Interconnects have been more important in high-performance computing systems and high-end servers beside its improvements in computing capability. Recently, active optical cables (AOCs) have started being used for this purpose instead of conventionally used copper cables. The AOC enables to extend the transmission distance of the high-speed signals dramatically by its broadband characteristics, however, it tend to increase the cost. In this paper, we report our developed quad small form-factor pluggable (QSFP) AOC utilizing cost-effective optical-module technologies. These are a unique structure using generally used flexible printed circuit (FPC) in combination with an optical waveguide that enables low-cost high-precision assembly with passive alignment, a lens-integrated ferrule that improves productivity by eliminating a polishing process for physical contact of standard PMT connector for the optical waveguide, and an overdrive technology that enables 100 Gb/s (25 Gb/s × 4-channel) operation with low-cost 14 Gb/s vertical-cavity surfaceemitting laser (VCSEL) array. The QSFP AOC demonstrated clear eye opening and error-free operation at 100 Gb/s with high yield rate even though the 14 Gb/s VCSEL was used thanks to the low-coupling loss resulting from the highprecision alignment of optical devices and the over-drive technology.
A 25-Gbps high-sensitivity optical receiver with a 10-Gbps photodiode (PD) using inductive input coupling has been
demonstrated for optical interconnects. We introduced the inductive input coupling technique to achieve the 25-Gbps
optical receiver using a 10-Gbps PD. We implemented an input inductor (Lin) between the PD and trans-impedance
amplifier (TIA), and optimized inductance to enhance the bandwidth and reduce the input referred noise current through
simulation with the RF PD-model. Near the resonance frequency of the tank circuit formed by PD capacitance, Lin, and
TIA input capacitance, the PD photo-current through Lin into the TIA is enhanced. This resonance has the effects of
enhancing the bandwidth at TIA input and reducing the input equivalent value of the noise current from TIA. We
fabricated the 25-Gbps optical receiver with the 10-Gbps PD using an inductive input coupling technique. Due to the
application of an inductor, the receiver bandwidth is enhanced from 10 GHz to 14.2 GHz. Thanks to this wide-band and
low-noise performance, we were able to improve the sensitivity at an error rate of 1E-12 from non-error-free to -6.5
dBm. These results indicate that our technique is promising for cost-effective optical interconnects.
In this work, we develop a simple and high-speed VCSEL driving technique with "virtual back termination" for optical
interconnect applications. For achieving compact and high-speed optical interconnects, an optical module with the flipchip
bonding structure is effective. To realize flip-chip mounting, the development of the VCSEL driving technique,
which can perform impedance matching with the transmission line, is a critical issue. Back termination has to be
implemented to reduce signal reflection via the transmission line. Additionally, back termination must have a simple dc
coupling. Introducing a virtual GND to the circuit ensures that these requirements are met. The virtual GND is made by a
dummy load connected to a complementary output and dc-coupled 50-Ω resisters between output and complementary
output. The dummy load has characteristics similar to the load VCSEL. As a result of the virtual GND, the resisters act
as the back termination. When we drove the VCSEL with this technique, clear eye opening without the reflectance
effects was obtained up to 28-Gb/s despite using a 10-cm transmission These results show that our driving technique is
suitable for high-speed optical interconnect applications.