A novel method for fabricating a single mode optical interconnection platform is presented. The method comprises the miniaturized assembly of optoelectronic single dies, the scalable fabrication of polymer single mode waveguides and the coupling to glass fiber arrays providing the I/O’s. The low cost approach for the polymer waveguide fabrication is based on the nano-imprinting of a spin-coated waveguide core layer. The assembly of VCSELs and photodiodes is performed before waveguide layers are applied. By embedding these components in deep reactive ion etched pockets in the silicon substrate, the planarity of the substrate for subsequent layer processing is guaranteed and the thermal path of chip-to-substrate is minimized. Optical coupling of the embedded devices to the nano-imprinted waveguides is performed by laser ablating 45 degree trenches which act as optical mirror for 90 degree deviation of the light from VCSEL to waveguide. Laser ablation is also implemented for removing parts of the polymer stack in order to mount a custom fabricated connector containing glass fiber arrays. A demonstration device was built to show the proof of principle of the novel fabrication, packaging and optical coupling principles as described above, combined with a set of sub-demonstrators showing the functionality of the different techniques separately. The paper represents a significant part of the electro-photonic integration accomplishments in the European 7th Framework project “Firefly” and not only discusses the development of the different assembly processes described above, but the efforts on the complete integration of all process approaches into the single device demonstrator.
Photopatternable optical silicone materials have been developed that can be fabricated into flexible polymer waveguides using conventional film processing and photolithography techniques. Waveguides designed for multimode applications have demonstrated loss of 0.05 dB/cm at 850 nm. These waveguides have proven stable in 85% relative humidity and 85°C storage conditions for greater than 2000 hours to date with no degradation in optical performance. Further, this polymer waveguide system has shown sufficient adhesion to flexible polyimide substrates without any curling, and passed bending tests to a 1 mm radius without damage.
For the realization of a polymer waveguide based optical backplane link for computing applications, we developed a
method to passively align multiple layers of polymer waveguide flex sheets in a single MT compatible ferrule. The
minimal feature forming the backplane is a 192 channel link. This link is equipped with four MT connector at each end,
and is performing a shuffling of the channels.
We describe the passive alignment used to realize the connectors. The achieved accuracy demonstrated in a 48 channels
connector consisting of 4 polymer sheets carrying 12 waveguides each, is shown to be better than ±5μm. The connection
losses between a 48 channel MT fiber connector and the realized polymer waveguide connector were found to be about
2dB.
Compared to fiber connectors, the presented concept using polymer waveguides has several advantages. The most
relevant are that only few assembly steps are needed, it is based on a totally passive alignment scheme and it can easily
be executed by standard pick and place tools.
The design, implementation and characterisation of an electro-optical backplane and an active pluggable optical
connector technology are presented. The connection architecture adopted allows line cards to mate and unmate from a
passive electro-optical backplane with embedded polymeric waveguides. The active connectors incorporate photonics
interfaces operating at 850 nm and a mechanism to passively align the interface to the embedded optical waveguides. A
demonstration platform has been constructed to assess the viability of embedded electro-optical backplane technology in
dense data storage systems. The electro-optical backplane is comprised of both copper layers and one polymeric optical
layer, whereon waveguides have been patterned by a direct laser writing scheme. The optical waveguide design includes
arrayed multimode waveguides with a pitch of 250 μm, multiple cascaded waveguide bends, non-orthogonal crossovers
and in-plane connector interfaces. In addition, a novel passive alignment method has been employed to simplify high
precision assembly of the optical receptacles on the backplane. The in-plane connector interface is based on a two lens
free space coupling solution, which reduces susceptibility to contamination. The loss profiles of the complex optical
waveguide layout has been characterised and successful transfer of 10.3 Gb/s data along multiple waveguides in the
electro-optical backplane demonstrated.
