Special Issue on Enabling Technology in Optical Fiber Communications: From Device, System to Networking

Abstract

It is well known that optical fiber communications support the global communication networks nowadays, which originates from Charles K [...]

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sensors
Editorial
Special Issue on Enabling Technology in Optical Fiber
Communications: From Device, System to Networking
Yang Yue 1 ,*, Jian Zhao 2, Jiangbing Du 3and Zhaohui Li 4


Citation: Yue, Y.; Zhao, J.; Du, J.; Li,
Z. Special Issue on Enabling
Technology in Optical Fiber
Communications: From Device,
System to Networking. Sensors 2021,
21, 1969. https://doi.org/10.3390/
s21061969
Received: 2 March 2021
Accepted: 9 March 2021
Published: 11 March 2021
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1Institute of Modern Optics, Nankai University, Tianjin 300350, China
2School of Precision Instruments and Optoelectronics Engineering, Tianjin University, Tianjin 300072, China;
enzhaojian@tju.edu.cn
3State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong
University, Shanghai 200240, China; dujiangbing@sjtu.edu.cn
4State Key Laboratory of Optoelectronic Materials and Technologies, School of Electronics and Information
Technology, Sun Yat Sen University, Guangzhou 510275, China; lzhh88@mail.sysu.edu.cn
*Correspondence: yueyang@nankai.edu.cn
It is well known that optical fiber communications support the global communication
networks nowadays, which originates from Charles K. Kao’s proposal of using optical
fiber as a light transmission medium in 1966 [
1
]. By utilizing different degrees of freedom
of the photon, society has made tremendous progress over the past half century. Revolu-
tionary technologies debut one after another, including wavelength-division multiplexing
(WDM), coherent detection, space-division multiplexing (SDM), and so forth [
2
,
3
]. Recently,
the emerging 5G, cloud computing, and high-definition video have been driving more
bandwidth and power-hungry applications. To better serve these needs, the optical fiber
communications community has been escalating the research and development efforts in
device, system, and networking to the next level.
This Special Issue aims to explore the enabling technology in optical fiber commu-
nications. It focuses on the state-of-the-art advances from fundamental theories, devices,
and subsystems, to networking applications, as well as future perspectives of optical fiber
communications. The collected papers have well accomplished these goals by contributing
leading-edge derivation, analysis, and experiments with significant results. The topics
cover integrated photonics, fiber optics, fiber and free-space optical communications, and
optical networking. The special issue consists of one review paper, nine research articles,
and eight letters.
More specifically, from the integrated device perspective, Fang, Y. et al. proposed
an Si
3
N
4
/SiO
2
horizontal-slot-waveguide-based polarization beam splitter (PBS) [
4
]. Its
coupling length can be effectively reduced due to the slot design, and the extinction
ratios (ER) of the fundamental modes for two orthogonal polarizations are both >20 dB.
Furthermore, it features low nonlinearity, which is critical for on-chip high-power systems.
For fiber-based devices, Rong, J. et al. numerically simulated shaping the supercontinuum
(SC) using the fiber cascading method to significantly increase the SC spectral width and
flatness in silica photonic crystal fiber (PCF) [
5
]. To characterize linear frequency-modulated
continuous-wave (FMCW) lasers, Yang, J. et al. [
6
] proposed a scheme for measuring the
mapping relationship between instantaneous frequency and time of a FMCW laser based
on a modified coherent optical spectrum analyzer (COSA) and digital signal processing
(DSP) method. The authors demonstrated precisely measuring an FMCW laser at a large
fast sweep rate of 5000 THz/s, while maintaining <100 kHz uncertainty.
Regarding the system-level technologies for optical communication, Tseng, S. et al.
developed a bipolar optical code division multiple access (Bi-OCDMA) technique based
on spectral amplitude coding for the formation and transmission of optical-polarized and
coded signals over wireless optical channels [
7
]. The proposed free-space optics commu-
nication system used a dual electro-optical modulator design, which could improve the
Sensors 2021,21, 1969. https://doi.org/10.3390/s21061969 https://www.mdpi.com/journal/sensors

