This PDF file contains the front matter associated with SPIE Proceedings Volume 8630, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Super computing is reaching out to ExaFLOP processing speeds, creating fundamental challenges for the way that computing systems are designed and built. One governing topic is the reduction of power used for operating the system, and eliminating the excess heat generated from the system. Current thinking sees optical interconnects on most interconnect levels to be a feasible solution to many of the challenges, although there are still limitations to the technical solutions, in particular with regard to manufacturability. This paper explores drivers for enabling optical interconnect technologies to advance into the module and chip level. The introduction of optical links into High Performance Computing (HPC) could be an option to allow scaling the manufacturing technology to large volume manufacturing. This will drive the need for manufacturability of optical interconnects, giving rise to other challenges that add to the realization of this type of interconnection. This paper describes a solution that allows the creation of optical components on module level, integrating optical chips, laser diodes or PIN diodes as components much like the well known SMD components used for electrical components. The paper shows the main challenges and potential solutions to this challenge and proposes a fundamental paradigm shift in the manufacturing of 3-dimensional optical links for the level 1 interconnect (chip package).

Next generation parallel optical interconnects that incorporate monolithic collimating lens arrays require new procedures and guidelines for performance certification and visual inspection. This paper, after reviewing design specifications, will report performance methodologies for a modular miniature photonic turn connector that is used as an interface into VCSEL and photodiode arrays. A new insertion loss test procedure is introduced that can be used to screen connector performance without the need for vendor-specific optical engines. Data will be presented to show how induced defects affect the connector performance and thus establish inspection criteria. Finally, recommended cleaning procedures will be addressed.

We developed a novel low-cost polish-less elastically deformable optical multifiber backplane connector. The ferrule deformation and fibers buckling were enabled to cancel the variation in the lengths of unpolished cut fibers. We confirmed the physical contact connection between unpolished cut fibers in the ferrule and a polymer waveguide in a connector for polymer-waveguides connected with mechanical transfer connector (PMT) ferrule. The connection loss is almost the same as that between a conventional polished mechanical transfer (MT) ferrule and the PMT ferrule. We prepared a connector housing to mount the ferrules on a backplane/midplane. The insertion loss and return loss results in the housing show no degradations during 200 mating/demating tests, and there were no fiber breakages or damage to the waveguide core.

We introduce a simple photo-mask-free fabrication method of multimode polymer optical waveguides (PPOWs): “the Mosquito method” that utilizes a micro-dispenser. In the Mosquito method, a viscous monomer for cores is directly dispensed from a thin needle into a cladding monomer that is coated on a substrate. Because of the monomer diffusion, graded-index (GI) circular core is formed. By adjusting several parameters to control the core shape and diameter, 15-cm long, 40-m core diameter, with 125-m pitch GI-circular core PPOWs (12 channels) are successfully fabricated by the Mosquito method. We experimentally demonstrate superior optical properties of the GI- circular core polymer waveguides: connection loss with GI multimode fibers, inter-channel crosstalk, etc. to SI-square core polymer waveguides composed of the same silicone polymer materials. Furthermore, we fabricate GI-core PPOW circuits in which perpendicularly curved waveguides are involved by applying the Mosquito method. The curved PPOWs with a 250-m pitch are obtained successfully and the curve-radius is varied from 3 mm to 20 mm by adjusting the scanning program of the needle. The insertion loss of the curved waveguides is measured for evaluating the bending losses. The waveguide with lager curve-radius shows lower insertion loss, namely low bending loss. Additionally, it is experimentally confirmed that the curved waveguides with higher numerical aperture (NA = 0.30) fabricated utilizing a different silicone resin for the cladding exhibit a bending loss as low as 0.5 dB even under 3-mm curve radius. The Mosquito-method would be a promising method for realizing highspeed and high-density on-board optical interconnects.

