“Unpaired Image-to-Image Translation using Cycle-Consistent Adversarial Networks”の学習報告

The arrival of the big data era has driven the rapid development of high-speed optical signaling and processing, ranging from long-haul optical communication links to short-reach data centers and high-performance computing, and even micro-/nano-scale inter-chip and intra-chip optical interconnects. On-chip photonic signaling is essential for optical data transmission, especially for chip-scale optical interconnects, while on-chip photonic processing is a critical technology for optical data manipulation or processing, especially at the network nodes to facilitate ultracompact data management with low power consumption. In this paper, we review recent research progress in on-chip photonic signaling and processing on silicon photonics platforms. Firstly, basic key devices (lasers, modulators, detectors) are introduced. Secondly, for on-chip photonic signaling, we present recent works on on-chip data transmission of advanced multi-level modulation signals using various silicon photonic integrated devices (microring, slot waveguide, hybrid plasmonic waveguide, subwavelength grating slot waveguide). Thirdly, for on-chip photonic processing, we summarize recent works on on-chip data processing of advanced multi-level modulation signals exploiting linear and nonlinear effects in different kinds of silicon photonic integrated devices (strip waveguide, directional coupler, 2D grating coupler, microring, silicon-organic hybrid slot waveguide). Various photonic processing functions are demonstrated, such as photonic switch, filtering, polarization/wavelength/mode (de)multiplexing, wavelength conversion, signal regeneration, optical logic and computing. Additionally, we also introduce extended silicon+ photonics and show recent works on on-chip graphene-silicon photonic signal processing. The advances in on-chip silicon photonic signaling and processing with favorable performance pave the way to integrate complete optical communication systems on a monolithic chip and integrate silicon photonics and silicon nanoelectronics on a chip. It is believed that silicon photonics will enable more and more emerging advanced applications even beyond silicon photonic signaling and processing.

On-chip silicon photonic signaling and processing: a review

In this paper, we review and provide additional details about our progress on multilayer silicon nitride (SiN)-on-silicon (Si) integrated photonic platforms In these platforms, one or more SiN waveguide layers are monolithically integrated onto a Si photonic layer This paper focuses on the development of three-layer platforms for the O- and SCL-bands for very large-scale photonic integrated circuits requiring hundreds or thousands of waveguide crossings Low-loss interlayer transitions and ultralow-loss waveguide crossings have been demonstrated, along with bilevel and trilevel grating couplers for fiber-to-chip coupling The SiN and Si passive devices have been monolithically integrated with high-efficiency optical modulators, photodetectors, and thermal tuners in a single photonic platform

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Monolithically Integrated Multilayer Silicon Nitride-on-Silicon Waveguide Platforms for 3-D Photonic Circuits and Devices

Silicon photonics is a technology based on fabricating integrated optical circuits by using the same paradigms of the dominating electronics industry. After twenty years of impetuous development, silicon photonics is entering the market with low cost, high performance and mass manufacturable optical devices. Up to now, most of the silicon photonic devices are based on linear optical effects, despite the many phenomenologies associated to nonlinear optics in both bulk materials and integrated waveguides. Silicon and silicon based materials have strong optical nonlinearities which are enhanced in integrated devices by the small cross section of the high index contrast silicon waveguides or photonic crystals. Here the photons are made to strongly interact with the medium where they propagate. This is the central argument of nonlinear silicon photonics. It is the aim of this review article to make the point of the state of the art in the field. Starting from the basic nonlinearities in a silicon waveguide or in optical resonator geometries, many phenomena and applications are described: from frequency generation, frequency conversion, frequency comb generation, supercontinuum generation, soliton formation, temporal imaging and time lensing, Raman lasing, up to comb spectroscopy. Emerging quantum photonics applications, as entangled photon sources, heralded single photon sources and integrated quantum photonic circuits, are also addressed at the end of the review.

Nonlinear silicon photonics

Simultaneous Kerr comb formation and second-harmonic generation with on-chip microresonators can greatly facilitate comb self-referencing for optical clocks and frequency metrology. Moreover, the presence of both second- and third-order nonlinearities results in complex cavity dynamics that is of high scientific interest but is still far from being well-understood. Here, we demonstrate that the interaction between the fundamental and the second-harmonic waves can provide an entirely new way of phase matching for four-wave mixing in optical microresonators, enabling the generation of optical frequency combs in the normal dispersion regime under conditions where comb creation is ordinarily prohibited. We derive new coupled time-domain mean-field equations and obtain simulation results showing good qualitative agreement with our experimental observations. Our findings provide a novel way of overcoming the dispersion limit for simultaneous Kerr comb formation and second-harmonic generation, which might prove to be especially important in the near-visible to visible range where several atomic transitions commonly used for the stabilization of optical clocks are located and where the large normal material dispersion is likely to dominate. Tuning the interactions between fundamental and second-harmonic waves can facilitate measurements of atomic-based optical clocks.On-chip microresonators have recently been developed that convert single-frequency laser sources into broadband, regularly spaced spectra — frequency combs — for portable metrology applications. Xiaoxiao Xue from Tsinghua University in China, Andrew Weiner from Purdue University in the United States, and colleagues have now found a way to convert a fraction of the frequency lines from a wideband comb into second-harmonic light that can potentially benchmark comb accuracy. The researchers located the resonant modes for second-harmonic generation in a silicon nitride microring using an adjustable infrared laser and then swept the microresonator over a range of frequencies to produce a comb. They identified a nonlinear coupling mechanism that may improve comb generation in the near-visible and visible spectral regions, where many atomic transitions in optical clocks occur.