Sensors 2021,21, 1969 2 of 4
transmission rate. For direct-detection optical communication systems, Zhang, W. et al.
compared the complexity, efficiency, and stability performance of pruned Volterra series-
based equalization (VE) and neural network-based equalization (NNE) for 112 Gbps
vertical-cavity surface-emitting laser (VCSEL)-enabled optical interconnects [
8
]. The ex-
perimental results showed that NNE has more than one order of magnitude bit error
rate (BER) advantage over VE at the same computation complexity, and pruned NNE
has around 50% lower computation complexity compared to VE at the same BER level.
For coherent-detection optical communication systems, Ding, J. et al. investigated the
impact of equalization-enhanced phase noise (EEPN) in Nyquist-spaced dual-polarization
quadrature amplitude modulation (DP-QAM) links [
9
]. It was found that EEPN-induced
distortions become more significant with the increase of the local oscillator (LO) laser
linewidth, and this results in degradations in BER, achievable information rate (AIR), and
AIR-distance product. Moreover, Wu, B. et al. proposed a blind discrete-cosine-transform-
based phase noise compensation (BD-PNC) to compensate the intercarrier interference (ICI)
in the coherent optical offset-quadrature amplitude modulation (OQAM)-based filter-bank
multicarrier (CO-FBMC/OQAM) transmission system [
10
]. The simulation results showed
that its BER performance is improved by more than one order of magnitude through the
mitigation of the ICI over traditional blind PNC scheme only aiming for common phase
error (CPE) compensation. For quantum communication applications, Wu, B. et al. pro-
posed and experimentally demonstrated a secure key generation and distribution system
that is compatible with optical amplifiers and standard WDM transmission systems [
11
].
The key generation system was tested in a 240 km bidirectional fiber-pair link with mul-
tiple optical amplifiers, and 38 WDM channels were transmitted together with the key
distribution channel.
On the networking level, Holik, M. et al. created an open-source software-based solu-
tion for monitoring traffic transmitted through gigabit passive optical network (GPON) [
12
].
The work described the issue of writing to the Mongo database system, showing that the
high processing speed is too high for Python processing and critical operations must be
implemented in the C# programming language. He, C. et al. proposed a FiWi broadband
access network, integrating the wireless mesh network (WMN) frontend subnetwork,
together with time and wavelength division multiplexed PON (TWDM-PON) optical back-
haul and adapting power over fiber (PoF) technology [
13
]. For elastic optical networks
(EONs), He, S. et al. proposed an advanced-reservation-based invalid-spectrum-aware
(AR-ISA) resource allocation algorithm to improve the networking performance and the
resource alignment [
14
]. Moreover, Rodrigues, E. et al. proposed a crosstalk-aware routing,
modulation, spectrum, and core allocation (RMSCA) algorithm that uses a multipath and
mapping scheme for improving resource allocation [
15
]. Simulation results show that
the algorithm decreases the blocking ratio by up to four orders of magnitude compared
with the other RMSCA algorithms in the literature. Furthermore, Zong, Y. et al. surveyed
the state-of-the-art works for the virtual network embedding (VNE) problem towards
multidomain heterogeneous converged optical networks, and discussed the future research
issues and challenges [16].
Additionally, there are several collected papers aimed at sensing applications. Wu, Z. et al.
demonstrated Bragg-grating-assisted Sagnac interferometer in SiO
2
-Al
2
O
3
-La
2
O
3
polarization-
maintaining fiber for strain–temperature discrimination [
17
]. Xu, X. et al. proposed and
demonstrated a temperature and humidity sensor based on a fluorinated polyimide film
and fiber Bragg grating [
18
]. Han, H. et al. proposed a surface plasmon resonance (SPR)
sensor based on a dual-side polished microstructured optical fiber (MOF) with a dual
core [
19
]. Wu, Y. et al. proposed a resolution enhancement and signal-to-noise ratio (SNR)
improvement scheme for digital optical frequency comb (DOFC)-based Brillouin optical
time-domain analysis (BOTDA) ultrafast distributed sensing employing a pump pulse
array [
20
]. Cheng, Y. et al. analyzed the source of the position deviation and proposed a
demodulation recursive compensation algorithm to ensure a submillimeter resolution in
improved optical frequency domain reflectometry (OFDR) [21].

Sensors 2021,21, 1969 3 of 4
It has just been over 50 years since the discovery of optical fiber as a low-loss light
transmission medium by Charles K. Kao and his coworkers. Within this fairly short
period of time, an extensive research community and industry have been established
globally. Nowadays, optical fiber communications is the backbone of our information
technology infrastructure, supporting voice, video, and data transmission through global
networks. One critical issue in its research and development is the challenge of meeting
the needs of increasing the data capacity without compromising size, weight, power, and
cost (SWaP-C) constraints. Especially during the past decade, photonics integration and
coherent detection technologies were booming tremendously. From this trend, we are
expecting the co-packaged optics (CPO) to dominate the next-generation optical fiber
communication systems. Hopefully, more and more novel technologies, such as SDM, can
be commercialized in the near future, to enable new growth for the industry and serve the
ever-growing data-traffic demand from society.
Conflicts of Interest: The authors declare no conflict of interest.
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