1White Paper

SEP 2019

ARRAYED WAVEGUIDE GRATING

Arrayed Waveguide
Grating
Dr. Bernard Lee, Ryo Konno, Mac Watanabe

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ARRAYED WAVEGUIDE GRATING

Arrayed Waveguide Grating

Contents

4Introduction

5

Operation Principle and Device Characteristics

7

Different AWG Technologies

9Applications

10 Mobile Fronthaul Network
11 Summary
12 Reference
12 Biography

4

ARRAYED WAVEGUIDE GRATING

Introduction
ArrayedopticalDue to their ability to multiplex large numbers of wavelengths into aplanar devices that are usually used as multiplexers/

WaveguideGratings(AWG)

aresingle optical fiber, AWGs are commonly used as optical multiplexers

demultiplexers. These design of these devices are based on an array ofand demultiplexers in a Wavelength Division Multiplexed (WDM)

waveguides with both imaging and dispersive properties. Althoughsystem. There are other areas of application such as signal processing,

AWGs are also known under different names such as Phased Arrays

measurement, and sensing. Silica-on-Silicon and Indium Phosphide(PHASARs), and Waveguide Grating Routers (WRGs), AWG is the most(InP) based semiconductor are the most deployed technologies

frequently used term in the telecommunications industry.

in the AWG market. The modal field matches well with that of anLight waves of different wavelengths interfere linearly with each other,

optical fiber, thus making them easy to couple with losses of less than

thus multiple optical channels with slightly different wavelengths

0.1dB. In addition, there is also a very low propagation loss of lesscan be transmitted over a single optical fiber with minimal crosstalkthan 0.05dB/cm. InP is the dominant technology for operation in thebetween the channels. Based on this principle, AWGs can be used

telecom window.

to multiplex multiple channels of different wavelengths on a single
optical fiber at the transmitter and also be used to demultiplex themback into their individual channels of different wavelengths at thereceiver.

5ARRAYED WAVEGUIDE GRATING

Operation Principle and Device Characteristics
An AWG is made up of mainly 3 parts, which are theinput/output fiber, the Free Propagation Region (FPR),

Typical spectrum of AWG
Arrayed Waveguides

and array waveguides.
Light wave of multiple different wavelengths enter the FPR throughthe input fiber. Within the FPR, the light wave is no longer confinedwithin an optical fiber and becomes divergent and enters a waveguidearray. The expanded light is then captured by the array waveguidewhich propagates it towards the output array aperture.
The individual waveguides have varying lengths with the inner guides

Free Propagation Region(FPR)

Input Fiberbeing shorter than the outer ones. The length difference of adjacent
waveguide paths is equal to the integer multiple of the central
wavelength of the DeMUX. This causes the wavelengths to arrive at theOutput Fiber

Constructive interference

Destructive interference++==

other end of the FPR at slightly staggered time, with the signals fromthe inner waveguide arriving first and the outer waveguide arriving
last. The length of the array waveguides is chosen such that the
optical path length difference between adjacent waveguides equals
an integer multiple of the central wavelength of the demultiplexer.
Hence, the wavelengths from the individual arrayed waveguides
arrive at the input aperture of the output coupler at different phases.
The multiple light beams interferes constructively and converges at asingle focal point at the output of the output coupler.

Image of AWG chip

Typical spectrum of AWG

6ARRAYED WAVEGUIDE GRATING

There are also AWG designed with multiple input and an equal
number of outputs. Such an AWG has a cyclical behavior with where aWavelength routersignal going into Input N will reappear at Output 1, if the frequency is

a1, a2, a3, a4

a4, b3, c2, d1

b1, b2, b3, b4

a3, b2, c1, d4

c1, c2, c3, c4

a2, b1, c4, d3

d1, d2, d3, d4

a1, b4, c3, d2

increased by an amount equal to the channel spacing.
This device is called a cyclical wavelength router. This type of AWG
can function as an add-drop multiplexers and wavelength switches.

Depending on the configuration of AWGs and wavelength switches,
add-drop multiplexers can be made.

Multiplexer with fixed add-drop channel

The most basic add-drop multiplexer can be made by using two 1xN

λ1

AWGs with identical wavelength response.
IN

By combining demultiplexers with switches, configurable add-drop

λ2
λ3

DMX

MUX

OUT

multiplexers can be made. This configuration allows the wavelengths
to be added and dropped with an external control signal. The more(de)multiplexers added to the configuration increases the insertion

DROP

λ4

ADD

loss of the multiplexer.
Add-drop multiplexers with lower insertion loss can be made bycombining a single (N+1) x (N+1) AWG with a wavelength router in a

Add-drop multiplexer with wavelength router

loop-back configuration.
The demultiplexed wavelengths are fed into switches where they can

λ1

be switched to the drop port or be looped back to the wavelength

λ2

router which will then multiplex them back into the output port.

