Tai Tsuchizawa

Monolithic Integration of Silicon-, Germanium-, and Silica-Based Optical Devices for Telecommunications Applications

This paper presents our recent progress with the integration of silicon (Si) photonic devices for optical telecommunications. To integrate Si wire waveguides, germanium (Ge) photodetectors (PDs) and silica waveguides, we have developed processes for the selective epitaxial growth of Ge on a Si waveguide core and for the low-temperature deposition of silica waveguide film and introduced spot size converters (SSCs) for coupling Si-wire and silica waveguide with low loss. Using these processes and SSCs, we have managed to monolithically integrate Si variable optical attenuators (VOAs) and Ge PDs, and Si VOAs and a silica arrayed waveguide grating (AWG). In the integrated VOA-PD, the Ge PD accurately detects the attenuation of light power in the Si VOA. The 3-dB cutoff frequency in VOA-PD synchronous operation is around 100 MHz, which is limited by the VOA. The integrated VOA-AWG provides high-speed power-level adjustment independently in every channel of the AWG with a response time of 15 ns. These integrated Si photonics devices exhibit sufficient performance for application to future telecommunications systems that combine WDM and burst-mode packets.

SOI-based photonic crystal line-defect waveguides

We have experimentally demonstrated single-mode light-wave transmission and tunable waveguiding characteristics in photonic crystal (PC) waveguides constructed on a silicon-on-insulator (SOI) substrate as is most likely to be used for the a large scale integration of photonic circuits. Although off-plane diffractive leakage has been a serious problem in SOI-PC waveguides, we have overcome this problem in our narrow line-defect and phase-shifted-hole line-defect waveguide structures. These devices were developed through intensive theoretical studies on PC line-defect waveguieds. We have also demonstrated low-loss mode profile converter that will enable efficient connection between conventional silica-based waveguides and PC line-defect waveguides. The converter features an inversely-tapered silicon wire waveguide with an ultra-thin tip constructed on an SOI substrate. In our experiments, this converter proved capable of coupling loss as low as 0.5dB per conversion. These SOI-based devices represent an important step towards practical large-scale integrated photonic crystal circuits.

Silicon wire waveguides and their applications for microphotonics devices

We report our recent progress in a Si wire waveguides, which promises size reduction and high-density integration of optical circuits. The application to functional devices and their nonlinear optical effects are also discussed.

Functional components in SOI photonic crystal slabs

We study various types of two-dimensional photonic crystal (PhC) waveguides (WG) on silicon-on-insulator (SOI) substrates for future photonic integrated circuit (PIC) applications. One of our goals is to realize a low-loss single-mode PhC WG. Off-plane diffractive leakage above the cladding layer light line is a serious problem in SOI-based PhC-WGs. We overcame this problem in width-varied line defect waveguides whose core-widths are changed by sliding PhC domains or deforming holes beside the waveguides. Narrow-core WGs have a wide transmission band below the cladding layer light line, and wide-core WGs can greatly suppress the diffractive leakage even above the cladding layer light line, and both types have a very low propagation loss. Another goal is to achieve a highly efficient coupling between SOI-based PhC WGs and single-mode fibers (SMFs). Normally, the loss of such a coupling system is very large, i.e. over 20 dB, because of the quite different mode profiles of the WGs and SMFs, and this loss is an obstacle to the development of PhC-based devices. Our system achieves a very small mode-profile-conversion loss of about 3-4 dB/connection from 1500 to 1600 nm wavelength.

Ultrasmall and Wideband Polarization Rotator Based on Silicon Wire Waveguides

We propose an ultrasmall and wideband polarization rotator consisting of a short silicon wire and an off-axis silicon oxinitride waveguide. The simulated extinction ratio is found to be over 20 dB for the C-band.