High precision approaches for active and passive alignment and assembly on optoelectronic micro benches have been realized at Fraunhofer IZM for various material systems and different scales. The alignment and reliable mounting of optical subcomponents such as semiconductor laser and photo diodes, micro lenses and micro prisms require far higher mounting and alignment accuracies than for micro-electronic parts. When connecting from silicon photonics chip level to single mode optical fibers, even higher precisions are called for (typically < 100 nm). Alignment and assembly commonly are performed on specialized lab equipment which needs manual operation, consuming a lot of time, with less possibilities for automation. To introduce a higher degree of automatized production, like it has become standard in large volume electronics, one can choose either active or passive alignment processes - or possibly a combination of both. In this article we will present examples of micro-optic benches and optical interconnections that include alignment structures for passive alignment - where the accuracy lies in the components to be assembled, and mounting takes place on a less accurate machine (“fit into place”). But there is also a lot of progress on optical "pick, measure and place" machines that realize a flexible and fully automated active alignment using vision systems and activated components of less cost, with machine and process robustness for usability in industrial environments. As connecting elements, passive optical components like optical fibers are commonly used. These fragile and flexible elements pose additional challenges in secure picking, placing and fixing, at long lengths vs. small diameters. A very recent and specific approach to use more robust plastic optical fibers (POF) for very short and cost effective optical interconnects by means of wire bonding machines will be presented.

The relationship between the propagation loss and roughness on multi-layered Si waveguides fabricated up to a 3^rd layer was investigated. By reducing the surface and sidewall roughness of the waveguides, a low propagation loss of 3.7 dB/cm for the 3 layer a-Si:H waveguides was demonstrated. Furthermore, vertical coupling between multilayer waveguides was demonstrated by use of a grating-type vertical coupler. A coupling efficiency of 22% was obtained for 10 pairs of gratings with a period of 640 nm, even with a layer distance of 1μm.

We demonstrate board-to-board optical communication utilizing 45 degree mirrors and print-on-demand micro-lenses as surface-normal coupler. The presented system which includes polymer waveguides, embedded 45 degree mirrors and micro-lenses, realizes back-to-back optical interconnects between two boards. The waveguide and mirrors are fabricated using molding method using a low-cost electroplated nickel mold. Micro-lenses (80 μm in diameter) are fabricated using a material inkjet-printer, on top of the 45 degree mirrors. Experimental results show that each 45 degree mirror contributes about 1.88 dB loss (65% coupling efficiency) to the total optical loss. When propagating in free space without a lens, a 2 mm separation between boards results in a 9.9 dB loss, which is reduced to 7.5 dB when a micro-lens is inserted in the path. High speed data transmission test is performed at various separations. At a separation of 1mm, the system can provide 10Gbps error free transmission. At 2mm separation, the maximum error free data rate was measured to be 3.5Gbps and 7.5Gbps, with and without the micro-lens, respectively. Printing the micro-lenses on both the input 45 degree mirror and the photodiode or the receiving 45 degree mirror in a confocal setup will significantly reduce the freespace propagation loss. To the best of our knowledge, this is the first report of free-space coupling between waveguides on separate boards.

We demonstrate the first sub-100 μm silicon Mach-Zehnder modulators (MZMs) that operate at <10 Gb/s, by exploiting low-dispersion slow-light in lattice-shifted photonic crystal waveguides (LSPCWs). We use two LSPCW-MZM structures, one with LSPCWs in both arms of the MZM, and the other with an LSPCW in only one of the arms. Using the first structure we demonstrate 10 Gb/s operation with a operating bandwidth of 12.5 nm, in a device with a phase-shifter length of only 50 μm. Using the second structure, owing to a larger group index as well as lower spectral noise, we demonstrate 40 Gb/s operation with a phase-shifter length of only 90 μm, which is more than an order-of-magnitude shorter than most 40 Gb/s MZMs.

In this paper, we present the design guidelines, fabrication challenges and device evaluation results of a surface-normal photonic crystal waveguide array for high-density optical interconnects. We utilize the slow light effect of photonic crystals to increase the effective interaction length between photons and medium, which in turn can be used to decrease the physical length and make compact devices. The effect of the structural parameters variations on the guided mode are studied in order to provide a guideline for fabrication. Photonic crystal waveguides are vertically implemented in a silicon-on insulator substrate. Our structure possesses advantages such as universal design, CMOS compatibility, and simple fabrication process, suitable for high dense on-chip applications. Transmission results show increase of power near 1.67 μm wavelength, which agrees with our simulation results.