IN

DMX

X
X

λ3

MUX

OUT

X

λ4

X
DMX

MUX

ADD

DROP

Reconfigurable add-drop multiplexer with 2x2 switch
OUTIN

X
X
X
X
DMX

ADD

MUXDROP7

ARRAYED WAVEGUIDE GRATINGDifferent AWG Technologies
There are many technologies deployed to develop AWGs. The two main technologies used for AWG manufacturing are the silica-on-silicon
technology and the Indium Phosphide based semiconductor technology.

Silica-on-Silicon (SoS) AWG

Silica-on-Silicon AWG chip

SoS AWG was introduced in the market in the early 1990’s and holds
the largest share of the AWG market. SoS is a type of Planar Lightwave
Circuit (PLC) that are fabricated on planar substrates by depositing
layers of glass with high silica content on the wafer. The compositionof the glass layers is very similar to an optical fiber which makes it easy
to couple with optical fibers due to its close modal field match.
This results in low coupling and propagation attenuation.
Another advantage of the PLC fabrication of the SoS AWG is its
excellent heat dissipation properties which makes it suitable for
deployment in an outside plant network environment. The majority
of commercially available SoS AWG has the majority share of the AWG
market due to its lower cost of manufacturing andInput partImage of cross section

AWG module

Output part8

ARRAYED WAVEGUIDE GRATINGIndium Phosphide (InP)-based AWG
InP-based AWG is a semiconductor-based AWG that can be integrated

The potential of InP-based AWG to be integrated in circuits withto multiple active devices such as optical amplifiers and switches on

multiple functionalities such as WDM transceivers, and opticala single chip. InP-based AWG can be manufactured in a compact

add-drop multiplexers is its biggest advantage. This allows forpackage due to the large index-contrast of InP-based waveguides.

manufacturers to integrate AWG functionalities onto active

The optical attenuation, coupling losses, and crosstalk performanceequipment to create InP-based Photonic Integrated Circuits (PICs) toof InP-based AWGs are not as good as silica based AWGs. Such alower network deployment cost. For example, add-drop multiplexing

disadvantage is a barrier for InP-based AWG to be more widely

functions can be done at the transceiver without the need for an

deployed as a stand-alone device.

external multiplexer. This reduces the component and installation
cost, as well as potential optical attenuation from multiple opticalconnectors.

9ARRAYED WAVEGUIDE GRATING

Applications
There are a large number of applications where AWGs canWith the increasing demand for higher bandwidth, AWGs have started

be deployed, ranging from complex telecommunication

to be used in the access network to allow multiple wavelengths

cross-connects to very simple add-drop multiplexers. In theto be transmitted from the exchange to the end user without

telecommunication industry, AWGs are mostly used as (de)significant changes to the existing optical fiber network. WDM-PON

multiplexers in WDM networks. This is traditionally deployed in long-is a technology where multiple WDM channels are transmitted overhaul networks such as international, national, and regional transport

the same optical network from an Optical Line Terminal (OLT) thatnetworks.

is housed in the exchange. WDM-PON channels can be overlaid on

The majority of Passive Optical Networks (PON) deployed globally uses

top of existing PON networks using demultiplexing and imaging

wavelength-independent optical splitter for power splitting and time

properties of an AWG to direct specific wavelength channels todivision multiplexing for upstream and downstream transmission.

individual subscribers. The upstream channels are multiplexed by theThis enables a lower cost of deployment and removes the need toAWG and directed to one output port.

manage wavelengths to individual connections downstream fromthe optical splitter.

AWG
imagingINλaλa , λ1

1/N

IN downstream
OUT upstream

λ1 ... λN..
.
λa , λNDeMUX

10ARRAYED WAVEGUIDE GRATING

Mobile Fronthaul Network
Mobile communications have evolved from a simple
voice application to high speed data networks. Mobilenetworks started with the deployment of GSM infrastructures using aTime Division Multiplexing (TDM) backhaul network which dominatedthe global mobile industry over CDMA. However, GSM is not designed
to support data heavy applications, thus data-optimized mobiletechnology such as 3G and WCDMA was created which uses theAsynchronous Transfer Mode (ATM) which replaced the TDM network.
This allowed the convergence of voice and data transmission as itsupported best-effort connectivity services as well as fixed bandwidth.
Soon, LTE (Long Term Evolution) was introduced which moved from anATM network to an IP based network. The mobile backhaul network
infrastructure started to transform to utilize technologies such as
Carrier Ethernet and MPLS which optimizes data management over L2and L3.
The aggregation of digitized RF signals from various antennas can bedone through the CPRI (Common Private Radio Interface) interface.
With an IP based network, there is higher demand in network
synchronization, jitter, and delay requirements. Higher inter-cellbandwidth demand also puts a high load onto the network, especially
if the network is based on the traditional centralized architecture.
Radio Base Stations (RBS) consists of a Radio Frequency Unit (RU) which
generates the radio signal, and a Baseband Unit (BBU), which processes
the user and control data. While the RU is mounted on a pole, the BBU
is usually installed at the base of the pole. The inter-cell transmission
latency between BBUs becomes a bottleneck with higher bandwidth
requirements. A new network architecture is required.

