Optical Interconnect Applications Research Articles

Terminated polymer waveguide with low coupling loss to single-mode fiber for 100 Gbps optical interconnects and beyond

We designed and fabricated terminated polymer waveguides with low coupling loss to a standard single-mode fiber (SSMF) for 100 Gbps optical interconnects application and beyond. The mode field diameter of the polymer waveguide is designed to perfectly match that of the SSMF with a theoretical coupling loss as small as 0.07 dB. The fabricated polymer waveguides on FR-4 substrates exhibit both a low propagation loss of 0.32 dB/cm and a low coupling loss of less than 0.17 dB to SSMF at a wavelength of 1310 nm. These low loss values indicate good compatibilities with both printed circuit boards and fiber optics. Both single-lane 56 Gbaud (112 Gbps) and 60 Gbaud (120 Gbps) 4-level pulse amplitude modulation (PAM4) optical transmission using terminated polymer waveguides have been successfully demonstrated. The bit error ratio (BER) for PAM4 transmission at both data rates reached a level of 10−4 which is well below the forward error correction (FEC) limitation with a power penalty of less than 1 dB. The bandwidth of the test equipment rather than the polymer waveguide itself limits the high-speed optical interconnect performances in our experiments. The results imply that the polymer waveguide is a promising media for single-lane 100 Gbps optical interconnects application and beyond.

Functionality Expansion of Guided Mode Radiation via On-Chip Metasurfaces.

On-chip metasurfaces play a crucial role in bridging the guided mode and free-space light, enabling full control over the wavefront of scattered free-space light in an optimally compact manner. Recently, researchers have introduced various methods and on-chip metasurfaces to engineer the radiation of guided modes, but the total functions that a single metasurface can achieve are still relatively limited. In this work, we propose a novel on-chip metasurface design that can multiplex up to four distinct functions. We can efficiently control the polarization state, phase, angular momentum, and beam profile of the radiated waves by tailoring the geometry of V-shaped nanoantennas integrated on a slab waveguide. We demonstrate several innovative on-chip metasurfaces for switchable focusing/defocusing and for multifunctional generators of orbital angular momentum beams. Our on-chip metasurface design is expected to advance modern integrated photonics, offering applications in optical data storage, optical interconnection, augmented reality, and virtual reality.

Optical amplification across the whole communication windows in PbS quantum-dot-doped fiber

The surge of big data, cloud computing, and AI bring in explosive requirement of data transmission capacities. However, the transmission capacity of traditional single-mode-fiber already reaches its limit due to the narrow emission bandwidth of rare-earth-ion. Here, we achieved ultra-broadband optical amplification in O + E + S + C + L bands for the first time in PbS quantum-dot-doped glass fiber with the maximum gain of 5 dB at 1370 nm under 100 mW pump. The mechanism of the broadband optical amplification, the advantages of quantum dot-doped fiber amplifier (QDFA), and a new measurement scheme for QDFA are systematically discussed. It is believed that in the future, the further optimization of the composition and architectures of the glass fiber, and the preparation method of quantum dot-doped fiber may lead to important applications in optical interconnection, artificial intelligence, Internet of things, mobile electronics and so on.

Overcoming oxygen inhibition in UV photolithography for the fabrication of low-loss polymer waveguides.

Perfluorinated acrylate polymer materials exhibit low absorption loss at 1310 and 1550 nm, but molecular oxygen inhibits their photocuring. We propose a novel, to our knowledge, UV photolithography method incorporating a pre-exposure process for fabricating low-loss perfluorinated acrylate polymer waveguides. During the pre-exposure process, a partially cured thin layer forms on the core layer, effectively overcoming oxygen inhibition in subsequent lithography. Furthermore, the functional group contents of the polymerized materials were characterized by a Raman spectrometer to analyze the development reaction under the pre-exposure layer. Utilizing this improved method, we fabricated a straight waveguide with a length of 21 cm. The experiments showed that the propagation losses are 0.14 dB/cm at 1310 nm and 0.51 dB/cm at 1550 nm. The inter-channel cross talk for a core pitch of 250 µm was measured as low as -49 dB at 1310 nm. Error-free NRZ data transmission over this waveguide at 25 Gb/s was achieved, showcasing the potential in optical interconnect and communication applications.

