Board-level Optical Interconnects Research Articles

Facile fabrication and optical properties of polymer waveguides with smooth surface for board-level optical interconnects

Facile fabrication and optical properties of polymer waveguides with smooth surface for board-level optical interconnects

A Chip-Level Optical Interconnect for CPU

With the rapid growth of electronic chip's performance, electric signal limits chip I/O in power consumption, reachability, and signal quality. In this letter, we propose a new chip-let architecture for chip-level optical interconnect. An optic I/O chip-let including an ultra-small optic transceiver and electronic components is implemented. Our analysis shows that the optical interconnect based on this architecture can achieve 1/3 power consumption and 1/2 area compared with traditional board-level optical interconnect in Ethernet NIC application. By adopting the architecture we proposed, the optic I/O chip-let can support any payload IC such as CPU, GPU, switch to have optic I/O.

Ball Lens Embedded Through-Package Via To Enable Backside Coupling Between Silicon Photonics Interposer and Board-Level Interconnects

Development of an efficient and densely integrated optical coupling interface for silicon photonics based board-level optical interconnects is one of the key challenges in the domain of 2.5D/3D electro-optic integration. Enabling high-speed on-chip electro-optic conversion and efficient optical transmission across package/board-level short-reach interconnections can help overcome the limitations of a conventional electrical I/O in terms of bandwidth density and power consumption in a high-performance computing environment. In this context, we have demonstrated a novel optical coupling interface to integrate silicon photonics with board-level optical interconnects. We show that by integrating a ball lens in a via drilled in an organic package substrate, the optical beam diffracted from a downward directionality grating on a photonics chip can be coupled to a board-level polymer multimode waveguide with a good alignment tolerance. A key result from the experiment was a 14 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m chip-to-package 1-dB lateral alignment tolerance for coupling into a polymer waveguide with a cross-section of 20 × 25 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^2$</tex-math></inline-formula> . An in-depth analysis of loss distribution across several interfaces was done and a −3.4 dB coupling efficiency was measured between the optical interface comprising of output grating, ball lens and polymer waveguide. Furthermore, it is shown that an efficiency better than −2 dB can be achieved by tweaking few parameters in the coupling interface. The fabrication of the optical interfaces and related measurements are reported and verified with simulation results.

Flexible Multimode Polymer Waveguide Arrays for Versatile High-Speed Short-Reach Communication Links

Optical interconnects play an important role in enabling high-speed short-reach communication links within next-generation high-performance electronic systems. Multimode polymer waveguides in particular allow the formation of high-capacity optical backplanes and cost-effective board-level optical interconnects. Their formation on flexible substrates offers significant practical advantages and enables their deployment in environments where shape and weight conformity become particularly important, such as in cars and aircraft. However, bending and twisting of flexible multimode waveguides can have a significant effect on their optical transmission performance due to mode loss and mode coupling. Moreover, the magnitude of the induced effects strongly depends on the launch conditions employed. In this paper, therefore, we present detailed loss, crosstalk, and bandwidth studies on flexible multimode waveguide arrays for different launch conditions regarding their use in real-world systems. The minimum radius to achieve a 1 dB excess bending loss is obtained for each launch condition as well as the crosstalk degradation due to bending. It is shown that for a 50 μm MMF input, a 6 mm bending radius is required for excess losses below 1 dB, while crosstalk below −35 dB is obtained in adjacent waveguides even under strong bending. Moreover, it is found that twisting has a small effect on the loss and crosstalk performance of the samples. Excess twisting loss less than 0.1 dB and crosstalk of −40 dB are obtained for three full 360° turns under a 50 μm MMF launch. Finally, the bandwidth studies carried out on the samples indicate than no significant bandwidth degradation is induced due to sample bending, with bandwidth-length product values larger than 150 GHz × m obtained for restricted launches. The set of results presented herein specify the deployment of this flexible polymer multimode waveguide technology in real-world systems and demonstrate its strong potential to implement low-loss high-bandwidth optical interconnects.

