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Polymer Waveguide-Coupled Solderable Optical Modules for High-Density Optical Interconnects
Abstract
To realize simple and low-cost VCSEL-based parallel-optical modules mounted onto a polymer-waveguide-embedded PCB in high density, we propose a new mounting technology using a guide pin-assisted optical coupling stabilization during a Sn-Ag-Cu solder -- reflow process. The guide holes of the 12-channel optical module and the guide pins of optical PCB are precisely shaped by a high-precision machining and photolithography technologies, respectively. The allowable optical axis displacement is +/-12 um in both x-and y-axes between the parallel-optical module and the arrayed polymer-waveguide. We fabricated VCSEL-based 14-Gb/s x 12-channel transmitter/receiver modules and 25-Gb/s x 4-channel transceiver modules based on the 12-channel packaging. The module was soldered on a polymer waveguide-embedded PCB to achieve a sufficiently low optical coupling loss. In the link tests, we successfully achieved a BER of 10^-12 for both 14-Gb/s x 12-channel and 25-Gb/s x 4-channel optical links.
... A polymer waveguide (PWG) is used for optical interconnects because the density of PWG arrays can be higher than that of optical fiber arrays. Therefore, optical interconnects using PWG arrays are expected to yield high-speed, low-power, and high-density on-board interconnects [12][13][14][15][16][17][18][19][20]. The longer PWG arrays, including mechanical transfer (MT) connectors, are designed to connect to the optical engines in transceiver modules [14,15]. ...
... The longer PWG arrays, including mechanical transfer (MT) connectors, are designed to connect to the optical engines in transceiver modules [14,15]. Optical signals from optical sources or signals fed into detectors are directly coupled to the shorter PWG arrays with MT connectors for connecting to the optical fiber ribbon [17][18][19][20]. The PWG arrays can also be embedded in printed circuit boards (PCBs) [14][15][16]. ...
... Therefore, the 400-Gb/s optical transmission through the 140 mm long PWG can be realized. Although active alignment was used for every channel, the good transmission performance observed here would still be expected if the misalignment displacement can be controlled within ± 12 μm in both x-axis and y-axis [19,20]. The performance of the 400-Gb/s optical receiver module was characterized using the experimental setup shown in Fig. 6. ...
We propose 400-Gb/s (16 × 25 Gb/s) optical transmitter and receiver modules for on-board interconnects using polymer waveguide (PWG) arrays. Four four-channel vertical cavity surface emitting laser and PIN photodiode arrays are used in the optical transmitter and receiver modules. A 16-channel PWG array with a length of 140 mm and a pitch of 62.5 μm is used as a short-reach optical signal transmission medium in the module. The 25-Gb/s optical eye diagrams of the all output channels utilize PWG transmissions and exhibit good performance in the transmitter module, with a signal-to-noise ratio (SNR) of over 8.01, peak-to-peak time jitter below 11.70 ps, and mask margin of over 31.5%. In the best-performance channel of the receiver module, the peak-to-peak time jitter and SNR of the eye diagram are 10.34 ps and 10.76, respectively, and the receiver sensitivity is approximately −2.6-dBm under a bit error rate of 10⁻¹². The optical transmitter and receiver modules utilizing PWG transmissions were designed and fabricated to realize 400-Gb/s on-board interconnects.
... The present datacenters most of the connections inside data centers presently are Optical, which includes server-to-switch and switch-to-switch connections. [5] The server boards are still electrical, i.e. the optical-electrical conversion is done at the board edge, which leads bandwidth limitations at the server board. Our idea was to extend optical fiber communication beyond cables i.e. optical-electrical conversion close to the processor, which can improve the bandwidth limitations. ...
... [11][12] Thus, in order to handle sustainably ever-increasing traffic demand it becomes of extreme importance to address the bandwidth constraints in intra-data center networks, which provide interconnections among various different servers within a data center as well as interfaces to the Internet. [5][10] ...
