The optical amplifier performance of Nd3+-doped polymer and amorphous Al2O3 channel waveguides with single-mode and multi-mode behavior around 880 nm is compared. Internal net gain in the wavelength range 865-930 nm is
investigated under continuous-wave excitation near 800 nm, for Nd3+ dopant concentrations typically in the range of 0.6-
1.0 × 1020 cm-3. A peak gain of 2.8 dB at 873 nm is obtained in a 1.9-cm-long polymer waveguide at a launched pump
power of 25 mW. The small-signal gain measured in a 1-cm-long sample is 2.0 dB/cm. In Al2O3, a peak gain of 1.57
dB/cm in a short and 3.0 dB in a 4.1-cm-long waveguide is obtained at 880 nm. Tapered multi-mode Nd3+-doped
amplifiers are embedded into an optical backplane and a maximum 0.21 dB net gain is demonstrated in a structure
consisting of an Al2O3:Nd3+ amplifier placed between two passive polymer waveguides on an optical backplane. The
gain can be further enhanced by increasing the pump power and improving the waveguide geometry, and the wavelength
of amplification can be adjusted by doping other rare-earth ions.
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.
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.
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.
Passive wavelength division (de)multiplexer (WDM) devices are required as basic building blocks for WDM-based on-chip
optical interconnects. In this application, many copies of the devices will be placed throughout a single die,
requiring that the device occupy as small a footprint as possible. Furthermore, it is critical that the demultiplexing
characteristics be very uniform from device to device, therefore the device must be tolerant to small fabrication
variations. There are various wavelength demultiplexer designs that lend themselves to on-chip integration with CMOS
integrated circuits and that could potentially reach the above specifications. In this presentation we will show the layout
and simulation of demultiplexer designs based on cascaded Mach-Zehnder wavelength splitters and on Echelle gratings
and compare these to measurement results of realized devices. The results on the Mach-Zehnder devices show that this
type of device is relatively sensitive to process variations. A fit of a device model to the measured curves shows that the
device variations result primarily from random phase errors in the optical delay lines, which are probably due to small
width variations in the waveguides. This problem should be strongly reduced in devices based on Echelle gratings,
because in this case the light does not propagate through channel waveguides in the part of the device that shapes the
optical response. This assumption is confirmed by the measurement results, which show good demultiplexer response
and excellent reproducibility between devices.
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.
With the ongoing progress in chip scaling, the data flow to and from chip packages is increasing accordingly. The
simultaneous increase of channel count and channel speed in an essentially constant form factor becomes a more and
more demanding challenge. The resulting I/O-bottleneck is considered to be a major limiting factor for the overall
performance of future chip packages and computing systems. Optical interconnects offer both increased channel density
as well as longer link reach at high frequencies.
Our current work focuses on integrating optical I/O with standard organic packages in order to maximize the aggregate
data flow to and from such packages. We present a novel approach for attaching an electro-optical conversion module
directly on top of the organic chip package, together with experimental results of a first prototype implementation.
We present ultra-compact integrated optical echelle grating WDM (de-)multiplexers for on-chip optical networks. These
devices are based on a design with two stigmatic points. The devices were fabricated using Silicon-On-Insulator (SOI)
photonic waveguide technology thus the smallest version of the (de-)multiplexer occupies an area of only 250x200 μm.
We will show measurement results on different variations of the echelle grating devices. In the measurements, we found
a channel to channel isolation of 19 dB. The minimum insertion loss, relative to a straight waveguide, is only 3 dB with a
channel to channel variation of 0.5 dB.nefit of the numerical reconstruction properties of DH in combination with
diffraction grating to get super-resolution. Various attempts have been performed and results are presented and
discussed. The approaches could be used for metrology and imaging application in various fields of engineering and biology.
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.
With the ongoing progress in chip scaling, the data flow to and from chip packages is increasing accordingly. The
simultaneous increase of channel count and channel speed in an essentially constant form factor becomes a more and
more demanding challenge. The resulting I/O-bottleneck is considered to be a major limiting factor for the overall
performance of future chip packages and computing systems. Optical interconnects offer both increased channel density
as well as longer link reach at high frequencies.
Our current work focuses on integrating optical I/O with standard organic packages in order to maximize the aggregate
data flow to and from such packages. We present a novel approach for attaching an electro-optical conversion module
directly on top of the organic chip package, together with experimental results of a first prototype implementation.
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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