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Second-harmonic-assisted four-wave mixing in chip-based microresonator frequency comb generation

Bistability (BS) and self-pulsation (SP) phenomena in silicon microring resonators (MRR) with intense CW light injection are studied. Several nonlinear optical effects including Kerr effect, two-photon absorption, free carrier absorption and free carrier dispersion are taken into account. The threshold optical intensity of BS and SP is derived from the coupled mode theory and a linear stability analysis method. The influences of MRR's parameters (carrier lifetime, linear loss and radius) and light injection conditions (input power, wavelength detuning) on the characteristics of SP (modulation depth and oscillating frequency) are analyzed and discussed. It is shown that, SP occurs only if the carrier lifetime ranges from several ps to several-hundred ps and the input light intensity is higher than 10⁶W/cm². The modulation depth of SP can be as large as 8dB and the associated oscillating frequency is in the range from several GHz to beyond 10 GHz.

Bistability and self-pulsation phenomena in silicon microring resonators based on nonlinear optical effects

Optical bistability (BI) and self-pulsation (SP) in high-$Q$ silicon microring resonators (MRRs) induced by thermo-optical (TO) effect and other nonlinear effects are theoretically studied with coupled mode theory and linear stability analysis method. It is found that the boundaries for both BI and SP are mainly restricted by two counteracting effects: free carrier dispersion effect and TO effect. If the refractive index changes of a MRR caused by these two effects are on the same order of magnitude, the output power will exhibit much more complicated dependence on the input power and wavelength, namely, input-power-dependent multi-BI and multi-SP regions will exist at certain input wavelength range. The controllability of multi-BI and multi-SP phenomena by the input power and input wavelength could be very useful in all-optical nonlinear devices.

Multibistability and self-pulsation in nonlinear high- Q silicon microring resonators considering thermo-optical effect

Optical bistability and self-pulsation (SP) in silicon microring resonators (MRRs) are experimentally observed. The waveforms and frequencies of SP can be controlled by changing input light power and its wavelength, and the region of SP can be modulated readily by applying a reverse voltage on the PN junction embedded in the MRR. These phenomena are theoretically studied by adopting the coupled-mode theory and linear stability analysis method for differential equations, with theoretical results fitting well with the experimental ones.

Experimental observations of thermo-optical bistability and self-pulsation in silicon microring resonators

An ultracompact polarization splitter-rotator based on an asymmetric directional coupler has been designed and analyzed. The device consists of one channel waveguide and one vertical slot waveguide, which couple to each other to form the asymmetric directional coupler. The coupling length is only 17.4 μm, and the polarization extinction ratio, with its largest value of 18 dB, is more than 15 dB in the operating wavelength range from 1515 to 1560 nm. The TM-to-TE conversion efficiency can reach up to 95% (0.02 dB) at 1550 nm wavelength.

Ultracompact polarization splitter-rotator based on an asymmetric directional coupler.

We propose and analyze a polarization rotator based on a bend asymmetric-slab waveguide on the silicon-on-insulator platform. The device can be fabricated using standard complementary metal-oxide-semiconductor process involving only two dry etching steps. Compared with the formerly reported polarization rotators based on two-step etching, our introduced device demonstrates a significant improvement for fabrication tolerance. Furthermore, an ultra compact structure of ~5 μm conversion length, an insertion loss of only 0.5 dB, and an extinction ratio of >40 dB for both TE to TM polarization conversion and TM to TE polarization conversion are exhibited. Operation wavelength and the influence of environmental temperature on our device are also discussed.

Libin Zhang

Papers

Bistability and self-pulsation phenomena in silicon microring resonators based on nonlinear optical effects

Multibistability and self-pulsation in nonlinear high- Q silicon microring resonators considering thermo-optical effect

Experimental observations of thermo-optical bistability and self-pulsation in silicon microring resonators

Ultracompact polarization splitter-rotator based on an asymmetric directional coupler.

Ultra-compact and fabrication-tolerant polarization rotator based on a bend asymmetric-slab waveguide.