16-ch Arrayed Waveguide Grating Module with 100-GHz Spacing
by Tsunetoshi Saito *, Toshihiko Ota *, Tomoaki Toratani *2 and Yoshimi Ono *2

Dense wavelength-division multiplexing is well on its way to dominating the fieldof fiber-optic communications, and creating the need for higher-capacity multiplexer/demultiplexers for the optical signals. The authors have developed an arrayed waveguidegrating (AWG) module that is capable of handling multiple wavelengths and based on planar
lightwave circuit (PLC) technology suitable for mass production. Modules are only 8.5 mm thickand come in a sealed package. A 100-GHz 16-ch AWG module has been fabricated and has
been confirmed to have outstanding optical characteristics and reliability.

ABSTRACT1.

INTRODUCTION

Explosive growth in the Internet and other multimedia
applications has created a need for larger communications capacity. Dense wavelength-division multiplexing(DWDM) represents a breakthrough in this area. DWDM
mainly uses the 1550-nm band, the gain window of
erbium-doped fiber amplifiers (EDFA), effecting simultaneous transmission of a number of different wavelengths ona single fiber. This provides a quantum increase in network capacity without the need for laying new lines. 1), 2)
Figure 1 is a schematic that shows a typical DWDM
communications system. Signals having wavelengths λ1
through λn from a plurality of transmitters (Tx) are passed
through a multiplexer and are aggregated on a single optical fiber. EDFAs (marked Amp) are provided according to
the length of the transmission path to amplify the power of
the attenuated signal. The path may also be provided withan add/drop multiplexer (ADM), which can add or drop
signals of any desired wavelength, or an optical crossconnect (OXC). The multiple signals transmitted on the
single optical fiber are then demultiplexed into signals of
the original wavelength. As Figure 1 makes clear, DWDM
transmission requires a multiplexer/demultiplexer for thevarious signal wavelengths. A device used for this purpose of a type using a dielectric filter has been developed, 3)
and has been applied in DWDM systems with 4 to 8 channels with 200-GHz (approximately 1.6 nm) spacing or
wider, but when the number of channels is increased,
devices using a dielectric filter are too bulky and expensive, and their reliability drops. Thus for systems having alarger number of channels, there is a need for a practicalmultiplexer/demultiplexer of the AWG type.
The AWG is a type of PLC in which optical waveguides
consisting of a silica-based cladding and core are fabricat**2

WP Project Team, Opto-technology Lab., R & D Div.
Production Technology Development Center, Plants andFacilities Dept.

ed on a substrate of silicon. 4), 5) AWGs have design flexibility with respect to number of channels and wavelength
spacing, and are superior in mass productivity, compactness and reliability. Accordingly they are expected to havea major role to play in DWDM systems. The authors havedeveloped a compact and highly reliable AWG module forthe multiplexer/demultiplexer of the 100-GHz 16-channel
DWDM system that is currently on the verge of commercial applicability.

2.

PRINCIPLE OF THE AWG

2.1 Multiplexing and Demultiplexing
Figure 2 shows the circuit structure of an AWG, in which
input waveguides, input slab waveguides, array waveguides, output slab waveguides and output waveguides
are fabricated on a substrate. The following description
relates to operation when used as a wavelength division
demultiplexer.
When a multiplexed signal consisting of a plurality ofwavelengths λ1 through λn is applied to an input waveguide, it is spread by diffraction at the input slab waveguide and input to the arrayed waveguides. The arrayedwaveguides consist of a plurality of optical waveguides
Tx
λ1
λ2
λ3
λ4

OXC
M
U
X

D
M
U
X

Amp

M
U
X

D
M
U
X

Amp

λn

Rx1
Rx2
Rx3
Rx4
Rxn

ADM
Tx

Rx

Tx

Rx

MUX: multiplexer
DMUX: demultiplexer
OXC: optical cross-connect
ADM: add/drop multiplexer

Figure 1

Schematic of DWDM system

47

Figure 2

λ1 λ
2 λ3

λn

Optical waveguide circuit structure of AWG

that propagate the signals from the input slab waveguide,
and which are arranged so that adjacent waveguides havea specific length difference ∆L. This means that at the output port of the arrayed waveguides, a phase offset corresponding to ∆L will be imposed on the signal propagatedin each waveguide. After passing through the arrayedwaveguides the signals reach the output slab waveguides
and are spread by diffraction, but due to the mutual interference between the signals from each waveguide, all the
wave fronts are diffracted, at a specified wavelength, in a
uniform direction.
Accordingly the signals of differing wavelength arefocused at different positions on the output side of the output slab waveguides, so that by positioning an output
waveguide at each of these positions, each of the signals
is sent to a different output waveguide, thereby extracting
the signals λ1 through λn.
Note, however, the signals passing through the input
and output waveguides propagate with spatial field. Thus
the transmission spectrum of the signals from the output
waveguides is generally Gaussian, as shown in Figure 3.
Recently it has been possible, by tinkering with the shapeof the input and output waveguides, to develop AWGs
with a flat-top transmission spectrum. 6)
2.2

