Si photonics technology is promising for reducing the size and cost of optical transmitters because we can use mature Si-CMOS technologies to fabricate compact Si photonics devices on a large-scale Si wafer. For the optical transmitters, integration of lasers and silicon photonic devices is essential. Recently, heterogeneously integrated devices consisting of InP-based lasers and silicon Mach-Zehnder modulators (MZMs) have been developed, where the thickness of the Si waveguide in the laser gain section needs to be ~500 nm for index matching. On the other hand, silicon waveguide thickness between 200 and 300 nm is typically used in Si photonic devices; therefore, a Si thickness transition is necessary between the laser gain section and silicon MZMs. For changing the Si thickness, additional etching, deposition, or growth of Si layers is needed. However, these are not suitable solutions because device performance would be degraded by increasing the surface roughness and thickness variations of the Si waveguide.
In this work, we proposed a novel technique for integrating lasers and Si photonic devices without a Si thickness transition. We use a lateral current-injection membrane buried heterostructure (BH) as a laser gain section. This structure enables us to reduce the total thickness of the III-V region, resulting in the reduction of its effective refractive index. Therefore, the effective refractive index of the membrane BH laser can be matched to that of a 200-nm-thick Si waveguide, and the laser is suitable for integration with Si photonic devices.
High-capacity optical transmitters with reduced size, cost, and power consumption are required to meet growing bandwidth requirements of network systems. A high-modulation-efficiency Mach-Zehnder modulator (MZM) on an Si platform is a key piece of equipment for these transmitters. Si-MZMs have been widely reported; however their performance is limited by the material properties of Si. To overcome the performance limitations of Si MZMs, we have integrated III-V materials on Si substrate and developed a heterogeneously integrated III-V/Si metal oxide semiconductor (MOS) capacitor phase shifter for constructing ultra-high efficient MZM, in which the n-InGaAsP, p-Si, and SiO2 film are used for constructing the MOS capacitor. The fabricated MZM with the MOS capacitor exhibited a VπL of 0.09 Vcm and insertion loss of ~2 dB. 32-Gbps modulation of the MZM was also demonstrated.
A high-efficiency and low-loss Mach-Zehnder modulator on a Si platform is a key component for meeting the demand for high-capacity, low-cost and low-power optical transceivers in future optical fiber links. We report a III-V/Si MOS capacitor Mach-Zehnder modulator with an ultrahigh-efficiency phase shifter, which consists of n-type InGaAsP and ptype Si. The main advantage of this structure is a large electron-induced refractive index change and low free-carrier absorption loss of the n-type InGaAsP. The heterogeneously integrated InGaAsP/Si MOS capacitor structure is fabricated by using the oxygen plasma assisted bonding method. The fabricated device shows VπL of 0.09 Vcm, a value over three-times smaller than that of the conventional Si MOS capacitor Mach-Zehnder modulator, without an increase in the insertion loss. This clearly indicates that the proposed III-V/Si MOS capacitor Mach-Zehnder modulator overcomes the performance limit of the Si Mach-Zehnder modulator.
Optical interconnects are expected to reduce the power consumption of ICT instruments. To realize chip-to-chip or chip-scale
optical interconnects, it is essential to fabricate semiconductor lasers with a smaller energy cost. In this context, we
are developing lambda-scale embedded active-region photonic-crystal (LEAP) lasers as light sources for chip-scale
We demonstrated the first continuous-wave (CW) operation of LEAP lasers in 2012 and reported a record low threshold
current and energy cost of 4.8 μA and 4.4 fJ/bit at 10 Gbit/s in 2013. We have also integrated photonic crystal
photodetectors on the same InP chip and demonstrated waveform transfer along 500-μm-long waveguides. Although
LEAP lasers exhibit excellent performance, they have to be integrated on Si wafers for use as light sources for chip-scale
In this paper, we give a brief overview of our LEAP lasers on InP and report our recent progress in fabricating them on
Si. We bonded the InP wafers with quantum-well gain layers directly on thermally oxidized Si wafers and performed all
process steps on the Si wafer, including high-temperature regrowth. After this process modification, we again achieved
CW operation and obtained a threshold current of 57 μA with a maximum output power of more than 3.5 μW at the
output waveguides. An output light was successfully guided through 500 × 250-nm InP waveguides.