Development of a Heater-Control AWG Module
by Jun-ichi Hasegawa *, Tsunetoshi Saito *, Hiroyuki Koshi * and Kazuhisa Kashihara *Arrayed waveguide gratings (AWGs) have an important role in the dense-wavelength division multiplexing (DWDM) systems that are coming into wider use bothin North America and elsewhere around the world. The authors have accordingly developed aheater-control AWG module that is only 12 mm thick and consumes less than 5 W of power. It
also achieves optical characteristics that are stable with respect to changes in ambient temperature and tension on optical fibers. Optical characteristics and reliability were confirmed with thefabrication of prototype 100-GHz, 40-channel AWG modules.

ABSTRACT1.

INTRODUCTION

In response to the need for fiber-optic systems of greater
capacity and higher bit rate, dense-wavelength divisionmultiplexing (DWDM) systems, in which multiple opticalsignals of differing wavelengths are aggregated, are in
actual service in North America and coming into wider use
elsewhere in the world.
Figure 1 is a schematic showing the structure of an optical communication network based on DWDM technology.
This has, until recently, been used mainly in long-haulsystems connecting large cities and ultralong-haul systems spanning oceans 1), 2). But it is expected to be introduced in medium-haul applications between smaller centers and in big-city metro rings. And with the growing needfor greater capacity, existing DWDM networks are expected to be given more channels and enhanced functions.
Wavelength-division multiplexer/demultiplexer will be key
in achieving this type of architecture for fiber-optic networks.
Multiplexers/Demultiplexers are devices that are capable of multiplexing optical signals of different wavelengths,
or demultiplexing a multiplexed signal. They come in avariety of designs--arrayed waveguide gratings (AWG),
fiber Bragg gratings (FBG), and thin-film filters (TFM).
AWGs are planar lightwave circuits in which opticalwaveguides are integrated on a substrate of silicon, silica,or the like, and they are fabricated by a glass depositionand patterning process involving photolithography andetching 3), 4). The AWG offers a high degree of design freedom in terms of number of channels and wavelength
spacing, so that more channels can be easily provided,and accommodated in a smaller space. And even when
there are more channels there is minimal change in module cost, so that costs per channel are lowered.
Furthermore in AWGs wavelength dispersion is very low* Optical Components Development Dept., FITEL-Photonics
Lab.

making it suitable for bit rates as high as 40-Gbps andholding great promise for an expanding role in DWDM
systems.

2.

PRINCIPLE OF THE ARRAYED WAVEGUIDE GRATING (AWG)

2.1 Multiplexing/Demultiplexing
Figure 2 shows the waveguide circuit structure of an
AWG, with the input waveguide, input slab waveguide,arrayed waveguide, output slab waveguide and output
waveguide formed on the substrate.
Following is a description of operation as an opticaldemultiplexer. When a WDM signal comprising a numberof optical signals having wavelengths λ1 through λn enters
the input waveguides it is diffracted and spread by the
input slab waveguides and is transmitted to the arrayedwaveguides. These are waveguides placed side-by-side inwhich each of the signals emitted by the input slab waveguides is propagated, and adjacent waveguides are dis-Long Haul
WDM
EDFA EDFA
post-amp in-line

OADM

EDFA
in-lineOADM

WDM
EDFA EDFA
in-line pre-amp

OADM

OADM

Metro Ring OADM

OADM

Metro
Ring

OADM

OADM

OADM
OXC

OADM

Metro Ring OADM
EDFA=Erbium-doped Fiber Amplifier
OXC=Optical Cross-Connect

Figure 1

Schematic of a DWDM optical communication network.

1

Arrayed waveguide

Output slab waveguide

Input slab waveguide
Input waveguide

Output waveguide
λ1 λ2

λ1, λ2, λ3...λn,Figure 2

λ3dλ
λ . dnc
λ . dL
=
+
dT
nc dT
L dT

λn

Waveguide circuit structure of AWG.

