Development of Wide Operating Temperature Range (-30~70℃)
Athermal AWG Module with High Reliability
by Junichi Hasegawa * and Kazutaka Nara *The authors have clarified the cause of the temperature dependence in athermal
Arrayed Waveguide Grating (AWG) modules of conventional type, and havedeveloped a new athermal AWG module of wide operating temperature range
having a reduced temperature dependence. The temperature dependence of the center wavelength of the developed module is not more than ±0.015 nm for all channels over a temperature
range of -30~70℃. A series of reliability tests have been carried out assuming outdoor installation of the module, and it has been confirmed that the module has extremely high reliability.

ABSTRACT1.

INTRODUCTION

Athermalization, or realization of temperature independence, of arrayed waveguide gratings (AWGs) which handle the function of wavelength multiplexing/demultiplexing
has become an indispensable requirement for constructing optical communications networks that are ever diversifying. To satisfy the demand, we have devised proprietary athermalization principles, and have developed anumber of athermal AWG modules 1)~3) . And now, in
recent years, the next-generation communications system
based on Wavelength Division Multiplexing – Passive
Optical Network (WDM-PON) scheme is being intensively
studied especially in Korea. In this system, it is possible tooffer each user an independent transmission method andspeed, because different wavelengths are assigned toeach user by using a wavelength multiplexer/demultiplexer. A number of approaches have been reported so far touse AWG modules as wavelength multi/demultiplexers 4), 5)
taking advantage of the ease of channel multiplicationand mass production of AWGs.
Figure 1 shows a schematic diagram of WDM-PON system. As shown, the AWG modules are installed outdoors,e.g. in a closure, like optical splitters used in ordinary
PON systems. For this reason, the AWG modules are
required to be provided with such characteristics as athermalization and high reliability in addition to a wide operation temperature range of -30~70℃. In response to therequirement of improving the operation temperature
range, we have theoretically clarified the cause of theslight temperature dependence in athermal AWGs of conventional type, and have developed a new athermal AWG
module that has achieved a reduced temperature dependence of the center wavelength. This paper reports on the* FITEL-Photonics Lab., R&D Div.

development together with the results of reliability tests
on the module in which, as a result of the tests conductedassuming outdoor installation, it has been confirmed thatthe module has high reliability.

AWG in a closure
CenterBuilding

Home
Oﬃce

AWG
in a closure

Figure 1

2.

Accessnetwork

Schematic diagram of WDM-PON system.

CAUSE CLARIFICATION OF
TEMPERATURE DEPENDENCE OF
CENTER WAVELENGTH IN
CONVENTIONAL ATHERMAL AWG
MODULE

2.1

Structure and Temperature Dependence ofConventional Athermal AWG Module
Figure 2 (a) is the simplified structural diagram of a conventional athermal AWG. The AWG chip is cut at one of
the slab waveguides, and each AWG chip is connectedwith a compensating plate made of copper. As temperature changes the copper plate expands and contracts,
causing the position of the output waveguides to shift.
Figure 2 (b) schematically shows the mechanism of temperature compensation. In a temperature-controlled AWG,
if not temperature controlled, the focus point will shift as

Furukawa Review, No. 30

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Development of Wide Operating Temperature Range (-30~70℃) Athermal AWG Module with High Reliability

the temperature fluctuates causing the center wavelength
to change. In an athermal AWG, on the other hand, thefocus point will also shift as the temperature fluctuates,
but the expansion or contraction of the copper platemoves the output waveguides to the new focus point, sothat the center wavelength is compensated. The shift distance is adjustable by the length of the copper plate.
Figure 3 shows the temperature dependence of thecenter wavelength of the conventional athermal AWG. The
temperature dependence is slightly convex downward,
and the temperature dependence of the center wavelength is less than ±0.015 nm over a temperature rangeof -5~70℃ specified in the general specification for officeapplications. The dependence will be greater when theoperation temperature range is expanded, becoming not
negligible in actual use. Thus, in order for the AWG module to be applied in WDM-PON systems where expansionof operation temperature ranges is needed, the temperature dependence of the AWG center wavelength has to be
reduced further.
（a）Structural diagram.
Output slab
waveguide

Input slab
waveguides

Compensatingplate (copper)

Light

（b）
Mechanism of temperature compensation.
Low temp.
Mid temp.

