations are above 90% in a 30nm wavelength range, namely [1525nm, 1555nm]. Further, in the wavelength range [1510nm, 1565nm], the transmission imbalance between the output fields is below 0.85dB for TE- or TM-polarized input light, respectively, while the phase errors in both cases are less than 4°.

Six‐wave mixing induced by free‐carrier plasma in silicon nanowire waveguides

AbstractNonlinear wave mixing in mesoscopic silicon structures is a fundamental nonlinear process with broad impact and applications. Silicon nanowire waveguides, in particular, have large third‐order Kerr nonlinearity, enabling salient and abundant four‐wave‐mixing dynamics and functionalities. Besides the Kerr effect, in silicon waveguides two‐photon absorption generates high free‐carrier densities, with corresponding fifth‐order nonlinearity in the forms of free‐carrier dispersion and free‐carrier absorption. However, whether these fifth‐order free‐carrier nonlinear effects can lead to six‐wave‐mixing dynamics still remains an open question until now. Here we report the demonstration of free‐carrier‐induced six‐wave mixing in silicon nanowires. Unique features, including inverse detuning dependence of six‐wave‐mixing efficiency and its higher sensitivity to pump power, are originally observed and verified by analytical prediction and numerical modeling. Additionally, asymmetric sideband generation is observed for different laser detunings, resulting from the phase‐sensitive interactions between free‐carrier six‐wave‐mixing and Kerr four‐wave‐mixing dynamics. These discoveries provide a new path for nonlinear multi‐wave interactions in nanoscale platforms. image

Rigorous analysis of numerical methods: a comparative study

For any photonic device simulation, the accuracy of the numerical solution not only depends on the methods being used but also on the discretization parameters used in that numerical method. In this work, Finite Element Method and Finite Difference Time Domain Method based on Maxwell’s equations were used to simulate optical waveguides and directional couplers. As the solution accuracy may also depend on the index contrast used in such photonic devices, the characteristics of low-index contrast Germanium doped Silica and high-index contrast Silicon Nanowire Waveguides were analyzed, evaluated and benchmarked. Numerical results to benchmark Directional Couplers are also reported in this paper.

Polymer taper bridge for silicon waveguide to single mode waveguide coupling

Coupling of optical power from high-density silicon waveguides to silica optical fibers for signal routing can incur high losses and often requires complex end-face preparation/processing. Novel coupling device taper structures are proposed for low coupling loss between silicon photonic waveguides and single mode fibers are proposed and devices are fabricated and measured in terms of performance. Theoretical mode conversion models for waveguide tapers are derived for optimal device structure design and performance. Commercially viable vertical and multi-layer taper designs using polymer waveguide materials are proposed as innovative, cost-efficient, and mass-manufacturable optical coupling devices. The coupling efficiency for both designs is determined to evaluate optimal device dimensions and alignment tolerances with both silicon rib waveguides and silicon nanowire waveguides. Propagation loss as a function of waveguide roughness and metallic loss are determined and correlated to waveguide dimensions to obtain total insertion loss for the proposed taper designs. Multi-layer tapers on gold-sputtered substrates are fabricated through photolithography as proof-of-concept devices and evaluated for device loss optimization. Tapered waveguide coupling loss with Si WGs (2.74dB) was experimentally measured with high correlation to theoretical results.

Three-Dimensional Finite-Element Mode-Solver for Nonlinear Periodic Optical Waveguides and Its Application to Photonic Crystal Waveguides

A 3-D finite-element mode-solver, based on a self-consistent algorithm, is newly developed for nonlinear periodic optical waveguides. The developed method makes it possible to calculate various nonlinear characteristics based on optical Kerr effect rigorously, including the nonlinear phase shift, the nonlinear refractive index distribution, and γ-value. Its validity is confirmed by analyzing a silicon-nanowire waveguide and a slot waveguide. To show the usefulness of the method, the nonlinear characteristics of photonic crystal line-defect waveguides and 1-D photonic crystal coupled resonator optical waveguides are investigated. Numerical results show that in some cases, γ-value calculated by the newly developed method and approximate linear analysis differs two-orders of magnitude. For instance, γ-value calculated by developed method is 510 000 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for a 1-D photonic crystal coupled resonator optical waveguide, whereas γ-value calculated by conventional method is only 5000 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . This difference indicates that, to estimate the nonlinear characteristics of 3-D waveguides precisely, the developed nonlinear guided mode solver is required.

