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Optoelectronics & Communications

Toward high-speed, low-cost, on-chip silicon optical interconnects

Improved design and simpler fabrication of a grating coupler results in fast, cost-efficient, silicon-based micro-ring resonator modulators and switches.
19 January 2011, SPIE Newsroom. DOI: 10.1117/2.1201012.003309

Computer performance is limited by the bandwidth and power consumption of the communication between processors and memory. As a result, chip-scale interconnections (ICs) that offer speed-of-light communication within and between microchips will have a profound impact on the speed of supercomputers. Silicon photonics offers low-cost, large-scale, and monolithic optoelectronic integration.1,2 Microelectronic ICs could be used to build on-chip, optical communication networks with TB/s bandwidth and fJ/bit power consumption. This, in turn, could lead to a supercomputer offering exascale (1018) floating point operations per second, which is ∼1,000 times faster than today's fastest supercomputers.

Silicon-based optical interconnects require compact, high-speed electro-optic modulators to translate the electrical signals into optical intensity signals and switches to control the propagation path of the optical signals. Modulators based on silicon-on-insulator (SOI) microresonators are of vital importance owing to their unique properties of wavelength selection and internal field enhancement. Modulators and switches must also be compatible with complementary metal-oxide-semiconductor (CMOS) manufacturing processes that are used to fabricate on-chip optical waveguides and photonic devices on high-quality silicon wafers. Recent progress on SOI microresonators has enabled high quality-factor (Q up to 104–106) micro-rings and microdisks within a micron-scale footprint. The high optical power confinement in the microresonators makes the power output very sensitive to the refractive index change and therefore helps to improve the modulation and switching speed by an order of magnitude, as well as to significantly reduce power consumption.2–5

However, most of these devices are limited by the traditional in-plane coupler, which either requires expensive CMOS processes below 0.13μm or complex preparation, such as sample cleaving or facet polishing. To reduce the costs of fabrication, measurement, and integration, we have developed cost-efficient, wafer-level-testable SOI micro-ring modulators and switches with high operation speed. We designed the devices to be compatible with 0.25μm-CMOS processes, with only a third of the fabrication cost of a 0.13μm-CMOS node. All of the device dimensions, including the waveguides, couplers, and active doping areas, are designed to be >300nm. We integrated CMOS-compatible grating couplers into the micro-rings, and this out-of-place coupling configuration enabled wafer-level online tests without any wafer cleaving and facet polishing (making them comparable to micro-electronic tests). We designed the etching depth of the grating coupler to be the same as that of the waveguides, so that it could be be simultaneously fabricated with the micro-ring using the same lithography and etching processes.

The SOI micro-ring modulator comprises aluminum ground-signal-ground (GSG) pads—the golden regions in Figure 1(a)—from which the electrical driving signals are loaded to the modulator. We fabricated the passive waveguides on an SOI wafer (with a 340nm thick silicon layer) using electron-beam lithography and inductively-coupled-plasma etching processes: see Figure 1(b). A p-i-n diode was embedded in to the ring waveguide by phosphorus and boron implantations: see Figure 1(b) and (c). When the p-i-n diode is forward biased, large numbers of carriers are injected into the micro-ring, resulting in a refractive index change of around 10−3 and a significant resonance blue shift. As a trade-off between the operation speed and the micro-ring's Q factor, the intrinsic width is designed as ∼1μm.

Figure 1.(a) Top-view image of the notch-type micro-ring modulator with ground-signal-ground (GSG) pads. (b) Scanning electron microscope image of the passive micro-ring. (c) Cross-section of the p-i-n diode embedded in the ring waveguide. Al: Aluminum. Si: Silicon.

We tailored the driving signal for high-speed operation. We generated pre-emphasis signals at each transition edge to accelerate the carrier injection and depletion speed while avoiding the over-injection of extra carriers. We did this by combining two channels of non-return-zero signals.6 We also used a microwave delay controller to freely adjust the pulse widths, which we found easier than an impulse generator network.7 In this way we demonstrated 10Gbit/s modulated driving signals5 and output (see Figure 2).

Figure 2.(a) Waveform of 24-bit electrical driving signal at 10Gbit/s with pre-emphasis pulses occurring at each transition edge. (b) Corresponding 10Gbit/s optical non-return-zero signal output from the modulator.5

We have also demonstrated a sub-nanosecond electro-optical switch based on the SOI micro-ring and embedded p-i-n diodes (see Figure 3). We found that an asymmetric dual-coupled micro-ring could greatly enhance the through-port extinction to >40dB,7 so we employed this asymmetric configuration to suppress the in-band crosstalk of the switch. After optimization, we measured ≤ -23dB crosstalk, indicating a 10dB improvement from the symmetric micro-ring.8

Figure 3.Top view microscope image of the 20μm-radius micro-ring 1×2 switch.

We accelerated the switch with pre-emphasis signals again and enlarged the widths of the forward and reverse pulses to 500ps for lower driving voltages: see Figure 4(a). We optimized the square-wave voltage levels to ensure large extinction ratios at the through port and the drop port. Finally, with the excitation of these driving signals, we obtained through/drop-port extinction ratios of 25 and 13dB while the on/off switch times were only 300 and 380ps, respectively: see Figure 4(b) and (c).8 To the best of our knowledge, this is the fastest silicon-based microresonator electro-optical switch yet demonstrated.

Figure 4.Dynamic optical responses of the micro-ring switch driven by pre-emphasis signals.8(a) Measured waveform of the pre-emphasis signal. (b) Measured optical response at the through port. (c) Measured optical response at the drop port. dB: Decibel.

For both of the aforementioned micro-ring devices, we integrated grating couplers at all the input and output ports for fiber-to-waveguide light coupling (see Figure 5). By using grating couplers, we were able to carry out all of the above characterizations by wafer-level measurements. We demonstrated high coupling efficiency of >50% by coating an index buffer layer on the gratings.9 Our next step is to develop 3D on-chip photonic ICs by hybrid bonding the laser source and photodetector on this SOI photonic chip. As a first step, a III-V photodetector with ∼0.8A/W responsivity has been successfully flip-chip bonded on the grating coupler.10 We hope to develop an on-chip silicon-based optical IC system consisting of laser, modulator, and photodetector for future high-performance computing.

Figure 5.Electron microscope images of silicon-on-insulator grating couplers integrated with the active micro-ring devices. (a) Top view. (b) Oblique view.

Xi Xiao, Haihua Xu, Yingtao Hu, Zhiyong Li, Yuntao Li, Yude Yu, Jinzhong Yu
State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors Chinese Academy of Sciences
Beijing, China

Xi Xiao received his PhD in 2010 from the Institute of Semiconductors of the Chinese Academy of Sciences. He is currently an assistant professor. His research interests include silicon-based optical modulators and switches, microresonator devices, nano/micro-scale fabrication, and integration technologies.

Yude Yu has been a professor and the deputy director at the institute since 2003. He graduated in 1977 from the Department of Physics at the University of Science and Technology of China. His present interests include silicon photonics, photoelectric biosensors, and the development of gene sequencing equipment.

Jinzhong Yu has been a professor at the institute since 1994. He graduated from the University of Science and Technology of China in 1965 and received his doctoral degree in electrical engineering from Osaka University (Japan) in 1991. He has undertaken research on laser diodes, detectors and waveguide devices since 1965 and has published more than 180 papers. His present research concentrates on silicon photonics and integrated optoelectronics.