Athermal
AWG

Focus point shifts.

� ��

Input
waveguides
High temp.

Waveguide is movedby thermal contraction.

(1)

�� � �� � � ���� � �� �� �(2)�� � �� � � ���� � �� �� �

These parameters are shown in Table 1.
The temperature dependence of the refractive index ofsilica-based material can be calculated using Equations(1) and (2) together with Table 1, and is found to bealmost linear as shown in Figure 4. This result means thatthe refractive index of silica-based material changes in aquadratic curve with respect to temperature.

Focus point shifts.

Sellmeier’s parameters considering temperaturedependence.

Waveguide is movedby thermal expansion.

i=1

Outline of athermal AWG.

0.1
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
-0.01
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Temperature dependence of conventional athermal
AWG center wavelength.

i=2

i=3

ai0

eV

228.7018

46.40806

0.014173

ai1

eV2/℃

4.930E-05−3.270E-05−1.704E-06

ai2

eV2/℃21.100E-07−3.780E-08−2.140E-09

bi0

eV

18.11163

10.671082

0.125

bi1

eV/℃

9.150E-06−2.991E-04

0

bi2

eV/℃2

7.478E-08−4.807E-07

02

1.2E-05
1.1E-05
1.0E-05��/��Temperature（℃）

Figure 3���� � ��� �� ����where: h is Planck’s constant; c is the light velocity; λ is
the wavelength; ai and bi are Sellmeier parameters considering temperature characteristics as shown in Equation(2). ai is dependent on the oscillator strength and numberof oscillators per unit volume, while bi is dependent on theresonance energy of the oscillators.

Table 1Figure 2

Center wavelength shift（nm）�� � �� �

Arrayedwaveguides

Outputwaveguides

Temp.control
AWG(No control)

perature dependence of the refractive index of silicabased material, which is supposed to contribute a great
deal to the temperature dependence. In general, the temperature dependence of the refractive index of silicabased material is expressed as dn/dT≒8 x 10-6, a constant.
However, it is reported that the temperature dependenceof the refractive index does not take a constant value in
actuality, but changes as a function of temperature T 6).
Thus, the refractive index n of silica-based material including its temperature dependence is expressed by the
Sellmeier polynomial formula as shown in Equation (1).

9.0E-06
8.0E-06
7.0E-06
6.0E-06

2.2

Theoretical Investigation Concerning the Causeof the Downward-Convex Temperature
Dependence of Center Wavelength
To begin investigation as to the cause why the temperature dependence of the center wavelength shows a downward-convex curve, we focused our attention on the tem-

5.0E-06-100 -80 -60 -40 -20

0

20

40

60

80

100

Temperature（℃）

Figure 4

Temperature dependence of refractive index of silicabased material.

Furukawa Review, No. 30

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Development of Wide Operating Temperature Range (-30~70℃) Athermal AWG Module with High Reliability

��
��
�

(3)

To understand the temperature characteristics of center
wavelength, the temperature change of nc has to be evaluated accurately. Accordingly, we calculated the temperature change of nc by the effective index method, letting therefractive index of the clad glass be the one obtained by
Equations (1) and (2), that of the core glass be the one
obtained by uniformly adding a ∆=0.8 % to that of theclad glass, and under the condition of the layer thickness
and the line width being equal to 6.5 µm.
On the other hand, the athermal AWG we developed
was temperature compensated almost linearly taking
advantage of thermal expansion and contraction of thecopper plate, where the copper plate was adjusted inlength to minimize the temperature dependence of thecenter wavelength over a design operation temperature
range of -5~70℃.
Based on the discussions described above, we havecalculated the temperature dependence of the center
wavelength of the AWG as well as the temperature compensation using the copper plate and, what is derived
from these, the temperature dependence of the center
wavelength of the conventional athermal AWG, and theresults are shown in Figure 5 (a). Each result is normalized so as to correspond to a designed wavelength at
20℃. The temperature dependence of the center wavelength of the AWG changes in a quadratic curve because
the temperature dependence of the refractive index of silica-based material is not constant in contrast to the factthat the temperature compensation by the copper platechanges almost linearly. This results in the temperaturedependence of the center wavelength after compensation
which shows a slightly downward-convex shape. It can beseen from this series of calculations that the downwardconvex temperature dependence of the center wavelength of the athermal AWG is caused by the compensation mechanism, where the temperature characteristics ofthe center wavelength of the AWG having a quadratic
curve is compensated by using the copper plate having
an almost linear temperature characteristic. Meanwhile,
as can be seen in Figure 5 (b), the temperature dependence obtained by calculation is in agreement with that of
the center wavelength of an athermal AWG module experimentally measured.