We report on the co-packaging of electrical CMOS transceiver and VCSEL chip arrays on a flexible electrical substrate
with optical polymer waveguides. The electro-optical components are attached to the substrate edge and butt-coupled to
the waveguides. Electrically conductive silver-ink connects them to the substrate at an angle of 90°. The final assembly
contacts the surface of a package laminate with an integrated compressible connector. The module can be folded to save
space, requires only a small footprint on the package laminate and provides short electrical high-speed signal paths.
With our approach, the electro-optical package becomes a compact electro-optical module with integrated polymer
waveguides terminated with either optical connectors (e.g., at the card edge) or with an identical assembly for a second
processor on the board. Consequently, no costly subassemblies and connectors are needed, and a very high integration
density and scalability to virtually arbitrary channel counts and towards very high data rates (20+ Gbps) become
possible. Future cost targets of much less than US$1 per Gbps will be reached by employing standard PCB materials and
technologies that are well established in the industry. Moreover, our technology platform has both electrical and optical
connectivity and functionality.
Scaling computing systems to Exaflops (1018 floating point operations per second) will require tremendous increases in
communications bandwidth but with greatly reduced power consumption per communicated bit as compared to today's
petaflop machines. Reaching the required performance in both density and power consumption will be extremely
challenging. Electrical and optical interconnect technologies that may be part of the solution are summarized, including
advanced electrical printed circuit boards, VCSEL-array based optical interconnects over multimode fibers or
waveguides, and singlemode silicon photonics. The use of optical interconnects will play an ever-larger role in
intrasystem communications. Although optics is used today primarily between racks, it will gradually migrate into
backplanes, circuit cards, and eventually even on-chip.
Keywords: optical interconnects, supercomputers, exascale,
Formulations containing silicon-based polymers have been used for the formation of planar waveguides on flexible substrates. The substrate of choice is compatible with the flexible waveguide and is made of materials commonly utilized in the printed circuit board industry. When the flexible waveguide material is combined with the chosen substrate using processes compatible with printed circuit board manufacturing techniques, the resultant optical interconnects display sufficient flexibility, low optical loss (<0.05 dB/cm at 850 nm), and high reliability.
Optical link technology will play an increasingly important role for board-level interconnects in servers and supercomputers as a means to keep pace with the increasing intra-system bandwidth requirements. Low-cost and high density optical packaging concepts are required. We describe the development of board-level interconnects based on polymer waveguide technology. In this paper, we focus on flexible optical waveguide sheets and the passive alignment of optical connectors.
The IBM Terabus program has developed parallel optical interconnects for terabit/sec-class chip-to-chip
communications through printed circuit boards with integrated optical waveguides. 16 TX + 16 RX channel transceiver
"Optomodules" were assembled and fully characterized, with fiber-coupled full links operating up to 15 Gb/s, for an
aggregate bi-directional data transfer rate of 240 Gb/s. Furthermore, we have demonstrated a complete link between two
Optomodules through polymer waveguides on a printed circuit board, with all 32 uni-directional links operating error-free
at 10Gb/s, for a 160 Gb/s bidirectional aggregate data rate. This is the fastest, widest, and most integrated
multimode optical bus ever demonstrated.
We present a novel approach for packaging high-speed opto-electronic 12x-array devices in a compact, low-cost package
for waveguide-based intra-system links. In order to avoid optical signal loss and crosstalk, the mutual alignment between
PCB-embedded multimode waveguides and the opto-electronic components needs to be in the order of 5-10 micrometer,
which is an order of magnitude tighter than standard PCB manufacturing tolerances. Our packaging concept uses a
combination of passive alignment steps, tolerance stackup reduction and a misalignment-tolerant coupling scheme in
order to bridge this gap in a cost competitive way.
Using flip-chip technology, the opto-electronic components are placed onto a very thin substrate with holes for the light
path. The top side of the 25 μm liquid crystal polymer (LCP) substrate not only provides fast and low-loss electrical
connections, but also serves as alignment reference plane for the entire module, avoiding alignment tolerance
accumulation over different assembly steps. Openings for the laser beams, passive lens alignment features, centering
holes for mechanical alignment pins between module and board and optional MT-guide receptacles are all laser-cut
within one single process step, with a precision better than 5 μm. A similar approach is used for the PCB-side optics, and
a lens-pair coupling scheme provides for a sufficiently large misalignment tolerance between the package and the PCB.