The drastic growth in internet services has led to
growth in data sharing between the users. This has led to need
for powerful and large data centers, where huge number of
storage and computing servers are interconnected. According to
the reports, annual data center traffic is growing at a very high
growth rate of 25%, of which the major contribution is due to the
servers within the data centers, i.e. Intra datacenters. The traffic
outside the data center is handled by high-speed optical fiber
network. However, within the datacenter the data is handled by
both optical and electrical interconnects. It is essential that the
fully network infrastructure is made optical to cater to the future
needs. To fulfill the intra- and inter-system bandwidth
requirements of data centers and HPC (high-performance
computers).Though the server racks are connected through
active optical cables, the server boards are still fully electrical, i.e.
the optical-electrical conversion is done at the board edge. This
conversion leads to bandwidth limitations at the server board. In
this work we proposed to extend the optical reach to the
processing unit i.e., microprocessors and memory unit from
board edge. This is achieved by designing an OEPCB (OpticalElectrical
Printed Circuit Board) using laser direct writing and
to define high-speed optical waveguide. In this work, we target to
demonstrate an on-board waveguide technology that can support
data rate of 10 Gbps/channel. The demonstrator will enable a
platform for further improvement in integration that will enable
the next generation High-speed connection in Datacenters.
... Meanwhile, a couple of techniques to utilize step-index (SI) core polymer optical waveguides have been proposed and developed [14][15][16][17]. For off-board links, multimode fibers (MMFs) have been frequently deployed because of the wide tolerance in their alignment in connections, namely low total link cost. ...
In this paper, we present a low-loss optical assembly utilizing a 90°-bent graded-index (GI) core polymer optical waveguide on vertical cavity surface emitting laser (VCSEL) based optical transceivers. The proposed assembly can replace conventional components such as mirrors and lenses for realizing subminiature optical engines applicable to on-board integration. To minimize the total insertion loss of the waveguide when connected to a high-speed VCSEL and a GI-core multimode fiber (MMF) at each end, the characteristics of the beam emitted from VCSELs are measured and taken into consideration for the waveguide design. In order to confirm the effect of insertion loss reduction by the waveguide numerical aperture control, 90°-bent GI-core polymer waveguides are fabricated applying the Mosquito method. The fabricated waveguide exhibits a total insertion loss as low as about 2 dB at 850-nm wavelength, which includes the coupling losses at both ends, bending, and propagation losses. We also investigate a way to reduce the insertion loss when a gap exists between the waveguide and VCSEL chip. We theoretically and experimentally confirm that filling the gap with a high index resin can reduce the coupling loss by 5 dB.
In this article, we propose a subminiature (10.9 mm
mm) vertical cavity surface-emitting laser (VCSEL)-based optical engine with a low crosstalk penalty for onboard applications. When applying optical engines to onboard interconnects, ICs [laser drivers and transimpedance amplifiers (TIAs)] and active optical devices (light sources and photodetectors) must be mounted densely to make the footprint as small as possible. It is a concern that such a high-density integration could increase the crosstalk between transmitter (Tx) and receiver (Rx) devices, which could be caused by the supply current difference between the circuit from laser drivers to light source and the circuit from photodetectors to TIAs. In this article, by inserting a gap in the ground electrode, a compact optical engine (less than half of the footprint of quad small form-factor pluggable-double density (QSFP-DD) compliant engines) enabling a 25.78-Gb/s error-free optical transmission is successfully fabricated. We optimize the gap width to decrease the crosstalk while maintaining efficient heat dissipation via the electrode. We compare the characteristics of the fabricated optical engine to the engine with the gap-less ground electrode structure formed in the same compact size. Then, we both theoretically and experimentally confirm a link power budget savings of about 1.8 dB, which is sustained even under high-temperature (
T_{c} = 70\,\,^{\circ }\text{C}
) operation. In addition, to realize further high-density assembly, we also represent a lens-less optical coupling by inserting a 90
o
-bent graded-index (GI) core polymer waveguide between the optical transmitter and multimode fiber. The transmission performance of the 90
o
-bent GI-core waveguide is preliminarily evaluated, and we successfully transmit 53.125-Gb/s PAM4 optical signals experimentally.
