Temperature-Insensitive (Athermal) AWG Modules
by Tsunetoshi Saito *, Kazutaka Nara *, Kanji Tanaka *2,
Yoshinobu Nekado *, Jun'ichi Hasegawa * and Kazuhisa Kashihara *3

High-performance temperature-insensitive (athermal) arrayed waveguidegrating (AWG) modules have been developed to meet the need to reduce theconsumption of DWDM systems and eliminate electric power for optical components. Employing
a unique temperature-insensitive technique that is applicable to all sorts of AWGs, we havefabricated athermal AWG modules of the 100-GHz × 32-ch (Gaussian) and 100-GHz × 42-ch(semi-flat) types. At temperatures from 0 to 70°C, center wavelength shift and insertion loss
variation were less than ±0.015 nm and ±0.1dB respectively, and it was confirmed that the opticalcharacteristics of the developed athermal AWGs showed high stability. It was also confirmed thatthey exhibit high reliability by reliability tests based on Telcordia GR-1209 and GR-1221.

ABSTRACT1.

INTRODUCTION

Dense wavelength division multiplexing (DWDM) systems,
which had their origins in long-haul communication
networks in North America, are now spreading to theentire world. They are also being used in Metro-networks
and other short-range communication networks. The
number of channels in DWDM systems is getting larger
due to the unlimited need for transmission capacity incommunication networks 1). In general, the greater thenumber of channels in a DWDM system, the greater the
power consumption of the system as a whole. Moreover,the increase in the number of optical components assystem functions increase, and the use of high-power
EDFAs 2) or Raman amplifier 3) means even greater
power consumption of the optical network system. For
this reason, there is an urgent need to reduce power
consumption or eliminate electric power for individual
optical components. AWGs are playing an important role
as multiplexer/demultiplexers in DWDM systems. The
AWG has design flexibility with respect to the numberof channels and wavelength spacing, and is superior inmass productivity and compactness. It has the further
advantage that chromatic dispersion is extremely low,making it suitable to suitable to high bit-rate (40 Gbps)
systems. This suggests that AWG is promising for use in
future DWDM systems.
In the AWG, the center wavelength is temperaturedependent due to the temperature dependence of the*

FITEL-Photonics Lab.
Development Dept., FITEL Products Div.
*3 Fiber Products Dept., Optical Fiber Div.
*2

refractive index of silica-based materials. This requires
temperature control by heater or thermoelectric (TE)device using the Peltier effect, an RTD or a thermistor
and an electric power supply. Therefore one of the
major requirements for AWGs has been to reduce power
consumption, and we developed a Peltier-type AWG
module and a heater-type AWG module with low power
consumption 4), 5). We have now developed novel athermal
AWG modules which need no electric power supply byusing a unique temperature insensitive technique. It meetsthe need for further reduction in the power consumption of
the system as a whole and is suitable to Metro networks,
in which eliminating the need for electric power is amajor advantage. These novel athermal AWGs almost
complete temperature compensation for AWGs with alarge number (32 or 42) of channels, which has been
hard to realize. In this paper, we report the principle ofthis temperature insensitive technique and the results offabrication of athermal AWG modules of the 100-GHz ×
32-ch (Gaussian) and 100-GHz × 42-ch (semi-flat) types.
We also report the results of reliability tests based on
Telcordia GR-1209 and GR-1221.

2.

PRINCIPLE OF TEMPERATUREINSENSITIVITY

2.1 Principle of AWG
Figure 1 shows the waveguide circuit structure of an
AWG, with the input waveguides, input slab waveguide,arrayed waveguide, output slab waveguide and output
waveguides formed on the substrate. Following is a
description of operation as an optical demultiplexer.
When a WDM signal comprising a number of optical

Furukawa Review, No. 24

2003

29

Temperature-Insensitive (Athermal) AWG Modules

Arrayed waveguide

Output slab waveguide

Input slab waveguide
Input waveguides
λ1, λ2, λ3

Output waveguides
λ1

λn ,

Figure 1

λ2 λ3

λn

Light-waveguide circuit structure of AWG.

