Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s intensity- and phase-modulated formats, and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Optical Modulation Formats

We review the recent progress of information theory in optical communications, and describe the current experimental results and associated advances in various individual technologies which increase the information capacity. We confirm the widely held belief that the reported capacities are approaching the fundamental limits imposed by signal-to-noise ratio and the distributed non-linearity of conventional optical fibres, resulting in the reduction in the growth rate of communication capacity. We also discuss the techniques which are promising to increase and/or approach the information capacity limit.

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Approaching the Non-Linear Shannon Limit

On a single chip, sources of entangled photons are combined with optical elements that can perform complex manipulations of quantum signals.

On-chip generation and manipulation of entangled photons based on reconfigurable lithium-niobate waveguide circuits.

A novel free-space coupling system combined with a multi-core fiber enables up-scaling to a record space-division-multiplexed (SDM) channel number of 19. We achieve 305-Tb/s transmission over 10.1 km using 19-SDM, 100-WDM PDM-QPSK signals.

19-core fiber transmission of 19×100×172-Gb/s SDM-WDM-PDM-QPSK signals at 305Tb/s

High-speed control of lightwave using electrooptic (EO) effect is investigated in this paper. Agile optical frequency shift can be achieved by optical single-sideband (SSB) and frequency-shift-keying (FSK) modulators, where high-speed optical phase-shift-keying (PSK) signals can also be generated by using FSK/SSB modulators. We also describe ultrahigh extinction ratio optical intensity modulation (IM) technique for two-tone lightwave signals with high spurious suppression, which is useful for photonic microwave and millimeter-wave generation. In addition, we investigated high-order optical sideband generation techniques: quadruple dual-sideband suppressed carrier (QDSB-SC) modulation and reciprocating optical modulation (ROM). Sub-tetrahertz signals can be obtained from lightwaves with high-order sidebands

High-Speed Control of Lightwave Amplitude, Phase, and Frequency by Use of Electrooptic Effect

We demonstrate record 25.6-Tb/s transmission over 240 km using 160 WDM channels on a 50-GHz grid in the C+L bands. Each channel contains two polarization-multiplexed 85.4-Gb/s RZ-DQPSK signals, yielding a spectral efficiency of 3.2 b/s/Hz.

25.6-Tb/s C+L-band transmission of polarization-multiplexed RZ-DQPSK signals

We investigated an integrated optical modulator consisting of two Mach-Zehnder interferometers. The modulator can generate optical signals in various types of modulation formats, which have advantages for long-haul transmission, optical labeling, etc. By using a fabricated versatile optical modulator having traveling-wave electrodes designed for high-speed signals, we demonstrated generation of optical 40Gb/s frequency-shift-keying signals, which can be demodulated by an optical filter. 80Gb/s optical differential quadrature-shift-keying modulation was also demonstrated, where 40Gb/s in-phase and quadrature data were, simultaneously, fed to the modulator.

High-speed optical DQPSK and FSK modulation using integrated Mach-Zehnder interferometers.

A method and device for adequately controlling the DC bias of each of the optical modulating sections of an optical modulator even while the optical modulator is operating in normal mode and even with a simple structure. An optical modulator bias controller (B) for controlling the DC bias of each optical modulating section of an optical modulator (1) is characterized by comprising DC bias application means (3) for applying a DC bias to each of the optical modulating sections, a low-frequency signal superimposing circuit (2) for superimposing a low-frequency signal fB with a specific frequency on a modulating signal b applied to each optical modulating section, optical sensing means (9) for sensing a change of the intensity of the light wave passing through the combining section, and bias control means (4) for extracting the change of the intensity of light corresponding to the low-frequency signal from the optical sensing means and controlling the DC bias application means according to the extracted change of the intensity of light.

Method and device for controlling bias of optical modulator

An optical SSB modulator capable of appropriately adjusting a phase difference of light in the MZ waveguides and automatically adjusting a voltage impressed to the bias adjustment electrodes is provided. The above-mentioned object is achieved by an optical SSB modulator ( 1 ) provided with a first sub Mach-Zehnder waveguide (MZ A ) ( 2 ), a second sub Mach-Zehnder waveguide (MZ B ) ( 3 ), a main Mach-Zehnder waveguide (MZ C ) ( 4 ), a first bias adjustment electrode (DC A electrode) ( 5 ), a second bias adjustment electrode (DC B electrode) ( 6 ), a first modulation electrode (RF A electrode) ( 7 ), a second modulation electrode (RF B electrode) ( 8 ) and a third bias adjustment electrode (DC C electrode) ( 9 ), wherein either one of or both of output portions ( 10, 11 ) of the MZ A and the MZ B have an X-branching form, and specifically having one of light paths of the X-branching connected to a photodetector.

Optical SSB modulator

A high-speed DQPSK optical modulator is fabricated with a thin lithium niobate substrate. Decrease in the propagation loss of electrodes and a low half-wave voltage, which can be realized by fabrication of a ridge-type optical waveguide structure, can help achieve 120-Gb/s (60-Gbaud) NRZ-DQPSK modulation with full electrical-time-division-multiplexing technique.

https://www.jstage.jst.go.jp/article/elex/7/11/7_11_817/_pdf

Kaoru Higuma

Papers

25.6-Tb/s C+L-band transmission of polarization-multiplexed RZ-DQPSK signals

High-speed optical DQPSK and FSK modulation using integrated Mach-Zehnder interferometers.

Method and device for controlling bias of optical modulator

Optical SSB modulator

120-Gb/s NRZ-DQPSK signal generation by a thin-lithium-niobate-substrate modulator