Valley photonic crystal (VPhC) waveguides have attracted much attention because of their ability to enable robust light propagation against sharp bends. However, their demonstration using a CMOS-compatible process suitable for mass production has not yet been reported at the telecom wavelengths. Here, by tailoring the photomask to suppress the optical proximity effect, VPhC patterns comprising equilateral triangular holes were successfully fabricated using photolithography. We optically characterized the fabricated VPhC devices using microscopic optics with NIR imaging. For comparison, we also fabricated and characterized line-defect W1 PhC waveguides, in which the transmission intensities decreased at some regions within the operating bandwidth when sharp turns were introduced into the waveguide. In contrast, the developed VPhC waveguides can robustly propagate light around the C-band telecommunication wavelengths, even in the presence of sharp bends. Our results highlight the potential of VPhC waveguides as an interconnection technology in silicon topological photonic ICs.

Here, we report on the increase of the quality-factors of photonic crystal nanocavities fabricated by a CMOS-compatible process. We fabricated nanocavities with the same cavity design but used either a binary photomask or a phase-shift photomask in the photolithography step to assess the impact of the photomask-type on the fabrication accuracy of the air holes. We characterized 62 cavities using time-resolved measurements and the best cavity had a quality-factor of 6.65 × 106. All cavities exhibited a quality-factor larger than 2 million and the overall average was 3.25 × 106. While the estimated magnitude of the scattering loss due to the air hole variations in the 33 cavities fabricated with the phase-shift photomask was slightly lower than that in the 29 cavities fabricated with binary photomask, the phase-shift photomask did not provide a significant improvement in the fabrication accuracy. On average, the scattering loss in these samples is more than 3 times larger than that of nanocavities fabricated using electron-beam lithography, which indicates room for further improvement.

We experimentally demonstrate the reconstruction of multiple wavelength light with an ultra-compact spectrometer. The spectrometer is based on a random photonic crystal and operates with a spectral resolution beyond the fabrication error.

The design of a novel 1 × M fold-back type wavelength selective switch (WSS), which has fewer waveguide crossings than a conventional integrated WSS, is reported. The WSS is composed of interleavers, 1 × M optical switches, and arrayed waveguide gratings (AWGs). Switches are combined with AWGs by fold-back waveguides, and each AWG works as both a demultiplexer and multiplexer thus avoiding center wavelength mismatch caused by fabrication errors. Waveguide crossings cause excess crosstalk and loss in lightwave circuits. By using a fold-back architecture the number of crossings can be reduced to less than half that of a conventional design. We discuss the operating principle, the design method, and the scalability of the fold-back type WSS. Furthermore, the switching operation of a 200-GHz spacing, 20-channel, 1 × 2 silicon WSS in a fold-back configuration on a 5 mm × 10 mm SOI chip is demonstrated. This has 15 waveguide crossings in a path, of which six are additional crossings with monitor waveguides. The average insertion loss and average extinction ratio are 29.6 dB and 10.9 dB, respectively.

We present applications of a CMOS-compatible SiN waveguide to an ultralow-loss fiber coupling structure and a Si-to-SiN waveguide on a Si photonics platform for C-band infrared light. We also evaluate waveguide properties for visible light.

Photonic crystal waveguide slow-light grating emits a free-space optical beam and steers it widely by changing the optical wavelength or waveguide refractive index. In the reverse process, returned light is coupled into the device again. We have proposed to use this optical transmission and reception antenna as a beam scanner for light detection and ranging (LiDAR). Ideally, a large-aperture antenna can narrow the transmission beam and enhance the reception efficiency. Actually, however, the transmission and reception performance is not scalable owing to waveguide loss even though the waveguide is simply lengthened. A serial array configuration in which the waveguide is divided into multiple antennas is effective for mitigating this problem. In this study, we fabricated such a device using Si photonics technology and obtained a small beam divergence of 0.02° at a telecom wavelength. Then, we observed the ranging operation by adding an optical setup of frequency-modulated continuous-wave (FMCW) LiDAR and confirmed that the divided antenna device improved the reception intensity by 12 dB. Moreover, we fabricated a FMCW LiDAR chip in which the serial array antennas were integrated in parallel with switch trees and Ge photodiodes and obtained point cloud images by two-dimensional beam scanning.

We demonstrated the fabrication of silicon valley photonic crystal structures with triangular air holes by using photolithography for the first time. The mask patterns we newly designed enable the formation of triangular air holes with reasonably high accuracy. This is an important step toward CMOS-compatible topological nanophotonics.

We fabricate and characterize a polarization-diversity 32 x 32 silicon photonics switch by newly introducing SiN overpass waveguides onto our nonduplicate polarization-diversity path-independent insertion-loss switch. The SiN overpass waveguides are used to simplify the optical paths with a uniform path length between the edge couplers and the switch matrix and significantly reduce the number of waveguide intersections. The switch chip is fabricated using a 300-mm silicon-on-insulator wafer pilot line. The fabricated switch comprises more than 7,600 components, making this the largest ever complementary-metal-oxide-semiconductor-based silicon photonics circuit. The switch chip is electrically and optically packaged and evaluated for a sampled port connection with 32 paths, with an average on-chip loss of similar to 35 dB and an average polarization-dependent loss of 3.2 dB where 75% of the measured paths exhibit a loss of less than 3 dB. The differential group delay is measured to be 1.7 ps. The performance can be further improved by optimizing the device design.

Fabricating silicon photonics devices by CMOS-compatible processes is important for applications. Here, we demonstrate a Raman silicon laser based on a heterostructure nanocavity that was fabricated by immersion photolithography using an argon fluoride excimer laser. The Raman laser confines the pump light and the Stokes Raman scattered light in two resonant modes of the nanocavity. By using the presented CMOS-compatible approach, sufficiently high quality-factors can be obtained for both modes. The sample whose frequency spacing of the two resonant modes closely matches the Raman shift of silicon, achieves continuous-wave oscillation with a lasing threshold of 1.8 µW at room temperature.