signals having wavelengths λ 1 through λ n enters the
input waveguides it is diffracted and spread by the
input slab waveguide and is transmitted to the arrayedwaveguide, which consists of waveguides placed sideby-side in which each of the signals emitted by the input
slab waveguide is propagated, and adjacent waveguides
are disposed at a fixed optical path length difference ∆L .
For this reason a phase difference appears in the signal
propagated in each waveguide. The signals passing
through the arrayed waveguide then enter the output slab
waveguide and are diffracted and spread, but the signals
passing though the respective waveguides interferewith each other so that in effect they are all diffracted ina direction such that their wave fronts are aligned. The
in-phase conditions in which the wave fronts are aligned
may be represented byns d sinφ +nc ∆L = mλ(1)where: nc and ns are the effective index of refraction
for the arrayed waveguide and slab waveguide,
φ is the angle of diffraction, and d is the
distance between arrayed waveguides, λ is the
wavelength, and m is an arbitrary integer andthe 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 eachdiffracted in a different direction. Accordingly the points
at which signals of differing wavelength converge at theoutput 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 λ 1through λn . The above description relates to the operationof an AWG as a wavelength-division demultiplexer,
but the same AWG can also be used as a multiplexer.
That is to say, when a signal of each wavelength fromthe output waveguide (when used as a wavelengthdivision multiplexer) is input, the signals from the input
waveguides (when used as a wavelength-divisionmultiplexer) 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 temperaturedependent. When there is a change in temperature, there
is a change in the index of refraction of the waveguidematerial and an expansion or contraction of the substrate
and waveguides, resulting in a difference in the lengthof the optical path. This results in a shift in the positionof the focal points at the output side of the output slab
waveguides, and a change in the wavelength of thesignals entering the output waveguides. By taking φ inEquation (1) as 0 and differentiating an equation solvedfor λ by temperature T , it becomes possible to represent
the magnitude of the temperature dependence of thecenter wavelength by

dλ
λ dnc
=
+ λαs
dT
nc dT

(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 thetemperature 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 obtaina change in center wavelength of approximately 0.011 nm/°C.
2.3 The Principle of Temperature Insensitivity
In order to compensate for this temperature dependence,
we focused on the linear dispersion of the AWG. Figure
2 shows the output slab waveguide portion of the AWG.
The symbol O corresponds to the position when φ =0 in
equation (1) and wavelength λ0 , and in this condition isrepresented by

λ0 =

nc ∆L
m

(3)

When the focus point with diffraction angle φp is definedas P and the distance between O and P is defined as
x , the relation between wavelength λ and x may be
represented by

dx
Lf ∆L
=
n
dλ
ns dλ0 g(4)

Output waveguides
x
O

P

Slab
waveguideφ
LfArrayed waveguide

Figure 2

Output slab waveguide of AWG.

Furukawa Review, No. 24

2003

30

Temperature-Insensitive (Athermal) AWG Modules

where: Lf is the focal length of the slab waveguideand n g is the group refractive index of thearrayed waveguide.
Equation (4) shows the linear dispersion of the
AWG, showing that it is possible to take out light witha wavelength that differs from λ0 by dλ when an output
waveguide is positioned at a point at a distance dx fromO . Thus, if the output waveguide is shifted by a distancecorresponding to wavelength change dλ caused by thetemperature change, the temperature dependence of thecenter wavelength should be compensated.
To realize this principle, we applied a novel concept inwhich one of the slab waveguides was separated. Figure
3 shows the structure of this novel athermal AWG. The
circuit of AWG is cut at one of the slab waveguides anddivided into a larger part and a smaller part. These twoparts are connected by a copper plate, which works as
a compensating plate. This copper plate is attached toeach part at both ends. Figure 4 shows the compensation
mechanism schematically. In the conventional AWG,
the focal point shifts as temperature changes and thecenter wavelength of the AWG is temperature-dependent.
In the athermal AWG on the other hand, the focal
point also shifts as the temperature changes, but thecenter wavelength is kept constant because the output
waveguides are moved to this shifted focal point by
thermal contraction and expansion of the copper plate.
This compensation is also applied when light is input fromthe output port.
This athermal AWG uses the technique of controllingthe focal point at the slab waveguide, so that the effect on
optical characteristics is very small by this temperature
insensitive operation. This realizes an athermal AWG witha large number of channels. It also has the advantage thatcompensation is very stable because the coefficient ofStructure of athermal AWG.

Low temp.

Mid temp.

High temp.

Conventional
AWG
Focus point shifts

Focus point shifts

Waveguide is moved

Waveguide is moved

Athermal AWG

Figure 4Compensation mechanism of athermal AWG.

