Flexible and localized initial alignment control of liquid crystal (LC) is important to enhance the performance and functionality of LC hybrid silicon photonic devices. This work proposes an initial ...LC alignment control technique based on integration of a nanometer-scale groove array in the buried oxide layer near a Si waveguide. We achieved control of the initial angle of LC director around the Si waveguide by selecting the required integrated groove direction and reduced the driving voltage by introducing a vertical groove array into the phase shifter. We then used the local and flexible LC initial alignment controllability to develop a Mach-Zehnder optical switch and ring-resonator wavelength filter. This approach will be helpful when integrating LC-loaded devices with various characteristics and functionalities into optical integrated circuits.
For input coupling from single-mode optical fibers to photoreceivers made with silicon photonics integrated circuits (Si-PICs), we constructed a polarization-insensitive optical surface coupler with ...a vertically curved silicon waveguide. The polarization insensitivity was attained by the equalization of the radiative transmission losses of TE and TM polarizations in the vertically curved waveguide (VCW) structure while keeping the straight structure of the spot-size converter (SSC) connected above the curved structure. Numerical simulations revealed that the radius of curvature of <inline-formula> <tex-math notation="LaTeX">6~\mu \text{m} </tex-math></inline-formula> or larger nearly enabled the equalization, and thus we developed a bending technology using ion implantation to realize simultaneously a VCW radius of curvature of <inline-formula> <tex-math notation="LaTeX">6~\mu \text{m} </tex-math></inline-formula> and a straight SSC of length <inline-formula> <tex-math notation="LaTeX">7~\mu \text{m} </tex-math></inline-formula> by varying the implantation angles. The bent silicon core was subsequently coated with SiO 2 by plasma-enhanced chemical vapor deposition to function as a collimation lens. The coupling properties of the fabricated coupler and the single-mode fiber with a mode-field diameter of <inline-formula> <tex-math notation="LaTeX">5~\mu \text{m} </tex-math></inline-formula> exhibited a polarization dependence loss of less than 0.25 dB, a 1-dB bandwidth of 180 nm, and coupling losses of about 1.6 dB at 1550 nm wavelength. A sharp inversely tapered Si waveguide that affects these characteristics significantly can be formed by applying ArF-immersion lithography on the 45-nm-node and hence fabrication is compatible with mass production processes.
A chip-surface optical coupler based on a vertically curved Si waveguide was demonstrated for coupling with high-numerical-aperture single-mode optical fibers with a mode-field diameter of 5 µm. This ...device features a dome-like SiO2 coupler cap, which acts as collimation lens. We succeeded in fabricating this structure using an isotropic SiO2 deposition technique employing plasma-enhanced chemical vapor deposition and obtained a light output that approximates that of a 5-µm-waist Gaussian beam. The fabricated coupler showed a coupling loss of less than 4.2 dB and a 0.5-dB-loss bandwidth above 150 nm for TE-polarized light.
We investigated the switching dynamics of optical modulators consisting of a Si waveguide with a VO
cladding layer by utilizing the photothermal effect, which induces a metal-insulator transition in ...VO
. The devices exhibited stable optical switching with a high extinction ratio exceeding 16 dB. The switching time of the insulator-to-metal transition (heating process) ranged from tens of nanoseconds to microseconds depending on the incident light power, and that of the metal-to-insulator transition (cooling process) was several microseconds regardless of the incident light power. The heat transfer in the devices was numerically simulated to reproduce the switching characteristics and revealed that the temperature change in the first few micrometers of the VO
/Si waveguide governed the switching time. The thermal structural design of the device is thus of key importance to improve the switching speed of the device.
The high-performance silicon optical coupler is one of the significant functional devices, which will be required to enable mass commercialization of silicon photonic integrated circuits (Si-PICs). ...In this work, we investigated the bending shapes for each device length and have successfully fabricated a very low-loss and broadband 5-μm-beam-spot surface optical coupler based on a lensed-top vertically curved silicon waveguide by adjusting the dose conditions used in the ion implantation process. The fabricated device showed a coupling loss of 2.4 dB, which included a waveguide loss of approximately 0.5 dB and a 0.5-dB-loss bandwidth of more than 220 nm for the transverse electric polarization in the 1.55-μm wavelength band. This coupling device will greatly contribute to the progress in the development of Si-PICs using the coarse wavelength division multiplexing technique.
To obtain high device fabrication uniformity and reproducibility for vertically curved silicon waveguide optical coupler, we developed the high accuracy ion implantation bending (IIB) method by ...incorporating a tungsten mask that can partially shield the ion implantation. The developed IIB method improved the positional accuracy of the origin of vertically curved Si wire bending from a wet etching-level of 1 m order to a dry etching-level of several 100 nm order, and we obtained a fabrication accuracy of about less than 0.4 m of the tip position of the Si-wire vertically curved structures.
Indoor free space optical (FSO) communication technology that provides high-speed connectivity to edge users is expected to be introduced in the near future mobile communication system, where the ...silicon photonics solid-state beam scanning device is a promising tool because of its low cost, long-term reliability, and other beneficial properties. However, the current two-dimensional beam scanning devices using grating coupler arrays have difficulty in increasing the transmission capacity because of bandwidth regulation. To solve the problem, we have introduced a broadband surface optical coupler, “elephant coupler,” which has great potential for combining wavelength and spatial division multiplexing technologies into the beam scanning device, as an alternative to grating couplers. The prototype port-selective silicon beam scanning device fabricated using a 300 mm CMOS pilot line achieved broadband optical beam emission with a 1 dB-loss bandwidth of 40 nm and demonstrated beam scanning using an imaging lens. The device has also exhibited free-space signal transmission of non-return-to-zero on-off-keying signals at 10 Gbps over a wide wavelength range of 60 nm. In this paper, we present an overview of the developed beam scanning device. Furthermore, the theoretical design guidelines for indoor mobile FSO communication are discussed.
We proposed and designed a compact Si interlayer polarization beam splitter (PBS) that is based on a crystalline Si and hydrogenated amorphous Si (a-Si:H) vertical asymmetrical directional coupler ...toward 3-D Si photonics integrated circuits. The changeable thickness of the upper a-Si:H waveguide enables the realization of the TM-mode-coupled PBS with simple-structured parallel Si wire waveguides. The device with a 260-nm × 220-nm first-layer waveguide and a 340-nm × 200-nm second-layer waveguide operates in a broadband width of 77 nm, with a crosstalk of less than -20 dB and an insertion loss of <;0.3 dB in the C-band. The device is robust in terms of alignment error between the two waveguides.
To realize three-dimensional (3D) optical interconnection on large-scale integration (LSI) circuits, layer-to-layer couplers based on Si-photonics platform were reviewed. In terms of optical cross ...talk, more than 1 µm layer distance is required for 3D interconnection. To meet this requirement for the layer-to-layer optical coupler, we proposed two types of couplers: a pair of grating couplers with metal mirrors for multi-layer distance coupling and taper-type directional couplers for neighboring layer distance coupling. Both structures produced a high coupling efficiency with relatively compact (∼100 µm) device sizes with a complementary metal oxide semiconductor (CMOS) compatible fabrication process.
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