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 onchip 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 highperformance 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
chipto- package 1-dB lateral alignment tolerance for coupling into a polymer waveguide with a cross-section of
. 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.
Multimode polymer waveguide crossings exhibiting the lowest reported excess loss of 0.006 dB/crossing and crosstalk values as low as -30 dB are presented. Their potential for use in high-speed dense optical interconnection architectures is demonstrated.
An optical backplane based on a meshed polymer waveguide architecture enabling high-speed board-to-board optical interconnection
is presented. This planar array of multimode polymer waveguides can provide passive strictly non-blocking links between server
line cards fitted with optical transmitter and receiver arrays. This architecture offers a scalable and low-cost solution
to the bandwidth limitations faced by electrical backplanes and is suitable for PCB integration. The reported backplane demonstrator
uses a matrix of 100 waveguides each capable of 10 Gb/s operation to interconnect 10 cards for a total capacity of a terabit
per second aggregate data rate in multicast mode. Characterisation of the backplane demonstrator reveals low link losses of
2 to 8dB for a multimode fibre input and crosstalk values below −35dB. Error free data transmission at 10 Gb/s is achieved
with a power penalty of only 0.2dB at a bit-error-rate of 10−9. Additionally, lossless operation of a Gigabit Ethernet link over the backplane is achieved even when using the worst-case
highest loss links.
Cost-effective multimode polymer waveguides, suitable for use in high-speed on-board optical interconnections, are presented. The fundamental light transmission properties of the fabricated waveguides are studied under different launch conditions and in the presence of input misalignments. Low loss (~0.04 dB/cm at 850 nm) and low crosstalk (<-30 dB) performance, relaxed alignment tolerances (plusmn20 mum) and high-speed operation at a 10-Gb/s data rate are achieved. No degradation in the high-speed link performance is observed when offset input launches are employed. Moreover, a range of useful waveguide components that add functionality and enable complex on-board topologies are presented. The optical transmission characteristics of the fabricated components are investigated and it is shown that excellent performance is achieved. Excess losses as low as 0.01 dB per waveguide crossing, the lowest reported value for such components, and bending losses below 1 dB for 90-degree and S-shaped bends are obtained even with multimode fiber launches. Moreover, high-uniformity power splitting and low-loss signal combining are achieved with Y-shaped splitter/combiners while a variable splitting ratio between 30%-75% is demonstrated with the use of multimode couplers. Overall, the devices presented are attractive potential candidates for use in on-board optical links.
We investigate the characteristics of crossing and branching nodes in monolayer soft-lithography-based polymer optical interconnects with experimental and theoretical analysis. The theoretical crosstalk, as calculated by a function of crossing angle, was determined for a set of interconnect pairs with varying cross-sections, and was compared with experimental measurements. Furthermore, a suitable branching angle was found for branching node and the effects of short-distance mode scrambling in highly multimode polymer waveguides were studied in detail. It was found that mode-filling occurred within a propagation distance of 1.5mm for a 50×50μm² cross-section for VCSEL coupling; however, complete scrambling of ray direction required a propagation distance of more than 5 mm.
