Development of a Variable Optical Attenuator (VOA)
Using MEMS Technology
by Masahito Morimoto *, Koji Morimoto *, Kouki Sato * and Shin’ichiro Iizuka *2

With the rapid increases in traffic on optical telecommunications systems, there isan active program for developing transmission devices for use in wavelength division multiplexing (WDM), which is becoming the mainstream technology for providing higher
transmission speeds and a larger number of signal channels. It has been proposed that in the
WDM systems of the future, variations in power due to wavelength could be reduced and thequality of transmission improved by adjusting the power after demultiplexing into individual signal
wavelengths. It is envisaged that the current method, in which the power of all the multiplexed
optical signals is adjusted by a single variable optical attenuator (VOA) would give way to amethod in which one VOA is used for each wavelength. Given the number of multiplexed wavelengths, this change will require VOAs that are considerably more compact. This paper reports on
the development of a VOA that has loss characteristics of low wavelength dependence and canalso be used with multiplexed signals, and employs micro-electromechanical system (MEMS)
technology, occupying only 1/25th the space of conventional devices.

ABSTRACT1.

INTRODUCTION

To cope with recent rapid increases in telecom traffic,
there has been a rapid deployment in the field of fiberoptic transmission using wavelength division multiplexing(WDM), in which a number of signals of differing wavelength are multiplexed for transmission on a single fiber. In
existing WDM transmission systems, transmission pathloss is compensated and relaying effected by erbiumdoped fiber amplifiers (EDFAs), which can batch amplify
the multiplexed signal made up of numerous individual
wavelengths. EDFA amplification gain, however, is wavelength-dependant in the bands used in WDM communication, and so to maintain a constant reception level for eachwavelength, the method most commonly used to maintaina flat transmission bandwidth is to use a gain equalizer(GEQ), having a wavelength dependence profile that is
the inverse of the EDFA 1).
But when there is a change in optical input power to the
EDFA due to a change in span loss in the transmission
path or add-drop multiplexing of the optical wavelength
signal (OADM), the wavelength dependence of the EDFA
gain profile changes, and since the profile of the GEQ is
fixed, there is a shift in the flattening effect it provides.
This makes it necessary to use an optical device known
as a variable optical attenuator (VOA), which can realize a*

Optical Components Development Dept., FITEL-Photonics
Lab., R & D Div.
*2 Process Development Dept., Production Technology
Development Center, Plants and Facilities Div.

flattened gain profile by applying to the EDFA and GEQ
an optical power adjusting function that has low wavelength dependence 2). Even with this method using a VOA,
however, the adjustment is applied to the multiplexed signal as a whole, so that in order to satisfy the needs forcommunication system upgrades and greater accuracy inthe future, it has become necessary to use VOAs for eachwavelength after the multiplexed signal has been demultiplexed. It is anticipated that in WDM systems, the numberof multiplexed wavelengths will increase from several
dozen at present to several hundred, so that to use a single VOA for each wavelength requires that each VOA be
extremely compact. It seems likely that realizing a VOA
that is both compact and low in wavelength dependence
will increase the degree of freedom in designing systems,
leading to greater demand.
Against this background we have developed a VOA
using micro-electromechnical system (MEMS) technology
with loss characteristics that have low wavelength dependence.

2.

DEVELOPMENT TARGETS

Table 1 shows the target values for the VOA development
project. With respect to the wavelength band, we aimed at
low wavelength dependence, envisaging application for
signals demultiplexed into single wavelengths, as well as
in current applications, in which the multiplexed signals
are batch adjusted. We have therefore taken account of
the fact that EDFA bandwidth has been extended from the

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Development of a Variable Optical Attenuator (VOA) Using MEMS Technology

Table 1

Target specifications for MEMS-type VOA.