Mechanical rigidity of the package and thermal protection are provided by an epoxy filled aluminum frame.
We will present our design considerations, the basic package concept, the actual experimental implementation and
characterization results of our first prototype package.
Polymer waveguides embedded in a printed circuit board offer a substantial increase in the achievable bandwidth density compared with today's electrical interconnects. We present our results on the polymer waveguide technology and the building blocks that perform the optoelectronic conversion. Specific challenges in integrating optics in a printed circuit board are addressed. Data transfer measurements are presented.
The development of optical interconnects in printed circuit boards (PCBs) is driven by the increasing bandwidth requirements in servers, supercomputers and switch routers. At higher data rates, electrical connections exhibit an increase in crosstalk and attenuation; which limits channel density and leads to high power dissipation. Optical interconnects may overcome these drawbacks, although open questions still need to be resolved. We have realized multimode acrylate-polymer-based waveguides on PCBs that have propagation losses below 0.04 dB/cm at a wavelength of 850 nm and 0.12 dB/cm at 980 nm. Transmission measurements at a data rate of 12.5 Gb/s over a 1-m-long waveguide show good eye openings, independent of the incoupling conditions. In the interconnect system, the transmitter and receiver arrays are flip-chip-positioned on the top of the board with turning mirrors to redirect the light. The coupling concept is based on the collimated-beam approach with microlenses in front of the waveguides and the optoelectronic components. As we aim for large two-dimensional waveguide arrays, optical crosstalk is an important parameter to be understood. Accordingly, we have measured optical crosstalk for a linear array of 12 optical channels at a pitch of 250 um. The influence of misalignment at the transmitter and the receiver side on optical crosstalk will be presented as a function of the distance between waveguide and transmitter/receiver.
This paper describes methods to control and manipulate birefringence in SiliconOxyNitride waveguides and devices.
Each method is demonstrated by measurements on example devices. The methods and devices that will be covered are:
Reduction of heater induced birefringence in a dynamic gain equalizer by heater design or etched trenches.
Reduction of polarization mode dispersion in a tunable dispersion compensator by UV trimming of residual
waveguide birefringence.
Polarization conversion using integrated optical half-wave-plates, fabricated by etching trenches at one side of
a waveguide.
Polarization splitting using waveguide sections with specified birefringence, obtained by etched trenches at
both sides of the waveguide.
Fast pulse-generating laser sources at 10 GHz are commercially available. For future communications system applications of these light sources at 40 GHz, we developed a passive, fully integrated optical 10 to 40 GHz time-domain multiplexer. This device is very compact (16×5 mm2) and robust, whereby its miniaturization and robustness are based on the high-index-contrast silicon-oxynitride (SiON) waveguide technology used. This 4X multiplexer consists of two cascaded asymmetric Mach-Zehnder structures. Thereby a total of three directional couplers and two delay lines of 50 ps and 25 ps, respectively, are cascaded. Because of the high SiO2-SiON index contrast of 3.8 % it was possible to realize a multiplexer device with bending radii of less than 1.0 mm in an ultra-compact double-folded design. The slightly unbalanced attenuation in the delay lines was pre-compensated by the directional coupler design, i.e. by detuning from 50 % : 50 % coupling ratio. We demonstrated experimentally that with a fundamentally mode-locked 10 GHz Er:Yb:glass laser source at the design wavelength of 1535 nm our 4X multiplexer produces a 40 GHz pulse train with < 0.22 dB pulse-to-pulse power variation and < 350 fs timing jitter. Although the current device is designed for 40 GHz, its principle can be applied to 160 GHz or higher, provided that suitable pulse sources are available.
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