Temperature Dependence of the Center
Wavelength
Since the AWG acts as a diffracting device, making use of
the length and length difference of the arrayed waveguidetransmission paths, precise control must be exerted overthe difference ∆L between the lengths of adjacent waveguides. In actual practice, however, the changes in therefractive index of the waveguide material caused bychanges in temperature, together with the thermal expansion and contraction of the substrate and waveguides,
result in changes in both the length and the ∆L of thetransmission path. This changes the focal point at the output side of the output slab waveguides as well as the
wavelength of the light entering the output waveguides.
This change in center wavelength in the 1550-nm band
amounts to approximately 0.011 nm/°C.
Furthermore, there might be, say, 100 waveguides in
the array, core shape and refractive index are bound todiverge from the designed values. Also even a minor discrepancy in the position of an output waveguide will meanthat the signal it produces will vary from the desired wavelength.
DWDM transmissions, however, use an international

48

Non-adjacent crosstalk(λn+1≤)

λ1, λ2, λ3...λn,

Outputwaveguide

λn+1Transmission

Input waveguideNon-adjacent crosstalk(≤λn-1)

Input slab waveguideλnAdjacent crosstalk (+1 ch)

λn-1

Output slab waveguideAdjacent crosstalk (-1 ch)

Arrayed waveguides

Wavelength

Figure 3

AWG transmission spectrum and crosstalk definitions

standard wavelength--called grid wavelength (represented
by λn in this paper), so that the center wavelength of the
AWG must be accurately matched to this grid wavelength.
Thus the initial temperature of the AWG is generally regulated so that the center wavelength matches the grid
wavelength, and some structure is required to assure that
this temperature is maintained despite changes in ambient
temperature.

3.

REQUIRED OPTICAL CHARACTERISTICS

In general, multiplexer/demultiplexers for DWDM are
required to have the following optical characteristics:• Small center wavelength offset from grid wavelength;• Low insertion loss;• Low channel crosstalk.
Since in DWDM the wavelength spacing between adjacent channels is extremely small, the optical line width
used in communication must be finer. Even a small offset
between the AWG center wavelength and the grid wavelength results in optical loss increase at the grid wavelength. This offset in center wavelength facilitates the passage of light from other channels, degrading propagation
characteristics. The permissible center wavelength offset
depends on the transmission spectrum of the AWG andthe transmission bit rate of the system, but is normally not
more than 0.05 nm.
As in the case of other fiber-optic transmission devices,
insertion loss should ideally be as low as possible, buthere the target was set as 5 dB or better.
Channel crosstalk in terms of a specific AWG channel n
is expressed as the difference between the insertion loss
at the grid wavelength λn of channel n and the insertion
loss at the grid wavelength of the respective channel.
Crosstalk with respect to the wavelengths of channels n±1(λn-1 and λn+1) is termed "adjacent crosstalk" (Figure 3).
"Non-adjacent crosstalk" is defined as the difference
between insertion loss when λ = λn and the maximum
value of insertion loss in the wavelength range λ ≤ λn-1
and λ ≥λ n+1. Channel crosstalk should be as low as possible, but here a target of -25 dB or better was set.

Furukawa Review, No. 19. 2000

Lid for chip
Thermistor
Fiber array

Fiber array

AWG chipHeat-dissipating plate
Peltier device

Figure 44.

