Ultra-Low-Loss Athermal AWG Module witha Large Number of Channels
by Junichi Hasegawa * and Kazutaka Nara *There is an urgent need for an arrayed waveguide grating (AWG), the device
that handles the function of wavelength multiplexer/demultiplexer, that is
unaffected by temperature, i.e., athermal. To satisfy this requirement we have applied proprietary
athermalizing technique, resulting in development of a multi-channel athermal AWG, which
had not been achieved previously. Also, by design of the circuit pattern it has been possible toachieve a 100-GHz 48-channel athermal AWG module that has an insertion loss of less than 2.8
dB. For all channels, temperature dependence is less than ±0.015 nm for center wavelengthand ±0.1 dB for insertion loss.

ABSTRACT1.

INTRODUCTION

With the increasing diversity of D-WDM systems in recent
years, one of the needs facing the AWG, which handles
the function of wavelength multiplexer/demultiplexer, isfor athermalization (temperature-independence) requiring
no power supply. The advantages of athermalization
are, most obviously, elimination of the need for a power
supply, plus the fact that there is no need to monitor
the temperature of the AWG module. This would makeit possible for the AWG module to be isolated fromtransmission devices inside an office and to be installed
in a variety of locations having no power supply, making
them able to satisfy the needs of the increasingly
diversified optical communications networks of the future.
Another point that may be mentioned is improving
module reliability. In conventional AWGs, where heat isregulated using a heater, etc., it is necessary, in order tomaintain the AWG center wavelength constant, to keep
the AWG chip at a temperature of 70°C or above, raisingproblems of thermal degradation of the adhesives usedin module construction. Thus if an athermal device canbe fabricated, temperature maintenance will be rendered
superfluous and reliability improved.
Various approaches to athermalizing AWGs have beenreported 1)~4), but to allow their use in actual D-WDM
systems there is a need for greater freedom in termsof number of channels, and a further reduction intemperature dependence.
The authors therefore have proposed a unique principleof athermalization, and using it have developed a
100-GHz 48-ch athermal AWG module with a semiflat spectrum profile that is mid-way between Gaussianand flat-top 5), 6). We therefore report here an ultra-low-

loss 100-GHz 48-ch AWG module, which thanks to lossreduction technique, achieves an insertion loss of 2.8 dB
or less. We also report the establishment of a proprietary
technique for center wavelength adjustment, which canlead to lower module cost.

2.

ATHERMALIZING TECHNIQUE

2.1 Principle of AWG Multiplexing/Demultiplexing
The arrayed waveguide grating (AWG) is a planar
lightwave circuit (PLC) in which the optical waveguideis formed by deposition of clad and core layers of silica
glass onto a silicon substrate. Figure 1 is a simplified
representation of AWG circuitry, comprising input
waveguides, input slab waveguide, arrayed waveguide,
output slab waveguide and output waveguides. The
following description relates to the function of an AWG
when used as a wavelength demultiplexer.
When multiplexed optical signals are input to the input
waveguides, they are spread by diffraction in the slab
waveguide to transmit to the arrayed waveguide. Since
adjacent waveguides of the array differ in length by afixed amount ∆ L, passage through the arrayed waveguide
imparts a phase difference, so that focusing occurs at
differing points at the end of the output slab waveguidedepending on the wavelength. Thus signals of differing
wavelength can be extracted by disposing the output
waveguides at their respective focus points.
Arrayed waveguide

Input waveguides

λ 1, λ 2, λ 3,

* FITEL-Photonics Lab., R&D Div.

Output slab waveguide

Input slab waveguide

λnFigure 1

Output waveguides

λ1

λ2

λ3

λn

Light-waveguide circuit structure of AWG.

