In the late nineteenth century, Heinrich Hertz demonstrated that the electromagnetic properties of materials are intimately related to their structure at the subwavelength scale by using wire grids with centimetre spacing to manipulate metre-long radio waves. More recently, the availability of nanometre-scale fabrication techniques has inspired scientists to investigate subwavelength-structured metamaterials with engineered optical properties at much shorter wavelengths, in the infrared and visible regions of the spectrum. Here we review how optical metamaterials are expected to enhance the performance of the next generation of integrated photonic devices, and explore some of the challenges encountered in the transition from concept demonstration to viable technology.

Subwavelength integrated photonics.

Driven primarily by cloud service and data-center applications, short-reach optical communication has become a key market segment and growing research area in recent years. Short-reach systems are characterized by direct detection-based receiver configurations and other low-cost and small form factor components that induce transmission impairments unforeseen in their coherent counterparts. Innovative signaling and digital signal processing (DSP) play a pivotal role in enabling these components to realize their ultimate potentials and meet data rate requirements in cost-effective manners. This paper presents an overview of recent DSP developments for short-reach communications systems and discusses future trends.

Digital Signal Processing for Short-Reach Optical Communications: A Review of Current Technologies and Future Trends

The realization of gigahertz bandwidth modulators out of silicon-based technology in the early 2000s marked a cornerstone of silicon photonics development. While modulation speeds have since progressed well above 50 GHz and satisfy the bandwidth requirements of current and emerging modulation formats, concurrently obtaining low drive voltages and low insertion losses remains a very active area of research. While modulators generally come in two categories, direct absorption and those relying on embedded phase shifters, the focus of this paper lies on the latter capable of supporting both complex-valued modulation and optically broadband operation. The paper provides an overview of the current state of the art, as well as of currently explored improvement paths. First, common phase shifter configurations, aspects related to electrical driving, and associated power consumption are reviewed. Slow-wave, resonant, and plasmonic enhancements are further discussed. The reader is familiarized with the optimization of these devices and provided with a sense of the limitations of current technology and the potential of novel hybrid material integration.

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High-Speed Silicon Photonics Modulators

Silicon Photonic Platform for Passive Waveguide Devices: Materials, Fabrication, and Applications

Responding to the growing bandwidth demand by emerging applications, such as fixed-mobile convergence for fifth generation (5G) and beyond 5G, 100 Gb/s/ λ access network becomes the next research focus of passive optical network (PON) roadmap. Intensity modulation and direct detection (IMDD) technology is still considered as a promising candidate for 100 Gb/s/ λ PON attributed to its low cost, low power consumption, and small footprint. In this paper, we achieve 100 Gb/s/ λ IMDD PON by using 20G-class optical and electrical devices due to its commercial availability. To mitigate the system linear and nonlinear distortions, neural network (NN) based equalizer is used and the performance is compared with feedforward equalizer (FFE) and Volterra nonlinear equalizer (VNE). We introduce the rules to train and test the data while using NN-based equalizer to guarantee a fair comparison with FFE and VNE. Random data have to be used for training, but for test, both random data and pseudorandom bit sequence are applicable. We found that the NN-based equalizer has the same performance with FFE and VNE in the case of linear distortion, but outperforms them in a strong nonlinearity case. In the experiment, to improve the loss budget, we increase the launch power to 18 dBm, achieving a 30-dB loss budget for 33 GBd/s PAM8 signal at the system frequency response of 16.2 GHz, attributed to the strong nonlinear equalization capability of NN.

Machine Learning for 100 Gb/s/ λ Passive Optical Network

We first optimize the design and compare the performance of thermo-optic phase-shifters based on TiN metal and N++ doped silicon, in the same SOI process. The designs don’t require special material processing, show negligible loss, and have very stable power consumption. The optimum TiN design has a switching powerPπ=21.4 mW and a time constantτ=5.6 µs, whereasPπ=22.8 mW andτ=2.2 µs for the best N++ Si design, enabling 2.5x faster switching compared to the metal heater. Doped-Si-based heaters are therefore the most practical and efficient on standard SOI. In addition, to optimize the layout density of highly integrated dies, we experimentally characterize internal and external thermal crosstalk for tunable Mach-Zehnder interferometers (MZIs) based on both heater designs for various power, distances, and etching patterns. Deep trenches are the best structures not involving special fabrication techniques to mitigate heat leakage affecting phase-sensitive devices close to heaters. Given the numerous applications of thermal tuners, this work is relevant to almost all silicon photonics designers.