Ultra-reliable quantum dot colliding pulse mode-locked laser as multi-wavelength source for integrated optical interconnects.

For optical interconnect applications, multi-wavelength comb sources require uniform comb spacings and high reliability at high operating temperature. Here, the high-temperature reliability measurements of a InAs quantum dot colliding pulse mode-locked (QD-CPML) laser with 100 GHz comb spacing are systematically investigated. Laser lifetime measurements are performed for over 1600 hours at 80 °C under constant stress current of 150 mA. The mean time to failure (MTTF) of the laser is approximately 38 years (336,203 hours), extracted from the threshold currents extrapolation method. The optical spectral revolutions are also monitored during the aging process, while the grids of comb laser are remarkably stable. The outstanding reliability and spectrum stability make this 100 GHz QD-CPML a promising candidate as a multi-wavelength laser source for datacom and optical I/O applications.

Graphene‐Based Silicon Photonic Devices for Optical Interconnects

AbstractThe exponential growth of global data traffic is driven by the unprecedented digitalization of the world. To meet the demands of next‐generation high‐capacity data communication systems, innovative solutions are required. Silicon photonics has gained significant attention due to its ability to integrate multiple core functions of optical interconnects into small‐scale chips, offering cost‐effective and scalable manufacturing capabilities. By leveraging silicon photonics, it becomes possible to address the challenge of scaling system complexity while reducing size, cost, and energy consumption. Despite its advantages, silicon has inherent limitations that restrict its application range, such as the weak plasma dispersion effect and relatively large bandgap. Recently, graphene has emerged as a promising solution for traditional silicon photonics because of its exceptional electrical and optical properties. For instance, graphene on a silicon chip enables nearly pure phase modulation and ultrafast photodetection beyond 500 GHz. Moreover, the demonstrated compatibility of graphene with wafer‐level integration paves the way for mass production of graphene‐based silicon photonic devices, offering improved yield, scalability, and cost‐effectiveness. In this work, a comprehensive review is provided, focusing on graphene‐based silicon photonic devices and their applications in optical interconnects, aiming to explore the potential of graphene in advancing silicon photonic technology.

Phase Diagram of High-Temperature Electron-Hole Quantum Droplet in Two-Dimensional Semiconductors.

Quantum liquids, systems exhibiting effects of quantum mechanics and quantum statistics at macroscopic levels, represent one of the most exciting research frontiers of modern physical science and engineering. Notable examples include Bose-Einstein condensation (BEC), superconductivity, quantum entanglement, and a quantum liquid. However, quantum liquids are usually only stable at cryogenic temperatures, significantly limiting fundamental studies and device development. Here we demonstrate the formation of stable electron-hole liquid (EHL) with the quantum statistic nature at temperatures as high as 700 K in monolayer MoS2 and elucidate that the high-temperature EHL exists as droplets in sizes of around 100-160 nm. We also develop a thermodynamic model of high-temperature EHL and, based on the model, compile an exciton phase diagram, revealing that the ionized photocarrier drives the gas-liquid transition, which is subsequently validated with experimental results. The high-temperature EHL provides a model system to enable opportunities for studies in the pursuit of other high-temperature quantum liquids. The results can also allow for the development of quantum liquid devices with practical applications in quantum information processing, optoelectronics, and optical interconnections.

Polarization independent Bragg gratings using tilted subwavelength grating waveguide Bragg gratings.

We propose and experimentally demonstrate a polarization independent subwavelength grating (SWG) waveguide Bragg grating (WBG) by using an SWG waveguide with tilted segments. By optimizing the tilting angle and other geometry parameters, such as the width and the length of the loading segments used to create the BG, we can obtain a zero birefringence tilted SWG waveguide and consequently, a polarization independent SWG WBG. In our simulations, the optimal tilting angle is ∼ 58°, whereas the optimal angle obtained in fabrication is ∼ 46°. This deviation is mainly due to fabrication errors, e.g., on the sidewall angle of the silicon segments. For the optimal tilting angle of 46°, the characterized Bragg wavelengths of the TE and TM modes are both ∼ 1517 nm. We believe that the proposed device can have applications in optical communications and interconnections.