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Ultra-Low-Loss Athermal AWG Module with a Large Number of Channels

An AWG can also function as a wavelength multiplexer.
In the reverse of the case described above: when opticalsignals of various wavelengths are input from the output
slab waveguide, they are multiplexed and output from the
input waveguides.
2.2 Temperature Dependence of the AWG
The index of refraction for the silica glass from which
the optical waveguides are made is temperaturedependent, causing the center wavelength of the
AWG to be temperature-dependent as well. Figure 2
illustrates the principle of temperature dependence.
Let us suppose that at room temperature (R.T.), thephase front of the optical signals propagated in thearrayed waveguide forms an arc at the end of the slab
waveguide, focusing the light at the center output
waveguide. When temperature rises above R.T., theeffective index of refraction neff increases, resulting in an
increase in the arrayed waveguide phase difference (2π /λ )(∆ L) neff, where λ is the center wavelength and ∆ L is thedifference in signal wavelength between adjacent arrayedwaveguides. When the phase difference in the arrayedwaveguide increases, the phase front becomes tilts, as
can be seen in Figure 2, and since light has the propertyof advancing perpendicular to the phase front, its focus
point will shift. That is to say light of differing wavelength
will be output from the center output waveguide, and awavelength shift will occur.
The temperature dependence of the center wavelengthof an AWG made of silica glass is 0.011 nm/°C, a value
too large to be neglected for applications to D-WDM
systems. Thus conventionally AWG modules have had
to be provided with Peltier effect devices or heaters tomaintain constant chip temperature.
T = R.T.

slab wa

veguide

Output waveguide

Arrayed waveguide

OutputPhase front

T > R.T.

slab wa

veguide

Output waveguide

Arrayed waveguide

OutputPhase front

Figure 2

Principle of temperature dependence.

2.3 Principle of Athermalization
In order to compensate for temperature dependence of
the AWG center wavelength without the use of Peltier
effect devices or heaters, the authors proposed theathermalizing principle described below. Figure 3a is a
simplified structural diagram of the athermal AWG. Eachof the AWG circuits is cut at one of the slab waveguides

into two pieces--one larger and one smaller, and thesechips are connected by a compensating plate madeof copper. As temperature fluctuates the copper plate
expands and contracts, causing the small piece toslide. Figure 3b is a cross-sectional representation of
the mechanism of temperature compensation, witheach frame showing an output slab waveguide in crosssection.
As shown by the arrows in Figure 3a, we may examine
a case in which an optical signal is input at point (1)and output at point (2). In a conventional temperaturedependent AWG, the focus point will shift as thetemperature fluctuates; that is to say, from the same
output waveguide light of a different wavelength will be
output. In an athermal AWG, on the other hand, the focus
point changes as the temperature fluctuates, but the
expansion or contraction of the copper plate moves theoutput waveguide to the new focus point, so that from thesame output waveguide light of the same wavelength canbe extracted.
(2)

(a)

(1)

Compensatingplate (Copper)(b)

Low temp.

High temp.

Mid.

Conventional
AWG

Athermal
AWG

Focus point shifts.

Focus point shifts.

Waveguide is moved
to the focus point.

Waveguide is moved
to the focus point.

Figure 3

Outline of athermal AWG.

2.4 Designing an Athermal AWG
To achieve precise compensation for the temperaturedependence of the center wavelength using theathermalizing principle described above, it is necessary toadjust the length of the copper compensating plate. The
relationship between the center wavelength shift due totemperature fluctuation dλ and the amount of positional
adjustment dx may be represented bydx

Lf ∆ L n d λ∆Tg
nsdλ odT(1)where:
Lf is the focal length of slab waveguide,∆ L is the difference in optical path length between
adjacent arrayed waveguides,
ns is the effective index of refraction of the slab
waveguide,ng is the grouped index of refraction of the arrayedwaveguide,d is the pitch of adjacent arrayed waveguides,
λ 0 is the center wavelength, and∆ T is the amount of temperature fluctuation.
Substituting the circuit parameters from Table 1 into Eq.
(1) yields

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Ultra-Low-Loss Athermal AWG Module with a Large Number of Channels(2)

dx 0.275 ∆ T

Process 1

indicating that shifting the focus point by 0.275 mm
for each 1°C will compensate for the temperaturedependence of the center wavelength. We may thencalculate that since the coefficient of linear thermal
expansion for the copper plate is 1.7 x 10–5, the plateshould be 16.2 mm in length.