Optimization of thermo-optic phase-shifter design and mitigation of thermal crosstalk on the SOI platform.

We demonstrate a novel polarization beam splitter (PBS) based on a subwavelength grating (SWG) multimode interference (MMI) coupler for the silicon-on-insulator platform The birefringence of the MMI coupler is engineered using SWG, which leads to a compact design footprint, with a device length of less than $100~\mu \text{m}$  Our PBS has simulated extinction ratios (ERs) better than 20 dB for both polarizations over the wavelength range from 1530 to 1625 nm that covers the entire $C$ - and $L$ -bands The fabricated device achieves the measured ERs larger than 20 dB at the wavelength of 1550 nm for both polarizations, and the insertion losses of 19 and 25 dB for the transverse electric (TE) and transverse magnetic (TM) polarizations, respectively, at 1550 nm In addition, the measured ERs are larger than 145 dB for the TE polarization and 117 dB for the TM polarization over an 84-nm spectral range covering the entire $C$ -band

Polarization Beam Splitter Based on MMI Coupler With SWG Birefringence Engineering on SOI

We experimentally demonstrate a compact broadband polarization beam splitter (PBS) based on a multimode interference (MMI) coupler with internal photonic crystal (PC) for the silicon-on-insulator platform. The internal PC structure is optimized to be reflective to the transverse electric polarization and transparent to the transverse magnetic polarization over a broad wavelength range. A detailed study of the device operation, including the photonic band gap and the influence of the internal PC structure on each mode of the MMI coupler, is presented. The designed PBS has been fabricated using electron beam lithography and the feature size used in our design is CMOS compatible. The fabricated device achieves measured extinction ratios higher than 20 dB and insertion losses lower than 2 dB for both polarizations over a 77 nm wavelength range from 1522 to 1599 nm that covers the entire C -band, with a device length of only 71.5 $\mu$ m.

Compact Broadband Polarization Beam Splitter Based on Multimode Interference Coupler With Internal Photonic Crystal for the SOI Platform

We experimentally and via simulations demonstrate ultracompact single-stage and cascaded optical add-drop multiplexers using misaligned sidewall Bragg grating in a Mach–Zehnder interferometer for the silicon-on-insulator platform. The single-stage configuration has a device footprint of 400  ${\mu }\text{m}\,\times$  90  ${\mu }\text{m}$ , and the cascaded configuration has a footprint of 400  ${\mu }\text{m}\,\times$  125  ${\mu }\text{m}$ . The proposed designs have 3-dB bandwidths of 6 nm and extinction ratios of 25 dB and 51 dB, respectively, and have been fabricated for the transverse electric mode. A minimum lithographic feature size of 80 nm is used in our design, which is within the limitation of 193 nm deep ultraviolet lithography.

A CMOS Compatible Ultracompact Silicon Photonic Optical Add-Drop Multiplexer with Misaligned Sidewall Bragg Gratings

We demonstrate a compact high-performance adiabatic 3-dB coupler for the silicon-on-insulator platform. The refractive index of the gap region between two coupling waveguides is effectively increased using subwavelength grating, which leads to high-performance operation and a compact design footprint, with a mode-evolution length of only 25 µm and an entire device length of 65 µm. The designed adiabatic 3-dB coupler has been fabricated using electron beam lithography and the feature size used in our design is CMOS compatible. The fabricated device is characterized in the wavelength range from 1500 nm to 1600 nm, with a measured power splitting ratio better than 3 ± 0.27 dB and an average insertion loss of 0.20 dB.

Zhenping Xing

Papers

Optimization of thermo-optic phase-shifter design and mitigation of thermal crosstalk on the SOI platform.

Polarization Beam Splitter Based on MMI Coupler With SWG Birefringence Engineering on SOI

Compact Broadband Polarization Beam Splitter Based on Multimode Interference Coupler With Internal Photonic Crystal for the SOI Platform

A CMOS Compatible Ultracompact Silicon Photonic Optical Add-Drop Multiplexer with Misaligned Sidewall Bragg Gratings

Compact high-performance adiabatic 3-dB coupler enabled by subwavelength grating slot in the silicon-on-insulator platform