Optical characteristics
Wavelength bands
Insertion loss
Return loss
Wavelength flatness

C (1530~1565 nm) and L (1565~1625 nm)< 1.0 dB
> 45 dB
0.3 dB (in C- or L-band)

Polarization-dependent loss

< 0.3 dB (less than 1% of loss)

Temperature dependence< 0.15 dB

Driver and other characteristics
Electrostatic drive system

Operating voltage< 200 V

Loss resolution< 0.1 dB

Repeatable setting accuracy< 0.1 dB

Dimensions (L × W × T)

10.0 × 2.0 × 0.6 mm

Package size (L × W × T)

33 × 6.5 × 4.5 mm

Sealing

Hermetic sealing

C-band (1530~1565 nm) to the L-band (1565~1625 nm),and made it applicable to both these bands. (Because ofmeasuring equipment limitations, 1530~1620 mn wasused in actual measurements and evaluations).
In setting the development targets in Table 1, we aimed
at broad equivalence with the characteristics of existing
VOAs.
We also aimed at an operating voltage of less than 200
V. For the MEMS chips themselves we adopted an electrostatic drive system, reducing the power consumption of
the chips themselves. The electrostatic drive system
requires a high voltage, but since there is fundamentallyno flow of current (although leakage current is present),
the typical power consumption is in the microwatt order.
Thus the power consumption of the module itself is determined more by the consumption of the voltage step-up circuit than by the MEMS chip. To prevent condensation to
the MEMS chip, a hermetically sealed package was used.

3.

STRUCTURE OF MEMS-TYPE VOA

Single-mode fiber (SMF) was used at the input and outputof the VOA developed here, with a graded index fiber(GIF) having the same diameter--125 µm--as the SMF
fusion spliced for a specified length, to form an optical
coupling with a lens function. An anti-reflection (AR) coating is applied to the tip of the GIF. The GIF tip is polishedat an angle so that the light beam emitted from the end of
the GIF is not aligned with the optical axis of the fiber, but
is at an angle to it. This angled optical beam is interrupted
by means of a shutter that has been formed by inductively-coupled plasma deep reactive ion etching (ICP-DRIE).
The MEMS chip uses a silicon-on-insulator wafer, withthe shutter, actuator and fiber grooves formed simultaneously on the chip by ICP-DRIE, followed by metal vapor
deposition over the whole chip.
The actuator of the MEMS chip is of the comb type, and
the GIF is held in the fiber grooves by means of adhesive.

Photo 1

SEM image of MEMS chip.

The MEMS chip with this GIF optical coupling system isfixed by adhesive within a casing, which is hermeticallysealed. Photo 1 is a SEM image of the MEMS chip.

4.

DESIGN

4.1 Optics
In designing this MEMS-type VOA, it was decided, inorder to take maximum advantage of the small size of the
MEMS chip, to make the collimator the same size as the
optical fiber thereby reducing the size of the module.
Accordingly a GIF coupling system was designed in whichan all-core GIF was given the function of lens and was
fusion spliced for a specified length to the tip of the SMF.
In this design there was some discussion as to whether
the VOA should be of the light shielding type or the tilted
mirror type. It was understood that with the light shielding
type the wavelength dependence of loss could be reducedby modifying the shape of the shutter 3), but with the tiltedmirror type, because of the wavelength dependence of themode field diameter of the fiber, the wavelength dependence of the loss would be greater than for the lightshielding type.
Figure 1 shows the result of simulations of wavelengthdependent loss for a tilted mirror type and a light shielding
type that interrupts the light beam from one side using
results of measurements of SMF mode field diameter.
(Taking loss at 1530 nm as the reference, the difference

Difference between losses
at 1530 nm and 1625 nm (dB)

MEMS actuator

3.5
3Tilted-mirror method

2.5

Light shielding method

2
1.5
1
0.5
0
-0.5
0

5

10

15

20

25

30

Loss at 1530 nm (dB)Figure 1

Result of simulations of wavelength-dependent loss.

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Development of a Variable Optical Attenuator (VOA) Using MEMS Technology