Calculation of the difference
between grid wavelengths
and measurement ofcenter wavelength

Process 4

Calculation of
curing temperature

Circuit parameters for the AWG.
Parameters

Channel spacing

Values100 GHz

Number of channels

48

Relative refractive index difference

0.8 %Focal length of slab waveguide: Lf

17.2 mm

Length difference of arrayed waveguide: ∆ L

31.0 µm

Pitch of adjacent channel waveguides: d13.75 µm

Diffraction order: m

Process 329

2.5 Techniques for Center Wavelength Adjustment
Generally speaking, for an AWG to be used in a D-WDM
system, the center wavelength of the AWG must be
matched to the grid wavelength, and in the fabrication
process for athermalization we have also developed aproprietary technique for matching center wavelength
to grid wavelength 6). This is a method in which thecenter wavelength can be easily changed by varying
the curing temperature of the adhesive used to connect
the copper plate to the chip. Figure 4 shows a flowchart
for center wavelength adjustment and the fabrication
process. The following description of the procedure forcenter wavelength adjustment is in accordance with thisflowchart.
First of all, before cutting the AWG chip, the center
wavelength of the chip is measured. From the value thus
obtained the shift from the grid wavelength is calculated
and the curing temperature is determined. We may thenproceed to fabrication, which comprises four processes.
Process 1: The AWG chip is temporarily attached tothe base plate and the chip is then cut into two at theslab waveguide.
Process 2: Adhesive is applied to the copper plate,which is connected to a specified position on thechip.
Process 3: The chip as a whole is kept uniformly ateach curing temperatures that have been calculated
while the adhesive cures. The plate will undergo
thermal expansion and it is connected to the chip sothat this length is maintained.
Process 4: The chip is returned to room temperature,at which time contraction of the copper plate willcause a change in the relative positions of the large
and small pieces. That is to say, the relative positionof the waveguides changes making it possible toshift the center wavelength. The chip is then removed
from the base plate.

Fabrication

Figure 4

Flowchart for adjustment of the center wavelengthand fabrication.

Let us now describe the method for calculating thecuring temperature, which relies on its relationship tocenter wavelength shift, as shown in Figure 5. This
relationship can be approximated by a straight line, theslope of which is virtually the same as the temperaturedependence of the AWG (0.011 nm/°C), and theoreticallyagrees with it. In the case, for example, that the difference
measured at 25°C between the center wavelength before
cutting and the grid wavelength was 0.4 nm, a curing
temperature of 63°C will result in matching with the grid
wavelength, and matching was achieved with a precisionof ±20 pm or better.
What is more, the range of center wavelength matching
is approximately 0.8 nm (far greater than the value of0.15 nm for conventional temperature-controlled AWGs),
making it possible to achieve a significant improvementin center wavelength yield during AWG chip fabrication.
For this reason the wavelength adjustment techniquedescribed here offers a major advantage in cost
reduction.
Center wavelength shift (nm)Table 1

Measurement ofcenter wavelengthof chip

Process 2

0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00

20

30

40

50

60

70

80

90

Curing temperature (˚C)

Figure 5

3.

Relationship between center wavelength shift andcuring temperature.

LOSS-REDUCTION TECHNIQUE

To extend the areas in which AWGs can be used in metro/
access systems, etc., the loss in the AWG itself must be
reduced. We have therefore examined techniques for
obtaining lower-loss AWGs.

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Ultra-Low-Loss Athermal AWG Module with a Large Number of Channels

3.1

Causes of AWG Losses and Methods ofReduction
In addition to propagation loss and the connection loss
between the input and output waveguides and opticalfibers, AWG loss also includes the mode-field mismatch
losses inherent in the circuits. To realize lower AWG
losses we decided to focus on reducing this mode-fieldmismatch loss.
Figure 6a shows the mechanism for generation ofmode-field mismatch loss. In the AWG when the arrayedwaveguide is excited from the slab waveguide, thephotoelectric field distribution at the end of the slab
waveguide is continuous, but, on entering the arrayedwaveguide, instantaneously converts to a discrete
field distribution. This is the point at which modefield mismatch loss is generated, and the loss that is
generated turns into emission loss and is observed as
AWG loss. That is to say, loss can be reduced if theemission loss can be suppressed. Accordingly we laidout a low-loss pattern, as shown in Figure 6b 7), in whichband-shaped waveguides are arranged so as to crossthe part of the arrayed waveguide that is connected to theslab waveguide. These band waveguides are disposed
at a constant pitch with waveguide width WB graduallydiminishing with increasing distance from the slab
waveguide. These parameters were optimized using thebeam propagation method (BPM).

waveguides, we can expect a reduction of better than 1.0
dB.
To achieve a further reduction in loss, identical linear
taper waveguides, as shown in Figure 8, were disposedat the input and output waveguide patterns.