posed at a fixed optical path length difference. For thisreason a phase difference appears in the signal propagated in each waveguide corresponding to ncλL. The signal
passing through the arrayed waveguides then enters theoutput slab waveguides and is diffracted and spread, but
the signals passing though the respective waveguides
interfere with each other so that in effect they are all diffracted in a direction such that their wave fronts arealigned. When the effective index of refraction for thearrayed waveguides and slab waveguides are nc and ns,
the angle of diffraction is θ, the distance between arrayedwaveguides is d and the wavelength is λ, the in-phaseconditions in which the wave fronts are aligned may be
represented bynsdθ + nc∆L = mλ(1)where m is an arbitrary integer and the order of diffraction. Since the angle of diffraction that gives the directionin which the wave fronts are aligned is dependent on
wavelength, signals of differing wavelengths are each diffracted in a different direction. Accordingly the points at
which signals of differing wavelength converge at the output ports of the output slab waveguides differ, and bypositioning an output waveguide at each of these points,
signals of differing wavelength can be sent to a different
waveguide, thereby extracting signals of wavelength λ1
through λn.
The above description relates to the operation of an
AWG as a wavelength-division demultiplexer, but thesame AWG can also be used as a multiplexer. That is tosay, when a signal of each wavelength from the output
waveguide (when used as a wavelength-division multiplexer) is input, the signals from the input waveguides(when used as a wavelength-division multiplexer) are all
output together.
2.2 Temperature-Dependence of Center Wavelength
As has been described, an AWG can be used to multiplex
and demultiplex signals of a set wavelength, but thiswavelength (the center wavelength) is temperature dependent.
When there is a change in temperature, there is achange in the index of refraction of the waveguide material and an expansion or contraction of the substrate andwaveguides, resulting in a difference in the length of the
optical path. This results in a shift in the position of the
focal point at the output side of the output slab wave-

2

guides, and a change in the wavelength of the signals
entering the output waveguides. By taking θ in Equation(1) as 0 and differentiating an equation solved for λ bytemperature T, it becomes possible to represent the magnitude of the temperature dependence of wavelength by(2)

where the first term on the right side of the equation represents the temperature dependence of the index ofrefraction and the second term the change in the index ofrefraction caused by stress on the waveguide due to thechange in the length of the waveguide path accompanying
expansion or contraction of the substrate. Since the temperature dependence of the index of refraction of silica
glass is 8✕10-6/°C and the coefficient of expansion of thesubstrate is 3✕10-6/°C (in the case of silicon), we obtain achange in center wavelength of approximately 0.011
nm/°C.
To make up for the temperature dependence of the center wavelength requires a means of keeping the AWG at aconstant temperature irrespective of changes in ambient
temperature. Generally this control is accomplished using
a Peltier device or a heater, and Furukawa Electric has
already developed an AWG module using a Peltier
device 5). In the present work, the authors have developed
a heater-cotrol AWG module.

3.

PERFORMANCE REQUIREMENTS FORAWG MODULES

In addition to its fundamental optical characteristics, an
AWG module must meet requirements with respect to:• low power consumption,• a compact package• stability with respect to changes in ambient temperature, and• high reliability.
Recently there has been a trend toward greater total
system power consumption, driven by the higher power of
erbium-doped fiber amplifiers (EDFAs), the introduction ofRaman amplifiers, and the adoption of new types of optical modules due to the enhanced performance and functionality of DWDM systems. The systems as a whole mustalso be larger to provide enough space to accommodate
these devices. This has resulted in an urgent need forindividual modules that are more compact and use lesspower.
There is, however, a trade-off between the size andpower consumption of optical modules. Figure 3 is a
generic representation (omitting the temperature control
mechanism) of an AWG module, which is an example ofan optical module with temperature control. Increasing
distance g between the AWG chip and the upper andlower extremities of the inside of the package improves
the heat insulating properties of the air and decreases
power consumption, but results in greater thickness t of
the package itself. If, on the other hand, distance g is

Furukawa Review, No. 22

2002

Optical fiber arraytgg
AWG chip

Cross-sectional view of AWG module.

decreased, package thickness is reduced but the thermal
conductivity of the AWG chip and package becomes
greater, and power consumption is increased.
It is therefore no easy matter to design an AWG modulethat is both small and low in power consumption. The
authors set targets of a package thickness of 12 mm orless and a power consumption, when used at ambient
temperatures of 0~70°C, of not more than 5 W, and proceeded to optimize AWG module package design, package materials, and the method of holding the AWG chip.

Appearance of 100-GHz, 40-ch heater-control AWG
module.