In the "Terabus" optical interconnect program, optical data bus technologies are developed that will support terabit/second chip-to-chip data transfers over organic cards within high-performance servers, switch routers, and other intensive computing systems. A complete technology set is developed for this purpose, based on a chip-like optoelectronic packaging structure (Optochip), assembled directly onto an organic card (Optocard). Vertical-cavity surface emitting laser (VCSEL) and photodiode arrays (4times12) are flip-chip bonded to the driver and receiver IC arrays implemented in 0.13-mum CMOS. The IC arrays are in turn flip-chip assembled onto a 1.2-cm2 silicon carrier interposer to complete the transmitter and receiver Optochips. The organic Optocard incorporates 48 parallel multimode optical waveguides on a 62.5-mum pitch. A simple scheme for optical coupling between the Optochip and the Optocard is developed, based on a single-lens array etched onto the backside of the optoelectronic arrays and on 45deg mirrors in the waveguides. Transmitter and receiver operation is demonstrated up to 20 and 14 Gb/s per channel, respectively. The power dissipation of 10-Gb/s single-channel links over multimode fiber is as low as 50 mW
To reduce the power consumption of 4-channel 28-Gb/s miniature optical modules, a 4-channel double intra-cavity 1060-nm InGaAs/GaAs quantum-well vertical-cavity surface-emitting laser (VCSEL) array with 5-mu m-wide apertures, a high differential gain, and a low temperature-insensitive differential resistance in conjunction with a specially optimized thermo-electric cooler heat sink for a reduced thermal resistance are used. Most important is the excellent resistance compatibility between the VCSEL and a low-power SiGe BiCMOS driver IC. Consequently, a 4-channel 28-Gb/s NRZ PRBS 2(31) - 1 link operation at a bit error rate of 10(-12) for an operational case temperature ranging from 25 degrees C to 70 degrees C was achieved at an unprecedentedly low VCSEL bias current of only 4 mA.
We propose a high-speed FPC connector, an inductive peaking line on a module substrate, and an integrated lens using inkjet technology for a small-diameter photodiode to extend the frequency bandwidth on electrical and optical packaging. These techniques enable high-speed operation over 25 Gb/s. We fabricated a 4-channel flexible printed circuit-based optical engine by adopting these technologies and demonstrated a clear eye opening and error-free operation up to 41.25 Gb/s.
This paper describes a unique solution to the next-generation high bit-rate and high channel-density parallel-optical links whereby the size of parallel-optical modules would have to be considerably reduced. Since the main problem concerning the size of optical-to-electrical conversion part is associated with the mounting of the parallel-optical module to the printed-circuit-board (PCB), this paper puts forward a remarkably compact parallel optical-module design that can be soldered directly onto the PCB. That is made possible by the high temperature tolerance of 250 °C required for Sn-Ag-Cu solder-reflow process. The miniature parallel-optical module measures only 10.5 mm in length, 6.5 mm in width, and 1.5 mm in height. The resulting parallel-optical modules are capable of four-channel error-free parallel pseudo-random binary sequence 231-1 transmission at 28 Gb/s without using clock and data recovery as well as feed-forward error correction (FEC); hence, these parallel-optical modules are applicable in next-generation high-density 100 Gb/s Ethernet applications that use FEC.
Multimode optical waveguides using low loss heat resistant fluoropolymer were demonstrated. We fabricated 10 cm-long film optical waveguides and measured insertion loss, tolerance, and heat resistant. These results showed good performance at 1310 nm.
We report here on the design, fabrication, and characterization of highly integrated
parallel optical transceivers designed for Tb/s-class module-to-module data transfer through
polymer waveguides integrated into optical printed circuit boards (o-PCBs). The parallel
optical transceiver is based on a through-silicon-via silicon carrier as the platform for
integration of 24-channel vertical cavity surface-emitting laser and photodiode arrays with
CMOS ICs. The Si carrier also includes optical vias (holes) for optical access to conventional
surface-emitting 850 nm optoelectronic devices. The 48-channel 3-D transceiver optochips are
flip-chip soldered to organic carriers to form transceiver optomodules. Fully functional
optomodules with 24 transmitter +24 receiver channels were assembled and
characterized with transmitters operating up to 20 Gb/s/ch and receivers up to 15 Gb/s/ch. At
15 Gb/s, the 48-channel optomodules provide a bidirectional aggregate bandwidth of 360 Gb/s.
In addition, o-PCBs have been developed using a 48-channel flex waveguide assembly attached to
FR4 electronic boards. Incorporation of waveguide turning mirrors and lens arrays facilitates
optical coupling to/from the o-PCB. Assembly of optomodules to the o-PCB using a ball grid
array process provides both electrical and optical interconnections. An initial demonstration
of the full module-to-module optical link achieved > 20 bidirectional links at 10 Gb/s. At 15 Gb/s,
operation at a bit error ratio of < 10-12 was demonstrated for 15 channels in
each direction, realizing a record o-PCB link with a 225 Gb/s bidirectional aggregate data
rate.
