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 report a silicon photonic modulator based on the use of dual parallel microring modulators (MRMs) inserted in a Mach-Zehnder interferometer (MZI) It is operated in a push-pull configuration for low-chirp transmission at approximately 1550 nm The chirp parameters of the device are measured using 10 Gb/s on-off keying (OOK) transmission over 20 km of standard single mode fiber (SSMF), and they are less than 001, showing the low-chirp characteristic of the modulator We further demonstrate four-level pulse amplitude modulation (PAM-4) transmission at 92 Gb/s over 1 km of SSMF and at 40 Gb/s over 20 km of SSMF The measured bit error rates (BERs) are below the hard-decision (HD) forward error correction (FEC) threshold of 38 × 10−3

High-speed low-chirp PAM-4 transmission based on push-pull silicon photonic microring modulators.

The performance of faster-than-Nyquist intensity modulation and direct detection (IM/DD) systems can be severely degraded by the inter-symbol interference (ISI) due to the component bandwidth limitation and the faster-than-Nyquist filtering. In addition, nonlinearities caused mainly by optical modulation and square-law detection can further deteriorate signal quality. Here, we propose to incorporate transmitter-side Tomlinson-Harashima precoding to mitigate the ISI and a Volterra nonlinear equalizer (VNLE) to mitigate the nonlinear distortions for PAM4 faster-than-Nyquist systems. We experimentally demonstrate a single wavelength 145 Gb/s PAM4 IM/DD transmission with bandwidth limited commercial components and sub-Nyquist sampling at 70 GSa/s. The optimal VNLE filter length is also investigated to balance the implementation complexity and system performance. In addition, compared with the scheme using transmitter-side duobinary precoding, receiver-side VNLE and maximum likelihood sequence estimation, both performance improvement and implementation complexity reduction are achieved.

Single-Lane 145 Gbit/s IM/DD Transmission With Faster-Than-Nyquist PAM4 Signaling

We design and experimentally demonstrate an ultra-broadband and compact two-mode multiplexer based on subwavelength-grating-slot-assisted adiabatic coupler for the silicon-on-insulator platform. The introduction of subwavelength-grating slot effectively increases the refractive index of the gap region between two strip waveguides and enhances the coupling between them, leading to high modal transmission over a short mode-evolution region, particularly at the short-wavelength regime. As a result, our two-mode multiplexer has a compact design footprint, with a mode-evolution region length of only 25  $\mu$ m and an entire device length of only 55  $\mu$ m. Simulation shows that the mode-division multiplexing (MDM) link formed by the designed two-mode multiplexers has a record bandwidth of 740 nm spanning from 1260 nm to 2000 nm, where the crosstalk is lower than −18.5 dB and the insertion loss (IL) is lower than 0.32 dB. The MDM link has been fabricated using electron beam lithography and achieves measured crosstalk lower than −18.8 dB and IL lower than 2.6 dB, over a 100 nm bandwidth from 1260 nm to 1360 nm that covers the O-band and a 130 nm bandwidth from 1500 nm to 1630 nm that covers the C-band and the L-band.

Ultra-Broadband and Compact Two-Mode Multiplexer Based on Subwavelength-Grating-Slot-Assisted Adiabatic Coupler for the Silicon-on-Insulator Platform

We propose 4D modulation with direct detection employing a novel Stokes vector Kramers Kronig transceiver. It shows that employing a Stokes vector receiver, transmitting a digital carrier and using Kramers Kronig detection offer an effective way to de-rotate polarization multiplexed complex double side band signal without using a local oscillator at receiver. The impact of system parameters and configurations including carrier-to-signal-power ratio, guard band of the digital carrier, digital oversampling ratio and real MIMO is experimentally investigated. Finally, a record net data rate of 400 Gb/s (a raw rate of 480 Gb/s including FEC overhead) over 80 km SSMF is achieved in a 60 Gbaud PDM-16QAM single carrier experiment with a BER below the threshold of 2.0x10−2.

Single wavelength 480 Gb/s direct detection over 80km SSMF enabled by Stokes vector Kramers Kronig transceiver

We present an O-band SiP MZM design enabling net transmission of 212.5 (200) Gbit/s over 2 (10) km using PAM-8 modulation and 20% SD-FEC, and net 200 Gbit/s back-to-back using PAM-6 and 6.7% HD-FEC.

Zhenping Xing

Papers

High-speed low-chirp PAM-4 transmission based on push-pull silicon photonic microring modulators.

Single-Lane 145 Gbit/s IM/DD Transmission With Faster-Than-Nyquist PAM4 Signaling

Ultra-Broadband and Compact Two-Mode Multiplexer Based on Subwavelength-Grating-Slot-Assisted Adiabatic Coupler for the Silicon-on-Insulator Platform

Single wavelength 480 Gb/s direct detection over 80km SSMF enabled by Stokes vector Kramers Kronig transceiver

Net 212.5 Gbit/s Transmission in O-Band with a SiP MZM, One Driver and Linear Equalization