Compact and broadband mode multiplexer and demultiplexer based on asymmetric plasmonic-dielectric coupling.

A compact and broadband mode (de)multiplexer utilizing the asymmetrical directional coupling between a hybrid plasmonic waveguide and a silicon nanowire waveguide is proposed. The coupling length is 13.6 μm, which is the shortest in TE0 and TE1 mode multiplexers reported, to the best of our knowledge. It shows a high mode conversion efficiency of 99.2% with excess loss of only 0.35 dB at 1550 nm. The mode multiplexing circuit exhibits a low crosstalk of less than -17 dB over a large bandwidth of 100 nm.

Non-instantaneous optical nonlinearity of an a-Si:H nanowire waveguide.

We use pump-probe spectroscopy and continuous wave cross-phase and cross-amplitude modulation measurements to study the optical nonlinearity of a hydrogenated amorphous silicon (a-Si:H) nanowire waveguide, and we compare the results to those of a crystalline silicon waveguide of similar dimensions. The a-Si:H nanowire shows essentially zero instantaneous two-photon absorption, but it displays a strong, long-lived non-instantaneous nonlinearity that is both absorptive and refractive. Power scaling measurements show that this non-instantaneous nonlinearity in a-Si:H scales as a third-order nonlinearity, and the refractive component possesses the opposite sign to that expected for free-carrier dispersion.

Parallel-core-type polarization rotator for silicon wire waveguide platform

We describe a polarization rotator based on a parallel-core structure consisting of a silicon nanowire waveguide and a silicon-nitride waveguide. The 60-μm-long rotator provides a polarization extinction ratio of more than 10 dB with excess loss of less than 1 dB. In addition, the extremely wide bandwidth of more than 150 nm expected from calculations is confirmed in experiments. A study of the fabrication tolerance of our rotator indicates that fabrication error of around 25 nm is allowable.

Low-loss and low-crosstalk 8 × 8 silicon nanowire AWG routers fabricated with CMOS technology.

Low-loss and low-crosstalk 8 × 8 arrayed waveguide grating (AWG) routers based on silicon nanowire waveguides are reported. A comparative study of the measurement results of the 3.2 nm-channel-spacing AWGs with three different designs is performed to evaluate the effect of each optimal technique, showing that a comprehensive optimization technique is more effective to improve the device performance than a single optimization. Based on the comprehensive optimal design, we further design and experimentally demonstrate a new 8-channel 0.8 nm-channel-spacing silicon AWG router for dense wavelength division multiplexing (DWDM) application with 130 nm CMOS technology. The AWG router with a channel spacing of 3.2 nm (resp. 0.8 nm) exhibits low insertion loss of 2.32 dB (resp. 2.92 dB) and low crosstalk of -20.5~-24.5 dB (resp. -16.9~-17.8 dB). In addition, sophisticated measurements are presented including all-input transmission testing and high-speed WDM system demonstrations for these routers. The functionality of the Si nanowire AWG as a router is characterized and a good cyclic rotation property is demonstrated. Moreover, we test the optical eye diagrams and bit-error-rates (BER) of the de-multiplexed signal when the multi-wavelength high-speed signals are launched into the AWG routers in a system experiment. Clear optical eye diagrams and low power penalty from the system point of view are achieved thanks to the low crosstalk of the AWG devices.

Cross-correlation frequency-resolved optical gating and dynamics of temporal solitons in silicon nanowire waveguides

We demonstrate the evolution of picosecond pulses in silicon nanowire waveguides by sum frequency generation cross-correlation frequency-resolved optical gating (SFG-XFROG) and nonlinear Schrödinger equation (NLSE) modeling. Due to the unambiguous temporal direction and ultrahigh sensitivity of the SFG-XFROG, which enable observation of the pulse accelerations, the captured pulses' temporal and spectral characteristics showed remarkable agreement with NLSE predictions. The temporal intensity redistribution of the pulses through the silicon nanowire waveguide for various input pulse energies is analyzed experimentally and numerically to demonstrate the nonlinear contributions of self-phase modulation, two-photon absorption, and free carriers. It indicates that free carrier absorption dominates the pulse acceleration. The model for pulse evolution during propagation through arbitrary lengths of silicon nanowire waveguides is established by NLSE, in support of chip-scale optical interconnects and signal processing.