R
R
H
R
R
H
S
F
PR
R
H
S
FP

S
FP

R
R
H
S
FPDW
DM

R
R
H
S
F
P

R
R
H
S
FPR
R
HS
FP

Rather than housing the BBUs with the RUs, the BBUs can be housed in acentral location which significantly increases inter-cell communication.
This architecture is crucial for the deployment of LTE-Advanced
networks due to its tight inter-cell coordination requirements. The RUs,
now referred to as RRH (Remote Radio Heads) can remain mounted ona pole. Another advantage of this architecture is the reduction in power
requirements since the processing unit of the network do not need tobe housed remotely.
The centralization of the BBUs is also known as C-RAN (CentralizedRadio Access Network or Cloud Radio Access Network). With higher
bandwidth and transmission distance demands between the C-RANand RRHs, optical fiber is the natural transmission medium of choice.
The optical fiber network between the C-RAN and RRH is often referred
to as the mobile fronthaul network.
A simple fronthaul network can be designed as a point-to-point fiber
network. However this requires a high number of optical fiber run
from the central office to each RRH. An effective way of deploying ahigh bandwidth fronthaul network with minimal access network fiber
is by implementing C/DWDM technology to maximize fiber bandwidth
utilization while minimizing installation cost. Each RRH can be assignedwith a dedicated wavelength which essentially creates a “point-topoint” connection between the BBU and RRH. Such a network can evenbe overlaid on an existing GPON access network, provided there issufficient attenuation overhead to support the additional losses fromthe C/DWDM (de)multiplexers.

RR
H
S
FP

DW
DM
A
DM

RR
H
S
F
P

DW
DM

DW
DM

SFP

BBU

SFP

BBU

SFP

BBU

SFP

BBU

SFP

BBU

SFP

BBU

SFP

BBU

SFP

BBU

SFP

BBU

11ARRAYED WAVEGUIDE GRATING

Summary
Understanding the need of the market, SENKO has embarked on the mission to makes good and reliable AWGs since the early 2000s.
Currently, Senko offers Athermal AWG integrated optical circuit built by Polymer approach (SoS substrate) that exceeds industrial
requirements by ensuring a more stable and reliable performance with temperature ranging from -40°C to 85°C. Other than guaranteeing
excellent optical performance coupled with high reliability. SENKO’s AWG offers customizable specifications and a high degree of uniformity
across high channel counts (DWDM spectrum). Working with end-users, SENKO is also able to offer customized packaging and integration.

12ARRAYED WAVEGUIDE GRATINGReferencest Xaveer J. M. Leijtens, Berndt Kuhlow and Meint K. Smit, Arrayed Waveguide Gratings, Eindhoven University of Technology, 2006t Self-Seeded RSOAs WDM PON field trial for business and mobile fronthaul applications, Optical Society of Americat Enabling Cloud-RAN with Mobile Fronthaul, Mobile operators to migrate small and micro cells to a cloud-RAN architecture, Infinera.
t XXXTVCNBSJOFDBCMFNBQDPN

BiographyDr. Bernard HL Lee is currently the Regional Technology Director at SENKO Advanced Components. He started his career in opticalcommunications when he was appointed as a Senior Research Office for the European Union IST project known as DAVID in 2000. In
2003, he joined Telekom Malaysia R&D where he has held various technical and management positions there including the Head of
Photonic Network Research and also Head of Innovation and Communications. Bernard then joined the parent company, Telekom
Malaysia (TM) in 2010 as the Assistant General Manager of the Group Business Strategy Division where he oversees the company’sbusiness direction. Bernard obtained his RCDD accreditation in 2016. Bernard is also a member of the International ElectrotechnicalCommission (IEC), the Institute of Engineering and Technology (IET) and was also the Director of the Board of the Fiber-To-The-Home
Council APAC. He is currently the Malaysia Country Chair for BICSI Southeast Asia.

Ryo Konno is currently the Product Line Manager at SENKO Advanced Components. With the extensive sales experience in Japan, UK,and USA, he took lead on sales strategy and established great relationships with major equipment companies. He also gained uniqueview of developing new Fiber Optical components which contribute to space saving, higher density and maintain high reliability
serving critical role in next generation optical module to support 5G and 400G+. Ryo is also a member of Gen-Z and Open ComputeProject (OCP).

Mac Watanabe, master of engineering, senior product engineer at SENKO advanced Components. He has more than 25years
experiences for fiber optics industry, researched and established for PLC splitter assembly technologies in early stage on 1990s
before FTTH movement rises. He had unique working experiences in Japan, United states and China, linked as Learned backgroundtechnologies, Grown with the latest marketing and Great tact of mass manufacturing. Through several years freelance engineer
experiences were also helping his all balanced skills. Currently is investigating with new product development of fiber optics related
for passive and active components.

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