Large-scale routers, supercomputers and datacenters all have internal networks for chip-to-chip communications with bandwidth, density, and power requirements that can surpass the capabilities of copper solutions. We review current state-of-the-art technologies, such as Avago's MicroPOD parallel optical modules.
We have studied technologies to design and fabricate high-bit-rate chip-to-chip optical interconnects on printed circuit boards (PCBs) using board-embedded polymer waveguides and surface-mounted component packages or modules. In order to demonstrate the developed technologies, a 4times10 Gb/s optical interconnect was completely integrated on a standard FR4 PCB. The optical link demonstrator consists of 4-channel BGA-mounted transmitter and receiver modules as well as of four parallel multimode optical waveguides fabricated on top of the solder mask of the PCB using lithographic patterning. The transmitters and receivers built on low-temperature co-fired ceramic (LTCC) substrates include 4times10 Gb/s flip-chip mounted VCSEL or photodiode array, wire-bonded driver and receiver ICs, and optical coupling structures. Two microlens arrays and a micro-mirror enable optical coupling between the optoelectronic devices and the waveguide array. The passive optical alignment was based on the marks and structures fabricated both in the LTCC process and in the patterning of the optical layers. The characterized optical alignment tolerances are in the limits of accuracy of the surface-mount technology. The demonstrated technology is suitable for interconnecting CBGA-packaged ICs or multi-chip-modules
We report on advances in polymeric waveguide technologies
developed worldwide for the telecom and datacom markets, and we describe
in detail one such technology developed at AlliedSignal. Optical
polymers are versatile materials that can be readily formed into planar
single-mode, multimode, and microoptical waveguide structures ranging in
dimensions from under a micrometer to several hundred micrometers. These
materials can be thermoplastics, thermosets, or photopolymers, and the
starting formulations are typically either polymers or oligomers in
solution or liquid monomers. Transmission losses in polymers can be
minimized, typically by halogenation, with state-of-the-art loss values
being about 0.01 dB/cm at 840 nm and about 0.1 dB/cm at 1550 nm. A
number of polymers have been shown to exhibit excellent environmental
stability and have demonstrated capability in a variety of demanding
applications. Waveguides can be formed by direct photolithography,
reactive ion etching, laser ablation, molding, or embossing.
Well-developed adhesion schemes permit the use of polymers on a wide
range of rigid and flexible substrates. Integrated optical devices
fabricated to date include numerous passive and active elements that
achieve a variety of coupling, routing, filtering, and switching
functions
A 120-Gb/s optical link (12 channels at 10 Gb/s/ch for both a transmitter and a receiver) has been demonstrated. The link operated at a bit-error rate of less than 10-12 with all channels operating and with a total fiber length of 316 m, which comprises 300 m of next-generation (OM-3) multimode fiber (MMF) plus 16 m of standard-grade MMF. This is the first time that a parallel link with this bandwidth at this per-channel rate has ever been demonstrated. For the transmitter, an SiGe laser driver was combined with a GaAs vertical-cavity surface-emitting laser (VCSEL) array. For the receiver, the signal from a GaAs photodiode array was amplified by a 12-channel SiGe receiver integrated circuit. Key to the demonstration were several custom testing tools, most notably a 12-channel pattern generator. The package is very similar to the commercial parallel modules that are available today, but the per-channel bit rate is three times higher than that for the commercial modules. The new modules demonstrate the possibility of extending the parallel-optical module technology that is available today into a distance-bandwidth product regime that is unattainable for copper cables.
High-density, high-count mechanically transferable (MT)-type and multifiber push-on (MPO)-type connectors were constructed by using a monolithic 60-fiber ferrule that aligns five stacked 12-fiber ribbons. The MT- and MPO-type connectors have high packaging densities of 286 and 40 fibers/cm2, respectively. We fabricated both types of connector. The average insertion loss of the MT type was 0.30 dB and that of the MPO type 0.70 dB, with and without index-matching material, respectively.