(a) Without low loss pattern(b) With low loss pattern

Discrete electric field
Band
Arrayedwaveguide

Figure 7

waveguide

Input slab
waveguide sideΛOutput slab
waveguide side

WB

Continuous electric field

Figure 6

Results of simulation by BPM.

Slab waveguideSlab waveguide

(a)

(b)

The same taper waveguides

Figure 8

Patterns at the input and output slab waveguides.

Mechanism of loss generation and a low-loss pattern.

3.2 Result of BPM Simulation
Figure 7 compares the results of simulation for aconventional circuit without a low-loss pattern (7a)and for a circuit incorporating a low-loss pattern after
parameter optimization (7b). It can be seen that in theconventional circuit, the light emerging from the slab
waveguide does not connect smoothly to the arrayedwaveguide, and considerable emission loss is generated.
In the circuit incorporating a low-loss pattern, on the other
hand, no significant emission loss is seen. Estimatesof the emission loss in the two circuits come to 0.70 dBfor (a) and 0.17 dB for (b). Thus it is seen that use of
the low-loss pattern effected a loss reduction of morethan 0.5 dB. Since in an AWG there are two areas ofconnection between the arrayed waveguide and the slab

4.

FABRICATION

Incorporating the low-loss pattern described above, wefabricated a 100-GHz 48-ch AWG using PLC fabrication
technology combining flame hydrolysis deposition(FHD), photolithography, and reactive ion etching (RIE).
The chip was then cut at the slab waveguide and its
center wavelength was matched to the grid wavelength
using the proprietary technique for center wavelength
adjustment described above. Optical fibers were thenconnected to the AWG chip, which was then packaged.
Figure 9 shows the appearance of the athermal AWG
module that was fabricated. The package was slim in
profile, measuring 130 x 80 x 8.5 mm.

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Center wavelength shift (nm)

0.10
0.08
0.06
0.04
0.02
0
--0.02--0.04--0.06--0.08--0.10
--10

0

10

20

30

40

50

60

70

80

Temperature (˚C)

Figure 11 Temperature dependence of center wavelength.

Appearance of athermal AWG module.

Figure 10 shows the spectrum of the ultra-lowloss 100-GHz 48-ch athermal AWG. Excellent opticalcharacteristics were obtained, with insertion loss of lessthan 2.8 dB (1.8~2.8 dB), adjacent crosstalk less than
–30 dB, non-adjacent crosstalk less than –30 dB,
and total crosstalk less than –22 dB. There wasno degradation of optical characteristics due toathermalization.
Figure 11 shows the temperature-dependence of center
wavelength. In all channels the center wavelength shift
over a temperature range of –5 to 70°C was ±0.015 nm,
indicating that the temperature dependence of center
wavelength was compensated for almost completely.
The change in insertion loss was ±0.1 dB. Figure 12
shows the temperature-dependence of the spectrum.
No spectrum degradation was observed at any of thetemperatures, confirming that other optical characteristics
were also stable.
Reliability has been confirmed using a 100-GHz 32-ch
athermal AWG module of identical structure.

0

Transmittance (dB)Figure 9

--5

--5˚C

--10

70˚C

--15

20˚C

--20
--25
--30
--35
--40
--45
--50
1540

1542

1544

1546

1548

1550

Wavelength (nm)Figure 12 Temperature dependence of transmission spectrum.

5. CONCLUSION
We have developed a 100-GHz 48-ch AWG module withinsertion loss of less than 2.8 dB using techniques ofathermalization and loss reduction. On all channels, the
AWG module had a center wavelength shift of less than
±0.015 nm and an insertion loss change of ±0.1 dB overtemperatures ranging from –5 to 70°C.

0Transmittance (dB)

--5
--10
--15

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--20
--25
--30
--35
--40
--45
--50
1522

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1567

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Figure 10 Spectrum of ultra-low-loss 100-GHz 48-ch athermal
AWG.

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