Design of Spontaneous Emission Enhancement Based on Si Ring Resonators

Electronics and Photonics convergence on Si CMOS platform will breakthrough the interconnection bottleneck in Si LSIs, such as saturation of clocking speed, multilevel masks, and so on. However, there has been a missing element on a chip, i.e., light emitters. III-V based emitters have been around but do not have compatibility with CMOS technologies. On the other hand, Si-based emitters remain a poor efficiency of light emission because of its band structure. Many of the works have therefore intended to employ spontaneous emission enhancement in terms of the Purcell’s effect. The present paper systematically shows the Purcell’s effect based on resonator structures is actually independent of the resonator volume. First let us recall the definition of quality factor Q of resonators [1]. Q is a measure of how well a resonator retains its energy, and is defined as the ratio of the energy (Ws) stored in the resonator to the loss of the energy from the resonator per unit radian of the cycle of the wave, where Ws can be expressed by: Ws = 1 2 e ⋅ E dV

Integrated photonic devices based on silicon photonic wire waveguide platform

Silicon photonic wire waveguides, featuring very strong optical confinement and compatibility with silicon electronics, provide a compact photonic platform on which passive, dynamic, and active photonic devices can be integrated. We have already developed a low-loss waveguide platform and integrated various photonic devices. For passive devices, we have developed polarization-independent wavelength filters using a monolithically integrated polarization diversity circuit, in which waveguide-based polarization manipulation devices are implemented. The polarization-dependent loss of a ring resonator wavelength filter with polarization diversity is less than 1 dB. For dynamic devices, we have developed compact carrier-injection-type variable optical attenuators (VOAs). The length of the device is less than one millimeter, and the response time is nanosecond order. The device has already been made polarization independent. We have recently monolithically integrated these fast VOAs with low-dark-current germanium photodiodes and achieved synchronized operation of these devices. For nonlinear devices, a free-carrier extraction structure using a PIN junction implemented in the waveguide can increase the efficiency of nonlinear functions. For example, in a wavelength conversion based on the Four-wave-mixing effect, the conversion efficiency can be increased by 6 dB.

Ultrahigh efficiency III-V on Si MOS capacitor optical modulator

A high-efficiency and low-loss Mach-Zehnder modulator on a Si platform is a key component for meeting the demand for high-capacity, low-cost and low-power optical transceivers in future optical fiber links. We report a III-V/Si MOS capacitor Mach-Zehnder modulator with an ultrahigh-efficiency phase shifter, which consists of n-type InGaAsP and ptype Si. The main advantage of this structure is a large electron-induced refractive index change and low free-carrier absorption loss of the n-type InGaAsP. The heterogeneously integrated InGaAsP/Si MOS capacitor structure is fabricated by using the oxygen plasma assisted bonding method. The fabricated device shows VπL of 0.09 Vcm, a value over three-times smaller than that of the conventional Si MOS capacitor Mach-Zehnder modulator, without an increase in the insertion loss. This clearly indicates that the proposed III-V/Si MOS capacitor Mach-Zehnder modulator overcomes the performance limit of the Si Mach-Zehnder modulator.

Ultra-high efficiency III-V on Si MOS capacitor Mach-Zehnder modulator

High-capacity optical transmitters with reduced size, cost, and power consumption are required to meet growing bandwidth requirements of network systems. A high-modulation-efficiency Mach-Zehnder modulator (MZM) on an Si platform is a key piece of equipment for these transmitters. Si-MZMs have been widely reported; however their performance is limited by the material properties of Si. To overcome the performance limitations of Si MZMs, we have integrated III-V materials on Si substrate and developed a heterogeneously integrated III-V/Si metal oxide semiconductor (MOS) capacitor phase shifter for constructing ultra-high efficient MZM, in which the n-InGaAsP, p-Si, and SiO2 film are used for constructing the MOS capacitor. The fabricated MZM with the MOS capacitor exhibited a VπL of 0.09 Vcm and insertion loss of ~2 dB. 32-Gbps modulation of the MZM was also demonstrated.

Polarization diversity circuit based on a double-core structure consisting of silicon photonic wire and silicon-oxinitride waveguide

We devised a silicon photonic circuit with polarization diversity. The circuit consists of polarization splitters and rotators. The splitter is based on simple 10-micrometer-long directional couplers. The polarization extinction ratio is 23 dB and excess loss is less than 0.5 dB. The rotator consists of a silicon waveguide embedded in an off-axis siliconoxynitride waveguide. A 35-micrometer-long rotator gives a rotation angle of more than 72 degrees and excess loss of about 1 dB. Both devices can be made by using planar fabrication technology and do not require a complex structure such as three-dimensional forming. Using these devices, we developed a polarization diversity circuit for a ringresonator wavelength filter. The polarization dependent loss of the filter with polarization diversity is about 1 dB. A 10- Gbps data transmission with scrambled polarization is demonstrated.