Structure of AWG module

STRUCTURE OF THE AWG MODULE

4.1 Structure and Fabrication Procedures
Furukawa Electric has amassed broad experience in the
fabrication of a wide variety of PLC modules, including 1 x
n star couplers, multi-channel wavelength-insensitive couplers (WINCs), and the like, and this expertise wasapplied in AWG module fabrication as well. Figure 4
shows AWG module structure.
In fabricating the AWG module, a chip is cut according
to the dimensions of the waveguide circuits formed on thesubstrate. A lid is affixed over the input and output ports,
and the end-faces of the chip are polished to an angle of
8° to reduce reflection. The optical fibers for input and output are connected using fiber arrays, in which the fibers
are laid out accurately on the V-groove of a glass substrate and bonded in place. The end-faces of fiber arrays
are also polished to an angle of 8°. The arrays are thenaligned to the AWG chip while monitoring the opticalpower, and the facets are bonded in place. The use of an
automated alignment unit having a precision travelingstage in aligning the light axes achieves low connection
loss in a short time.
4.2 Controlling AWG Module Temperature
As has been mentioned above, accurate control of thetemperature of the PLC chip is essential, and this isaccomplished by a Peltier device and a thermistor (seeFigure 4). The Peltier device acts as a heater or cooler
depending on the direction of current flow. Temperature
signals from the thermistor are fed to an external controller which controls the current to the Peltier device, providing precise temperature control. The module is also
provided with a heat-dissipating plate. The temperature
control system was designed to maintain AWG chip temperature at 40 to 50°C, and is effective at ambient temperatures of 0 to 70°C.
Normally, of course, when a Peltier device is used forcooling, heat is produced on the opposite surface, so that,in addition to providing accurate control of AWG chip temperature, it was necessary to provide for the effective dissipation of the heat thus generated. By paying carefulattention to Peltier device selection, package design andoptimization of the package cover material, it was possible
to design an AWG module with a thickness of only 8.5
mm. As can be seen in Photo 1, the package of the slim-

Photo 1100-GHz 16-ch AWG module

line AWG module is provided with heat-dissipating fins. It
is extremely power-efficient, with a consumption of not
more than 5 W at ambient temperatures of 0 to 70°C.
4.3 Sealed Package
When a temperature-controlled AWG is operated in ahigh-temperature, high-humidity environment, moisture
will condense on the AWG chip because it is at a lower
temperature. Not only is the heat capacity of the chipincreased but accurate temperature control is impossible,
and as moisture builds up, heat dissipation cannot keep
up with the heat produced by the Peltier device, and temperature control failure occurs. The authors therefore
decided to eliminate this problem by using a sealed package arrangement with an O-ring gasket between the package and the cover and sealing compound at the outlet forthe optical fibers and power cord, thereby improving performance with respect to high temperature and highhumidity.

5.

100-GHZ 16-CH AWG MODULE

Using the sealed package structure described above, an
AWG module was fabricated, based on an AWG chip providing 16-channel WDM with a channel spacing of approximately 0.8 nm. Single-mode fibers were used to connect
to the AWG chip. The module was extremely compact,measuring only 120 x 60 x 8.5 mm, including the heat-dissipating fins. The results of optical measurements aredescribed below.
5.1 Optical Characteristics
5.1.1 Establishing Operating Temperature andMeasuring Center Wavelength
To measure the optical characteristics of the AWG module, it was first necessary to establish the operating temperature. This was accomplished by a technique thatinvolved: 1) measuring the center wavelength at each port
at a given temperature; 2) calculating the temperature atwhich the average center wavelength offset was smallest;and 3) repeating measurements of center wavelength at
the calculated temperature. The optical characteristics are

49

160

160
Max. : 0.029 nm
Avg. :0.009 nm
σ: 0.0057 nm
n: 496 ports

140

100
80
60

100
80
60

40

40

20

20

0

≤0.005≤0.010≤0.015≤0.020≤0.025≤0.030λn ±0.8 nm

120Frequency

Frequency120

λn ±0.7 nm λn ±0.8 nm
Max. :-25.3 dB -27.2 dB
Avg. :-33.4 dB -35.0 dB:3.78 dB
3.29 dBσ:496 ports x 2 = 996nλn ±0.7 nm

1400>0.030

≤-40

≤-38

≤-36

Center wavelength offset (nm)Figure 5

≤-34

≤-32

≤-30

≤-28

≤-26

>-25

Adjacent crosstalk (dB)Histogram for center wavelength offset of AWG module

Figure 8

Histogram for adjacent crosstalk of AWG module0
Temperature
controller-5

Variable-wavelength
light source

Polarization
controller

AWG
moduleOptical power
meter

Wavelength meter

Figure 6

Transmission (dB)

-10

Setup for insertion loss measurement

-15
-20
-25
-30
-35
-40
-45
1546

1548

1550

120

1554

Max. : 4.17 dB
Avg. : 3.14 dB
σ: 0.332 dBn: 496 ports

80

Figure 9

1558

1560

1562

Typical transmission spectrum of AWG module

140

60

Max. :-25.0 dB
Avg. :-33.6 dB
σ:3.78 dBn:496 ports x 2 = 996

120

40

≤2.6

≤2.8

≤3.0

≤3.2

≤3.4

≤3.6

≤3.8

≤4.0

≤4.2

≤4.2

Insertion loss (dB)Figure 7

Histogram for insertion loss of AWG module

Frequency

100
20
0

1556

Wavelength (nm)100

Frequency

1552

80
60
40
20
0

≤-40

≤-38

≤-36

≤-34

≤-32

≤-30

≤-28

≤-26

>-25

Non-adjacent crosstalk (dB)the results of measurements made at room temperaturewhen set to the operating temperature established in thisway.
Figure 5 is a histogram of the absolute values measured
for center wavelength offset of 31 AWG modules (496ports). The maximum was 0.029 nm, and the average was
0.009 nm--fully satisfactory for a DWDM multiplexer/
demultiplexer.

