Interlayer Coupler Research Articles

Inverse design of ultra-compact high-efficiency broadband interlayer couplers based on random gratings

Ultra-compact high-efficiency broadband interlayer couplers based on random gratings are inversely designed by particle swarm optimization algorithm. The optimal structure is obtained by adjusting the pixel state to achieve a maximum coupling efficiency in 1550 nm band. The two-layer coupler consists of two vertically overlapping inverse taper waveguides with random gratings. With a short coupling length of 4 μm and interlayer gap of 200 nm, the coupling efficiency reaches 93% at a wavelength of 1550 nm, and exceeds 90% over a broadband wavelength range of 1503∼1600 nm, for both TE and TM mode. The coupling efficiency can maintain no less than 90% at a large waveguide misalignment of >100 nm, exhibiting high process error tolerance. The performance of the 4 μm-long inverse-designed coupler is higher than the conventional waveguide with a much large length of 10 μm. To achieve longer transmission distance, a tri-layer interlayer coupler is proposed by cascading two bi-layer couplers, which exhibits a remarkable coupling efficiency of 85% at a gap of 600 nm, more than 30 times higher than the bi-layer structure. This work may pave the way for the development of ultra-compact high-performance interlayer couplers supporting high-integration Si-based photonic integrated circuits.

Multilayer stacked crystalline silicon switch with nanosecond-order switching time.

To realize compact and denser photonic integrated circuits, three-dimensional integration has been widely accepted and researched. In this article, we demonstrate the operation of a 3D integrated silicon photonic platform fabricated through wafer bonding. Benefiting from the wafer bonding process, the material of all layers is c-Si, which ensures that the mobility is high enough to achieve a nanosecond response via the p-i-n diode shifter. Optical components, including multimode interferences (MMIs), waveguide crossing, and Mach-Zehnder interferometer (MZI)-based switch, are fabricated in different layers and exhibit great performance. The interlayer coupler and crossing achieve a 0.98 dB coupling loss and <-43.58 dB cross talk, while the crossing fabricated in the same layer shows <-36.00 dB cross talk. A nanosecond-order switch response is measured in different layers.

Low-loss 3-dimensional waveguide crossing with a parabolic taper interlayer coupler based on a SiN-SiN-Si three-layer platform.

We demonstrated a SiN-SiN-Si three-layer silicon waveguide crossing with low-loss crossings and interlayer couplers. The underpass and overpass crossings exhibited ultralow loss (<0.82/1.16 mdB) and cross talk (<-56/-48 dB) in the wavelength range of 1260-1340 nm. To reduce the loss and length of the interlayer coupler, a parabolic interlayer coupling structure was adopted. The measured interlayer coupling loss was less than 0.11 dB from 1260 to 1340 nm, which is, to the best of our knowledge, the lowest loss reported for an interlayer coupler based on a SiN-SiN-Si three-layer platform. The total interlayer coupler length was only 120 µm.

Broadband 32 × 32 Strictly‐Nonblocking Optical Switch on a Multi‐Layer Si3N4‐on‐SOI Platform

AbstractLarge‐scale silicon optical switches are essential to support the ever‐increasing data traffic. Among the various switching architectures, the well‐known switch‐and‐select (S&amp;S) architecture has the advantages of low crosstalk and strict non‐blocking. However, it suffers from high path‐dependent losses as the waveguide crossings increase dramatically with the number of ports. In this paper, a large‐scale 32 × 32 S&amp;S optical switch on a multi‐layer Si3N4‐on‐SOI platform is reported. The optical switch chip incorporates 1984 broadband thermo‐optic Mach‐Zehnder interferometer (MZI)‐based switch elements, 246 016 three‐dimensional (3D) waveguide crossings, and 2048 interlayer couplers. Both ≈1‐mdB‐loss 3D waveguide crossings and ≈0.3‐dB‐loss interlayer couplers are realized, significantly reducing the overall insertion loss and footprint of the switch chip. The measured average fiber‐to‐fiber insertion loss is 12.88 dB at the 1580 nm wavelength. In addition, the crosstalk is less than −20.7 dB over the 110‐nm wavelength range. The power consumption of the entire switch chip is only ≈0.98 W due to the air trenches and substrate undercut. High‐fidelity optical transmission of a 50 Gb s−1 quadrature phase‐shift keying signal verifies the high‐performance routing capability of this chip. These results indicate that the large‐scale optical switch with broadband, low crosstalk, and high‐power efficiency is promising for datacenter optical network applications.