We propose a surface plasmon multilevel coupler based on the orthogonal junction coupling technique between silicon nanowires and plasmonic slot waveguides (PSWs). It couples light of different polarizations from a silicon nanowire into multilevel plasmonic networks. Two orthogonal PSWs are employed to guide each polarization to its respective port. The proposed structure splits the polarizations and allows for simultaneous processing at different horizontal layers. Our device overcomes inherent polarization limitation in plasmonic structures by providing multilevel optical signal processing. This ability of controlling polarization can be exploited to achieve 3-D multilevel plasmonic circuits and polarization controlled chip to chip channel. Our device is of a compact size and a wideband operation. The device utilizes both quasi-TE and quasi-TM polarizations to allow for increased optical processing capability. The crosstalk is minimal between the two polarizations propagating in two different levels. We achieve -4.5 dB transmission efficiency at a wavelength of 1.55 μm for the different polarizations in the respective ports. A transmission efficiency of -21 dB is achieved in the subsidiary port. We analyze and simulate the structure using the FDTD method. The proposed device can be utilized in integrated chips for optical signal processing and optical computations.

The increasing data rates and processing on board satellites call for the use of photonic interconnects providing high-bitrate performance as well as valuable savings in mass and volume. Therefore, optical transmitter and receiver technology is developed for aerospace applications. The metal-ceramic-packaging with hermetic fiber pigtails enables robustness for the harsh spacecraft environment, while the 850-nm VCSEL-based transceiver technology meets the high bit-rate and low power requirements. The developed components include 6 Gbps SpaceFibre duplex transceivers for intra-satellite data links and 40 Gbps parallel optical transceivers for board-to-board interconnects. Also, integration concept of interchip optical interconnects for onboard processor ICs is presented.

Polymeric waveguides with dynamic coupler array are fabricated by vacuum assisted microfluidic technique for card-to-backplane optical interconnect applications. The dynamic optical coupler on backplane consists of 45° integrated waveguide mirrors and a perpendicular microfluidic channel for flow motion of index matching liquid and air bubbles. Controlled positioning of the air bubble or index matching liquid in contact with the integrated mirror can effectively turn on or off each surface normal waveguide coupler for card-to-backplane optical interconnection. The size of the integrated mirror determines the coupler efficiency. Effective surface normal optical coupling is demonstrated.

Interconnect technology has been progressed at a very fast pace for the past decade. The signaling rates have steadily increased from 100:Mb/s to 25Gb/s. In every generation of interconnect technology evolution, optics always seems to take over at first, however, at the end, the cost advantage of copper wins over. Because of this, optical interconnects are limited to longer distance links where the attenuation in copper cable is too large for the integrated circuits to compensate. Optical interconnect has long been viewed as the premier solution in compared with copper interconnect. With the release of Thunderbolt technology, we are entering a new era in consumer electronics that runs at 10Gb/s line rate (20Gb/s throughput per connector interface). Thunderbolt interconnect technology includes both active copper cables and active optical cables as the transmission media which have very different physical characteristics. In order for optics to succeed in consumer electronics, several technology hurdles need to be cleared. For example, the optical cable needs to handle the consumer abuses such as pinch and bend. Also, the optical engine used in the active optical cable needs to be physically very small so that we don't change the looks and feels of the cable/connector. Most importantly, the cost of optics needs to come down significantly to effectively compete with the copper solution. Two interconnect technologies are compared and discussed on the relative cost, power consumption, form factor, density, and future scalability.

An integrated silicon light source still remains the Holy Grail for integrated optical communication systems. Hot carrier luminescent light sources provide a way to create light in a standard CMOS process, potentially enabling cost effective optical communication between CMOS integrated circuits. In this paper we present a 1 Mb/s integrated silicon optical link for information transfer, targeting a real-world integrated solution by connecting two PCs via a USB port while transferring data optically between the devices. This realization represents the first optical communication product prototype utilizing a CMOS light emitter. The silicon light sources which are implemented in a standard 0.35 μm CMOS technology are electrically modulated and detected using a commercial silicon avalanche photodiode. Data rates exceeding 10 Mb/s using silicon light sources have previously been demonstrated using raw bit streams. In this work data is sent in two half duplex streams accompanied with the separate transmission of a clock. Such an optical communication system could find application in high noise environments where data fidelity, range and cost are a determining factor.