板级波导光互联技术研究现状及发展趋势

板级波导光互联技术研究现状及发展趋势

Bandwidth Studies on Multimode Polymer Waveguides for &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;\({\ge }25\) &lt;/tex-math&gt;&lt;/inline-formula&gt; Gb/s Optical Interconnects

Multimode polymer waveguides constitute a promising technology for use in board-level optical interconnects. However, the continuous improvements in high-speed performance of VCSELs raise important questions about their ability to support such high data rates due to their inherent highly multimoded nature. Thorough experimental studies on the bandwidth of a 1.4-m-long multimode spiral waveguide are presented in this letter, indicating a bandwidth-length product of at least 35 GHz×m even in the case of an overfilled launch. No significant transmission impairments are observed for spatial input offsets, while error-free (BER <; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> ) data transmission over the 1.4-m-long spiral waveguide is demonstrated at 25 Gb/s.

Laser Direct Writing of Single-Mode Polysiloxane Optical Waveguides and Devices

Using a custom-built laser direct writing system, single-mode polymer optical waveguides, and devices for board-level optical interconnects were fabricated. A novel photopatternable polysiloxane material was developed that combines low-loss, simple, and large-area processability, and reliability during manufacturing and system operation. The polysiloxane waveguides were designed with quadratic cross sections of 5.5 $\times$ 5.5 µm2 and a refractive index contrast of 0.0086 between core and cladding polymer for single-mode operation at the wavelength of 1.3 µm. A straight waveguide propagation loss of 0.28 dB/cm was achieved. A wide range of passive optical devices, including Y-splitters, directional couplers, and Mach–Zehnder interferometers were successfully fabricated and characterized. The results prove that the presented combination of material and process technology is a viable implementation for short distance board-level optical links.

Polymer-Based Hybrid-Integrated Photonic Devices for Silicon On-Chip Modulation and Board-Level Optical Interconnects

The accelerating increase in information traffic demands the expansion of optical access network systems that require the cost reduction of optical and photonic components. Low cost, ease of fabrication, and integration capabilities of low optical-loss polymers make them attractive for photonic applications. In addition to passive wave-guiding components, electro-optic (EO) polymers consisting of a polymeric matrix doped with organic nonlinear chromophores have enabled wide-RF-bandwidth and low-power optical modulators. Beside board level passive and active optical components, compact on-chip modulators (a few 100 μm to a few millimeters) have been made possible by hybrid integration of EO polymers onto the silicon platform. This paper summarizes some of the recent progress in polymer-based optical modulators and interconnects. A highly linear, broadband directional coupler modulator for use in analog optical links and compact, and low-power silicon/polymer hybrid slot photonic crystal waveguide modulators for on chip applications are presented. Recently, cost-effective roll-to-roll fabrication of electronic and photonic systems on flexible substrates has been gaining interest. A low-cost imprinted/ink-jet-printed Mach–Zehnder modulator and board-to-board optical interconnects using microlens integrated 45° mirror couplers compatible with the roll-to-roll fabrication platforms are also presented.

Compact Multimode Polymer Waveguide Bends for Board-Level Optical Interconnects

Multimode polymer waveguides are promising for use in board-level optical interconnects. In recent years, various on-board optical interconnection architectures have been demonstrated making use of passive routing waveguide components. In particular, 90° bends have played important roles in complex waveguide layouts enabling interconnection between non co-linear points on a board. Due to the dimensions and index step of the waveguides typically used in on-board optical interconnects, low-loss bends are typically limited to a radius of ~ 10 mm. This paper therefore presents the design and fabrication of compact low-loss waveguide bends with reduced radii of curvature, offering significant reductions in the required areas for on-board optical circuits. The proposed design relies on the exposure of the bend section to the air, achieving tighter light confinement along the bend and reduced bending losses. Simulation studies carried out with ray tracing tools and experimental results from polymer samples fabricated on FR4 are presented. Low bending losses are achieved from the air-exposed bends up to 4 mm of radius of curvature, while an improvement of 14 μm in the 1 dB alignment tolerances at the input of these devices (fibre to waveguide coupling) is also obtained. Finally, the air-exposed bends are employed in an optical bus structure, offering reductions in insertion loss of up to 3.8 dB.

