Ultra-wideband optical communication is the frontier of research to increase the capacity of fiber infrastructure. As the capacity of C-band systems with coherent technologies is approaching the Shannon limit, extending frequency bands (from C band to C + L or C + L+S band) becomes the path to further scale the capacity of single mode fiber. Realizing ultra-wideband optical communication requires cooperatively developed components and algorithms such as multi-band optical amplifiers, multi-band reconfigurable optical add-drop multiplexers (ROADM), algorithms for fiber impairment compensation, flexible coded modulation schemes, and joint signal processing, end-to-end system optimization and control, and so forth. Meanwhile, to maintain a compact transponder size for ultra-wideband systems, high interface rate per wavelength is needed. Currently, the interface data rate per wavelength is approaching Tb/s, enabled by advanced digital signal processing (DSP), coded modulation and high bandwidth components such as ultra-fast optical modulator. Superchannel transceiver is also being developed to realize multi-Tb/s throughput. Utilizing the ultra-high speed transceivers, along with multi-band components and algorithms, data rate per fiber may achieve 100 Tb/s and beyond.

This Feature Issue aims to present some of the most recent advances on these topics, and in the following, we provide a summary of 12 articles that appear in this issue.

The key component of generating ultra-wideband signals with a baud rate beyond 100 Gbaud is electro-optic modulator. M. Xu and X. Cai present a comprehensive review of the recent advances in integrated ultra-wideband electro-optic modulators [1]. Design guidelines, structures and material platforms are discussed and a future roadmap is provided. X. Liu and co-authors propose a wideband thin-film lithium niobate modulator for sub-terahertz operation [2]. Realized by periodic capacitively loaded travelling-wave electrodes (CL-TWEs) with an undercut structure, the 3-dB bandwidth of this modulator is estimated to be beyond 300 GHz for a 10-mm modulation length.

For systems with a high baud rate and high order modulation, channel impairment modeling and compensation need to be revisited. N. Cui and co-authors investigate polarization mode dispersion (PMD) in a wideband fiber channel beyond 100 GHz [3], and find that it manifests a more complicated effect with an all-order manner. As a result, channel equalization becomes more difficult. In [4], J. Yang and co-authors propose a pilot-based multiple-input multiple-output (MIMO) equalizer for 64QAM signals, which can track fast state of polarization (SOP) change and is immune to cycle slips. A tolerance to a SOP speed of 600 krad/s and laser linewidth of 2 MHz is achieved for 56 GBaud 64QAM systems.

Superchannel transmission is an approach to scale single channel capacity beyond the electro-optic bandwidth of optical transceiver. G. Liu and co-authors demonstrate an aggregate 12.544 Tbit/s superchannel transmission capacity using an InAs/InP quantum dot mode-locked laser as a chip-scale optical frequency comb source [5]. This result shows the feasibility of building low-cost comb sources to realize ultra-high capacity systems.

To increase the bandwidth of current fiber links, multi-band transmission is exploited and optical amplifiers play a key role in such systems. Y. Ososkov and co-authors report a design of bismuth-doped fiber amplifier for E + O bands, and demonstrate >23 dB gain from 1325 nm to 1441 nm with flatness <3 dB and a noise figure (NF) of <6.8 dB [6]. This result features a new record for fiber amplifier in this spectral region. In addition to optical amplifiers, other optical devices are required to support multi-band systems. In [7], J. Zhang and co-authors propose an all-silicon multi-band TM-pass polarizer based on one-dimensional gratings for O-, S-, C-, and L- bands. This simple and compatible design is suitable for the development of practical multi-band silicon photonic integrated circuits.

When using multiple bands for transmission, link optimization and network management become much more complex and critical. To address the stimulated Raman scattering effect in multi-band transmission, H. Luo and co-authors propose a simulated annealing algorithm based power control scheme [8], and show that it can improve the capacity of a C + L+S-band system at a fast searching speed. In [9], N. Shevchenko and co-authors compare three optimization strategies: optimizations of non-uniformly and uniformly distributed launch power per channel, and optimization based on the optimal ratio of linear and nonlinear noise, with the target of achieving the maximization of ultra-wideband fiber channel capacity. Regarding network management, I. Khan and co-authors first present a wideband optical switch design based on photonic integrated circuits and then propose a software-defined network (SDN) based model that can determine the control state and evaluate QoT degradation of the system with the photonic switching fabric [10].

Finally, two works report demonstrations of optical transmission systems that improve link capacity through exploiting a wider spectrum of fiber. In [11], B. Puttnam and co-authors successfully transmit a signal with a bandwidth exceeding 157 nm in S-, C-, and L-bands over a standard single mode fiber. Link amplification is realized using both doped fiber amplifiers and distributed Raman amplifiers. The total capacity is 244.3 Tb/s and the distance is 54 km. In [12], A. Zhang and co-authors report a field trial of real-time 24 Tb/s transmission over a 1910km deployed G.654.E fiber link with 60 channels of 400 Gb/s signals. In this demonstration, the C-band is widened to 6 THz (1524 nm∼1572 nm), which is essential to achieve this result.

We would like to thank all the authors who have contributed to this issue. We would also like to thank Optics Express Peer Review Manager Carmelita Washington for the continuous help during the preparation of this Feature Issue.