Remote transfer of high-resolution video information finds more applications in detached display applications for large facilities such as theaters, sports complex, airports, and security facilities. Active optical cables (AOCs) provide a promising approach for enhancing both the transmittable resolution and distance that standard copper-based cables cannot reach. In addition to the standard digital formats such as HDMI, the high-resolution, long-distance transfer of VGA format signals is important for applications where high-resolution analog video ports should be also supported, such as military/defense applications and high-resolution video camera links. In this presentation we present the development of a compressionless, high-resolution (up to WUXGA, 1920x1200), long-distance (up to 2 km) VGA extenders based on serialized technique. We employed asynchronous serial transmission and clock regeneration techniques, which enables lower cost implementation of VGA extenders by removing the necessity for clock transmission and large memory at the receiver. Two 3.125-Gbps transceivers are used in parallel to meet the required maximum video data rate of 6.25 Gbps. As the data are transmitted asynchronously, 24-bit pixel clock time stamp is employed to regenerate video pixel clock accurately at the receiver side. In parallel to the video information, stereo audio and RS-232 control signals are transmitted as well.

This paper presents a switchless, optical interconnect module that serves as a node in a network of identical distribution modules for large-scale systems. Thousands to millions of hosts or endpoints may be interconnected by a network of such modules, avoiding the need for multi-level switches. Several common network topologies are reviewed and their scaling properties assessed. The concept of message-flow routing is discussed in conjunction with the unique properties enabled by the optical distribution module where it is shown how top-down software control (global routing tables, spanning-tree algorithms) may be avoided.

Optical modulators are key components in optical data links and are, currently, the subject of much research. Here we combine a comb laser with a series of cascaded photonic crystal modulators. A silicon nitride waveguide is used as the bus providing a near ideal means of routing light around a photonic chip as well as low insertion losses from optical fibre. This is vertically coupled to the photonic crystal cavities, the large free spectral range of which provides very high scalability along with very low (~femtojoule) switching energies. Finally, we show a route to integrating optics with electronics at the frontend. Combined with the low silicon area consumed by vertically coupled photonic crystal cavities, this is very promising for future optical interconnects.

A silicon (Si)-based, large-scale optical I/O chip will be a key device for a large-bandwidth, low-cost optical interconnection employed in future high-performance computing systems. For these Si optical I/O chips, a significant improvement in energy cost is strongly expected, hence, the use of micro ring-resonator (RR) based modulator is assumed to be a promising approach. In order to handle a narrow and temperature-dependent operation bandwidth of the RR-based modulator, we have proposed a novel Si transmitter that uses a cascaded RR MZ modulator and RR-based Si hybrid laser. The RR-based Si hybrid laser is an external cavity laser integrating an InP SOA and a Si mirror chip comprising a RR and DBR mirror. The SOA is flip-chip bonded to the Si mirror chip utilizing a precise flip-chip bonding technology. The fabricated Si hybrid laser exhibited a low threshold current of 9.4mA, a high output power of <15 mW, and a large wall-plug efficiency of 7.6% at 20°C. In addition, the device maintained a stable single longitudinal mode lasing and a low RIN level of <-130 dB/Hz for 20-60°C. We also fabricated an integrated Si transmitter combining a cascaded RR MZ modulator and RR-based Si hybrid laser. The 20-RR cascaded MZ modulator exhibited a 1-nm operation bandwidth using multiple low-Q RRs. The modulator was driven with 10Gbps PRBS signal. For a temperature range between 25 and 60°C, the lasing wavelength exhibited a red-shift of 2.5 nm, nevertheless, we confirmed clear eye openings without adjusting the operating wavelength of the modulator.