Circuit parameters for 100-GHz × 32-ch athermal
Parameter

Value

Channel spacing

100 GHzChannel number

32

Relative refractive index difference

0.8 %Focal length of slab waveguide: L f

13160 µ m

Length difference between adjacent arrayedwaveguides : ∆ L
Pitch between adjacent arrayed waveguides at theend of arrayed waveguide: dDiffraction order: m

40.5 µ m
13.75 µ m
38

expansion of copper is constant and very stable.
In order to compensate accurately for the temperaturedependence of the center wavelength, it is necessary toadjust the length of the copper plate, which is determined
as follows. The change in positional compensating valuedx may be expressed as a function of temperature change
T by

dx =

Lf ∆L
dλ
ng
T
nsdλ0
dT

(5)

Using as an example the circuit parameters shown in
Table 1, we may derivedx = 0.275T(6)

That is to say when the temperature changes 1°C thefocal point must change 0.275 μm at the output portion of
the output slab waveguide. Therefore this is the value forwhich the copper plate must compensate.
Since the thermal expansion coefficient of copper is 1.7
× 10-5, the length of the copper plate may be calculated
to be 16.2 mm, depending on the design of the AWG,
and in most case, is around 15 to 25 mm. Assuming acenter wavelength compensating accurately of 0.005
nm or better (over 0~70°C) an error of 0.2 mm in copper
plate length is permissible, well within manufacturing
tolerances.

3.

Copper plate

Figure 3

Table 1

EXPERIMENTAL RESULTS

Using the principle described above, we fabricatedathermal AWG modules of 100-GHz × 32-ch and 100-GHz
× 42-ch, with spectra that are Gaussian and semi-flat
type respectively. The semi-flat type has a configuration
midway between the Gaussian type and the conventional
flat type, and is recently in demand for its flatness, andlower insertion loss than the conventional flat type.
Figure 5 is a photo of the 100-GHz × 32-ch athermal
AWG module. The package is very compact and slim-line
measuring only 57 × 97 × 8.5 mm. The package size ofthe 100-GHz × 42-ch AWG module is 80 x 130 x 8.5 mm.
Figures 6 and 7 show the spectra over all channels
for 100-GHz × 32-ch and 100-GHz × 42-ch athermal
AWG modules. Spectrum profiles were substantially the

Furukawa Review, No. 24

2003

31

Temperature-Insensitive (Athermal) AWG Modules

Center wavelength change (nm)

0.10
0.08
0.06
0.04
0.02
0.00
-0.02
-0.04
-0.06
-0.08
-0.10

-10

0

10

20

30

40

50

60

70

80

Temperature (°C)

Appearance of athermal AWG module(100-GHz × 32-ch).

Figure 8

Temperature dependence of center wavelengthof 100-GHz × 42-ch athermal AWG module.

0

0

-5

-5

70°C

-10

-10

20°C

-15

0°C

Transmittance (dB)

Transmittance (dB)Figure 5

-15
-20
-25
-30
-35

-20
-25
-30
-35

-40

-40

-45

-45

-50
1527

1532

1537

1542

1547

1552

1557

-50
1542

1562

1543

1544

Wavelength (nm)Figure 9Spectrum of 100-GHz × 32-ch Gaussian typeathermal AWG module.

0

0
-5

0°C

-10

-10

20°C

-15

70°C

-15
-20
-25
-30
-35
-40

-20
-25
-30
-35

-45
1527

1532

1537

1542

1547

1552

1557

1562

1567

Wavelength (nm)Figure 7

1547

-40

-45
-50
1522

1546

Temperature dependence of spectrum
for 100-GHz × 32-ch Gaussian type.

-5

Transmittance (dB)

Transmittance (dB)Figure 61545

Wavelength (nm)Spectrum of 100-GHz × 42-ch semi-flat typeathermal AWG module.

same as its chip state and no deterioration attributable toathermal operation was observed.
Figure 8 shows the temperature dependence of thecenter wavelength for the 100-GHz × 42-ch athermal
AWG module. Over a temperature range from 0 to 70°C,
temperature dependence of the center wavelength wasless than ±0.01 nm and almost complete compensationof temperature dependence was confirmed. The 100-GHz
× 32-ch athermal AWG module also exhibited the samecharacteristics.
Figure 9 shows an example of the temperature

-50
1543

1544

1545

1546

1547

1548

Wavelength (nm)Figure 10 Temperature dependence of spectrum
for 100-GHz × 42-ch semi-flat type.

dependence of spectrum for the 100-GHz × 32-ch
Gaussian-type AWG, using the results for the as
representative of all ports. There was no change inspectrum within the 0 to 70°C range, confirming thehigh stability of the optical characteristics. Insertion losschange from 0 to 70°C also was less than 0.1dB. Figure
10 shows an example of the temperature dependence ofspectrum for the 100-GHz × 42-ch semi-flat type AWG.
This was also very stable, confirming that this athermal
principle was applicable to AWGs with a larger number ofchannels.