119.6

STRUCTURE OF HEATER-CONTROLAWG MODULE

Following is a description of the structure of a heater-control type AWG module. Optical fiber arrays were connected to the AWG chip, and a heater and resistance temperature device (RTD) were mounted within the package.
Photo 1 shows a 100-GHz, 40-ch heater-control AWG
module. As can be seen in Figure 4 the package has afootprint of only 119.6 ✕ 65.5 mm, and is only 12 mm
thick. This package can also accommodate other types of
AWGs (50-GHz, 40-ch, etc.).
To keep down the power consumption of this module
the package was made using a plastic of low thermal conductivity, and measures were taken to minimize the areaof contact between the AWG chip and the package.
The decision to use a plastic package, however, raised
a number of problems. Plastics normally have a largecoefficient of thermal expansion making the package highly responsive to changes in ambient temperature, so that
if the optical fibers are attached to the package they are
subjected to tension. This applies stress to the joint interface between the chip and the optical fiber array, resulting
in an increase in insertion loss and even in breakage ofthe module. For this reason when plastic is used for the
package the normal practice is either to leave the opticalfibers unattached, or to use an adhesive that is resilient.
This means, however, that if tension is applied to the optical fibers it is transmitted to the joint interface between the
optical fiber array and the AWG chip, resulting in a major
increase in insertion loss. In the heater-control AWG module described here, a structure was adopted that overcomes this problem, modifying the point of fiber attachment so that even when ambient temperature changes or
tension is applied to the fiber, the stress is not transmitted
to the chip-optical fiber array joint.

12.0

4.

Photo 1

65.5

Figure 3

Package

1 2 3 4 5 6 7 8 9 10

53.5

Figure 4

5.

Pins for a Heater and RTDs
2.54 mm Pitch

Dimensions of 100-GHz, 40-ch heater-control AWG
module.

PROTOTYPE 100-GHz, 40-ch HEATERCONTROL AWG MODULES

Using the structure described above, seven prototype
100-GHz, 40-ch heater-control AWG modules were fabricated, and evaluated for the desired characteristics.
Generally speaking the center wavelength of an AWG
will vary as a result of fabrication error, and to make surethat the center wavelength agrees with the grid wavelength, the control temperature must be set separately foreach device. In the case of a heater-control type AWG
module this control temperature must be set higher thanthe operating temperature range (0~70°C), and here atemperature of 70 to 80°C was adopted. In order to evaluate the suitability of the module structure adopted, thecontrol temperature was set at 80°C for these prototypes.
5.1 Temperature Dependence
The operation of the prototype heater-control AWG modules was confirmed at ambient temperatures of from 0 to
70°C. Figure 5 shows the temperature dependence ofinsertion loss and center wavelength during ambient tem-3

perature cycling of 20°→70°C→0°C→20°C. Fluctuations
are represented as a change from the value at the initial
setting of 20°C. It can be seen that fluctuation was

0.50
8 port0.40

16 portInsertion loss (dB)

0.30

24 port0.20

32 port0.10

40 port

0.00
-0.10
-0.20
-0.30
-0.40
-0.50
20°C

70°C

0°C

20°C

extremely small for both insertion loss and center wavelength.
Table 1 summarizes the change in insertion loss andcenter wavelength at 70 and 0°C referenced to the value
at 20°C. The maximum fluctuation in insertion loss was
only 0.05 dB, confirming that the fiber anchoring structure
was satisfactory. Similarly the maximum fluctuation in center wavelength was only 0.006 nm, demonstrating that
despite changes in ambient temperature, the optical characteristics showed excellent stability.
Figure 6 shows a typical transmission spectrum of a
prototype module. With an insertion loss of 5 dB or below
and crosstalk of -26 dB or below, it was confirmed that
optical characteristics were equivalent to those of
Furukawa Electric's Peltier-control type AWG modules.

a) Insertion loss
0.05
8 portCenter wavelength (nm)0.04

16 port0.03

24 port0.02

32 port0.01

40 port

0.00
-0.01
-0.02
-0.03
-0.04
-0.05
20°C

70°C

0°C

20°C

5.2 Power Consumption
The power consumption of the prototype heater-control
AWG modules was measured at ambient temperatures of0, 20 and 70°C. The control temperature was 80°C for all
modules. Figure 7 shows typical results for measurementof power consumption, which varied in linear proportion totemperature with maximum consumption at 0°C. Table 2
summarizes the results of measurements for the sevenprototype modules. The maximum value of power consumption was 4.38 W, comfortably lower than the target
value of 5 W.

b) Center wavelength

Figure 5

Temperature dependence of selected characteristics.

Table 1

Summary of temperature dependence of selectedcharacteristics.

5.3 Optical Fiber Retention (Tension) Tests
The prototype heater-control AWG modules were subjected to optical fiber retention (tension) tests in conformity
5.0

temperature (°C)

700Insertion loss (dB)Center wavelength (nm)Ave : 0.01

Ave : 0.001

Worst : 0.05

Worst : 0.006

σ : 0.023

σ : 0.0030

n=4

n=7

Ave : 0.01

Ave : 0.001

Worst : 0.05

Worst : 0.003

σ : 0.020

σ : 0.0011

n=4

n=7

Power consumption (W)Change from value at 20°C

Ambient

4.0

3.0

2.0

1.0

0.0
0

10

20

30

40

50

60

70

Ambient temperature (°C)

Figure 7

Table 2

Typical power consumption.

