Compact, cost-effective monolithic photonic platform for telecom applications
The explosive increase in telecommunications traffic means we will soon need a huge number of photonic devices with very high functionality. The devices need to be compact, energy efficient, and cost-effective. Because of their complicated fabrication processes and fragility, conventional photonic technologies based on free-space optics and III-V semiconductors cannot meet these requirements. Indium phosphide and other III-V semiconductors can be neither compact nor cost-effective. And free-space optics consisting of discrete lenses, prisms, and mirrors require a large area and costly assembly.
On the other hand, silicon/germanium-based (Si/Ge) photonics technology can produce devices that are small and sturdy. Silicon is a semiconductor and is suitable for micro-fabrication. We can easily build compact electronic structures from it with optical waveguides and various active optical devices, such as optical modulators. Germanium is also a semiconductor. It absorbs IR light and generates photo-carriers, allowing us to construct photodiodes on silicon. Computers and mobile phones are everyday demonstrations of the robustness of Si/Ge devices. Silicon has a large refractive index, which means that photonic devices made of it can be quite small. And since the devices can be small, the driving power for the devices can also be small. Waveguide cores on a scale of 100nm and micrometer-order bending of silicon photonic wire waveguides provide us ultra-compact integrated photonic devices with fast operation speed.1
However, the hurdles to telecommunications applications of the silicon photonics platform are still very high. In telecommunications applications, where long-distance optical transmission is essential, photonic devices with a large dynamic range, low optical losses, and low polarization dependence are required. The present silicon photonic platform, however, cannot provide such high-performance photonic devices. In particular, the performance of passive photonic devices, such as wavelength filters, must be improved.
To overcome these obstacles, we developed a photonic platform on which silicon-, germanium- and silica-based photonic devices are monolithically integrated (see Figure 1). On this platform, silicon photonic wire waveguides with 100-nm-scale cores and a germanium layer grown on them are mainly used for dynamic and active devices, which require compactness and high operation speed. Silica waveguides are used for high-performance passive devices. Thanks to the large fabrication tolerance of silica-based waveguide devices, they can still work effectively with up to 10nm geometric error in the core of a medium-index silica waveguide. This contrasts with the mere angstroms of error that is tolerable in a silicon photonic wire. As a result, we can build passive devices with low loss, low crosstalk, and low polarization dependence.
We connected a silicon photonic wire waveguide and silica waveguide with a spot-size converter (SSC) using an inverse silicon taper with a 80-nm tip.2 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 silicon and germanium. Low-temperature silica deposition is also very important in the monolithic integration of photonic devices on electronics circuits. For this purpose, we developed silicon-rich silica (SiOx) film deposited by the electron-cyclotron-resonance plasma-enhanced chemical vapor deposition method.3 Using this technology, we deposited SiOx films while keeping the wafer temperature below 200°C, which did not damage other devices underneath the films. Since the refractive index of SiOx can be controlled from 1.46 to 1.72 by adjusting the deposition condition, this material can be applied as the core of various kinds of optical waveguides. Using this technology, we developed a SiOx waveguide with a 3μm-square core. The fabricated waveguides exhibit propagation loss of about 0.6dB/cm near 1550nm wavelength, which is low enough for making practical photonic devices on a small chip.
On this Si-Ge-silica monolithic photonic platform, we developed various photonic devices, such as SiOx-based arrayed-waveguide-grating (AWG) wavelength filters, silicon-based electrically-driven modulation devices, and germanium-based photodetectors. As mentioned previously, the SiOx-based AWG wavelength filters must exhibit high performance in optical loss, crosstalk, and polarization dependence. The performance of active/dynamic devices has also been improved in order to meet severe telecommunications standards. For example, the responsivity, frequency bandwidth, and polarization-dependent loss of fabricated germanium photodetectors (PDs) are 1A/W, ∼20GHz, and 1dB, respectively.4 These values are almost as good as those of indium-phosphide-based devices. In the silicon-based variable optical attenuator (VOA), a kind of modulation device, both optical loss and polarization dependent loss have been reduced to less than 1dB.5
These photonic devices can be monolithically integrated as AWG-PDs and AWG-VOAs.6, 7 Integration of a 16-channel AWG and germanium PD (Ge-PD) is shown in Figure 2(a).6 We designed the AWG for wavelength demultiplexing of optical signals near 1550nm wavelength. The channel separation is 1.6nm. Each output of the AWG is connected to a Ge-PD via an SSC. The photocurrent spectrum of the Ge-PDs connected to the AWG is shown in Figure 2(b). We can see a beautiful multichannel wavelength filtering function with crosstalk of −23dB, which is significantly lower than those based on silicon photonic wire waveguides. An eye pattern of one output channel of the device for a 22-Gbps optical data input is shown in the inset of Figure 2(b). The clear eye pattern guarantees a high-speed optical receiving operation with a low bit error rate.
These photonic circuits based on the Si-Ge-silica photonic platform are compact, robust, and compatible with complementary metal-oxide-semiconductor (CMOS) fabrication. The size of the integrated device is typically less than 1cm square, which will fit in a USB memory stick. The CMOS compatible fabrication process simplifies the assembly and reduces cost. The platform can also endure the flip-chip bonding of electronic circuits, and electronic circuits have already been integrated on a small photonic chip, as shown in Figure 3.8
The optical transmission modules for these devices require wavelength filters, detectors, modulators, and light sources on a single monolithic chip. So far, we have integrated just two per chip, either a filter-detector or filter-modulator or modulator-detector. The next step will be to integrate all of them together. When we can do that, we will be able to construct compact, highly functional, high-density photonic-electronic integration devices for sustainable growth of the telecommunications industries.
Microsystem Integration Laboratories
Koji Yamada is a group leader of the Nano Silicon Technology Research Group. He has worked with synchrotron light sources at NTT since 1988. His current research focuses on silicon photonics.