3.

LINEAR EXPANSIONCHARACTERISTICS OFCOMPENSATING PLATE FORCOMPLETE ATHERMALIZATIONThe calculation results above mentioned implies that it ispossible to athermalize an AWG if there exists a compen-

Circuit parameters of AWG.
100

Channel spacing (GHz)40

Channel number
Focal length of slab waveguide (mm)

Lf

17.2

Diffraction orderm29

Pitch of adjacent arrayed waveguide (µm)

d

13.8

Path length difference of arrayed waveguide (µm)

∆L

31.0

Group index of arrayed waveguide at R.T.

ng

1.4760Effective index of slab waveguide at R.T.

ns

1.537

Effective index of arrayed waveguide at R.T.

nc

1.4514Center wavelength shift（nm）

��

Table 2

1.5
1
0.5
0
-0.5
-1
-1.5
-100 -80 -60 -40 -20

Conventional AWG
Compensation by copper plate
After compensation

0

20

40

60

80

100

Temperature（℃）（a）Calculation result.
Center wavelength shift（nm）

Next let us calculate temperature dependence of thecenter wavelength of an AWG using circuit parameters
shown in Table 2.
Using these parameters, the center wavelength of the
AWG is give by Equation (3).

0.1
0.08
0.06
0.04
0.02
0
-0.02
Calculation-0.04
Measurement
-0.06
-0.08
-0.1
-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100
Temperature（℃）（b）Comparison with measurement result.
Figure 5

Temperature dependence of the center wavelength ofconventional athermal AWG.

sating plate which matches the AWG in terms of nonlinear
temperature dependence of the center wavelength. We
calculated the linear expansion property required for sucha compensating plate. The amount of positional compensation dx to be used in the athermal AWG is given by
Equation (4) using the circuit parameters shown in Table2.
�� �� � � �� � �� �� � �� � ����� � �� � �� �

(4)

where the temperature is expressed by a change from
20℃, and the temperature dependence of the center
wavelength dλ/dT is given by Equation (5).
�� � � ��� ���� � ����� �� � �� �

(5)

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Development of Wide Operating Temperature Range (-30~70℃) Athermal AWG Module with High Reliability

where αs is the linear expansion coefficient of silicon
which constitutes the substrate of the AWG, and is equal
to 3.0 x 10-6/℃. Figure 6 shows the amount of positional
compensation dx for complete athermalization, which is
calculated using Equations (4) and (5) together with
Equation (3). From the Figure, it can be seen that thepositional compensation dx required shows a quadrature
characteristic with respect to temperature. The temperature dependence of dx, i.e. d(dx)/dT is calculated asshown in Equation (6).
� ���� � ���� �� � ��  �����
��(6)

Selecting a material that satisfies the rate of change ofd(dx)/dT for a compensating plate will result in complete
athermalization. Figure 7 compares the calculated rate ofchange, i.e. d(dx)/dT with those of common metal materials, where the calculated rate of change is seen to be
extremely higher than those of metal materials.
Consequently, we selected an aluminum material JIS
A1050 that has the highest rate of change of d(dx)/dT
among metal materials.

30

��（μm）

20

a 100-GHz 48-ch AWG chip. The chip was cut at the slab
waveguide, and was athermalized whereby a pure aluminum plate of JIS A1050 was connected as a compensating plate. Optical fibers were then connected to the AWG
chip, which was then packaged. Figure 8 shows theappearance of the athermal AWG module that was fabricated. The package was compact and slim, measuring
130 x 65 x 8.5 mm.
Figure 9 shows the spectrum of the 100-GHz 48-ch
athermal AWG module. Excellent optical characteristics
were obtained, with insertion loss of less than 2.5 dB(1.8~2.5 dB), and crosstalk less than -30 dB. There wasno degradation of optical characteristics due to athermalization.
Figure 10 shows the temperature dependence of thecenter wavelength and, for the sake of comparison, thatof conventional athermal AWG module using a compensating plate of copper. The temperature dependence of
the AWG module using a pure aluminum compensatingplate was less than ±0.015 nm over a temperature rangeof -30~70℃, achieving a significant reduction over theconventional. The change in insertion loss was less than±0.1 dB. Figure 11 shows the temperature dependenceof the spectrum. No spectrum degradation was observedat any of the temperatures, confirming that other opticalcharacteristics were also stable.