Summary of power consumption. (n=7)

Ambient temperature (°C)

Change in power consumption (W)0Worst : 4.38

Ave : 4.01

σ : 0.228
Ave : 3.11
20

Worst : 3.37

σ : 0.145
Ave : 0.57
70

Figure 6

4

Typical transmission spectrum of 100-GHz, 40-chheater-control AWG module.

Worst : 0.62

σ : 0.024

Furukawa Review, No. 22

2002with Telcordia GR-1209. Tests were carried out on the
input- and output-side fibers independently, with the module held fast, using a tensile load of 15 N on the input sideand both 15 N and 30 N on the output-side, applied for
periods of 1 minute. Table 3 shows typical results forchanges in insertion loss, measured immediately before
and 1 minute after release of the load at three ports--1, 20
and 40, shown in terms of change from the value before
testing. As can be seen, the tensile stress produced nochange whatever in insertion loss confirming that thesemodules offer excellent stability with respect to tension on
the optical fibers.

6.

RELIABILITY TESTS

Reliability tests, comprising vibration, impact, temperature
cycling and damp heat tests were carried out in conformity
with Telcordia GR-1209 and 1221. During the tests temperature control of the AWG modules was not carried out.
Further, the respective optical characteristics were measured at an ambient temperature of 20°C, with moduletemperatures controlled at 80°C. Table 4 summarizes thevarious test conditions and the results that were obtained

Table 3

Typical results for change in insertion loss under
optical fiber retention (tension) tests.
Load (N)

Input

Immediately before 1 min after releasePortof load (dB)measured release of load (dB)

15

side

1

0.00

0.00

20

0.00

0.00

40

0.00

0.00

1

0.00

0.00

20

0.00

0.00

Output

40

0.00

0.00

side

1

0.00

0.00

20

0.00

0.00

40

0.00

0.00

15

30

Table 4
Type of test(for insertion loss and center wavelength). Results areshown in terms of the change from the value before testing. As can be seen, the maximum value of change in
insertion loss was 0.13 dB and the maximum change incenter wavelength was 0.005 nm. Degradation of performance did not occur in either test, confirming that theheater-control AWG modules developed here had a highlevel of reliability.

7.

CONCLUSIONS

The authors have developed a heater-control type AWG
module that can be used in DWDM systems. This module
has the following features:• Slim-line package only 12 mm thick;• Maximum power consumption of not more than 5 W(at ambient temperatures of 0~70°C);• Fluctuation in center wavelength of ±0.01 nm or less(at ambient temperatures of 0~70°C);• Fluctuation in insertion loss of 0.1 dB or less (at ambient temperatures of 0~70°C);• A structure that withstands tension on the opticalfibers;• Optical characteristics equivalent to those of Peltiercontrol AWG modules manufactured by Furukawa
Electric; and• High reliability (conforming to Telcordia GR-1209 and1221).
The heater-control design developed in this work can, in
addition to the 100-GHz, 40-ch modules described in thispaper, also be applied to 50-GHz, 40-ch modules.

REFERENCES
1) H. Onaka et al.; OFC '96, PD-19-1, (1996)
2) Y. Yano et al.; ECOC '96, ThB. 3.1, (1996)
3) H. Takahashi et al.; Electron Lett., vol. 26, (1990), 874) C. Dragone ; IEEE Photon. Technol. Lett., vol. 3, (1991), 812
5) Tsunetoshi Saito et al.; Furukawa Review, No. 19, (2000), 47

Results of reliability tests. (n=3)
Change in characteristic

Conditions

Insertion loss (dB)Vibration

ImpactTemperature cycling

20 G, 20~2000 Hz

Ave : 0.00

Ave : -0.001

4 min/cycles

Worst : 0.06

Worst : 0.004

4 cycles/axis

σ : 0.018

σ : 0.0016

500 G

Ave : 0.00

Ave : 0.000

5 times/direction

Worst : 0.05

Worst : -0.002

6 directions

σ : 0.023

σ : 0.0009-40°C~85°C
49 cycles

Damp heat

Center wave-length (nm)85°C • 85 %RH
336 hrAve : 0.02

Ave : 0.000

Worst : 0.06

Worst : -0.002

σ : 0.015

σ : 0.0005

Ave : 0.05

Ave : 0.001

Worst : 0.13

Worst : 0.005

σ : 0.049

σ : 0.0023

5