IET Optoelectronics
Research Article
Digital dispersion pre-compensation based on
intensity modulator and phase modulator for
PAM-4 in short-reach fibre communications
ISSN 1751-8768
Received on 26th June 2016
Revised 13th September 2016
Accepted on 29th March 2017
E-First on 7th July 2017
doi: 10.1049/iet-opt.2016.0088
www.ietdl.org
Bo Xu
1
, Kun Qiu
1
1
Key Laboratory of Optical Fibre Sensing and Communications (Education Ministry of China), School of Communication and Information
Engineering, University of Electronic Science and Technology of China, People's Republic of China
E-mail: xubo@uestc.edu.cn
Abstract: High-speed PAM-4 is a promising way for future low-cost short-reach fibre communications. However, for 1550 nm
transmission with the lowest attenuation for standard single-mode fibre (SSMF), the fibre dispersion-induced inter-symbol
interference (ISI) becomes non-linear in direct detection. Dispersion pre-compensation at the transmitter side is necessary to
avoid such non-linear ISI. Different from previous methods to generate the vector signal from dispersion pre-compensation
using IQ modulators, in this study a digital dispersion pre-compensation (DDPC) scheme based on an intensity modulator (IM)
plus a phase modulator (PM) is studied. Taking into account the digital-to-analogue converter (DAC) bandwidth limit on the
vector signal generation, anti-aliasing filtering on both the magnitude signal and the phase signal should be applied. The error
rate performance of the IM–PM based DDPC scheme is analysed in detail using numerical simulations. For 50 Gb/s PAM-4
transmission at 1550 nm over SSMF, over 50 km fibre transmission can be supported if the two DACs have enough bandwidth
larger than 17.5 GHz. If the DAC bandwidth is limited to 15 GHz, it is also possible to transmit over 20 km SSMF. Meanwhile, it
is verified that practical 5-bit DAC resolution is enough for the IM–PM based DDPC scheme to work for a wide range of SSMF
lengths.
1 Introduction
Short-reach fibre communication technologies are the keys to meet
the increasing bandwidth demand for data centre and optical
interconnects. For this purpose, carrier-less amplitude and phase
modulation (CAP) and discrete multi-tone (DMT) modulation have
been studied as two possible choices. Both CAP-32 and other
higher order CAP systems have been experimentally demonstrated
[1, 2]. With the help of high-speed arbitrary waveform generator
and electro-absorption modulated laser (EML), multi-band CAP
can be used to achieve 102 Gb/s signal transmission over 15 km
[3]. DMT can be seen as another form of orthogonal frequency
division multiplexing with direct detection for low cost. A 52.8
Gb/s DMT signal transmission over 20 km standard single-mode
fibre (SSMF) has been achieved with a distributed feedback laser
[4]. A 101 Gb/s DMT signal generated with high-speed digital-to-
analogue converter (DAC) chips has been verified by experiment
to be able to transmit over 10 km of SSMF [5]. Though both CAP
and DMT can achieve good system performance with high
spectrum efficiency and flexible multi-level modulating,
complicated signal processing is required at both transmitter side
and receiver side.
Pulse amplitude modulation (PAM) is another promising choice
that may provide high data rate transmission with low-cost
implementation. In [6], an SiP Mach–Zehnder modulator working
at 1310 nm was demonstrated for the generation of 112 Gb/s
PAM-4 signals. The authors in [7–9] demonstrated analytical and
experimental results for PAM-4 transmission with different
wavelengths using either multi-mode fibre or single-mode fibre.
However, if 1550 nm is used for its lowest attenuation for SSMF,
then equalisation at the receiver side is prerequisite to recover
PAM-4 signals with 50 Gb/s rate and above due to serious inter-
symbol interference (ISI) caused by fibre dispersion. Feed-forward
equalisation (FFE), decision-feedback equalisation (DFE) and
maximum likelihood sequence estimation (MLSE) are the three
most commonly used equalisation techniques [10–13]. Compared
with FFE and DFE, MLSE can achieve a better performance at the
cost of higher complexity.
Dispersion pre-compensation at the transmitter side provides
instead another possibility to reduce the detrimental effects from
the dispersion-induced ISI especially for systems with direct
detection [14, 15]. However, for a real signal like PAM, the signal
becomes a vector signal after dispersion pre-compensation and an
IQ modulator is often used for the generation of the dispersion pre-
compensated PAM signal [16, 17]. Here in this paper, a different
approach to generate the dispersion pre-compensated signal using
an intensity modulator (IM) plus a phase modulator (PM) is
studied. The IM can be implemented with an external electro-
absorption modulator (EAM) or an EML. Compared with the IQ
modulator scheme, the IM–PM based scheme has the advantage of
low cost which is especially important for short-reach fibre
communications. Similar IM–PM based schemes have been
successfully applied to optical signal processing like phase noise
cancellation in a microwave photonic link [18] and ultra-broadband
radio-frequency signal generation [19]. It also came to our
attention that such an IM–PM based approach to generate the
dispersion pre-compensated PAM signal has been experimentally
demonstrated and reported by a different group in a recent paper to
be published [20].
However, no matter whether the vector signal after dispersion
pre-compensation is generated using an IQ modulator or the IM–
PM based scheme, high-speed DACs are required. In the
experiments of [20], high-resolution 8-bit DAC from signal
generator is used with large enough bandwidth and the system's
bandwidth is limited by the IM. However, instead in this paper, we
focus on the DACs non-ideal properties, especially the limited
DAC bandwidth and resolution which might have great influence
on the performance of the generated signals [21–23]. The IM–PM
based digital dispersion pre-compensation (DDPC) scheme is
introduced in the next section. Then, numerical simulations are
used to study the error rate performance of the system under
limited DAC bandwidth and resolution.
2 Principle of the IM–PM based DDPC scheme
Fig. 1 gives the schematic of a short-reach fibre communication
system with PAM-4 modulation at the transmitter side and direct
IET Optoelectron., 2017, Vol. 11 Iss. 4, pp. 163-168
© The Institution of Engineering and Technology 2017
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