The compelling demand for higher performance and lower power consumption in electronic systems is the main driving force of the electronics industry's quest for devices and/or architectures based on new materials. Here, we provide a review of electronic devices based on two-dimensional materials, outlining their potential as a technological option beyond scaled complementary metal-oxide-semiconductor switches. We focus on the performance limits and advantages of these materials and associated technologies, when exploited for both digital and analog applications, focusing on the main figures of merit needed to meet industry requirements. We also discuss the use of two-dimensional materials as an enabling factor for flexible electronics and provide our perspectives on future developments.

Electronics based on two-dimensional materials

We review the current status of single-photon-source and single-photon-detector technologies operating at wavelengths from the ultraviolet to the infrared. We discuss applications of these technologies to quantum communication, a field currently driving much of the development of single-photon sources and detectors.

/pdf/invited-review-article-single-photon-sources-and-detectors-5bsn8auo37.pdf

Invited review article: Single-photon sources and detectors

The current status of lithium-niobate external-modulator technology is reviewed with emphasis on design, fabrication, system requirements, performance, and reliability. The technology meets the performance and reliability requirements of current 2.5-, 10-, and 40-Gb/s digital communication systems, as well as CATV analog systems. The current trend in device topology is toward higher data rates and increased levels of integration. In particular, multiple high-speed modulation functions, such as 10-Gb/s return-to-zero pulse generation plus data modulation, have been achieved in a single device.

/pdf/a-review-of-lithium-niobate-modulators-for-fiber-optic-1s6rskx8q3.pdf

A review of lithium niobate modulators for fiber-optic communications systems

Digital filters underpin the performance of coherent optical receivers which exploit digital signal processing (DSP) to mitigate transmission impairments. We outline the principles of such receivers and review our experimental investigations into compensation of polarization mode dispersion. We then consider the details of the digital filtering employed and present an analytical solution to the design of a chromatic dispersion compensating filter. Using the analytical solution an upper bound on the number of taps required to compensate chromatic dispersion is obtained, with simulation revealing an improved bound of 2.2 taps per 1000ps/nm for 10.7GBaud data. Finally the principles of digital polarization tracking are outlined and through simulation, it is demonstrated that 100krad/s polarization rotations could be tracked using DSP with a clock frequency of less than 500MHz.

Digital filters for coherent optical receivers.

Optical fiber transmission is impacted by linear and nonlinear impairments. We study the use of digital backpropagation (BP) in conjunction with coherent detection to jointly mitigate dispersion and fiber nonlinearity. We propose a noniterative asymmetric split-step Fourier method (SSFM) for solving the inverse nonlinear Schrodinger equation (NLSE). Using simulation results for RZ-QPSK transmitted over terrestrial systems with inline amplification and dispersion compensation, we obtain heuristics for the step size and sampling rate requirements, as well as the optimal dispersion map.

Compensation of Dispersion and Nonlinear Impairments Using Digital Backpropagation

The spectral spread of the best stabilised semiconductor lasers has been reduced to several megahertz. Conventional spectroscopy techniques cannot offer a spectral resolution fine enough for measuring such a sharp spectrum. The letter proposes a novel method by which 50 kHz resolution can be obtained. The principle, experimental set-up and results are described.

Novel method for high resolution measurement of laser output spectrum

The recently developed digital coherent receiver enables us to employ a variety of spectrally efficient modulation formats such as $M$ -ary phase-shift keying and quadrature-amplitude modulation. Moreover, in the digital domain, we can equalize all linear transmission impairments such as group-velocity dispersion and polarization-mode dispersion of transmission fibers, because coherent detection preserves the phase information of the optical signal. This paper reviews the history of research and development related to coherent optical communications and describes the principle of coherent detection, including its quantum-noise characteristics. In addition, it discusses the role of digital signal processing in mitigating linear transmission impairments, estimating the carrier phase, and tracking the state of polarization of the signal in coherent receivers.

Fundamentals of Coherent Optical Fiber Communications

This paper describes a coherent optical receiver for demodulating optical quadrature phase-shift keying (QPSK) signals. At the receiver, a phase-diversity homodyne detection scheme is employed without locking the phase of the local oscillator (LO). To handle the carrier phase drift, the carrier phase is estimated with digital signal processing (DSP) on the homodyne-detected signal. Such a scheme presents the following major advantages over the conventional optical differential detection. First, its bit error rate (BER) performance is better than that of differential detection. This higher sensitivity can extend the reach of unrepeated transmission systems and reduce crosstalk between multiwavelength channels. Second, the optoelectronic conversion process is linear, so that the whole optical signal information can be postprocessed in the electrical domain. Third, this scheme is applicable to multilevel modulation formats such as M-array PSK and quadrature amplitude modulation (QAM). The performance of the receiver is evaluated through various simulations and experiments. As a result, an unrepeated transmission over 210 km with a 20-Gb/s optical QPSK signal is achieved. Moreover, in wavelength-division multiplexing (WDM) environment, coherent detection allows the filtering of a desired wavelength channel to reside entirely in the electrical domain, taking advantage of the sharp cutoff characteristics of electrical filters. The experiments show the feasibility to transmit polarization-multiplexed 40-Gb/s QPSK signals over 200 km with channel spacing of 16 GHz, leading to a spectral efficiency as high as 2.5 b/s/Hz.

Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation

Coherent Optical Fiber Communications

In coherent optical communication schemes, the FM noises of the transmitter and/or the local oscillator seriously deteriorate the receiver sensitivity. A theory is developed describing the effect of the FM noise on the bit-error rate (BER) performance of various heterodyne and homodyne schemes. To verify the theory, the BER of a phase-shift keying (PSK) heterodyne system is measured. The feasibility of the PSK scheme using semiconductor lasers is also discussed on the basis of analysis and experimentation.

Kazuro Kikuchi

Papers

Novel method for high resolution measurement of laser output spectrum

Fundamentals of Coherent Optical Fiber Communications

Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation

Coherent Optical Fiber Communications

Degradation of bit-error rate in coherent optical communications due to spectral spread of the transmitter and the local oscillator