Furukawa Review, No. 24

2003

32

Results of reliability test on 100-GHz ×
32-ch AWG modules.
ResultTest

Conditionsn

Insertion loss Center wavelengthchange (dB)change (nm)Vibration

20 G, 20~2000 Hz
Ave
0.00
4 min/cycles 14 Worst -0.10
4 cycles/axis
Std
0.031

Ave
0.002
Worst -0.015
Std0.0056

Impact

500 G
Ave -0.01
5 times/direction 14 Worst 0.11
6 directions
Std
0.025

Ave
0.000
Worst 0.011
Std
0.0046Ave
0.00
14 Worst 0.17
Std
0.051

Ave -0.003
Worst 0.013
Std0.0060Damp heatAve
0.10
85°C·85 %RH
15 Worst 0.30
2000 hours
Std
0.087

Ave
0.012
Worst 0.022
Std
0.0050

Low temperaturestorage-40°C
2000 hours

3

Ave -0.06
Worst -0.11
Std
0.023

Ave -0.011
Worst -0.018
Std
0.0075Temperature-humidity
cycling-40~85°C
20~85 %RH
42 cycles

3

Ave
0.02
Worst 0.09
Std
0.018

Ave -0.014
Worst -0.020
Std0.0057

Cable retention

Ave
0.00
1.5, 0.45 kgf
11 Worst 0.00
60 sec
Std
0.000

-

Side pull

0.23 kgf
Ave
0.00
5 sec
11 Worst 0.00
90°2 directions
Std
0.000

-

Temperature cycling-40~85°C
500 cycles

0.10
0.08
0.06
0.04
0.02
0.00
-0.02
-0.04
-0.06
-0.08
-0.10

0

100

200

300

400

500

600

500

600

Cycle
(a) Center wavelength variation

Insertion loss variation (nm)Table 2Center wavelength variation (nm)Temperature-Insensitive (Athermal) AWG Modules

1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0

0

100

200

300

400

Cycle
(b) Insertion loss variation

Figure 11 Results of temperature cycling test(-40/85C, n=14 modules).

4.

RESULTS OF RELIABILITY TESTS

Reliability tests were carried out on 100-GHz × 32-ch
athermal AWG modules in accordance with Telcordia
GR-1209 and GR-1221, and Table 2 shows the results
obtained. The values in this table show the amount ofchange from the initial state.
As representative of all tests, Figures 11(a) and11(b) show the shift in center wavelength and insertion
loss resulting from temperature cycling tests. Thesegraphs show the worst values over all cannels for each
module. Through 500 cycles of -40 to 85°C, both center
wavelength and insertion loss were are very stable. No
failure was recognized in any of the tests, confirming thehigh reliability of the developed athermal AWG modules.

5.

REFERENCES
1) G. Charlet, J.-C. Antona, S. Lanne, P. Tran, W. Idler, M.
Gorlier, S. Borne, A. Klekamp, C. Simonneau, L. Pierre, Y.
Frignac, M. Molina, F. Beaumont, J.-P. Hamaide, S. Bigo:“6.4 Tb/s (159~42.7Gb/s) capacity over 21~100 km using
bandwidth-limited phase-shaped binary transmission” Proc.
European Conference on Optical Communication, (2002), PD
Paper 4.1.
2) Y. Tashiro, S. Koyanagi, K. Aiso and S. Namiki, “1.5W
erbium doped fiber amplifier pumped by the wavelength
division multiplexed 1480 nm laser diodes with fiber Bragg
grating”, Tech. Dig. OAA’98, Vail CO, July ’98, paper WC2.
3) Namiki: ECOC2001, We. M. 2. 1
4) Tsunetoshi Saito et al.; Furukawa Review, No. 19, (2000), 47
5) Junichi Hasegawa et al.; “Development of a Heater-Control
AWG Module”, Furukawa Review, No. 22, (2002)

CONCLUSIONSAn athermal AWG using a novel principle has beendeveloped to fill the need for reductions in the electric
power consumption of DWDM systems and for
applications to Metro-networks, and the performanceof athermal AWG modules of the 100-GHz × 32-ch(Gaussian) type and 100-GHz × 42-ch (semi-flat) typewas demonstrated. The center wavelength shift under thetemperature from 0 to 70°C was less than ±0.015 nm andother optical characteristics were also stable. Reliability
tests confirmed the reliability of these athermal AWG
modules. This athermal principle is applicable to various
type of AWG because athermal operation causes virtually
no deterioration in optical characteristics.

Furukawa Review, No. 24

200333