Yutaka Urino

Silicon photonics for next generation system integration platform

New semiconductor technologies such as many-core processors and 3D memories are being researched in order to overcome the limitations of electronics in the near future. Here, we first discuss some drawbacks of current technologies, and then show that silicon photonics will solve those interconnection problems. Next, we describe our studies toward realizing a system integration platform based on photonics and electronics convergence, and show that an optical interposer is the most efficient way to cope with the various problems that a purely electronic system may encounter. Our recent advances in silicon photonic devices are also described, and their integration into the hybrid interposer is reported through an early prototyping result. Finally, a surface mounted components approach for silicon photonics technology is discussed, which may prove useful in the computer and communication markets.

First demonstration of high density optical interconnects integrated with lasers, optical modulators, and photodetectors on single silicon substrate

Optical interconnects integrated with lasers, silicon optical modulators and germanium photodetectors on a single silicon substrate were demonstrated for the first time. A 5 Gbps line bit rate and 3.5 Tbps/cm2 transmission density were achieved.

A Hybrid Integrated Light Source on a Silicon Platform Using a Trident Spot-Size Converter

This paper reports a hybrid integrated light source fabricated on a Si platform using a spot-size converter (SSC) with a trident Si waveguide. Low-loss coupling for 1.55 μm and 1.3 μm wavelengths was achieved with merely the simple planar form of a Si waveguide with no use of complicated structures such as vertical tapers or an extra dielectric core overlaid on the waveguide. The coupling loss tolerance up to a 1 dB loss increase was larger than the accuracy of our passive alignment technology. The coupling efficiency was quite robust against manufacturing variations in the waveguide width compared with that of a conventional SSC with an inverse taper waveguide. A multi-channel light source with highly uniform output power and a high-temperature light source were fabricated with a 1.55 μm quantum well laser and a 1.3 μm quantum dot laser, respectively. The integration scheme we report can be used to fabricate light sources for high-density, multi-channel Si optical interposers.

High density hybrid integrated light source with a laser diode array on a silicon optical waveguide platform for inter-chip optical interconnection

A hybrid integrated light source was developed with a novel configuration in which a laser diode (LD) array was mounted on a silicon optical waveguide platform for inter-chip optical interconnection. An output power uniformity of 1.3 dB was observed at the 13-channel integrated light source with 30-µm pitch. Use of a SiON waveguide with a spot size converter resulted in an optical coupling loss of 1 dB between an LD and a SiON waveguide. The integrated light source including 52 output ports was demonstrated to reduce the footprint per channel. These integrated light sources are attractive candidates for use with photonic integrated circuits for high density optical interconnection.

First Demonstration of Athermal Silicon Optical Interposers With Quantum Dot Lasers Operating up to 125 °C

We previously proposed a photonics-electronics convergence system to solve bandwidth bottleneck problems among large-scale integrations (LSIs) and demonstrated a high bandwidth density with silicon optical interposers at room temperature. For practical applications, the interposers should be usable under high-temperature conditions or rapid temperature changes so that they can cope with the heat generated by the mounted LSIs. We designed and fabricated athermal silicon optical interposers integrated with temperature-insensitive components on a silicon substrate. An arrayed laser diode (LD) chip was a flip-chip bonded to the substrate. Each LD had multiple quantum dot layers with a 1.3-μm lasing wavelength. The output power was higher than 10 mW per channel up to 100 °C. Silicon optical modulator and germanium photodetector (PD) arrays were monolithically integrated on the substrate. The modulators were structured as symmetric Mach-Zehnder interferometers, which were inherently insensitive to temperature and wavelength. The phase shifters composed of p-i-n diodes were stable against temperature change under a constant bias-current condition. The PD photocurrent was also temperature insensitive, and the photo-to-dark current ratio was higher than 30 dB up to 100 °C. We achieved error-free data links at 20 Gbps and a high bandwidth density of 19 Tbps/cm2 operating up to 125 °C without adjusting the LDs, modulators, or PDs. The interposers are tolerant of the heat generated by the mounted LSIs and usable over the extended industrial temperature range without complex monitoring or feedback controls. The bandwidth density is sufficient for the needs of the late 2010s.