High efficiency and compact vertical interlayer coupler for silicon nitride-on-silicon photonic platform

In recent years, three-dimensional photonic integrated circuits have attracted extensive attention due to the possibility of combining the advantages of many different material systems. However, efficient coupling between multilayers is challenging. In this work, high efficiency and compact vertical interlayer coupler with nonlinear taper was proposed for silicon nitride-on-silicon photonic platform. The coupling efficiency can reach 99.2% at the wavelength of 1550 nm with a small size of 55 µm in length. The crosstalk is below −40 dB in the wavelength range between 1460 nm and 1620 nm. This promotes low-loss and high-dense integration of optical functionalities in hybrid material platforms.

Interlayer Slope Waveguide Coupler for Multilayer Chalcogenide Photonics

The interlayer coupler is one of the critical building blocks for optical interconnect based on multilayer photonic integration to realize light coupling between stacked optical waveguides. However, commonly used coupling strategies, such as evanescent field coupling, usually require a close distance, which could cause undesired interlayer crosstalk. This work presents a novel interlayer slope waveguide coupler based on a multilayer chalcogenide glass photonic platform, enabling light to be directly guided from one layer to another with a large interlayer gap (1 µm), a small footprint (6 × 1 × 0.8 µm3), low propagation loss (0.2 dB at 1520 nm), low device processing temperature, and a high bandwidth, similar to that in a straight waveguide. The proposed interlayer slope waveguide coupler could further promote the development of advanced multilayer integration in 3D optical communications systems.

Three-dimensional wavelength-division multiplexing interconnects based on a low-loss SixNy arrayed waveguide grating.

We fabricate three-dimensional wavelength-division multiplexing (3D-WDM) interconnects comprising three SixNy layers using a CMOS-compatible process. In these interconnects, the optical signals are coupled directly to a SixNy grating coupler in the middle SixNy layer and demultiplexed by a 1 × 4 SixNy array waveguide grating (AWG). The demultiplexed optical signals are interconnected from the middle SixNy layer to the bottom and top SixNy layers by four SiOxNy interlayer couplers. A low insertion loss and low crosstalk are achieved in the AWG. The coupling losses of the SiOxNy interlayer couplers and SixNy grating coupler are ∼1.52 dB and ∼4.2 dB, respectively.

High efficiency, small size, and large bandwidth vertical interlayer waveguide coupler

Since the advent of three-dimensional photonic integrated circuits, the realization of efficient and compact optical interconnection between layers has become an important development direction. A vertical interlayer coupler between two silicon layers is presented in this paper. The coupling principle of the directional coupler is analyzed, and the traditional method of using a pair of vertically overlapping inverse taper structures is improved. For the coupling of two rectangular waveguide layers, a pair of nonlinear tapers with offset along the transmission direction is demonstrated. For the coupling of two ridge waveguide layers, a nonlinear taper in each layer is used to achieve high coupling efficiency. The simulation results show that the coupling efficiency of the two structures can reach more than 90% in a wavelength range from 1500 nm to 1650 nm. Moreover, the crosstalk is reduced to less than –50 dB by using multimode waveguides at intersections. The vertical interlayer coupler with a nonlinear taper is expected to realize the miniaturization and dense integration of photonic integrated chips.

Heterogeneous integration of InP and Si3N4 waveguides based on interlayer coupling for an integrated optical gyroscope.

In this study, we demonstrate a novel, to the best of our knowledge, integrated indium phosphide (InP) and silicon nitride (Si3N4) waveguide platform, which is based on interlayer coupling, to achieve heterogeneous integration of a photodetector and waveguide ring resonator firstly. In order to improve the gyro bias stability, the Si3N4 and InP waveguides were designed with a high polarization extinction ratio and ultra-low loss. Three-dimensional finite difference time domain methods are used to optimize the InP taper dimensions to provide efficient optical coupling between the Si3N4 and InP waveguides. The optical coupler with a length of 100 µm is designed to achieve optical coupling between the Si3N4 and InP waveguides while maintaining its state of polarization all the way from the taper waveguides. The coupling efficiency of the optimized interlayer coupler has been improved to about 99.5%.