from loss at 1625 nm is plotted; for other light shielding
types, see reference 3.) With the light shielding type, however, to reduce the influence of diffraction at a portion of
the shutter, the diameter of the light beam must be
increased, necessitating a larger movement of the MEMS
actuator, while with the tilted mirror type a large loss canbe achieved with a small movement. Since the objectiveof this work was to develop a VOA with lower wavelength
dependence of loss, the light beam shielding type wasadopted.
The spot size from the light beam emitted was designed
to be the maximum spot diameter obtainable from the GIF
optical coupling system used. And for the GIF facet toachieve a return loss of -45 dB or better, angled polishing
was used. In this way the light beam is emitted from theend facet of the GIF at an angle calculated by Snell’s law.
Thus the GIF optical coupling system must not be
opposed in a straight line, but in an offset position. The
fiber grooves to the left and right are thus fabricated with
an offset that is determined from the angle of emission of
the beam and the opposed distance. These offset fiber
grooves were formed in one operation on the MEMS chip.
As a result of measurements of the beam actually emitted from the GIF using a near field pattern (NFP) system,
it was confirmed that the beam diameter and opposed distance were in accordance with the design. Figure 2 shows
an image of the near-field pattern of the GIF specially fabricated in this work, by a process in which an all-core GI
preform was drawn to the same diameter as SMF.
4.2 MEMS Chip
The MEMS chip is fabricated by micromachining of a silicon wafer, but even so, it is still a mechanical structurewith moving parts that could be damaged by mechanicalvibration or impact.
Accordingly simulations were run to confirm that moving
parts were not subject to breakdown by the application ofvibration or in impact tests.
Specifically, we considered that in the Telcordia test forvibration (GR-1221) breakdown did not occur under 6axis, 20 G, 20 to 2000 Hz. Similarly with respect to impact,
considering that breakdown did not occur when subjected
to 6 axis, 500 G impacts in the Telcordia impact test, it
was confirmed by simulation that during application of a

Figure 2

Image of near-field pattern of GIF.

500 G impact, the weakest part of the comb did not madecontact with the structure. In the simulation, under an
impact of 500 G the moving parts moved in the directionof movement of the shutter but the teeth of the comb did
not made contact with each other, and with respect to the
opposite direction, the rigid portion of the truss structure of
the spring made contact with the electrode, but this portion was strong enough that it was not damaged. Further
the movement in the two planes perpendicular to thedirection of movement of the shutter was less than 1 µm
and design was such that the moving parts made no contact whatever with other structural components.

5.

RESULTS FROM PROTOTYPES

Photo 2 shows the appearance of the MEMS-type VOA
module developed in this work. The development targets
in terms of dimensions were achieved: 6.5 mm in width,
33 mm in length (fiber retaining boot length 5 mm), and
4.5 mm in height. The module was sealed by means of
nitrogen gas encapsulation.
5.1 Optical Characteristics
5.1.1 Insertion Loss
The insertion loss of the prototype MEMS-type VOA module using a 1550-nm light source had an average value of
0.6 dB, easily satisfying the design target of 1 dB. This
loss depends on the accuracy of the alignment between
the opposed collimators, but these results demonstratethat the fiber grooves formed in a single operation on thesilicon substrate were very accurately made.
5.1.2 Return Loss
Figure 3 shows the wavelength dependence of return loss
at several values of attenuation by the insertion of the
shutter at the incidence port. Since it is anticipated that ina structure in which the light beam is interrupted by the
shutter some reflection from the shutter will occur in theattenuation range, the return loss was measured at an
attenuation of 10, 20 and 30 dB. Over the whole range ofattenuation the levels of return loss obtained presented nopractical problems.

Photo 2

Appearance of MEMS-type VOA module.

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Development of a Variable Optical Attenuator (VOA) Using MEMS Technology

Return loss (dB)

70
0dB
10dB
20dB
30dB

65
60
55
50
45
40
1530

1550

1570

1590

1610

Wavelength (nm)Figure 3

Wavelength dependence of return loss at several values of attenuation by shutter. (at incidence port)

5.1.3 Wavelength Characteristics
Figure 4 shows the wavelength dependence of loss atselected values of attenuation. It was confirmed that thetarget value for band flatness (maximum loss minus minimum loss) for an attenuation range of up to 25 dB for bothC- and L-bands was within 0.3 dB. Further, at an attenuation range of up to 20 dB, even across a broad band
including both the C- and L-bands, it was confirmed thatflatness came within 0.3 dB. Also no periodic ripples wereobserved in this spectrum. This flatness showed good
agreement with the results of simulations of the wavelength dependence of SMF mode field diameter. The simulation yielded the fact that this good wavelength flatness
is not obtainable with a collimator having an ordinary
aspheric lens, and in actual measurement as well, thewavelength unevenness obtained using the GIF couplingsystem of this work was approximately 1/3 the results witha collimator using an ordinary aspheric lens.

