Physics 464/564
Research Project: AWG Technology in DWDM System
By: Andre Y. Ma
Date: 2-28-03
Abstract:
The ever-increasing demand for bandwidth poses a serious limitation for the existing
telecommunication carrier technology. However, this extraordinary growing demand,
coupled with the introduction of dense wavelength division multiplexing (DWDM) fiber
optic systems, has sparked a revolution in the optical component and networking
industry. Wavelength division multiplexing (WDM) systems are based on all optical
multiplexing (MUX) and demultiplexing (DMUX) technology. The advantage of optics
as the only transmission medium has allowed WDM systems to be free from limits of
electronic processing speed, such as time division multiplexing (TDM) systems that are
widely adopted in our existing electrical-to-optical-to-electrical networks today. Over the
short years of WDM deployment, it has already been proven to be one of the most
capable technologies in this highly competitive telecommunication market. Performance
wise, WDM has already improved from 4 to 128 channel counts, and channel spacing has
shrunk from 500GHz to 50GHz. In short, WDM technology can manifold the capacity of
our existing networks by transmitting multiple channels simultaneously on a single fiber
optic line. This capacity boost has been enabled by the advancement in fiber optic
components, arrayed waveguide gratings, and advanced packaging technology. The
primary focus of this report is based on my investigation and research on AWG
technology in DWDM system, and how AWG works in DWDM system that may realize
my dream of media-on-demand some day.
Introduction:
While the silicon technology is highly saturated in the electronics field, optical engineers
and physicists have found a new way to utilize this mature technology that can further
enhance today’s telecommunication network. These so called planar lightwave circuits
(PLCs) are optical waveguides formed on silicon substrate that can be custom designed to
incorporate multiple optical signal processing capabilities. The PLC chips, designed for
multiplexing (MUX) and demultiplexing (DMUX) of optical signals in DWDM
networks, are known as arrayed waveguide gratings (AWGs). In the few years of
AWGs’ introduction, it has already revolutionized the optical telecommunication system;
AWGs are the building blocks for even more complicated systems such as variable
optical attenuator (VOA), thermo-optic switch, DWDM channel monitor, dynamic gain
equalizer, etc. A common AWG module assembly is shown below.
Compact in size, highly integrated with essential active and passive components on a
single substrate, and its volume manufacturability on a fab that has been well developed
thru the years in the semiconductor industry; these strengths and characteristic of AWGs
have successfully lead to worldwide recognition and adoption of AWG technology in this
rapidly expanding all-optical DWDM networks. An AWG optical chip is composed of
an input waveguide, an input slab, array of waveguides, output slab, and output
waveguides. All of these are fabricated on a single substrate, forming a PLC. A
schematic of an AWG is shown below.
Content:
The heart of the DWDM system is the optical multiplexing (MUX) and demultiplexing
(DMUX) functionality that are embedded in AWGs. AWG’s multiplexer is known as
wavelength division multiplexer (WDM) and AWG’s demultiplexer is known as the
wavelength division demultiplexer (WDDM). Optical signals are generated by laser
diodes (LDs) in a series of monochromatic wavelengths 1, 2, …N, (within a standard
wavelength range) and sent through N fiber to a WDM. The WDM then combines these
input signals into a polychromatic output signal, a process known as multiplexing.
Multiplexing allows access to very large bandwidth that is available in an optical fiber.
The multiplexed, polychromatic signal is then launched into a single optical fiber for
transportation. At the destination end, WDDM separates the polychromatic signal into
individual constituent wavelengths, identified as a series of narrow band channels; this
process is known as demultiplexing. A basic concept of a WDM is illustrated in the
diagram below.
The WDDM is purposely designed such that the center wavelengths among all the
channels are the same as the original wavelengths of the system (1310 or 1550 nm). The
WDDM channels must also have spectral widths, N, that are large enough to
accommodate system tolerances, yet small enough to avoid overlapping of the channels.
In general, the WDM and the WDDM are spectrogram devices that are not tunable; as a
result, the performance of DWDM system depends entirely on the perfection of AWG’s
design and its silicon-based waveguide fabrication.
Depending on the application needs, different types of WDM systems are deployed
throughout the network; the list includes point-to-point long distance transmission, local
access network, reconfigurable network, etc. Each of these systems needs different
WDM components. At the transmitter end, a laser array is used as the signal source that
has predetermined wavelength settings with fixed channel parameters. These
wavelengths are standardized by the international telecommunication union (ITU) for
DWDM network. Other critical components of WDM networks are optical add/drop
multiplexers (OADM), optical cross connect switches (OCX), and optical amplifier such
as erbium doped fiber amplifiers (EDFAs). A basic configuration of a point-to-point
transmission system is show below.
The ITU has adopted the standard for optical telecommunication that specifies certain
standard frequencies be used to identify and specify WDM channels. ITU channels start
at 190.00 THz (channel 0, 1577.86 nm) and increments by 0.10 THz for each subsequent
channel. It usually spans over the C-band (1520-1570 nm). The wavelength, , and
frequency, , of a wave traveling in a medium are related by the following equation:
n = c, where n is the refractive index of the medium
WDMs must be designed such that the center wavelength of each channel coincides with
an ITU standard channel. For instance, a 40-channel AWG with 100 GHz spacing used
for DWDM application has its center wavelength coincide with the ITU channel 30
(193.00 THz, 1553.33 nm). The channel wavelengths and corresponding ITU
frequencies can be calculated from the above equation. The ITU channels’ frequencies
are given by the following equation:
N = 190.000 + 0.1N (THz), N = 0, 1, 2, ….
Thus, ITU channels are spaced at a frequency of 100 GHz; the operating frequencies are
called ITU grid frequencies. The corresponding wavelength spacing is given by the
following equation:
 = (2) / c, where c = 299792.458 THz.nm, and  = 0.1 THz.
From the above equation, one can see that  ~ 0.89 nm, however, it increases slightly
with  ( 2). WDMs can be designed to operate at ITU grid frequencies as well as their
multiples (e.g., 200GHz, 500 GHz, etc.) and sub-multiples (e.g., 50 GHz). The laser
outputs are modulated by individual electronic signals, either by direct or external
electro-optic method.
In long-haul network (which spans over hundreds to thousands of kilometer), optical
amplification becomes a necessity because of attenuation losses due to exceedingly long
distance transmission. However, the addition of optical amplifiers significantly increases
the overall network cost, complicates the network design, and at the same time it can
reduce the available channel counts. In long-haul transport, these additional complicacies
and cost factors can be justified, while in metro networks this would not be the case. In a
metro optical network (typically up to 100 kilometers), it is likely that a traffic channel
will transmit through many add/drop locations before reaching its destination. Therefore,
equipment related attenuations become a critical factor in DWDM network; in metro’s
design, a fine balance between fiber and component losses is very critical. If the
integrated WDM components such as AWGs are designed efficiently, they can avoid the
used of optical amplifiers in the metro design. A simple schematic of optical network is
shown below.
WDM systems use different wavelengths for different channels. Each channel
may transport homogeneous or heterogeneous traffics, such as SONET/SDH
(synchronous optical network / synchronous digital hierarchy) over one wavelength,
ATM (asynchronous transfer mode) over another, and yet another may be used for TDM
voice, video or internet protocol. WDM also make it possible to transfer data at different
bit rates. Thus, it offers one channel carrying traffic at OC-3, OC-12, OC-48, OC-192 or
up to OC-768 rate, and another channel may carry a different rate transmission.
Resulting multiple transmission rates on one single optical fiber. These functions are all
accomplished by a MUX at the transmitter end and a DMUX at the receiver end.
Conclusions:
Although there are several technologies currently deployed to manufacture optical
MUX/DMUXes, and while each has its own strength and weaknesses, the search for
better performing, cost reducing and more reliable technology is still ongoing. Thus far,
AWGs have proven to be the most prominent technology for DWDM network; based on
the matured silicon fabrication technique, its ability for custom design to integrate
multiple active and passive components, and the functionality of MUX/DMUX on a
single substrate.
Working as a project manager at Flextronics Photonics, I never had the
opportunity to understood the technology side of the optoelectronic products we package
for our customer. Specifically PLC devices such as splitters, dynamic-gain-equalizers,
and AWGs are common products that I encounter everyday for the past two and half
years in the photonics industry. From the investigation of this research project, I have
gained valuable knowledge and understanding of DWDM network, use of AWGs to
MUX and DMUX multiple signals to enhance higher capacity data transmission on a
single optical fiber. The worldwide acceptance of AWG’s in DWDM system can
manifold the capacity of our existing networks, thus my dream of media-on-demand may
realize in the next few years.
References:

