For a multi mode fiber optical link, a high speed silicon photonics receiver based on a highly alignment tolerant
vertically illuminated germanium photodiode was developed. The germanium photodiode has 20 GHz bandwidth and
responsivity of 0.5 A/W with highly alignment tolerance for passive optical assembly. The receiver achieves 25 Gbps
error free operation after 100 m multi mode fiber transmission.
Germanium light-emitting devices on silicon for very-short-reach interconnect were investigated theoretically and experimentally. Our approach to enhance light emission is by applying process-induced strain to the germanium active layer. According to first-principles calculations, larger optical gain in germanium with lower carrier density is obtained at a larger tensile strain. In addition to the thermally induced strain caused by the difference of the thermal expansion ratios, process-induced stress was applied to the germanium active layer by fabricating a SiN stressor on it. As for practical light-emitting devices, a laterally injected light-emitting device was fabricated and tested. In the case of this device, the current is laterally injected into the germanium active layer through a thin silicon layer. In this device structure, mode loss caused by free carrier absorption is expected to be small, since the guided mode overlaps slightly with the heavily doped silicon layer. The electroluminescence (EL) property of the device showed a superlinear increase in integrated EL intensity with increasing injection current, indicating that direct recombination is enhanced by L-valley filling. An increase in intensity and red-shift of the EL peaks of the device with a SiN stressor indicate that additional tensile strain was successfully applied to the germanium active layer.
We have developed an ultra compact dispersion compensator based on multiple one dimensional coupled-defect-type
photonic crystals, utilizing large optical group velocity dependence on the wavelength without polarization mode
dependence. The photonic crystal of the compensator consists of a SiO2/Ti2O5 multi-layer thin-film structure and SiO2
defect layers and was designed for a 1.55-μm, 40-Gbit/s optical communication system. The thin-film structure is
substrate-free, which enables the compensator to be small, that is, a 1.4-mm-edge cube. To obtain a large group-velocity
difference, 60 substrate-free films are stacked to form the compensator. The passband is 2 nm, and the group delay time-difference
within the band is more than 100 ps. A 40-Gbit/s non-return-to-zero optical transmission experiment was
carried out with the compensator, demonstrating dispersion-compensation operation over a 10-km standard single-mode
fiber, which corresponds to dispersion of 170 ps/nm.
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