5.1.5 Temperature Dependence
Figure 6 plots data showing the temperature dependence
of insertion loss at temperatures of from -45 to +85°C.
Only about 50 hours are covered by the graph shown but
in fact tests were carried out for two weeks yielding anequivalent results. It can be seen that the variation in
insertion loss was held to within ±0.10 dB, which was lessthan the development target of 0.15 dB. The temperaturedependence is attributed to the method used to anchor
the GIF, and good results for GIF anchoring wereachieved by adjusting the amount of epoxy adhesive
used, the position of attachment, and the area of attachment.
5.2 Operating Characteristics
5.2.1 Voltage Dependence of Attenuation
Figure 7 shows the relationship between voltage andattenuation during operation of the MEMS-type VOA. The
relationship obtained in simulations is also plotted on thesame graph. As can be seen the simulation and actual
measurements are in extremely good agreement, andsince voltage setting accuracy can be obtained to the millivolt order, the error in attenuation adjustment is 0.1 dB or
less. Note that the MEMS chip itself is capable of analog
operation, so that the accuracy of attenuation setting isdependent on the setting accuracy of the voltage circuit.
5.2.2 Switching TimeIn terms of measuring switching times, assuming an application for the adjustment of gain level inside the EDFA,
several decibels may be considered an appropriate varia0.5

0.4
0.35

PDL (dB)

0.3
0.25
0.2
0.15
0.1
0.05
0
0

10

15

Figure 5

10dB

10

15dB
20dB

5

Loss variation (dB)15

Sample 2

Temperature

90

0.40

25

5dB

70

0.30
50

0.20
0.10

30

0.00
10

-0.10
-0.20

-10

-0.30
-30

-0.40
25dB
0
1530

1545

1560

1575

1590

1605

-0.50
0

1620

Wavelength dependence of loss at selected values ofattenuation.

10

20

30

40

-50
50

Time (hours)

Wavelength (nm)Figure 430

Attenuation dependence of PDL.

Sample 1

20

25

0.50

IL
3dB

20

Attenuation (dB)30

Loss (dB)5Temperature (°C)

5.1.4 Polarization-dependent Loss (PDL)
Figure 5 shows the attenuation dependence of polarization-dependent loss (PDL) in terms of average values of
PDL measured at 5-nm intervals from 1530 to 1620 nm.
At an attenuation of up to about 10 dB PDL deteriorated
sharply, while at higher attenuation results of approximately 0.3 dB were obtained. In fundamental experiments
using the same GIF optical coupling system and a razor
with an extremely sharp blade as the shutter, PDL characteristics were extremely good--less than 1% of attenuation.

0.45

Figure 6

Temperature dependence of insertion loss.

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Development of a Variable Optical Attenuator (VOA) Using MEMS Technology

30

0.5

Attenuation (dB)25

Insertion loss (dB)Actual values
Simulated values

20
15
10

0.4

Before and after test No. 1

0.3

Before and after test No. 2

0.2
0.1
0
-0.1
-0.2
-0.3

5

-0.4
-0.5
1530

0
0

50

100

150

200

1550

Applied voltage (V)Figure 7

1570

1590

1610

Wavelength (nm)

Voltage dependence of attenuation.

Figure 10 Change in insertion loss before and after
vibration tests.

Voltage (a.u.)

Insertion loss (dB)

0.5

Applied voltage

510Before and after test No. 2

0.2
0.1
0
-0.1
-0.2

15

-0.4
-0.5
1530

20

1550

1570

1590

1610

Time (ms)Wavelength (nm)Switching time of insertion loss to 3-dB attenuation.
(in air)

Figure 11 Change in insertion loss before and after
impact tests.

can be suppressed by injecting into the case a liquid suchas matching oil, which offers viscous resistance. Figure 9shows measured values for switching time when matching
oil was introduced. In these results switching time wasapproximately 2 ms.

Voltage (a.u.)

