Yuriko Maegami

Low-loss and low wavelength-dependence vertical interlayer transition for 3D silicon photonics.

This paper presents optimized design and measurement results for a low-loss broadband vertical interlayer transition (VIT) device located between lower and upper Si nano-photonic waveguides. The device comprises the lower c-Si taper, the upper a-Si:H taper, and a wide and thin SiON secondary core with a 0.6-μm-thick SiO₂ interlayer. The device structure facilitates the low loss VIT, giving insertion losses of 0.87 and 0.79 dB for quasi-TE and TM modes, respectively, at 1550 nm. Also, the evanescent coupling nature of the VIT device renders it wavelength- and polarization-insensitive, leading to loss variation of within 0.5 dB in the C-band.

Spot-size converter with a SiO(2) spacer layer between tapered Si and SiON waveguides for fiber-to-chip coupling.

We experimentally demonstrate low-loss and polarization-insensitive fiber-to-chip coupling spot-size converters (SSCs) comprised of a three dimensionally tapered Si wire waveguide, a SiON secondary waveguide, and a SiO(2) spacer inserted between them. Fabricated SSCs with the SiO(2) spacer exhibit fiber-to-chip coupling loss of 1.5 dB/facet for both the quasi-TE and TM modes and a small wavelength dependence in the C- and L-band regions. The SiON secondary waveguide is present only around the SSC region, which significantly suppresses the influence of the well-known N-H absorption of plasma-deposited SiON at around 1510 nm.

Strip-loaded waveguide-based optical phase shifter for high-efficiency silicon optical modulators

We propose a novel silicon optical phase shifter structure based on heterogeneous strip-loaded waveguides on a photonic silicon on insulator (SOI) platform. The features of an etchless SOI layer and loaded strip would enhance the performance and uniformity of silicon optical modulators on a large-scale wafer. We implemented the phase shifter by loading an amorphous silicon strip onto an SOI layer with a vertical PN diode structure. Compared to the conventional lateral PN phase shifter based on half-etched rib waveguides, this phase shifter shows a >1.5 times enhancement of modulation efficiency and provides >20  GHz high-speed operation.

Completely CMOS compatible SiN-waveguide-based fiber coupling structure for Si wire waveguides.

For Si wire waveguides, we designed a highly efficient fiber coupling structure consisting of a Si inverted taper waveguide and a CMOS-compatible thin SiN waveguide with an SiO2 spacer inserted between them. By using a small SiN waveguide with a 310 nm-square core, the optical field can be expanded to correspond to a fiber with a 4.0-μm mode field diameter. A coupled waveguide system with the SiN waveguide and Si taper waveguide can provide low-loss and low-polarization-dependent mode conversion. Both losses in fiber-SiN waveguide coupling and SiN-Si waveguide mode conversion are no more than 1 dB in a wide wavelength bandwidth from 1.36 μm to 1.65 μm. Through a detailed analysis of the effective refractive indices in the coupled waveguide system, we can understand mode conversion accurately and also derive guidelines for reducing the polarization dependence and for shortening device length.

Demonstration of photonic digital-to-analog conversion (DAC) utilizing a single silicon Mach-Zehnder modulator (Conference Presentation)

Digital-to-analog converters (DAC) are indispensable functional units in optical signal transmission and processing. The photonic DAC that converts electrical digital signals to an optical analog one will offer advantages in lowering system complexity, power, and cost. Especially with the required bandwidth increasing, it could mitigate the problems faced by its electrical counterparts in dealing with higher sampling rate. Achieving such a photonic DAC in silicon photonics is promising due to the integration capability of both electronics and photonics and large scale DAC-based photonic circuits can be further realized for on-chip optical signal processing. In this work, we demonstrate 2-bit D/A conversion for the simple proof of concept utilizing only one single silicon Mach-Zehnder modulator (MZM), which is much simpler than previously reported segmented MZM and microring resonator based DACs. One-single MZM capable of 2-bit DAC merits future higher bit resolution design and meanwhile guarantees wide spectral bandwidth. One arm of MZM is used for the MSB bit input, while the other for the LSB, both of them being accomplished by only one phase shifter. For each bit input, we utilize amplitude modulation, instead of phase modulation, by applying the carrier injection induced absorption in the phase shifters. For principle, by setting different bias points for two phase shifters, we can produce the condition at which the amplitude weighting ratio of LSB to MSB is 1/2 in order to obtain the linear amplitude DAC output. In other words, the output optical field has the analog linear amplitude levels (0,1,2,3) which corresponds to the power levels of (0,1,4,9) at the full extinction condition. For fabrication, this device was fabricated on a 220-nm SOI wafer with a 3-m buried-oxide layer at the AIST SCR 300-mm CMOS foundry. The 430-nm-wide fully etched channel waveguide was used for the components except for the pn phase shifter which adopted the shallow-etched rib waveguide structure with a slab thickness of about 110 nm and a width of about 600 nm. The doping density in the weak p/n regions was about 1.61018 cm-3. This MZM was arm-balanced with 2-mm-long phase shifters, adopting GSGSG configuration. Two 50- terminators were also integrated on-chip at the ends of two signal electrodes. For measurement, a two-channel pulse pattern generator produced bit sequences at various frequencies for both MSB and LSB which was applied to the signal electrodes through bias-tees and high-speed probes. The 1.55-m cw light at TE polarization was coupled into the chip via a tapered fiber and the optical output passed an EDFA and a bandpass filter and then was sent to a high-speed oscilloscope for examining DAC analog output. Using this device, we successfully achieved correct D/A conversions with the sampling rates up to 3 GS/s with <1 V peak-to-peak voltages. Note that this speed can be further enhanced to <10 GS/s by constructing the pn phase shifter into a MZM structure or replacing it with a SiGe electro-absorption modulator. In summary, this work verified the feasibility to realize high-sampling-rate 2-bit D/A conversion utilizing a single silicon MZM modulator.