Two-Dimensional Elliptical Microresonator Arrays for Wide Flat Bandwidth and Boxlike Filter Response

Based on two-dimensional elliptical microresonator arrays, we designed and fabricated a compact filter on the silicon-on-insulator platform with potential applications for on-chip optical interconnects. The fabricated optical filter exhibits a wide flat bandwidth of 951 GHz with the shape factor of 0.57 at the through port for the 3×20 arrays. The out-of-band rejection is as high as 50 dB. The crosstalk is also very low (−46 dB). The spectral shows a boxlike response. Although there are sixty rings used in the array, the insertion loss is still very small (≤1.36 dB).

Polycrystalline silicon PhC cavities for CMOS on-chip integration

In this work, we present an on-chip 2D and 3D photonics integration solution compatible with Front End of Line integration (FEOL) using deposited polycrystalline silicon (poly:Si) for optical interconnects applications. Deposited silicon integration on a bulk silicon wafer is here discussed in all its processing steps and configurations. Moreover, results of deposited silicon high-Q Photonic Crystal (PhC) resonators are shown, demonstrating the possibility to employ optical resonators patterned on this material in the next generation of 2D and 3D integrated optical interconnects.

Wavelength-Division Multiplexing on an Etchless Lithium Niobate Integrated Platform

Arrayed waveguide gratings (AWGs) are widely used as (de)multiplexers in wavelength-division-multiplexed optical communication systems and as integrated spectrometers in optical sensing and imaging systems. Lithium niobate integrated photonics is a burgeoning field due to many excellent optical properties of lithium niobate and rapid development of wafer processing technologies. Here, we introduced a new strategy of low-loss waveguiding on an etchless lithium niobate integrated platform for realizing high-performance AWGs thereon. We experimentally demonstrated 4-, 8-, and 16-channel AWGs operating in both telecom and near-visible wavelength bands on the etchless lithium-niobate-on-insulator platform. All the channels have insertion losses below 9.1 dB and crosstalks less than −12.8 dB in the designed wavelength bands. With advantages of an ultrawide transparency window and strong electro-optic and nonlinear optical coefficients of lithium niobate, the demonstrated AWGs will find wide applications in on-chip high-capacity optical interconnects, ultrafast bioimaging, LiDAR, and astronomical spectrographs.

High-speed performance evaluation of ultra-flexible polymer waveguides supporting meter-scale optical interconnects.

We demonstrate bandwidth measurement and high-speed data transmission of meter-scale connectorized ultra-flexible multimode waveguide links with a maximum length of 180 cm. The pulses propagating through the waveguides broadened linearly with the increase of the length from 20 cm to 240 cm and the estimated mode delay from the pulse broadening was 0.093 ps/cm. The corresponding waveguide bandwidth decreased inversely with the increase of waveguide length, leading to a bandwidth-length product of 42 GHz·m. Degradation in bandwidth due to the introduction of bending or twisting was small when the samples were bent with a bending radius as small as 1 mm for 3 turns or twisted for 4 full turns, respectively. Error-free transmission of 30 Gb/s non-return-to-zero (NRZ) signal was achieved with a record link length up to 140 cm to the best of our knowledge. Our results show that the demonstrated flexible waveguides have both excellent optical and mechanical properties and are ideal for high-speed optical interconnects application especially those have a strict requirement on flexibility.

Y-branch wavelength demultiplexer based on topological valley photonic crystals

Inspired by topological insulators in the condensed matter physics, topological photonics is a new paradigm, which is free of light backscattering at sharp waveguide bends due to the topological protection, and shows unidirectional light propagation based on the valley-chiral locking. Herein, we propose a two-channel Y-branch wavelength demultiplexer operating at the telecom band using the topological photonic integrated circuits for the first time. The proposed Y-branch wavelength demultiplexer is designed as adjoining and separating of two different topological interfaces at its input and output ports, respectively. Topological edge states with spectral interleaving can be excited in the two different topological interfaces, and be split into two output ports of the demultiplexer in short and long wavelengths. The Y-branch topological waveguide can be used as a dichroic mirror or a wavelength filter for separating light at two different wavelengths. The proposed topological integrated circuit is immune to structural defects. It is promising for the practical applications of optical interconnects and signal processing on a silicon photonic chip.