W.A. Shurcliff, “Polarized Light: Production and Use,” Harvard University Press,
Cambridge, MA, 1966

Dennis Derickson, “Fiber Optic Test and Measurement,” Prentice Hall, Upper
Saddle River, NJ, 1998.

Toshihiko Ota, Tsunetoshi Saito, Tomoaki Toratani, and Yoshimi Ono, “16-ch
Arrayed Waveguide Grating Module with 100-GHz Spacing,”
www.furukawa.co.jp/review/fr019/fr19_09.pdf

Hiroyuko Toda, Tsukasa Yamashita, Ken-ichi Kitayama, “A Demultiplexing
Scheme Using an Arrayed Waveguide Grating for a DWDM MM-Wave Fiber
Radio System by Optical Frequency Interleaving. Graduate School of
Engineering, Osaka University, Japan.

H. Ehlers, M. Biletzke, B. Kuhlow, G. Przyrembel and U.H.P. Fischer,
“Optoelectronic Packaging of Arrayed-Waveguide Grating Modules and their
Environmental Stability Tests.” Heinrich-Hertz-Institut fur Nachrichtentechnik
GmbH, 15.11.2000.

Anis Rahman, Ph. D., “AWG Parameters Definition and Discussion”

Anis Rahman, Ph. D., “Heart of Optical Networks”

Dr. Martin Amersfoort, “Arrayed Waveguide Grating,” Concept to Volume b.v.,
15 June 1998, Application note A1998003.

Michael C. Parker, “Arrayed-Waveguide Grating Based WDM Access Networks:
An Evolutionary Multi-Gv/s Upgrade Path,” Fujitsu Telecommunications Europe
Ltd.