A novel spot size convertor for hybrid integrated light sources on photonics-electronics convergence system

We have developed a spot size converter (SSC) with a trident silicon (Si) waveguide for the hybrid integrated light source in a photonics-electronics convergence system. Low loss coupling was achieved with just the simple planar form of a Si waveguide with no use of complicated structures such as vertical tapers or extra dielectric core overlaid on the waveguide. The coupling loss was 2.3 dB and the tolerance of misalignment up to a 1-dB loss increase was about ± 0.9 μm. We also confirmed that the coupling efficiency was quite robust against manufacturing fluctuations of the waveguide width compared with a conventional SSC with an inverse taper waveguide. We were able to fabricate a 52-channel light source with highly uniform output power by using the proposed trident SSC.

Holographic Optical Element with Analyzer Function for Magneto-Optical Disk Head

A new holographic optical element, made of a birefringent LiNbO3 substrate, has been developed for magneto-optical, disk heads. The element includes the analyzer function required for magneto-optical signal detection, in addition to focusing and tracking error signal detection functions. The analyzer function was realized by introducing a birefringent grating structure, using proton exchange method. More than 15dB extinction ratios were obtained for the fabricated element.

Ultra-small one-chip color-less multiplexer/demultiplexer using silicon photonic circuit

Using silicon waveguides, an arrayed waveguide grating and switches are integrated into one chip. Low-power and fast-speed reconfigurability of optical paths within a 2-mm device is demonstrated.

Optical interconnect technologies for high-speed VLSI chips using silicon nano-photonics

Optoelectronic and electrooptic elements are integrated on VLSI chips. The junction capacitance of a nano-photodiode is extremely low (<10aF), which permits a high load resistance to be used, resulting in higher output voltage at high frequencies. A ceramic Pb(,ZrTi)O3 film with average crystallite diameter below 20nm has a high electro-optical coefficient (>150pm/V) suitable for on-chip modulators. This paper introduces a new approach for realizing high-speed optical interconnects on silicon chips. This concept uses nano-photodiodes on silicon with extremely low parasitic capacitance (less than 10aF) enabling robust communication at very high frequencies. The results demonstrate 5GHz clocking with the promise of up to 20GHz. The authors will also discuss how the silicon nano-photodiode can be used for wavelength-division multiplexing and low-voltage electro-optic modulators for on-chip and off-chip optical communications

Silicon photonic-wire waveguide devices

Silicon photonic-wire waveguide is one of the most promising platforms in constructing compact optical devices, since the waveguide can be bent with a radius of less then several microns. Recently, we have demonstrated various optical devices based on silicon photonic-wire waveguides, which include a directional coupler, a tunable optical add-drop multiplexer, and some ultra-compact 1 x N optical switches. The optical coupling length of the directional coupler was just around 10 microns, due to its high coupling efficiency. The tunable optical add-drop multiplexer was constructed with Bragg grating waveguides. It was about 700-μm-long, and was controlled through thermo-optic effect. The maximum center-wavelength shift of the tunable optical add-drop multiplexer was 6.6 nm, which was obtained at a tuning power of 0.82 W. The 1 x N optical switches were Mach-Zehnder interferometer types and were also thermally controlled. The 1 x 2 switch was compact with a footprint of 85 x 30 µm2. Its maximum extinction ratio exceeded 30 dB. The switching power and switching time was about 90 mW and 100 μsec, respectively. The 1 x 4 optical switch was constructed based on the 1 x 2 switch. Its operation was successfully demonstrated. The 1 x 4 optical switch was believed to be the smallest switch in the world. A 1 x 8 optical switch was also demonstrated with its switching operations. Further, we are fabricating a compact packaged switch module with a size of 15 x 8 x 5 (height) mm3, which includes a 1 x 4 optical switch and the input and output fiber couplers assembly.