Insertion loss results are excellent, with a maximum of
4.14 dB, an average of 3.14 dB, and a standard deviationof 0.33 dB.

5.1.2 Insertion Loss
Figure 6 shows the setup for measuring insertion loss,
using a variable-wavelength light source accurately controlled by a wavelength meter. The use of a polarization
controller allows simultaneous measurement of the polarization dependent loss of the module. Figure 7 is a histogram showing insertion loss at each grid wavelength.
The measured value includes polarization dependent loss.

5.1.3 Crosstalk
Adjacent crosstalk was measured at grid wavelength ± 0.7
nm and grid wavelength ± 0.8 nm. Figure 8 is a histogram of
the results. Crosstalk at grid wavelength ± 0.7 nm was -25.3
dB maximum and -33.4 dB average, and at grid wavelength ± 0.8 nm was -27.2 dB maximum and 35.0 dB average.
Non-adjacent crosstalk was measured using an ampli-

50

Figure 10 Histogram for non-adjacent crosstalk of AWG module

Furukawa Review, No. 19. 2000

σ

n

Max.

Avg.

Center wavelength offset (nm)0.029

0.009 0.0057

496

Insertion loss at λ n (dB)

4.17

3.14

496

Ripple at λ n ± 0.1 nm (dB)

1.23

0.82

0.093

496

PDL at λ n (dB)

0.34

0.06

0.036

496

-25.3

-33.4

3.78

496 x 2=992

λ n ± 0.8 nm

Non-adjacent crosstalk at λ n- 0.8 nm≤ λ ≤ λ n + 0.8 nm (dB)
Return loss at λ n (dB)

0.3

60

0.2
0.1

40

0
-0.1

20

-0.2
-0.3

0

-0.4
-0.5
0

2

4

6

8

10

-20
12

Temp. (°C)

Time (hr)

-27.2

-35.0

3.29

496 x 2=992

-25.0

-33.8

3.78

496 x 2=992a) Insertion loss (dB)

In

42.4

51.6

2.06

31 x 16=496

Out

42.4

51.6

2.05

496

fied spontaneous emission (ASE) light source and optical
spectrum analyzer (OSA). Values were calculated basedon the measurement results (Figure 9) as the difference
between the insertion loss when λ = λn, and the minimum
insertion loss when λ = λn-0.8 nm and the minimum insertion loss when λ = λn+0.8 nm. The range measured was
the range of grid wavelengths for the 16 channels ± 0.8
nm (λ1-0.8 nm ≤λ ≤ λ16+0.8 nm). Figure 10 shows theresults: -25.1 dB maximum and -33.7 dB average.

0.05

80

0.03

60

0.02
40

0.01
0

20

-0.01
-0.02

0

-0.03
-0.04
-0.05
0

2

4

6

8

10

-20
12

Temp. (°C)

Time (hr)b) Center wavelength (nm)
5

80

4
3

60

V, A, W3

5.1.4 Summary of Optical Characteristics
Table 1 summarizes the optical characteristics of the
AWG modules. Ripple (λn±0.1 nm) signifies the deviation
in insertion loss in the range of grid wavelength + 0.1 nm.
Return loss was measured at input and output ports foreach grid wavelength.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

0.04

Temperature (°C)

crosstalk (dB)

λ n ± 0.7 nm

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

0.4

1

40

0
-1

20

-2
-3

Temperature (°C)

Adjacent

0.332

Insertion loss change (dB)

Result

Item

80

0.5

Temperature (°C)

Optical characteristics of 31 AWG modules

Center wavelength change (nm)Table 10-4Voltage (V)Current (A)Power (W)

-5
0

2

4

6

8

10

-20
12

Temp. (°C)

Time (hr)c) Voltage, current and power consumption

Figure 11 Temperature dependence of AWG module characteristics

10

Power consumption (W)5.2 Temperature Dependence
It was confirmed that the AWG modules can operate atambient temperatures of 0 through 70°C, and Figure 11
shows the changes in insertion loss, center wavelengthand power consumption during ambient temperature
cycling of 20°C→70°C→0°C→20°C. The AWG module
tested had an operating temperature of 44.4°C. The variation in insertion loss was less than 0.1 dB and that in center wavelength less than 0.01 nm; power consumption
also showed normal variation. This confirmed that the
AWG module is stable with respect to fluctuations in ambient temperature.

