Lightwave communications is a necessity for the information age. Optical links provide enormous bandwidth, and the optical fiber is the only medium that can meet the modern society's needs for transporting massive amounts of data over long distances. Applications range from global high-capacity networks, which constitute the backbone of the internet, to the massively parallel interconnects that provide data connectivity inside datacenters and supercomputers. Optical communications is a diverse and rapidly changing field, where experts in photonics, communications, electronics, and signal processing work side by side to meet the ever-increasing demands for higher capacity, lower cost, and lower energy consumption, while adapting the system design to novel services and technologies. Due to the interdisciplinary nature of this rich research field, Journal of Optics has invited 16 researchers, each a world-leading expert in their respective subfields, to contribute a section to this invited review article, summarizing their views on state-of-the-art and future developments in optical communications.

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Roadmap of optical communications

The unitraveling-carrier photodiode (UTC-PD) is a novel photodiode that utilizes only electrons as the active carriers. This unique feature is the key for its ability to achieve excellent high-speed and high-output characteristics simultaneously. To date, a record 3-dB bandwidth of 310 GHz and a millimeter-wave output power of over 20 mW at 100 GHz have been achieved. The superior capability of the UTC-PD for generating very large high-bit-rate electrical signals as well as a very high RF output power in millimeter/submillimeter ranges can lead to innovations in various systems, such as broadband optical communications systems, wireless communications systems, and high-frequency measurement systems. Accomplishments include photoreceivers of up to 160 Gb/s, error-free DEMUX operations using an integrated UTC-PD driven optical gate of up to 320 Gb/s, a 10-Gb/s millimeter-wave wireless link at 120 GHz, submillimeter-wave generation at frequencies of up to 1.5 THz, and photonic frequency conversion with an efficiency of -8 dB at 60 GHz. For the practical use, various types of modules, such as a 1-mm coaxial connector module, a rectangular-waveguide output module, and a quasi-optic module, have been developed. The superior reliability and stability are also confirmed demonstrating usefulness of the UTC-PD for the system applications.

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High-speed and high-output InP-InGaAs unitraveling-carrier photodiodes

We discuss optical multi-band orthogonal frequency division multiplexing (OFDM) and show that by using multiple parallel OFDM bands, the required bandwidth of the digital-to-analogue/ analogue-to-digital converters and the required cyclic prefix can significantly be reduced. With the help of four OFDM bands and polarization division multiplexing (PDM) we report continuously detectable transmission of 10 times121.9-Gb/s (112.6-Gb/s without OFDM overhead) at 50-GHz channel spacing over 1,000-km standard single mode fiber (SSMF) without any inline dispersion compensation. In this experiment 8 QAM subcarrier modulation is used which confines the spectrum of the 121.9 Gb/s PDM-OFDM signal within a 22.8 GHz optical bandwidth. Moreover, we propose a digital signal processing method to reduce the matching requirements for the wideband transmitter IQ mixer structures required for PDM-OFDM.

/pdf/121-9-gb-s-pdm-ofdm-transmission-with-2-b-s-hz-spectral-2dovklbj9g.pdf

121.9-Gb/s PDM-OFDM Transmission With 2-b/s/Hz Spectral Efficiency Over 1000 km of SSMF

A waveguide to microstrip T-junction includes a microstrip transmission line structure having a ground plane separated from a strip conductor by a dielectric layer, the ground plane defining an aperture; a waveguide channel having a conductive periphery being electrically coupled to the ground plane to provide a waveguide short circuit wall located at the end of the waveguide channel; at least one conducting ridge inside the waveguide channel; and an end of the ridge being electrically coupled with the ground plane.

Waveguide to microstrip transition

This paper reviews ultrahigh-speed data transmission in optical fibers based on optical time division multiplexing (OTDM) transmission technology. Optical signal processing in the transmitter and receiver as well as the requirements on ultrahigh-speed data transmission over a fiber link are discussed. Finally, results of several OTDM-transmission experiments, including 160-Gb/s transmission over 4320 km, 1.28-Tb/s transmission over 240 km, and 2.56-Tb/s transmission over 160-km fiber link, are described

Ultrahigh-Speed OTDM-Transmission Technology

In an optical receiver of the prior art, a multistage fiber-optic amplifier in which optical signal components propagating along different paths in the same direction beat with one another generate noise in addition to the noise due to spontaneous emission in the amplifier. A two-stage fiber-optic amplifier, according to the invention is provided in which a first section of optical fiber (F1) is pumped opposite to or in the direction of propagation of the optical signal and a second section of optical fiber (F2) is pumped in or opposite to the direction of propagation. A pump-light source (1), the sections of optical fiber (F1, F2), and couplers (K1, K2, K3) are arranged in such a way that any signal components propagating along different paths in the same direction will not beat with one another.

Multistage fiber-optic amplifier

We have demonstrated an advanced 160 Gbit/s OTDM transmission system with a wavelength transparent transmitter concept suitable for DWDM operation. Latest 40 Gbit/s ETDM technology is used for the first time both in the 160 Gbit/s transmitter and receiver along with PLL clock recovery. 160 Gbit/s OTDM transmission has been demonstrated over 2 /spl times/ 100 km of TeraLight/spl trade/ fibre without an error floor. The experiments clearly show that long TeraLight/spl trade/ fibre spans can be bridged without the need for Raman-amplification.

Advanced 160 Gbit/s OTDM system based on wavelength transparent 4 /spl times/ 40 Gbit/s ETDM transmitters and receivers

To compensate for nonlinear signal distortions in analog optical communication transmission systems, caused by laser chirps and the chromatic dispersion of the optical fiber, an equalizer in the form of an LC component is known, whose capacitance is formed by a variable capacitance diode. However, the known equalizer functions only when the capacitance has the proper polarity, which cannot be predicted because of possible polarity inversion during signal transmission. According to the invention, the variable capacitance diode (C a ) has another variable capacitance diode (C b ), with the opposite polarity, connected in parallel, and is equally biased in the high-resistance direction. By adjusting the bias voltage of both capacitances, it can be achieved that one of the two variable capacitance diodes takes over the equalization function, and the other is practically inoperative. Further developments of this solution concern keeping the frequency response constant during adjustment of the equalization, by means of a capacitance connected in parallel to the variable capacitance diodes (C a , C b ), and the equalization of large bandwidth signals by an LC chain circuit with LC components according to the invention.

Equalizer for optical communication systems

http://ieeexplore.ieee.org/iel5/5760047/5760048/05760057.pdf

Bernhard Junginger

Papers

Multistage fiber-optic amplifier

Advanced 160 Gbit/s OTDM system based on wavelength transparent 4 /spl times/ 40 Gbit/s ETDM transmitters and receivers

Equalizer for optical communication systems

8 Tbit/s (80x107 Gbit/s) DWDM ASK-NRZ VSB transmission over 510 km NZDSF with 1bit/s/Hz spectral efficiency

8x107 Gbit/s Serial Binary NRZ/VSB Transmission over 480 km SSMF with 1bit/s/Hz Spectral Efficiency and without Optical Equalizer