1) Coherent optical receivers can meet the high bandwidth requirements of next generation optical fiber networks in an efficient hardware manner by digitally processing both the amplitude and phase of received optical signals. 2) Coherent receivers offer advantages over direct detection, including advanced digital signal processing capabilities to compensate for distortions and flexibility to support different modulation formats through software changes alone. 3) Coherent receiver technologies are necessary for next generation 100Gb/s optical transmission systems to meet future bandwidth demands and are being adopted by telecommunications providers like Sri Lanka Telecom to upgrade their transport networks.

1. Hardware efficient optical receiver for coherent digital
transmission system
H.W.R.D. Wanniarachchi
Engineer- Sri Lanka telecom
Abstract
The world is moving forward to a new era in
telecommunication field as the demand of
data, voice as well as other services have
been increased. More computer networks
came out as well as more technologies were
developed. The idea of Next Generation
Network causes the development in optical
fiber networks rather than old copper wire
network. Even though it wants elaborate
and different types of transmitters &
receivers, it will give more profits to users
by enabling a very efficient utilization of the
available bandwidth. The development of
coherent optical receivers thereby profits
from advancements in integrated circuit
technologies that allow the digital
realization of the required signal
processing.
This paper mainly describes how coherent
optical receivers can meet the requirements
addressed by optical fiber network in
hardware efficient manner.
Introduction
The continuous increase of the worldwide
data traffic demands new concepts for data
transmission in the optical fiber backbone
network. One promising way to increase
the capacity of the existing fiber
infrastructure is to use multilevel
modulation formats in combination with
polarization-multiplexing and coherent
detection. Though elaborate transmitters
and receiver are required to transmit
multiple bits per symbol, but this also
enables a very efficient utilization of the
available bandwidth.
Multi-level Phase shift keying (BPSK, QPSK,
8-PSK, 16-PSK, 64-PSK) and Quadrature
Amplitude Modulation (QAM) are most
result oriented modulation techniques that
compromise the bandwidth efficiency
verses hardware efficiency.
Coherent Optical Receiver
A coherent receiver system, which forms
the foundation of a digital coherent system,
is capable of achieving a reception
sensitivity that is higher than the direct
detection scheme conventionally used. It is
a method in which information on both
optical amplitude and phase is converted
into a current for reception.
Figure 01: Coherent optical receiver
structure.

2. Compared with the conventional coherent
receiver, coherent optical receiver front end
is mostly same. The difference is that the
output is analogue to digital converted and
then subject to waveform processing by a
digital signal processing unit.
This difference brings about the following
specific advantages.
a. DSP circuit allows the realization of
an advanced waveform distortion
equalizer, which is difficult to realize
with an analogue circuit.
b. Flexible combination with various
modulation schemes (multi-valued
PSK, QAM etc.) or intra wavelength
channel multiplexing (polarized
wave multiplexing, OFDM etc.) can
be achieved by making only the
minimum changes to the hardware
architecture.
Carrier phase estimation and frequency
offset compensation will lead to gain more
results discussed above. Researchers have
proposed many circuits to achieve those
two conditions.
Although optical transmission system have
advantages that of a conventional cupper
system still it may causes losses due to
polarization crosstalk and polarization
dependent loss, chromatic dispersion,
phase noise and attenuation. Many
algorithms and hardware designs have
proposed to eliminate those issues and get
maximum advantages out of the
technology.
Industrial Application
Telecommunication industry is now moving
towards 100Gb/s systems to cater the
future traffic demands. For the realization
of a 100Gb/s optical transmission system
that meets future demands, a digital
coherent receiver is a technology expected
to serve practical purposes.
Figure 02: Application of 100Gb/s
optical transport.
As Sri Lanka’s premier telecommunication
service provider Sri Lanka Telecom also
involving the project to upgrade their
transport network to cater 100G traffic by
implementing DWDM network which
enable to enhance NBN to cover whole
island. On top of that LTE, FTTH, Smart
Home applications will implement to fulfill
future demands.
Conclusion
Optical transmission network which
comprise of coherent digital receivers
enable to transport high traffic volume to
satisfy future demands including both voice
and data.