Silicon-Germanium-Silica Monolithic Photonic Integration Platform for High-Performance Optical Data Communication Systems
Authors: Yamada Koji , Tsuchizawa Tai , Nishi Hidetaka , Kou Rai , Hiraki Tatsurou , Takeda Kotarou , Fukuda Hiroshi , Okazaki Kota , Ishikawa Yasuhiko , Wada Kazumi , Yamamoto Tsuyoshi 
Published: February 2020

Recently, data communication systems are now facing an explosive traffic increase. In order to cope with such an explosive traffic increase, densely-integrated photonic circuits, which would provide compact, high-functionality, energy-efficient and cost-effective network modules, are required. The silicon (Si) photonic platform, which enables monolithic integration of compact photonic devices made of Si and germanium (Ge) on Si wafer, is one of the breakthrough technologies that satisfy these requirements. Waveguide cores on a scale of 100 nm and micrometer-order bending of silicon wire waveguides provide us ultra-compact integrated photonic devices with fast operation speed.
However, the hurdles to practical applications of the Si photonic platform are still very high. In middle and long distance applications, for example, photonic devices with a large dynamic range, low optical losses, and low polarization dependence are required. The present Si photonic platform, however, can not provide such high-performance photonic devices. This is because the device performances are extremely sensitive to geometric errors. For example, the geometric tolerance required for passive photonic devices, such as wavelength filters for wavelength division multiplexing (WDM) systems, is far below that of the errors in the present fabrication technology.
To overcome these obstacles, we have developed a photonic platform on which Si-, Ge- and silica-based photonic devices are monolithically integrated[1]. Figure 1 shows the conceptual structure of the Si-Ge-silica monolithic photonic platform. On this platform, Si wire waveguides and a Ge layer grown on them are mainly used for dynamic and active devices, which require compactness and high operation speed, while silica waveguides are used for high-performance passive devices. Thanks to the large fabrication tolerance of silica-based waveguide devices, we can build passive decvices with low loss, low crosstalk and low polarization dependece. The silica waveguide can also serve as an interface to external optical fiber. In the Si-Ge-silica monolithic integration, it is important to deposit index-controllable silica at a low temperature so as not to damage active/dynamic devices based on Si and Ge.
On this platform, we have developed various photonic devices, such as silica-based arrayed-waveguide-grating wavelength filters (AWG), Si-based electrically-driven modulation devices, and Ge-based photodetectors. The silica-based AWG exhibits high performance in optical loss, crosstalk and polarization dependence. Moreover, the performance of active/dynamic devices has also been improved in order to meet practical applications. For example, the responsivity, frequency bandwidth and polarization-dependent loss of fabricated Ge photodetectors (PDs) are almost comparable to those of indium-phosphide-based high-performance devices.
These photonic devices can be monolithically integrated on a Si chip. As an example, Fig. 2 shows an integrated AWG-PD for a 16-channel WDM receiver with a 1.6-nm channel spacing. Inter-channel crosstalk of the device is -22 dB, which is significantly lower than those based on silicon wire waveguides. Each channel works at a data rate of 25 Gbps, and total receiver capacity reaches 400 Gbps.
Features of the photonic circuits based on the Si-Ge-silica photonic integration platform are compactness, CMOS compatible fabrication and robustness. The size of the integrated device is typically less than 1-cm square, which can be installed in a small package. The CMOS compatible fabrication process simplifies the assembly processes and reduces fabrication cost. The platform is also endurable against the flip-chip bonding of electronic circuits. For example, we have already mounted 4-channel high-speed electronic amplifiers for PDs on a photonic chip consisting of 4-channel wavelength filters and PDs.
Thus, using the Si-Ge-silica monolithic photonic platform, we can construct compact, highly-functional high-density photonic-electronic integration devices for sustainable growth of data communications industries.
[1] T. Hiraki et al., IEEE Photonics Journal, vol. 5, 4500407 (2013).