A simple low-loss fiber coupling structure consisting of a Si inverted-taper waveguide and a 435 nm wide and 290 nm thick SiN waveguide was fabricated with fully complementary metal-oxide semiconductor (CMOS)-compatible processes. The small SiN waveguide can expand to the optical field corresponding to a fiber with a mode-field diameter of 4.1 µm. The fiber-to-chip coupling losses were 0.25 and 0.51 dB/facet for quasi-TE and quasi-TM modes, respectively, at a 1550 nm wavelength. Polarization-dependent losses of the conversion in the Si-to-SiN waveguide transition and the fiber-to-chip coupling were less than 0.3 and 0.5 dB, respectively, in the wavelength range of 1520-1580 nm.

We demonstrate a polarization-diversity 32×32 Si-photonics switch by newly introducing a SiN overpass circuit onto our non-duplicate polarization-diversity path-independent insertion-loss switch circuit. An average PDL in a sampled connection setting was evaluated as 3.2 dB.

We report characteristics of 200 GHz-spaced 20-channel 1 × 2 wavelength selective switch using single arrayed-waveguide grating with fold-back waveguides. The average insertion loss is 29.6 dB and the average extinction ratio is 10.9 dB.

A wavelength selective switch (WSS) can route optical signals into any of output ports by wavelength, and is a key component of the reconfigurable optical add/drop multiplexer. We propose a wavefront control type WSS using silicon photonics technology. This consists of several arrayed waveguide gratings sharing a large slab waveguide, wavefront control waveguides and distributed Bragg reflectors. The structure, design method, operating principle, and scalability of the WSS are described and discussed. We designed and fabricated a 1 × 2 wavefront control type WSS using silicon waveguides. This has 16 channels with a channel spacing of 200 GHz. The chip size is 5 mm × 10 mm. The switching operation was achieved by shifting the phase of the light propagating in each wavefront control waveguide, and by controlling the propagation direction in the shared large slab waveguide. Our WSS has no crossing waveguide, so the loss and the variation in loss between channels were small compared to conventional waveguide type WSSs. The heater power required for switching was 183 mW per channel, and the average extinction ratios routed to Output#1 and Output#2 were 9.8 dB and 10.2 dB, respectively.

We report on a novel design of a wavelength selective switch (WSS) using silicon photonics technology. It comprises of 1 × 4 interleavers, arrayed-waveguide gratings (AWGs) connected to fold-back waveguides, and 1 × 2 optical Mach-Zehnder interferometer switches. In the proposed WSS, fold-back waveguides enable the AWGs to be used for both demultiplexing and multiplexing. Therefore the WSS has less waveguide crossings than a conventional configuration. Moreover, a 20-channel, 200-GHz spacing, 1 × 2 fold-back type WSS was fabricated on 5 mm × 10 mm SOI chip using CMOS technology.

Electron-beam lithography allows precise photonic crystal (PC) fabrication, but is not suitable for mass production. Therefore, the development and optimization of CMOS-compatible processes is necessary to implement unique nanocavity technologies in optoelectronic circuits. We investigated the quality factors (Q) and the resonant wavelengths (lambda) of PC heterostructure nanocavities fabricated by the 193-nm argon fluoride immersion lithography on a 300-mm-wide silicon-on-insulator wafer. We measured 30 cavities distributed over nine chips at various positions of the wafer. An average Q of 1.9 million was obtained for the 30 cavities, and the highest value was 2.5 million, which is the highest Q reported so far for a nanocavity fabricated by photolithography. Such high Q were realized by the improvements of the nanocavity design and the fabrication process. All nanocavities exhibited a Q of larger than 1 million and the fluctuation of the chip-averaged Q was independent of the chip location. On the other hand, the measured A tended to shift to shorter wavelengths as the distance between the nanocavity and the substrate center increased. Among the nine chips, the difference of the chip-averaged A was as large as 8.0 nm. We consider that a systematic shift of the average air-hole radius by several nanometers is responsible for the large fluctuation of the chip-averaged A. These statistical studies provide important hints to accelerate the application study of PC high-Q nanocavities.

Fumi NAKAMURA1*, Keijiro SUZUKI2, Ken TANIZAWA2, Minoru OHTSUKA2, Nobuyuki YOKOYAMA2, Kazuyuki MATSUMARO2, Miyoshi SEKI2, Keiji KOSHINO2, Kazuhiro IKEDA2, Shu NAMIKI2, Hitoshi KAWASHIMA2, Hiroyuki TSUDA1 1Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama, Kanagawa, 223-0061, Japan 2National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki, 305-8569, Japan * f_nakamura@tsud.elec.keio.ac.jp

We fabricated photonic crystal high-quality factor (Q) nanocavities on a 300-mm-wide silicon-on-insulator wafer by using argon fluoride immersion photolithography. The heterostructure nanocavities showed an average experimental Q value of 1.5 million for 12 measured samples. The highest Q value was 2.3 million, which represents a record for a nanocavity fabricated by complementary metal-oxide-semiconductor (CMOS)-compatible machinery. We also demonstrated an eight-channel drop filter with 4 nm spacing consisting of arrayed nanocavities with three missing air holes. The standard deviation in the drop wavelength was less than 1 nm. These results will accelerate ultrahigh-Q nanocavity research in various areas.

A 300-mm-wafer silicon photonics technology with fine fabrication accuracy and uniformity provides large-scale photonic integrated circuits with low power consumption and operation flexibility, as well as key photonic devices for an ultra-low-energy optical path network.