9

Unsealed package

8

Sealed package

7
6
5
4
3
2
1

6.

SEALED PACKAGE

As stated above the AWG modules have a sealed package structure allowing them to operate even under conditions of high temperature and humidity. To confirm theeffectiveness of the seal, temperature control of the modules was carried out at an ambient temperature of 70°C
and relative humidity of 90%, and power consumption wasmonitored. Figure 12 plots the power consumption for
AWG modules in sealed and unsealed packages. In thecase of unsealed modules, power consumption began to

0
0.1

1

10

100

1000

Time (hr)Figure 12 Damp heat performance of AWG in sealed andunsealed packages

rise immediately after the start of the test, and control failure occurred in less than an hour. AWGs in sealed packages, on the other hand, were found to be stable evenafter 200 hours of operation.

51

Table 2

Results of reliability tests
Changes in

Item

Vibration

Thermal shock
Low temperatures

Conditions

7.

σ

Avg.

Worst

σ

3

0.03

0.20

0.06

-0.001-0.004

0.002

3

0.01

0.24

0.12

0.001

0.004

0.002

3

-0.04

-0.24

0.09

-0.002-0.007

0.003

4

-0.02

-0.18

0.06

-0.004-0.0070.002

85°C, 90% RH, 5000 hours 4

0.43

0.60

0.09

-0.004-0.0300.016

10~55 Hz, 1.52 mmp-p
3 axes, 2 hours/axes-15~85°C, Slope :2 mins
Keep :30 mins
-40°C, 336 hours

RELIABILITY TESTSb) Slim-line profile, only 8.5 mm thickc) Equipped with heat-dissipating fins, and consumes
no more than 5 W at ambient temperatures of 0 to
70°Cd) Sealed package structure for improved performance
under high temperatures and humiditye) Reliability demonstrated under Bellcore standards
GR-1209 and GR-1221.
From these results, we are confident that the AWG
modules developed here will have a role to play as highperformance multiplexer/demultiplexers in DWDM systems.
In addition to the AWG modules described in this paper,
development work is going forward on AWG modules for
up to 42 channels, and flat-top broadband transmission
characteristics. Furukawa Electric is also able to provide
temperature controllers, optical power monitors for AWG
modules, as well as MUX/DEMUX modules with alarmfunctions.

CONCLUSIONSAn AWG module with 16 channels at 100-GHz spacing
capable of being used in DWDM systems has been developed and fabricated. The AWG module has the following
characteristics:a) Superior optical characteristics• Center wavelength offset: ≤0.03 nm• Insertion loss: ≤4.5 dB• Crosstalk: ≤-25 dB• PDL: ≤0.4 dB• Return loss: ≥40 dB

52

Center wavelength (nm)

Worst

Reliability tests were carried out in accordance with
Bellcore standards GR-1209 and GR-1221, covering
vibration, thermal shock, low temperatures, temperature
cycling and damp heat. Table 2 shows the test conditions
and results obtained in each of the tests. To verify AWG
module reliability, fluctuations in insertion loss and center
wavelength were measured. To accelerate the damp heattest, a temperature of 85°C was used--10°C higher than
the temperature specified. AWG module temperature control was not activated during the tests, and optical characteristics before and after the test were measured with
AWG module temperature control at room temperature.
The variation in insertion loss was 0.6 dB maximum, andno failures were observed. This confirmed the high reliability of these AWG modules.

8.

Insertion loss (dB)
Avg.

Temperature cycling -40~75°C, 1000 cycles
Damp heatn

REFERENCES

1) H. Onaka et al.; OFC '96, PD-19-1, (1996)
2) Y. Yano et al.; ECOC '96, ThB. 3.1, (1996)
3) M. A. Scobey and D. E. Spock ; OFC '96, ThK1, (1996)
4) H. Takahashi et al.; Electron Lett., vol. 26, (1990), 87
5) C. Dragone ; IEEE Photon. Technol. Lett., vol. 3, (1991), 812
6) K. Okamoto and Yamada ; Opt. Lett., vol. 20, no. 1, (1995), 43
Manuscript received on November 8, 1999.

Furukawa Review, No. 19. 2000