10
0
-10
-20
-30
-100 -80 -60 -40 -20

0

20

40

60

80

100

Temperature（℃）

Figure 6

Calculation result of dx for complete athermalization.

7.0E-04

Figure 8

6.0E-04

Appearance of 100-GHz 48-ch athermal AWG module.

5.0E-04
4.0E-040

3.0E-04-5

2.0E-04
1.0E-04
0.0E+00

Figure 7

Calculation result
for complete
athermalization

Aluminum

CopperComparison of the rate of change of dx/dT with metalmaterials.

Transmittance（dB）

Rate of change' �
（��）
/��

8.0E-04

-10
-15
-20
-25
-30
-35
-40
-45
-50

1526 1531 1536 1541 1546 1551 1556 1561 1566
Wavelength（nm）

4.

FABRICATION RESULTS

Figure 9Spectrum of 100-GHz 48-ch athermal AWG module.

Using the planar lightwave circuit (PLC) fabrication technology that combines flame hydolysis deposition (FHD),
photolithography and reactive ion etching, we fabricatedFurukawa Review, No. 30

2006

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Development of Wide Operating Temperature Range (-30~70℃) Athermal AWG Module with High Reliability

Center wavelength shift（nm）

0.045
0.04

Pure aluminum plate

0.035

Copper plate

0.03
0.025
0.02
0.015
0.01
0.005
0
-0.005-30 -20 -10

0

10

20

30

40

50

60

70

Temperature（℃）

immersion tests (43±2℃, PH5±0.5, 336 hr), in whichchanges from the initial values are plotted. After water
immersion for 336 hours, virtually no change wasobserved in terms of both the center wavelength and theinsertion loss. Figures 15 and 16 show the results ofdump heat test (85℃, 85%RH, 5000 hr) and heat cycle
test (-40~85℃, 1650 cycles), respectively. The results
show that, even against these harsh tests, both the center
wavelength and the insertion loss are very stable. Fromthese test results, it has been confirmed that the athermal
AWG module has extremely high reliability and that themodule is rugged enough allowing for outdoor installation.

Figure 10 Temperature dependence of center wavelength.

0Transmittance（dB）

Wavelength meter

Tunable light source

Module under test

-5
-10

-30℃
20℃
70℃

-15
-20
-25

Optical multimeter-30

Vibration,
impact test equipment

-35
-40
-45

Direction A

Direction B

Direction C-50
1541 1542 1543 1544 1545 1546 1547 1548 1549
Wavelength（nm）

Figure 11 Temperature dependence of spectrum.
Figure 12 Outline of insertion loss monitoring test and load
application directions.