Embedded Photonics Interconnect Eco-system for Data Center Applications

The exponential increase in data center system bandwidth densities necessitates the migration of optical connectivity into the system enclosure. This is reflected by the emergence of a new technology eco-system including board-mountable optical engines and board-level optical interconnect. We consider how data center architectures are evolving and investigate the relative maturity and viability of various optical interconnect technologies. We also discuss current European research activities to develop disruptive optical interconnect solutions across all hierarchical levels of the data system from rack-to-rack, board-to-board, chip-to-chip and ultimately on-chip photonic interconnect. An early outcome of these activities has been the successful demonstration of an optically enabled data storage system.

Right choice for optical bus interconnect: metallic-hollow-core-waveguides or multimode polymer waveguides?

The performance of metallic-hollow-core-waveguides (MHCWs) and multimode polymer waveguides (MMPWs) are numerically evaluated using ray-tracing method for complex board level optical interconnects sys- tems such as optical bus. Based on an ideal model neglecting the wave- guide surface roughness, we find that MHCWs will provide lower optical loss for small curvature bending waveguides, 45 deg micro-mirror couplers and optical beam splitters, while MMPWs shows a better optical transmis- sionfor straight waveguides. The conclusion is thatMHCWs will be a better choice than MMPWs for complex optical bus architectures in terms of per- formance, cost and stability. © 2012 Society of Photo-Optical Instrumentation Engineers

Regenerative polymeric bus architecture for board-level optical interconnects

A scalable multi-channel optical regenerative bus architecture based on the use of polymer waveguides is presented for the first time. The architecture offers high-speed interconnection between electrical cards allowing regenerative bus extension with multiple segments and therefore connection of an arbitrary number of cards onto the bus. In a proof-of-principle demonstration, a 4-channel 3-card polymeric bus module is designed and fabricated on standard FR4 substrates. Low insertion losses (≤ -15 dB) and low crosstalk values (< -30 dB) are achieved for the fabricated samples while better than ± 6 µm -1 dB alignment tolerances are obtained. 10 Gb/s data communication with a bit-error-rate (BER) lower than 10(-12) is demonstrated for the first time between card interfaces on two different bus modules using a prototype 3R regenerator.

Impact of Data Transmission over 10 Gbps on High-Density and Low-Cost Optoelectronic Module with Polynorbornene Waveguides

We proposed a rigid/flex optoelectronic (O/E) module with 48-channel polymeric waveguides for short-distance board-level optical interconnection. A flexible O/E test module was fabricated in the following two steps by using standard packaging processes. First, two vertical cavity surface emitting laser diodes (VCSELs) and one VCSEL driver (VD) were flip-chip bonded to a completed flexible printed circuit board (PCB), and two photodiodes (PDs) and one transimpedance amplifier/limiting amplifier (TIA/LA) to another flexible PCB. Second, the two flexible PCBs were attached with a polynorbornene (PNB) sheet in which high-density PNB waveguides were formed by UV exposure. Active areas of VCSELs and PDs on the flexible PCBs were aligned to micromirrors of the waveguides with -6 µm offset toward the signal propagation direction. We successfully demonstrated data transmission over 10 Gbps and low inter-channel crosstalk of less than -20 dB was achieved in the flexible O/E test module with 120-mm-long and 62.5-µm-pitch waveguides.

Manufacturable Polymeric Optical Waveguide based Bus Structures for Board Level Optical Interconnects

Abstract:In this paper, we studied the optimization of preparation for polymeric optical waveguide based bus structures with embedded 45 degree micro-mirrors by metallic hard mold method. The 45º facets on the metallic hard mold, which were used to create the 45 degree micro-mirrors, were studied by the atomic force microscopy (AFM). The surface roughness of the 45 degree facets was reduced from 70nm to be 2nm by a photopolymer coating step. High speed test on the waveguide shows the low loss and high Q-factor performance of the waveguide structures. A backplane bus with 10 Gbits/sec channel will be reported.

