We introduce a suite of modular test cards designed and developed on the H2020 COSMICC project to allow test and validation of a silicon photonics transceiver at up to 56 Gb/s. The modular test cards include large and small mezzanine cards to house the silicon photonics transceiver under test and a power distribution and sense card, which allows real time measurement of power consumption in a data center environment. The test modules can be driven stand-alone or incorporated into different data center platforms.
We introduce a universal test and measurement system allowing comparative characterisation of optical transceivers, board-to-board optical connectors and both embedded and passive optical circuit boards. The system comprises a test enclosure with interlocking and interchangeable test cards, allowing different technologies spanning different Technology Readiness Levels to be both characterised alone and in combination with other technologies. They form part of the open test design standards portfolio developed on the FP7 PhoxTroT and H2020 COSMICC projects and allow testing on a common test platform.
The field of silicon photonics is attracting a lot of attention due to the prospect of low-cost and compact circuits that integrate photonic and microelectronic elements on a single chip. Such silicon chips have applications in optical transmitter and receiver circuits for short-distance communications as well as for long-haul optical transmissions. Silicon photonics has proven to be a successful platform for many functional elements such as low-loss waveguides, filters, multiplexers/demultiplexers, optical modulators and Ge-on-Si photodiodes. On-going developments for advanced photonic integrated circuits include compact and energy-efficient silicon modulators, temperature-insensitive passive devices and hybrid III-V on Silicon lasers.
The European COSMICC project gathers key industrial and research partners in the field of silicon photonics, CMOS electronics, printed circuit board packaging, optical transceivers and datacenters, aiming at developing low-cost and low-energy consumption 50 Gb/s 4-wavelength coarse wavelength division multiplexing optical transceivers that will be packaged on-board. Combining CMOS electronics and Si-photonics with innovative high-throughput fiber attachment techniques, the developed solutions will be scalable beyond 1 Tb/s to meet the future data-transmission requirements in data-centers and super computing systems.
We report on a converged optically enabled Ethernet storage, switch and compute platform, which could support future disaggregated data center architectures. The platform includes optically enabled Ethernet switch controllers, an advanced electro-optical midplane and optically interchangeable generic end node devices. We demonstrate system level performance using optically enabled Ethernet disk drives and micro-servers across optical links of varied lengths.
Embedded optical waveguide technology for optical printed circuit boards (OPCBs) has advanced considerably over the past decade both in terms of materials and achievable waveguide structures. Two distinct classes of planar graded index multimode waveguide have recently emerged based on polymer and glass materials. We report on the suitability of graded index polymer waveguides, fabricated using the Mosquito method, and graded index glass waveguides, fabricated using ion diffusion on thin glass foils, for deployment within future data center environments as part of an optically disaggregated architecture. To this end, we first characterize the wavelength dependent performance of different waveguide types to assess their suitability with respect to two dominant emerging multimode transceiver classes based on directly modulated 850 nm VCSELs and 1310 silicon photonics devices. Furthermore we connect the different waveguide types into an optically disaggregated data storage system and characterize their performance with respect to different common high speed data protocols used at the intra and inter rack level including 10 Gb Ethernet and Serial Attached SCSI.
The RMS surface roughness of an optical polymer waveguide end facet cut by a milling router and measured by AFM is
investigated for a range of rotation speeds and translation speeds of the router. It was found that 1 flute (cutting edge)
routers gave significantly less rough surfaces than 2 or 3 flute routers. The best results were achieved for a 1 flute router
when the milling bit was inserted from the copper layer side of the board with a rotation speed of 15,000 rpm and a
translation speed of 0.25 m/min which minimized the waveguide core end facet RMS roughness to 183 ± 8 nm and gave
input optical coupling loss of 1.7 dB ± 0.5 dB and output optical coupling loss of 2.0 dB ± 0.7 dB. The relationship
between optical coupling loss at the input and output of the waveguides and waveguide end facet roughness is also
investigated in this paper. The ratio of RMS roughness to autocorrelation length of the roughness is shown to have a
quantified linear relationship with experimental measurements of optical insertion loss, input optical coupling loss and
output optical coupling loss. A new fabrication technique for cut waveguide end facet treatment has been proposed and
demonstrated which reduces the insertion loss by 2.60 dB ± 1.3 dB which is more than that achieved by the closest
available index matching fluid which gave 2.23 dB ± 1.2 dB and which is far more robust for use in commercial products.
Optical interconnects for data transmission at board level offer increased energy efficiency, system density, and
bandwidth scalability compared to purely copper driven systems. We present recent results on manufacturing of electrooptical
printed circuit board (PCB) with integrated planar glass waveguides. The graded index multi-mode waveguides
are patterned inside commercially available thin-glass panels by performing a specific ion-exchange process. The glass
waveguide panel is embedded within the layer stack-up of a PCB using proven industrial processes. This paper describes
the design, manufacture, assembly and characterization of the first electro-optical backplane demonstrator based on
integrated planar glass waveguides. The electro-optical backplane in question is created by laminating the glass
waveguide panel into a conventional multi-layer electronic printed circuit board stack-up. High precision ferrule mounts
are automatically assembled, which will enable MT compliant connectors to be plugged accurately to the embedded
waveguide interfaces on the glass panel edges. The demonstration platform comprises a standardized sub-rack chassis
and five pluggable test cards each housing optical engines and pluggable optical connectors. The test cards support a
variety of different data interfaces and can support data rates of up to 32 Gb/s per channel.