Insertion losschange（dB）

0.10
0.05
0.00
-0.05
-0.10
-0.15
0

Direction A

60
50
40
�
30
20
10
0
--10
50 100 150 200 250 300 350 400 450 500��Time（ms）

0.05
0
-0.05
-0.1
-0.15

60
50
40
�
30
20
10
0
-10
50 100 150 200 250 300 350 400 450 500
��

0.1

0

Impact（G）

Direction B

0.15
Insertion losschange（dB）

Time（ms）
Direction C

0.15
0.1
0.05
0
-0.05
-0.1
-0.15

60
50
40
�
30
20
10
0
-10
50 100 150 200 250 300 350 400 450 500
��0Impact（G）

We have heretofore carried out a series of tests in conformity with Telcordia GR-1221 and GR-1209 to evaluate thereliability performance of this athermal AWG module,
thereby confirming that no problems occur 1). However,when used outdoors, the modules may be exposed toharsh environmental conditions, so that extremely highreliability is required for the modules. Accordingly, assuming all possible situations that can occur during outdoor
installation, the module was tested by insertion loss monitoring tests under application of impact and vibration,water immersion tests, in addition to long-term dump heattests and heat cycle tests lasting longer than Telcordia
GR-1221.
Figure 12 shows the measuring setup for loss monitoring test. Light was input to the module using a tunable
light source and a wavelength meter, and the insertion
loss change was monitored for each test at a specified
sampling period in the load application directions A, B
and C as indicated in the Figure. Figures 13 (a) and 13 (b)show the results of impact-based loss monitoring test (50G, 5 times, 6 directions) and vibration-based loss monitoring test (20 G, 20~2000 Hz, 4 min/cycle, 4 cycle/axis),
respectively. It can be seen that no change in insertion
loss occurs in each direction under the application of
impact and vibration. Figure 14 shows the results of water

0.15

Impact（G）

RELIABILITY TESTS ASSUMINGOUTDOOR INSTALLATIONS

Insertion losschange（dB）

5.

Time（ms）

Figure 13 (a) Results of impact-based loss monitoring test.

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Development of Wide Operating Temperature Range (-30~70℃) Athermal AWG Module with High Reliability

0
-0.05
-0.1
-0.15
1.2

Center wavelenght
change（nm）

Insertion loss change
（dB）

0.1
0.05

2000
1800
1600
1400
1200
1000
800
��
600
��
400
200
0
7.5 16.5 20.0 20.0 20.0 20.0 20.0

Vibration frequency（Hz）

Direction A

0.15

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

0

1000

-0.1
-0.15
1.2

0.05
0
-0.05
-0.1
-0.15
1.2

4000

5000

0.75
0.5
0.25
0
-0.25
-0.5
-0.75

-1

-1.25
-1.5

0

1000

Figure 15 Results of ultra long-term dump heat test.

Direction C

2000
1800
1600
1400
1200
1000
800
��
�� 600400
200
07.5 16.5 20.0 20.0 20.0 20.0 20.0

Vibration frequency（Hz）

Insertion loss change
（dB）

0.1

2000
3000
Time（h）1(b) Insertion loss.

Vibration amplitude（G）
0.15

Insertion loss change
（dB）

0
-0.05

2000
1800
1600
1400
1200
1000
800
��
600
��
400
200
0
7.5 16.5 20.0 20.0 20.0 20.0 20.0

5000

1.5
1.25

Vibration amplitude（G）

Center wavelength change
（nm）

0.05

Direction B

Vibration frequency（Hz）

Insertion loss change
（dB）

0.1

4000

(a) Center wavelength.

Vibration amplitude（G）
0.15

2000
3000
Time（h）

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

Figure 13 (b) Results of vibration-based loss monitoring test.

0

200

400

600

800 1000 1200 1400 1600
Cycle

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

Insertion loss change（dB）

Center wavelength change（nm）(a) Center wavelength.

0

50

100

150

200

250

300

Time（h）

Insertion loss change
（dB）

0

200

400

600

800 1000 1200 1400 1600
Cycle(a) Center wavelength
1.50
1.25
1.00
0.75
0.50
0.25
0.00
-0.25
-0.50
-0.75
-1.00
-1.25
-1.50

1.5
1.25
1
0.75
0.5
0.25
0
-0.25
-0.5
-0.75
-1
-1.25
-1.5

(b) Insertion loss.

Figure 16 Results of ultra long-term heat cycle test.

0

50

100

150
200
Time（h）

250

300

installation. The developed module has achieved excellent performance with a temperature dependence of thecenter wavelength of not more than ±0.015 nm for all
channels over a temperature range of -30~70℃, in addition to extremely high reliability.

(b) Insertion loss.

Figure 14 Results of water immersion test.

6.

CONCLUSIONS

We have clarified the cause of the temperature dependence in the athermal AWG module of conventional type,and have developed a new athermal AWG module with areduced temperature dependence that permits outdoor

REFERENCES
1) T. Saito et al.: OFC2003, MF47, (2003), 57.
2) J. Hasegawa et al.: OFC2005, OTuD5, (2005).
3) J. Hasegawa et al.: OECC2005, 7E3-3, (2005).
4) D. J. Shin et al.: OFC2005, PDP36, (2005).
5) Soo-Jin Park et al.: Journal of Lightwave Tech., 22 (2004), 2582.
6) J. Matsuda et al.: Journal of Non-Crystalline Solids, 135 (1991), 86.

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