Fibers in Printed Circuit Boards With Passively Aligned Coupling

Several types of printed circuit boards with embedded optical glass fibers for board level optical interconnects are presented. Transmitter and receiver coupling elements were developed which are solder temperature resistant and align passively on the fiber ends of the embedded glass fibers. Further a cost-effective demonstrator system using FPGA, serializer/deserializer, VCSEL driver, and photodiode amplifiers to test the optical interconnect was set up and is described.

High-Density 2$\,\times\,$4 Channel Polymer Optical Waveguide With Graded-Index Circular Cores

We propose to introduce a densely aligned 2times4 channel polymer parallel optical waveguide with graded-index circular cores for board-level optical interconnection, which is fabricated using the preform method. It is verified that this novel waveguide has sufficiently low loss (0.028 dB/cm at 850 nm and 0.061 dB/cm at 980 nm) and high bandwidth, due to the specific characteristic of the graded-index profile. This paper discusses insertion loss sensitivity to the displacement of the launched beam, various inter-channel crosstalk properties, and inter-channel skew, taking into account the real implementation of waveguides on printed circuit boards, especially for high-speed optical interconnects.

Polymer-Waveguide-Based Board-Level Optical Interconnect Technology for Datacom Applications

On the basis of a realized 12times10 Gb/s card-to-card optical link demonstrator, the capabilities of a polymer-waveguide-based board-level optical interconnect technology are presented. The conception and realization of the modular building blocks required for this board-level optical interconnect technology are described in detail. In particular, we report on the fabrication and characterization of board-integrated optical low-loss polymer waveguides that are compatible with printed circuit board (PCB) manufacturing processes. We also explain our fully passive alignment technique, superseding time-consuming active positioning of components and connectors. To realize optical transceiver modules comprising vertical cavity surface emitting laser (VCSEL) arrays with laser drivers and photodetector arrays with transimpedance amplifiers (TIAs), a mass-production concept based on wafer-level processing has been elaborated and successfully implemented.

Chip- and board-level optical interconnections using rigid flexible optical electrical printed circuit boards

A new optical interconnection scheme based on a rigid flexible optical electrical printed circuit board (RFOE-PCB) is suggested. The easily installed RFOE-PCB can be universally applied for both chip- and board-level optical interconnections. This letter describes the detailed fabrication process, optical properties, and heat-resisting property of the RFOE-PCB. The fabricated RFOE-PCB was also successfully demonstrated with a 2.5-Gb/s data transmission through a 45 degrees-ended optical waveguide embedded in the RFOE-PCB.

Fully Embedded Board-Level Optical Interconnects From Waveguide Fabrication to Device Integration

This paper presents the latest progress toward fully embedded board-level optical interconnects in the aspect of waveguide fabrication and device integration. A one-step pattern transfer method is used to form a large cross-section multimode waveguide array with 45deg micromirrors by silicon hard molding method. Optimized by a novel spin-coating surface smoothing method for the master mold, the waveguide propagation loss is reduced to 0.09 dB/cm. The coupling efficiency of the metal-coated reflecting mirror, which is embedded in the thin-film waveguide, is simulated by an M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> factor revised Gaussian beam method and is experimentally measured to be 85%. The active optoelectronic devices, vertical surface emitter lasers and p-i-n photodiodes, are integrated with the mirror-ended waveguide array and successfully demonstrate a 10 Gbps signal transmission over the embeddable optical layer.

Discrete Out-of-Plane Coupling Components for Printed Circuit Board-Level Optical Interconnections

We propose discrete out-of-plane coupling components as a versatile alternative to current approaches used to couple light in and out of the propagation plane in waveguide-based printed circuit board (PCB)-level optical interconnections. The out-of-plane couplers feature a 45deg micromirror and are fabricated using deep proton writing as a rapid prototyping technology. Their fabrication is compatible with replication techniques and shows all the potential of low-cost mass fabrication. In a first configuration, we use the component in a fiber-to-fiber coupling scheme. Coupling losses as small as 0.77 dB were achieved. In a second configuration, the out-of-plane coupler is plugged into a laser ablated cavity in optical waveguides integrated on a PCB. Here a total link loss between out-of-plane fiber and in-plane fiber of 3.00 dB was achieved when using it at the transmitter side and 5.69 dB when using it at the receiver side.