Takatoshi Yagisawa

Cost-effective low-loss flexible optical engine with microlens-imprinted film for high-speed on-board optical interconnection

There is a strong demand for optical interconnection technology to overcome bandwidth bottlenecks in high-end server systems. The interconnection speed in present systems is approaching 10 Gb/s, and higher-speed interconnections over 25 Gb/s are being discussed. To achieve such optical interconnections in commercial production, it is necessary to develop lower-cost and higher-speed optical transceiver modules. We propose a flexible printed circuit optical engine (FPC-OE) with a microlens-imprinted film and a polymer waveguide to achieve low-cost and high-speed operation. The microlens-imprinted film can be produced at low cost by using nanoimprint technology and can drastically reduce the optical loss of the FPC-OE with polymer waveguide. We successfully demonstrated error-free operation at 25 Gb/s with the fabricated optical transceiver that contains an FPC-OE, microlens-imprinted film, and a polymer waveguide.

30-Gb/s VCSEL transmitter fabricated on flexible printed circuit substrate

A VCSEL transmitter was fabricated on an flexible printed circuit substrate. The transmitter achieved clear eye opening with an extinction ratio of 4.6 dB and error-free operation at 30-Gb/s, demonstrating the applicability of the FPC for high-speed transmitter applications.

FPC-based compact 25-Gb/s optical transceiver module for optical interconnect utilizing novel high-speed FPC connector

A compact (22.6 × 9.1 × 0.86 mm), 25-Gb/s, flexible printed circuit based optical transceiver module was developed. The module consists of a 25-μm-thick FPC substrate on which a vertical-cavity surface-emitting laser array, driver, photodiode array, and a transimpedance amplifier are mounted by flip-chip bonding. The polymer waveguide is laminated from the backside of the FPC via microlens-imprinted film for the optical coupling. Its compactness and high-speed characteristics were realized by using a 0.35-mm-pitch novel FPC connector accurately designed by using a three-dimensional electromagnetic simulation. It was confirmed that the bandwidth of the FPC connector was more than 40 GHz. Also the measured results of the frequency response corresponded with the simulation results. Clear eye openings and error-free operation of all eight channels at 25 Gb/s were obtained with the fabricated optical transceiver.

Cost-effective optical transceiver subassembly with lens-integrated high-k, low-Tg glass for optical interconnection

Optical interconnection is a key technology to overcome bandwidth bottlenecks in high-performance computing and high-end servers. An optical subassembly that consists of a transparent substrate and optical devices is attractive for optical interconnection owing to the ease of optical coupling between an integrated lens and optical devices. However, there are difficulties associated with the fabrication of the substrate. To solve these problems, we propose an optical transceiver subassembly using high-k, low-Tg glass. The transceiver subassembly helps in overcoming not only the difficulty in processing conventionally used substrates but also the limitation on high-speed signal transmission by the coplanar waveguide on those substrates. We successfully demonstrate error-free operation at 10 Gb/s and 14 Gb/s with the fabricated optical transceiver subassembly that contains a vertical-cavity surface-emitting laser (VCSEL) array, a driver array, a photodiode (PD) array, and a transimpedance amplifier (TIA) array.

Novel trace design for high data-rate multi-channel optical transceiver assembled using flip-chip bonding

We propose a trace design for high data-rate multichannel optical transceiver assembled using flip-chip bonding technology. The trace, which is formed on a transceiver substrate, compensates for bandwidth degradation due to the parasitic capacitance of the flip-chip bonding pad for optical and electrical devices. The trace was adopted to the receiver side of a flexible-printed-circuit-based optical engine that consists of a 25-μm-thick flexible printed circuit substrate on which a vertical-cavity surface-emitting laser array, driver, photodiode array, and transimpedance amplifier are mounted by flip-chip bonding. The polymer waveguide is laminated from the backside of the substrate via microlens-imprinted film for optical coupling. The sensitivity degradation caused by the parasitic capacitance was successfully suppressed to 0.6 dB at 40 Gb/s by using our trace design compared to that with a minimum length wire bonding connection.

Double-sided-mount card-edge-connected optical transceiver module using cost-effective FPC-OE optical engine

We developed an 8-channel optical transceiver that achieves high density and low cost by adopting cost-effective FPC-OEs on both surfaces of a card-edge module. Clear eye-openings and error-free operation up to 14 Gb/s were obtained.

Novel packaging technologies for FPC-based optical transceiver for high-speed optical interconnect

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.

24-ch microlens-integrated no-polish connector for optical interconnection with polymer waveguides

We successfully developed a new 24-ch optical connector for polymer waveguides. The connector consists of a transparent thermoplastic resin that has two rectangular slits on one side for alignment of the waveguide films and integrated microlens arrays on the other side for coupling to the MT connector. Two 12-ch waveguide films were cut to a 3-mm width. The thickness of each waveguide film was controlled at 100 μm. The waveguide films were inserted into the slits until they touched the bottom face of the slit. Ultraviolet curing adhesive was used to achieve a short hardening process. The expanded beam in the transparent material is focused by the microlens arrays formed on the connector surface. This lens structure enables assembly without the need for a polishing process. We designed the lens for coupling between a step-index 40-μm rectangular waveguide and a graded-index 50-μm fiber. We achieved low-loss optical coupling by designing a method of providing asymmetric magnification between the horizontal and vertical directions in order to compensate for the asymmetric numerical aperture of the waveguide. The typical measured coupling losses from/to the waveguide to/from the fiber were 1.2 dB and 0.6 dB, respectively. The total coupling loss was as small as that of a physical contact connection.

High-Bandwidth Optical Interconnect Technologies for Next-Generation Server Systems

We present optical interconnect technologies to realize next-generation server systems. We propose blade server architecture with optical bus-extensions for expanding consolidation, and demonstrate our cost-effective optical transceiver technologies for high-bandwidth interconnects.

Cost-effective transceiver technologies for high-bandwidth optical interconnection in high-end server systems

We have developed cost-effective technologies for optical transceivers with over 10-Gb/s operations in high bandwidth interconnection. These include optical engines on flexible printed circuit and 25Gb/s operation with pre-emphasis overdrive of conventional VCSEL for 10Gb/s.