An important challenge that remains to date in board level optical interconnects is the coupling between the optical
waveguides on printed wiring boards and the packaged optoelectronics chips, which are preferably surface mountable on
the boards. One possible solution is the use of Ball Grid Array (BGA) packages. This approach offers a reliable
attachment despite the large CTE mismatch between the organic FR4 board and the semiconductor materials.
Collimation via micro-lenses is here typically deployed to couple the light vertically from the waveguide substrate to the
optoelectronics while allowing for a small misalignment between board and package. In this work, we explore the
fabrication issues of an alternative approach in which the vertical photonic connection between board and package is
governed by a micro-optical pillar which is attached both to the board substrate and to the optoelectronic chips. Such an
approach allows for high density connections and small, high-speed detector footprints while maintaining an acceptable
tolerance between board and package. The pillar should exhibit some flexibility and thus a high-aspect ratio is preferred.
This work presents and compares different fabrication methods and applies different materials for such high-aspect ratio
pillars. The different fabrication methods are: photolithography, direct laser writing and deep proton writing. The
selection of optical materials that was investigated is: SU8, Ormocers, PU and a multifunctional acrylate polymer. The
resulting optical pillars have diameters ranging from 20um up to 80um, with total heights ranging between 30um and
100um (symbol for micron). The aspect-ratio of the fabricated structures ranges from 1.5 to 5.
In an effort to address the need for robust optical chip I/O interconnects, we describe the fabrication and testing of
microscopic polymer pillars for use as a flexible optical bridge between the chip and the substrate. The polymer pillars
are photoimaged using the polymer Avatrel to a height of up to 350 &mgr;m. The photodefinable polymer Avatrel was used
for the fabrication of the optical pillars due to its ease of processing and its unique material properties that include high
Tg and low modulus. To evaluate the performance of the polymer pillars, the optical coupling efficiency from a light
source to an optical aperture with and without an optical pillar is measured. For a light source with 12o beam divergence,
a 30x150 &mgr;m polymer pillar improves the coupling efficiency by 3 to 4.5 dB compared to pillar-free (free-space) optical
coupling. Due to the high mechanical compliance of the optical pillars, we also demonstrate that polymer pillars enhance
the optical coupling efficiency between the chip and the substrate when they are misaligned in the lateral direction and
that the displacement tolerance can be doubled from 15 to 30 &mgr;m for a 1dB power loss budget.
Integration technologies for board-to-board optical interconnect systems are presented. In the module architecture, optical transmitters and receivers are placed on the line cards and the signals are routed to the optically passive backplane through optical jumpers. The backplane contains a light guiding layer with embedded polymer waveguides (WGs) and 45-deg reflector micromirrors. The WGs are fabricated by direct lithographic patterning and have propagation losses as low as 0.05 dB/cm. The wedge dicing technology is developed for fabrication of the 45-deg micromirrors with 0.5-dB excess losses. The pluggable optical connectors with microlens adaptors couple the light from the optical jumpers into the backplane WGs. Evaluation of the connector alignment tolerances demonstrates a very weak dependence of the coupling efficiency on the axial displacement and a more significant effect of the radial shifts. The presented results show that the displacement tolerances can be substantially improved with auxiliary lenses formed on the substrate. Prototype optical interconnect modules with integrated channel WGs, mirrors, and assembled connectors are fabricated with insertion losses of 5 to 6 dB. The modules successfully pass the high-speed transmission tests at data rates up to 11 Gbits/s.
Fabrication and assembly technologies for high-speed board-to-board optical interconnect (B2OI) systems are presented. In the system architecture, the transmitters and receivers are placed on the linecards and the optical signals are routed to the optically passive backplane through the optical jumpers with MTP connectors. The backplane contains an optical layer with embedded polymer waveguides and 45° reflector micromirrors. The waveguides are fabricated by direct lithographic patterning and have propagation losses as low as 0.05 dB/cm at 850 nm. Hot-embossing is also evaluated for the waveguide fabrication demonstrating the waveguide propagation losses in the range of 0.06-0.1 dB/cm but rather poor channel-to-channel uniformity. The wedge dicing technology is developed for fabrication of the 45° reflector micromirrors with 0.5 dB losses. The pluggable optical connectors with microlens adaptors are used to couple the light from the optical jumpers into the backplane waveguides. The fabricated prototype optical interconnect modules with integrated channel waveguides, mirrors, and assembled connectors demonstrate insertion losses of 5-6 dB. The modules successfully pass high-speed transmission tests at data rates up to 11 Gb/s.
Planar waveguides and embedded microelements such as 45o vertical mirrors, lateral mirrors, bends, and microlenses comprise main building blocks of the waveguide-based optical printed circuit boards (PCB) for board-level optical interconnects (OI). These microelements enable a variety of three dimensional (3D) routing architectures which are required to support high density interconnects in optical boards. Optical polymers have proved to be the materials of choice for large-scale OI modules with propagation dimensions exceeding 100 mm. In order to meet the loss budget available for the integrated OI modules, the polymers are expected to have optical losses less than 0.05 dB/cm. Both channel and slab waveguides can be used to transmit the signals between the input and output ports. In the case of channel waveguides, the critical issues are the waveguide core shaping, propagation losses and ability to form various passive elements such as bends, crossings and reflective mirrors. In the case of slab waveguides, two dimensional waveguide microlenses have to be designed to collimate the light beams for propagation at longer distances with the controllable beam divergences. The 45o micromirrors can be used to couple the light signal in and out of the waveguiding layer and enable 3D routing of the optical signal in the waveguiding layers. In this work, we present the experimental and computational results on the development of different waveguide devices and microelements for the board level OI.