Atomic scale memristive photon source

Memristive devices are an emerging new type of devices operating at the scale of a few or even single atoms. They are currently used as storage elements and are investigated for performing in-memory and neuromorphic computing. Amongst these devices, Ag/amorphous-SiOx/Pt memristors are among the most studied systems, with the electrically induced filament growth and dynamics being thoroughly investigated both theoretically and experimentally. In this paper, we report the observation of a novel feature in these devices: The appearance of new photoluminescent centers in SiOx upon memristive switching, and photon emission correlated with the conductance changes. This observation might pave the way towards an intrinsically memristive atomic scale light source with applications in neural networks, optical interconnects, and quantum communication.

Low-crosstalk laser-direct-writing FI/FO device for 8×100-Gbps optical interconnection.

Fan-in/fan-out (FI/FO) device with low crosstalk is essential for weakly coupled short-reach optical interconnect based on multicore fibers (MCF), for which the laser-direct-writing (LDW) technique is one of the preferred fabrication schemes. In this paper, the influence of FI/FO crosstalk on short-reach intensity-modulation/direction-detection MCF optical interconnection is firstly evaluated, and the crosstalk related to different refractive-index profiles of waveguides and misalignment is analyzed for LDW-FI/FO devices. Then low-crosstalk compact LDW-FI/FO devices matching 8-core MCF are fabricated, adopting multiple-scan method for waveguides with a flat-top refractive-index profile and aberration correction method for precise alignment. Owing to the low crosstalk, 8×100-Gbps optical interconnection over 10-km MCF is experimentally demonstrated with only 0.5-dB penalty compared to 10-km G.652D single-mode fiber transmission. Simulation results indicate that the transmission reach can be further extended to over 40 km. The proposed prototype system with low crosstalk is promising for high-speed optical interconnection applications.

Contact Geometrical Study for Top Emitting 980 nm InGaAs/GaAsP Vertical-Cavity Surface Emitting Lasers

Geometrical contacts of a double mesa structure with 16 rows ×15 columns arrays of top emitting GaAs based 980 nm vertical cavity surface emitting lasers (VCSELs) are fabricated and characterized. In this paper, 5 strained In0.22Ga0.78As/Ga0.9AsP0.1 quantum wells (QWs) within λ/2 thick cavity have been employed. The top and the bottom epitaxially grown mirrors are based on the linear graded Al0.9Ga0.1As/GaAs distributed Bragg reflectors (DBRs) with 20.5 and 37 periods, respectively. Static parameters including threshold currents, rollover currents, maximum optical output power and wall-plug efficiency are extracted from light out power-current-voltage (LIV) of VCSELs with fixed oxide aperture diameter of ∅~ 6 μm and various mesa2 diameters. In addition, spectral emission for 980 nm VCSELs of oxide aperture between ∅~ 6 and 19 μm and with fixed ∅~ 6 μm and different bias currents are analyzed. The highest optical output power of around 33 dBm is observed at bias current of 0.8 mA for short−reach optical interconnect applications.

Optical interconnects on a flexible substrate by multi-material hybrid additive and subtractive manufacturing

The integration of flexible photonic components into flexible electronics provides new opportunities for applications in imaging, sensing, and optical interconnection between devices. While most photonic devices are currently printed on rigid substrates, their integration on flexible polymer substrates has also been demonstrated in recent years. However, current technology of contact and non-contact processes limits the type and performance of optical devices on these flexible substrates and requires special processes or product design to get the entry and exit of optical interconnect end facets out of the substrate plane. Here, we show that by utilizing recent advances in additive manufacturing, we can print photonic interconnects directly on flexible substrates. For our proof-of-concept devices, our results demonstrate propagation losses of about 0.35 dB for the six cm samples, negligible changes in transmission losses through substrate bending at moderate bend radii down to about 5 mm, and no change in optical losses during fatigue testing up to 1000 cycles. These results demonstrate how flexible photonic devices can be produced using additive manufacturing techniques. A type of shape deposition manufacturing, laser enhanced-direct print additive manufacturing, is used to manufacture these devices. This system of micro-dispensing, fused filament fabrication, and laser subtraction has a unique ability to produce flexible optoelectronics in a single automated process. We anticipate flexible integrated optoelectronics devices based on this new technology to be a starting point in the development of more sophisticated devices.