Silicon traveling-wave Mach–Zehnder modulator under distributed-bias driving

The silicon traveling-wave (TW) Mach-Zehnder modulator (MZM) is one of the most important devices in silicon photonic transceivers for high-speed optical interconnects. Its phase shifter utilizes carrier depletion of pn diodes for high speed, but suffers low modulation efficiency. Extensive efforts have been made on pre-fabrication optimizations, including waveguides, doping, and electrodes to enhance high-frequency modulation efficiency. Instead, we here propose an adaptive post-fabrication distributed-bias driving method that enables 20%∼30% high-frequency efficiency enhancement at both 10 and 25 Gbps without doing any optimizations for a silicon TW-MZM. This method explores the bias nonlinearity of index modulation which, to the best of our knowledge, is utilized for the first time in driving silicon modulators to improve the efficiency. We demonstrated the viability of this adaptive driving concept to achieve better performance, and this Letter could open new avenues for silicon traveling-wave modulator design and performance trade-off.

Novel adaptive driving method enabling better high-frequency performance for silicon Mach-Zehnder modulator

We propose an adaptive distributed-bias driving method for silicon travelling-wave Mach-Zehnder modulators and achieved ∼25% modulation efficiency enhancement at both 10 and 25 Gb/s with <3.5 Vpp, without optimizing horizontal PN-diodes. This method also suggests a new modulator scheme allowing efficiency improvement and design flexibilities.

High-efficiency silicon Mach-Zehnder modulator with vertical PN junction based on fabrication-friendly strip-loaded waveguide

We demonstrate a vertical p-n junction silicon Mach-Zehnder modulator constructed with hydrogenated amorphous silicon strip-loaded waveguides on a flat SOI platform. A 3-mm-long phase shifter shows 0.80- to 1.86-Vcm modulation efficiency, 7.3- to 16.9-dBV loss-efficiency product, 3-dB bandwidth of 17 GHz, and 25-Gb/s operation.

High-efficiency strip-loaded waveguide based silicon Mach-Zehnder modulator with vertical p-n junction phase shifter

We demonstrate a silicon Mach-Zehnder modulator (MZM) based on hydrogenated amorphous silicon (a-Si:H) strip-loaded waveguides on a silicon on insulator (SOI) platform, which can be fabricated by using a complementary metal-oxide semiconductor (CMOS) compatible process without half etching of the SOI layer. Constructing a vertical p-n junction in a flat etchless SOI layer provides superior controllability and uniformity of carrier profiles. Moreover, the waveguide structure based on a thin a-Si:H strip line can be fabricated easily and precisely. Thanks to a large overlap between the depletion region and optical field in the SOI layer with a vertical p-n junction, the MZM provides 0.80- to 1.86-Vcm modulation efficiency and a 12.1- to 16.9-dBV loss-efficiency product, besides guaranteeing a 3-dB bandwidth of about 17 GHz and 28-Gbps high-speed operation. The αVπL is considerably lower than that of conventional high-speed modulators.

High-efficiency and high-speed silicon optical modulator based on amorphous silicon loaded SOI waveguide

We proposed a silicon optical modulator using an amorphous silicon loaded SOI waveguide without forming a shallow rib waveguide by etching. This novel modulator shows a >1.5-times efficiency enhancement in comparison to the lateral PN shallow-rib modulator while having a >20 Gbps modulation capability.