Figure 80

Before and after test No. 1

0.3

-0.3

O/E conversion voltage

-5

0.4

Applied voltage
O/E conversion voltage

-5

0

5

6.
10

Time (ms)Figure 9

RELIABILITY TESTING

Switching time of insertion loss to 3-dB attenuation.
(in matching oil for damping)tion in attenuation. Measurements were therefore made of
the switching time required for an attenuation variation of
3 dB. Measurements were made using a 2-channel digital
storage oscilloscope, with the input to one channel being
the O/E conversion of the light from the output port of the
MEMS-type VOA module, and the input to the other channel being the voltage applied to the MEMS chip. Figure 8
shows the results.
During application of the voltage, attenuation oscillated
in the neighborhood of 3 dB, and it required about 15 to
20 ms for the oscillation to stabilize. We attribute this tothe lively movement occurring at the point of equilibrium
between the spring and electrostatic force. This oscillation

6.1 Telcordia Tests
The following sections present the results of Telcordia
tests (GR-1209 and -1221) performed to test the reliabilityof the MEMS-type VOA developed here.
6.1.1 Mechanical Tests
Figures 10 and 11 show changes in insertion loss before
and after two modules were subjected to a vibration test
(20~2000 Hz, acceleration 20 G) and an impact test
(acceleration 500 G, 6 axes, 5 times).
The amount of change after the mechanical tests wasless than 0.1 dB. No damage was observed to the MEMS
chips which, as the simulation predicted, passed the
Telcordia mechanical tests.
6.1.2 Operating Behavior in Vibration Environment
To test device behavior when subjected to vibration during
operation as set forth in Telcordia GR-63 for office vibration, the change in attenuation was measured under an

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Development of a Variable Optical Attenuator (VOA) Using MEMS Technology

Attenuation (dB)

10

7.

A compact MEMS-type VOA has been developed which
offers outstanding attenuation-wavelength characteristics
and allows external control of attenuation by means of anelectrical signal to adjust for dynamic variation in loss inthe transmission path. It was confirmed that the following
characteristics, which were the values targeted in thedevelopment, were satisfied:

9.95

9.9

9.85

10 dB up
10 dB down

9.8
0

20

40

60

80

CONCLUSION

100

Frequency (Hz)Figure 12 Change in attenuation near 10 dB when accelerationand vibration added in Y direction.

attenuation of approximately 10 dB due to application ofvoltage, scanning in both directions for 10 min for 5~100
Hz in the direction of shutter movement with an acceleration of 0.1 G (19 Hz/min). Results are shown in Figure 12.
The “up” and “down” in the graph refer to an increase or
decrease in frequency. These results showed no difference between “up” and “down”, and the characteristics
were extremely stable--only about ±0.03 dB with an attenuation of 10 dB.
6.1.3 Environmental Tests
As tests under specific conditions of temperature andhumidity the following tests are scheduled to be conducted: temperature/humidity aging test (GR-1209: 85°C
RH85% for 14 days), temperature/humidity cycling test
(GR-1209: -40~+75°C, RH10~80%, 42 cycles for 14days), high-temperature/high-humidity storage (85°C,
RH85% for 2000 hr), high-temperature storage (85°C for2000 hr), and low-temperature storage (-40°C, RH85% for2000 hr). The modules are hermetically sealed, and judging by the results of temperature dependence tests presented above, no problems are anticipated.

Wavelength bands
Insertion loss
Return loss
Wavelength flatness

C (1530~1565 nm)
and L (1565~1625 nm)<1 dB>45 dB

0.3 dB (in C- or L-band @ 25 dB)
0.3 dB (in C- or L-band @ 20 dB)
Temperature dependence<0.15 dB
Loss resolution<0.1 dB
Repeatable setting accuracy <0.1 dB
Dimensions (L × W × T)
33×6.5× 4.5 mm
The reliability of the MEMS-type VOA was tested using
Telcordia tests, and it was confirmed that the results weresatisfactory.

REFERENCES
1) Mizuno et al.: Furukawa Review, No.19, 2000, 53.
2) S.Kinoshita, Y.Sugaya, H.Onaka, M.Takeda, C.Ohshima, andT.Chikama: Optical Amplifiers and Their Applications, (1997), 49.
3) Masahito Morimoto, Kouki Sato, Akira Mugino, Hirokazu
Tamura, Matt Neal, and Alan L. Sidman: NFOEC, Session E5(2001)

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