The manufacturing of electro-optical circuit boards (EOCB) is based to a large extent on established technologies. First products with embedded polymer waveguides are currently produced in series. The range of applications within the sensor and data communication markets is growing with the increasing maturity level. EOCBs require design flows, processes and techniques similar to existing printed circuit board (PCB) manufacturing and appropriate for optical signal transmission. A key aspect is the precise and automated assembly of active and passive optical components to the optical waveguides which has to be supported by the technology. The design flow is described after a short introduction into the build-up of EOCBs and the motivation for the usage of this technology within the different application fields. Basis for the design of EOCBs are the required optical signal transmission properties. Thereafter, the devices for the electro-optical conversion are chosen and the optical coupling approach is defined. Then, the planar optical elements (waveguides, splitters, couplers) are designed and simulated. This phase already requires co-design of the optical and electrical domain using novel design flows. The actual integration of an optical system into a PCB is shown in the last part. The optical layer is thereby laminated to the purely electrical PCB using a conventional PCB-lamination process to form the EOCB. The precise alignment of the various electrical and optical layers is thereby essential. Electrical vias are then generated, penetrating also the optical layer, to connect the individual electrical layers. Finally, the board has to be tested electrically and optically.

We successfully developed a new 24-ch optical connector for polymer waveguides. The connector consists of a transparent thermoplastic resin that has two rectangular slits on one side for alignment of the waveguide films and integrated microlens arrays on the other side for coupling to the MT connector. Two 12-ch waveguide films were cut to a 3-mm width. The thickness of each waveguide film was controlled at 100 μm. The waveguide films were inserted into the slits until they touched the bottom face of the slit. Ultraviolet curing adhesive was used to achieve a short hardening process. The expanded beam in the transparent material is focused by the microlens arrays formed on the connector surface. This lens structure enables assembly without the need for a polishing process. We designed the lens for coupling between a step-index 40-μm rectangular waveguide and a graded-index 50-μm fiber. We achieved low-loss optical coupling by designing a method of providing asymmetric magnification between the horizontal and vertical directions in order to compensate for the asymmetric numerical aperture of the waveguide. The typical measured coupling losses from/to the waveguide to/from the fiber were 1.2 dB and 0.6 dB, respectively. The total coupling loss was as small as that of a physical contact connection.

The increasing request for higher data speeds in the information and communication technology leads to continuously increasing performance of microprocessors. This has led to the introduction of optical data transmission as a replacement of electronic data transmission in most transmission applications longer than 10 meters. However, a need remains for optical data transmission for shorter distances inside the computer. This paper gives an overview of the Joint European project FIREFLY, in which new polymer based single mode waveguides are developed for integration with VCSELs, splitters and fibers that will be manufactured using multi-layer nanoimprint lithography (NIL). Innovative polymers, new applications of nano-technology, new methods for optical coupling between components, and the integration of all these new components are the technical ingredients of this ambitious project.

Electro-optical printed circuit boards (EOCB) based on planar multimode polymer channels are limited by dispersion in the step-index waveguide structures and increased optical absorption at the longer telecom wavelengths [1]. We present a promising technology for large panel EOCB based on holohedrally integrated glass foils. The planar multimode glass waveguides patterned into these glass foils have a graded-index structure, thereby giving rise to a larger bandwidthlength product compared to their polymer waveguide counterparts and lower absorbtion at the longer telecom wavelengths. This will allow glass waveguide based EOCBs to support the future bandwidth requirements inherent to large scale data centre and high performance computer subsystems while not incurring the same dispersion driven penalties on interconnect length or loss dependence on wavelength. To this end glass foil structuring technologies have been developed that are compatible with industrial PCB manufacturing processes. Established processes as well as new approaches were analysed for their eligibility and have been applied to the EOCB process. In addition a connector system has been designed, which would allow optical pluggability to glass waveguide EOCBs.

We have investigated the feasibility of multimode polysilicon waveguides to demonstrate the suitability of polysilicon as a candidate for multilayer photonic applications. Solid Phase Crystallization (SPC) with a maximum temperature of 1000°C is used to create polysilicon on thermally grown SiO₂. We then measure the propagation losses for various waveguide widths on both polysilicon and crystalline silicon platforms. We find that as the width increases for polysilicon waveguides, the propagation loss decreases similar to crystalline silicon waveguides. The difference in loss between the two platforms for a given waveguide width is due to the scattering from the polysilicon grain boundaries, which excites higher order modes. Depending on the waveguide width, these modes either propagate as higher order modes or are lost as radiation modes. Due to their different propagation constants, the presence of higher order modes is confirmed using sub-wavelength grating couplers. At a waveguide width of 10μm, polysilicon and crystalline silicon waveguides have propagation losses of 0.56dB/cm and 0.31dB/cm, respectively, indicating there is little bulk absorption from the polysilicon. This propagation loss is the lowest for polysilicon demonstrated to date. Modal conversion in multimode waveguides by polysilicon grain boundary scattering are investigated using a sub-wavelength grating coupler and discussed. These results vindicate the use of polysilicon waveguides of varying widths in photonic integrated circuits.