SiN-SOI Multilayer Platform for Prospective Applications at 2 μm

Silicon photonics at the 2 μm waveband, specifically the 1.9 μm wavelength region is strategically imperative. This is due to its infrastructural compatibility (i.e., thulium-doped fiber amplifier, hollow-core photonic bandgap fiber) in enabling communications, as well as its potential to enable a wide range of applications. While the conventional Silicon-on-Insulator platform permits passive/active functionalities, it requires stringent processing due to high-index contrast. On the other hand, SiN can serve to reduce waveguiding losses via its moderate-index contrast. In this work, by demonstrating SiN passives and Si-SiN interlayer coupler with favorable performance, we extend the Si-SiN platform to the 1.9 μm wavelength region. We report waveguide propagation loss of 2.32 dB/cm. Following, trends in radiation loss with regards to bending radius is analyzed. A high performance 3-dB power splitter with insertion loss and bandwidth of 0.05 dB and 55 nm (1935--1990 nm) respectively is introduced. Lastly, Si-SiN transition loss as low as 0.04 dB is demonstrated.

A multi-layer platform for low-loss nonlinear silicon photonics

We demonstrate four-wave mixing (FWM) interactions in a-Si:H waveguides in a multilayer integrated silicon photonic chip. The a-Si:H waveguides are accessed through interlayer couplers from waveguides composed of SiNx. The interlayer couplers achieve a coupling of 0.51 dB loss per transition at the target wavelength of 1550 nm. We observe greater idler power extraction and conversion efficiency from the FWM interaction in the interlayer-coupled multilayer waveguides than in single-material waveguides.

SiNx-Si interlayer coupler using a gradient index metamaterial.

In this work, we demonstrate a metamaterial (MM)-based SiNx-Si interlayer coupler. A gradient index (GRIN) MM was employed to achieve refractive index matching, balancing the large difference in the refractive index between Si (∼3.42) and SiNx (∼2). In addition, a two-layer SiNx interlayer coupler was proposed to further minimize the coupling loss and interlayer crosstalk. Finally, we demonstrated a three-dimensional (3D) SiNx-Si interlayer coupler with the -0.6 dB insertion loss per layer propagation, with the 40nm 1-dB-bandwidth (1530-1570nm) and <-45 dB interlayer crosstalk. These promising results demonstrate the great potential of a GRIN MM-based SiNx-Si interlayer coupler in 3D photonic integration.

Heterogeneous integration of LN and Si3N4 waveguides using an optical interlayer coupler

Abstract Integrated optical gyros (IOG) draw attention due to the remarkable promise of miniaturization, mass-manufacturability and high reliability. However, the wafer-scale integration of these devices is not available. A heterogeneous platform of lithium niobate (LN) and silicon nitride (Si3N4) waveguide is proposed in this study to enable the integration of phase modulator and resonator firstly, with large polarization intensity extinction ratio and low loss. The optical interlayer coupling between LN and Si3N4 waveguide is realized using Si3N4 bilevel taper above the lithium niobate on insulator (LNOI) substrate. The relationship between polarization intensity extinction ratio and lithium niobate thin film (LNTF) thickness of LN waveguide is developed based on polarization model. The optimal polarization intensity extinction ratio is achieved when the thickness of LNTF is 0. 2 μ m . At the start of the interlayer coupler, the Si3N4 strip with width=0. 9 μ m and thickness=0. 1 μ m is designed to maximize the fraction of optical power in the LNTF layer. The polarization intensity extinction ratio of the LN waveguide is 85.6 dB/mm. At the end of the coupler, the optical power in the Si3N4 strip with width of 0. 9 μ m and thickness of 0. 9 μ m reaches its maximum. Along the direction of light propagation from LN to Si3N4 waveguide, the optical interlayer coupler with length= 110 μ m is designed to achieve optical coupling between LN and Si3N4 waveguide while maintaining its state of polarization all the way from the feeder waveguides. The coupling efficiency of the optimized interlayer coupler has been improved to about 99 %.

Grating-assisted-cylindrical-resonant-cavities interlayer coupler

A grating-assisted-cylindrical-resonant-cavities (GARC) interlayer coupler made of Si/SiO2 is designed and simulated to achieve efficient and broadband interlayer coupling. This coupler consists of three cylindrical resonant cavities: two waveguide cavities in the horizontal direction and one cylindrical via cavity in the vertical direction. The resonant strengths of the two cylindrical waveguide cavities are enhanced by circular Bessel-function-defined gratings and distributed Bragg reflectors. The interlayer coupling efficiency of this Si/SiO2 GARC coupler is simulated as ηc=68%(-1.7 dB) for transverse electric polarization at 1.55μm wavelength, which is generally higher than those of conventional rectangular silicon-on-insulator gratings with additional features such as reflectors, overlayers, chirped periods, dual gratings, etc. The GARC couplers are predicted to have favorable attributes compared to previous couplers, including wider operational bandwidth (δλ,1 dB=270 nm), larger tolerance to inplane misalignment (±2 μm for 1dB extra loss), easier grating patterning (wider grating ridges), smaller footprint (20μm in diameter), and more flexible choices of interlayer distances (2-5μm). A sensitivity analysis is also provided as a guide in fabrication. In general, it is found that the vertical dimensions of the GARC couplers need to be carefully controlled while the horizontal dimensions are less critical.