Emerging light-emitting diodes for next-generation data communications

The continuing development of consumer electronics, mobile communications and advanced computing technologies has led to a rapid growth in data traffic, creating challenges for the communications industry. Light-emitting diode (LED)-based communication links are of potential use in both free space and optical interconnect applications, and LEDs based on emerging semiconductor materials, which can offer tunable optoelectronics properties and solution-processable manufacturing, are of particular interest in the development of next-generation data communications. Here we review the development of emerging LED materials—organic semiconductors, colloidal quantum dots and metal halide perovskites—for use in optical communications. We examine efforts to improve the modulation performance and device efficiency of these LEDs, and consider potential applications in on-chip interconnects and light fidelity (Li-Fi). We also explore the challenges that exist in developing practical high-speed LED-based data communication systems. This Review examines the development of emerging semiconductor materials—organic semiconductors, colloidal quantum dots and metal halide perovskites—for light-emitting diodes, considering efforts to improve modulation performance and device efficiency, as well as potential applications in on-chip interconnects and light fidelity (Li-Fi).

100 Gbaud On–Off Keying/Pulse Amplitude Modulation Links in C-Band for Short-Reach Optical Interconnects

We experimentally evaluate the high-speed on–off keying (OOK) and four-level pulse amplitude modulation (PAM4) transmitter’s performance in C-band for short-reach optical interconnects. We demonstrate up to 100 Gbaud OOK and PAM4 transmission over a 400 m standard single-mode fiber with a monolithically integrated externally modulated laser (EML) having 100 GHz 3 dB bandwidth with 2 dB ripple. We evaluate its capabilities to enable 800 GbE client-side links based on eight, and even four, optical lanes for optical interconnect applications. We study the equalizer’s complexity when increasing the baud rate of PAM4 signals. Furthermore, we extend our work with numerical simulations showing the required received optical power (ROP) for a certain bit error rate (BER) for the different combinations of the effective number of bits (ENOB) and extinction ratio (ER) at the transmitter. We also show a possibility to achieve around 1 km dispersion uncompensated transmission with a simple decision feedback equalizer (DFE) for a 100 Gbaud OOK, PAM4, and eight-level PAM (PAM8) link having the received power penalty of around 1 dB.

Ultra-Compact Low Loss Polymer Wavelength (De)Multiplexer With Spot-Size Convertor Using Topology Optimization

Polymer waveguides are considered to be good candidates for optical interconnects application due to their advantages such as flexibility, good compatibility, and versatility for realizing 3D and micro structures. However, constrained by the limited degrees of freedom provided by template-based design approaches, there lacks a satisfying solution which achieves small footprint and low loss simultaneously. Inverse design has been proposed to improve the performance. However, most inverse design methods target at silicon-based optical devices optimization which has a relatively high refractive index difference. In this paper, we demonstrate an ultra-compact low loss polymer wavelength (de)multiplexer with spot-size conversion structure to interposing Si and optical fiber using gradient-based topology optimization and downhill simplex method. The 2-channel wavelength (de)multiplexer displays an excess propagation loss of 0.84 dB and 1.06 dB at 1310 nm and 1550 nm, respectively, with a total footprint of 245 × 35 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The 4-channel wavelength (de)multiplexer for coarse wavelength division multiplexing applications with 20 nm spacing (1270, 1290, 1310, and 1330 nm) with an excess waveguide propagation loss of 1.72 dB within 271 × 50 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> has also been demonstrated and all the output ports of the polymer wavelength demultiplexing structure achieved a conversion efficiency over 90%.