Widespread adoption of optical circuit boards will herald substantial performance, environmental and cost benefits for
the data communications industry. Though optical circuit board technology has advanced considerably over the past
decade, commercial maturity will be gated by the availability of conformity standards to forge future quality assurance
procedures. One important prerequisite to this is a reliable test and measurement definition system, which is agnostic to
the type of waveguide system under test and therefore can be applied to different optical circuit board technologies as
well as being adaptable to future variants. A serious and common problem with the measurement of optical waveguide
systems has been lack of proper definition of the measurement conditions for a given test regime, and consequently
strong inconsistencies ensue in the results of measurements by different parties on the same test sample. We report on the
development of a new measurement identification standard to force testers to capture sufficient information about the
measurement conditions for a given optical circuit board such as to ensure consistency of measurement results within an
acceptable margin. Furthermore we demonstrate how the application of the measurement identification system can bring
about a dramatic improvement in results consistency, by comparative evaluation of the results on multimode polymer
waveguide based optical circuit test boards from a large selection of testing organisations.
Electro-optical printed circuit board technology (EOCB) based on integrated planar polymer optical waveguides has been the subject of research and development for many years to provide a cost viable, fully integrated system embedded optical interconnect solution, however a number of constraints of this technology have yet to be overcome. Optical coupling loss at the input and output of the waveguides is one of the major issues and waveguide end facet roughness is one of the main sources of the coupling loss which is investigated in this paper. The results of a comprehensive investigation of the end facet roughness of multimode polymer waveguides, fabricated on FR4 printed circuit boards, PCBs, and its effect of optical loss are presented theoretically and experimentally. The waveguide end facet roughness was measured using an atomic force microscope, AFM, when the waveguides were cut using a milling router with various numbers of cutting edges called flutes. The optimized cutting parameters are derived and the optical coupling loss, between the laser source and the waveguide, due to the different roughness magnitudes is measured by experiment for the first time. To improve the surface quality and decrease the waveguide optical loss, a new fabrication technique for reducing the end facet roughness after cutting is proposed and demonstrated. The insertion loss was reduced by 2.60 dB ± 1.3 dB which is more than that achieved by other conventional methods such as index matching fluid.
The evolution of data storage communication protocols and corresponding in-system bandwidth densities is set to impose prohibitive cost and performance constraints on future data storage system designs, fuelling proposals for hybrid electronic and optical architectures in data centers. The migration of optical interconnect into the system enclosure itself can substantially mitigate the communications bottlenecks resulting from both the increase in data rate and internal interconnect link lengths. In order to assess the viability of embedding optical links within prevailing data storage architectures, we present the design and assembly of a fully operational data storage array platform, in which all internal high speed links have been implemented optically. This required the deployment of mid-board optical transceivers, an electro-optical midplane and proprietary pluggable optical connectors for storage devices. We present the design of a high density optical layout to accommodate the midplane interconnect requirements of a data storage enclosure with support for 24 Small Form Factor (SFF) solid state or rotating disk drives and the design of a proprietary optical connector and interface cards, enabling standard drives to be plugged into an electro-optical midplane. Crucially, we have also modified the platform to accommodate longer optical interconnect lengths up to 50 meters in order to investigate future datacenter architectures based on disaggregation of modular subsystems. The optically enabled data storage system has been fully validated for both 6 Gb/s and 12 Gb/s SAS data traffic conveyed along internal optical links.
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 . 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.
In-plane bend loss represents the greatest commercial inhibitor to deploying multimode optical waveguides on densely
populated electro-optical printed circuit boards (OPCB) as the minimum bend radii currently possible are too large to
be practical in common designs. We present a concept and fabrication method for creating novel polymer optical
waveguide structures with reduced bend losses to enable higher density routing on an OPCB. These nested core
waveguide structures comprise a core surrounded by a thin shell of cladding, which allows for two-fold modal
containment by first a conventional low refractive index contrast (LIC) boundary followed by a secondary high
refractive index contrast (HIC) boundary. The purpose of this is to reduce the in-plane bend losses incurred on tightly
routed optical channels, while not incurring prohibitive dispersion, sidewall scattering and optical crosstalk penalties.
We have designed and fabricated nested core multimode polymer waveguides, evaluated their performance in
comparison to conventional step-index waveguides and simulated these structures using the beam propagation method.
Preliminary results are presented of our measurements and simulations.
As both data storage interconnect speeds increase and form factors in hard disk drive technologies continue to
shrink, the density of printed channels on the storage array midplane goes up. The dominant interconnect
protocol on storage array midplanes is expected to increase to 12 Gb/s by 2012 thereby exacerbating the
performance bottleneck in future digital data storage systems. The design challenges inherent to modern data
storage systems are discussed and an embedded optical infrastructure proposed to mitigate this bottleneck.
The proposed solution is based on the deployment of an electro-optical printed circuit board and active
interconnect technology. The connection architecture adopted would allow for electronic line cards with
active optical edge connectors to be plugged into and unplugged from a passive electro-optical midplane with
embedded polymeric waveguides.
A demonstration platform has been developed to assess the viability of embedded electro-optical midplane
technology in dense data storage systems and successfully demonstrated at 10.3 Gb/s. Active connectors
incorporate optical transceiver interfaces operating at 850 nm and are connected in an in-plane coupling
configuration to the embedded waveguides in the midplane. In addition a novel method of passively aligning
and assembling passive optical devices to embedded polymer waveguide arrays has also been demonstrated.
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.