MRONoC: A Low Latency and Energy Efficient on Chip Optical Interconnect Architecture

The circuit switched optical network on chip (ONoC) is popularly employed since the optical buffer is not available. However, this technique suffers from limited transmission bandwidth, high setup-time overhead, and high network resource contention, which consequentially induces long latency and degraded throughput. In this paper, we propose a new ONoC architecture aiming at ultralow setup cost, improved scalability, and contention-free communication. We first utilize wavelength division multiplexing (WDM) to introduce the basic version of this ONoC with efficient wavelength assignment. A series of potential versions are developed by using multiple waveguides to relieve the pressure on the number of wavelengths. These potential versions can make a tradeoff between required wavelengths and waveguides and improve the scalability. The new architectures employ two layers relying on the interlayer coupler, which contributes to the decrease of crossing losses. The simulation results show that the architecture can achieve 133% saturated bandwidth improvement compared with the traditional mesh ONoC employing WDM technology under the uniform traffic pattern.

High-quality silicon on silicon nitride integrated optical platform with an octave-spanning adiabatic interlayer coupler

Hybrid nanophotonic platforms based on three-dimensional integration of different photonic materials are emerging as promising ecosystems for the optoelectronic device fabrication. In order to benefit from key features of both silicon (Si) and silicon nitride (SiN) on a single chip, we have developed a wafer-scale hybrid photonic platform based on the integration of a thin crystalline Si layer on top of a thin SiN layer with an ultra-thin oxide buffer layer. A complete optical path in the hybrid platform is demonstrated by coupling light back and forth between nanophotonic devices in Si and SiN layers. Using an adiabatic tapered coupling method, a record-low interlayer coupling-loss of 0.02 dB is achieved at 1550 nm telecommunication wavelength window. We also demonstrate high-Q resonators on the hybrid material platform with intrinsic Q's as high as 3 × 10(6) for a 60 μm-radius microring resonator, which is (to the best of our knowledge) the highest Q observed for a micro-resonator on a hybrid Si/SiN platform.

A Novel Two-Layer Passive Optical Interconnection Network for On-Chip Communication

Passive optical interconnection network (OIN) plays a key role in optical Network-on-Chip (ONoC) architecture. Existing passive OINs based on wavelength division multiplexing (WDM) are popularly employed. However, the scalability of these passive OINs is limited by the number of wavelengths and large insertion loss induced by the waveguide crossings. In this paper, we propose a novel Passive OIN based on two-layer architecture, POINT, for ONoC architecture. POINT leverages space division multiplexing (SDM) to assist WDM in eliminating blocking. The inter-layer communication in POINT relies on the inter-layer coupler, which contributes to reduce crossing losses. POINT features a modular and scalable design, in which the proposed SDM-based cell (SBC) is used as the basic building block to construct POINT with efficient wavelength assignment. Furthermore, SBCs of different sizes provide different options for constructing POINT. Comparisons with existing passive OINs confirm that POINT can provide an optimal choice with the balance between the number of wavelengths, area overhead, and insertion loss for the same size.

A micro-architectural analysis of switched photonic multi-chip interconnects

Silicon photonics is a promising technology to scale offchip bandwidth in a power-efficient manner. Given equivalent bandwidth, the flexibility of switched networks often leads to the assumption that they deliver greater performance than point-to-point networks on message passing applications with low-radix traffic patterns. However, when optical losses are considered and total optical power is constrained, this assumption no longer holds. In this paper we present a power constrained method for designing photonic interconnects that uses the power characteristics and limits of optical switches, waveguide crossings, inter-layer couplers and waveguides. We apply this method to design three switched network topologies for a multi-chip system. Using synthetic and HPC benchmark-derived message patterns, we simulated the three switched networks and a WDM point-to-point network. We show that switched networks outperform point-to-point networks only when the optical losses of switches and inter-layer couplers losses are each 0.75 dB or lower; achieving this would require a major breakthrough in device development. We then show that this result extends to any switched network with similarly complex topology, through simulations of an idealized "perfect" network that supports 90% of the peak bandwidth under all traffic patterns. We conclude that given a fixed amount of input optical power, under realistic device assumptions, a point-to-point network has the best performance and energy characteristics.