In this paper, we demonstrated recent progress on Si based optical components for advanced optical communication systems. The polarization beam splitter with extinction ratio of more than 20dB and the optical 90° hybrid having phase deviation within ±5° were obtained using MMI structures. The 12Gb/s modulators and the 20GHz photodetectors were measured. Benefiting from the unique properties of silicon modulator, an error-free 80Km transmission of the signals generated by our silicon carrier-depletion Mach-Zehnder modulator was also demonstrated at 10Gb/s and the power penalty was as low as 1.15dB. These results show that silicon photonics has a great potential in advanced optical communication systems.

We review recent progress made on the hybrid silicon platform towards realizing an integrated high speed WDM transmitter on silicon. Using ion implantation enhanced quantum well intermixing, four band gaps are integrated on a single chip and used to demonstrate a DFB laser array operating over 200 nm from 1250 to 1450 nm. Results from an independent effort to improve on hybrid silicon EA modulator performance are also described. Together these demonstrations pave the way to realize a terabit transmitter on silicon.

Broadband thermo-optic switch based on an ultra-compact W2 photonic crystal waveguide (PCW) is demonstrated with an integrated titanium/aluminum microheater on its surface. The operating principle relies on shifting a transmission-dip caused by the enhanced coupling between the defect modes in W2 PCW. As a result, broadband switching functionality with larger extinction ratio can be attained. Moreover, microheaters with different width are evaluated by the power consumptions and heating transfer efficiency, and an optimized slab microheater is utilized. Finally, switching functionality with bandwidth up to 24 nm (1557~1581 nm) is measured by the PCW with footprint of only 8μm×17.6 μm, while the extinction ratio is in excess of 15 dB over the entire bandwidth. What’s more, the switching speed is obtained by the measurement of alternating current modulation. Response time for this thermo-optic switch is 11.0±3.0 μs for rise time and 40.3±5.3 μs for fall time, respectively.

In this paper we demonstrate a three-dimensional (3D) photonic integration scheme based on crystalline silicon. We develop a process using SU-8 based adhesive bonding to fabricate vertically stacked, double-layer silicon nanomembranes. A single-layer silicon photonic integrated circuit fabricated on a silicon-on-insulator (SOI) chip and a bare SOI chip are bonded together, followed by removal of the bare SOI chip’s silicon substrate and buried oxide layer, to form a silicon nanomembrane as a platform for additional photonic layer. We designed and fabricated subwavelength nanostructure based fiber-to-chip grating coupler on the bonded silicon nanomembrane, and also inter-layer grating coupler for coupling between two silicon nanomembranes. The fiber-to-chip grating coupler has a peak efficiency of -3.9 dB at 1545 nm operating wavelength with transverse-electric (TE) polarization. The inter-layer grating coupler has a peak efficiency of -6.8 dB at 1533 nm operating wavelength with TE polarization. The demonstrated approach serves as a potential solution for 3D photonic integration and novel 3D photonic devices.

We demonstrate efficient coupling of light into silicon nanomembrane (SiNM) based photonic devices using a subwavelength grating (SWG) coupler. Our designed 17.1 μm x 10 μm grating couplers are fabricated at the input and the output of 8 mm x 2.5 μm SiNM waveguides on a glass substrate. A high coupling efficiency of 39.2% is achieved at a wavelength of 1555.56 nm. The 1-dB and 3-dB bandwidths are measured to be 29 nm and 57 nm, respectively. Peak efficiency variation of 0.26 dB is observed from a measurement of 5 grating pairs. Such grating couplers open limitless possibilities for the development of high performance nanomembrane based photonic devices.