Prototypes of optical interconnect (OI) modules for backplane applications are presented. The transceivers attached to the linecards E/O convert the signals that are passed to and from the backplane by optical jumpers terminated with MTP-type connectors. The connectors plug into adaptors attached to the backplane and the microlens arrays mounted in the adaptors couple the light between the fibers and waveguides. Planar polymer channel waveguides with 30-50 μm cross-sections route the optical signals across the board with propagation losses as low as 0.05 dB/cm @ 850 nm. The 45º-tapered integrated micromirrors reflect the light in and out of the waveguide plane with the loss of 0.8 dB per mirror. The connector displacement measurements indicate that the adaptor lateral assembly accuracy can be at least ±10 μm for the excess loss not exceeding 1 dB. Insertion losses of the test modules with integrated waveguides, 45º mirrors, and pluggable optical jumper connectors are about 5 dB. Eye diagrams at 10.7 Gb/s have typical width and height of 70 ps and 400 mV, respectively, and jitter of about 20 ps.
Different types of planar waveguide microlenses are fabricated with PLC technologies from a variety of optical materials such as silica, photo-definable epoxy resins, and a number of other optical polymers. Hybrid microlenses are also fabricated in which the base of the lens, with a double concave gap, is formed from silica and the gap is filled with an optical polymer. The optimized lens structures provide the maximum coupling efficiencies between the input and output channels at distances up to 100 mm with a minimum channel pitch of 0.5-0.7 mm. Experimental and theoretical studies provide results on collimation and focusing properties of single and double microlenses made of silica, polymer, and silica/polymer. The evaluation of the temperature and wavelength effects on the collimation characteristics of the lenses demonstrate that the single lenses are more stable and, thus, more suitable for operations under varying conditions. Examples of the planar waveguide microlens applications are presented. In one application the microlens arrays are integrated in fast electrooptic photonic switching modules. In the other application the microlenses are embedded in the backplanes with nonblocking optical interconnects.
Nonblocking crossconnect photonic switches based on light beam deflection require planar optical modules with hybrid integration of active deflector chips. In this work we present optical modules with two dimensional silica microlens arrays and slab waveguides fabricated on silicon substrates. The 1.55 μm light is launched in the input microlens array, which collimates parallel beams propagating along the module. The slab waveguide vertically confines the light. The output microlenses focus the beams laterally into output fibers. Two chips are inserted in the light path after the input microlens and before the output microlens arrays. The input and output microlenses allow propagation of the light beams through modules up to 100 mm long with a beam width of less than 400 μm. A hybrid integration process flow is developed to place the deflector chips in the light path with high alignment accuracy. The chips are flip-chip bonded to the substrate with submicron accuracy in the vertical positioning. Various contributions can lead to the chip displacements such as, for example, standoff island height variations, aligner tolerances, substrate bow, etc. Experiments are conducted to evaluate the effect of chip displacement on the insertion losses of the hybrid-integrated modules. 100-mm long optical modules with input and output chips are fabricated with less than 4 dB insertion losses. The analysis of loss contributions and possibilities for improvements are discussed.
Two-dimensional (2-D) microlens arrays have been fabricated with silica-on-silicon planar lightwave circuit (PLC) technology. Several experimental techniques and computer simulation methods are applied to characterize properties of single and double microlens arrays, with one and two refracting surfaces, respectively. Systematic comparison of the measured and simulated beam propagation profiles enables optimization of the lens and module design resulting in higher input-output coupling efficiency. The insertion losses of the lens-slab-lens optical modules with 90-mm-long slab waveguides are measured to be 2.1 and 3.5 dB for the double and single lens modules, respectively. Comprehensive analysis reveals the major loss contributions. Excess losses of the modules caused by variations of the lens curvatures, material refractive indexes, light wavelength, etc., can be controlled within the acceptable limits. Further possibilities for the module loss reduction are discussed. Fairly weak wavelength dependence as well as overall stability of the module properties indicate that the microlens arrays are suitable for dense wavelength division multiplexing (DWDM) photonic networks.
We propose a new concept of optoelectronic (OE) interconnect hardware 'OE Scalable Substrate (OE-SS)' and 'Film Optical Link Module (FOLM)', which have potentiality to remove optics excess. The structure is as follows: OE-films, in which waveguides, thin-film OE devices, LSIs, capacitor chips etc. are integrated with via/pad/electrode, are stacked by electrical joints (Z-connections). This gives rise to standardized-interface capability and scalability. Using one basic technology 'film/Z-connection', all levels of interconnection will be achieved, including massive parallel optical link, inter-board optical connect, and 3D- stack-OE-MCM. We prose a new process 'Device Integration with Self-Organizing Transfer', which is essential for low- cost OE-SS and FOLM, especially for WDM applications.
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