Development of a Variable Optical Attenuator
by Kouki Sato *, Takuma Aoki *, Yasuhiro Watanabe *, Yuichiro Oguri *,Nobuyuki Shibata *2, Toshimitsu Nishiwaki *2, Mizuki Oike *2
and Toshihiko Ota *3

In response to higher levels of traffic on optical communications networks, wavelength-division multiplexing or WDM--the mainstream system now in use--isdeveloping in two directions: higher data rates and a larger number of channels. The erbiumdoped amplifier (EDFA) is an indispensable component of WDM systems, and, in order to provide dynamically flexible gain level adjustment, is often used in combination with variable opticalattenuators (VOAs), which have flat wavelength characteristics over a wide band range. Thispaper reports the development of a VOA able to withstand the 200-mW power produced by highoutput EDFAs.

ABSTRACT1.

INTRODUCTION

To respond to the recent rapid growth in demand fortelecommunications services, networks are being implemented using wavelength-division multiplexing (WDM)
technology, in which multiple wavelengths are multiplexed
on a single optical fiber. Erbium-doped fiber amplifiers(EDFAs) are indispensable to these WDM transmission
systems: their gain spectrum corresponds to the bands
used for WDM communications, enabling batch amplification of the many signals in the transmission band.
The EDFA makes use of the input signal amplification
effect due to stimulated emission of erbium ions, and its
gain profile is wavelength dependent. In most designs,
therefore, a gain equalizer (GEQ) having a reverse profile
to the EDFA gain spectra is used with the EDFA to flatten
the gain characteristics across the transmission band andthereby maintain a constant level of reception for the signals of each wavelength 1).
The GEQ however has a fixed profile, so that when
there is a change in span loss during transmission orwhen the signals are added or dropped (optical add-drop
multiplexing or OADM) the signal power to the EDFA
changes, altering the wavelength-dependence of its gain
profile and causing a deviation in the gain-flatteness provided by the GEQ. This makes it necessary to use an optical component known as a variable optical attenuator(VOA) to flatten the gain profile by applying to the EDFAand GEQ a signal-adjusting function that is substantiallyindependent of wavelength 2).

* WP Team, FITEL Photonics Lab.
*2 Production Technology Development Center, Plant andFacility Div.
*3 Optical Components Dept., FITEL Products Div.

VOAs now on the market may be divided into two types:
mechanical, having moving parts, and non-mechanicalwith no moving parts 3). Among non-mechanical VOAs aretypes that make use of the magneto-optic effect and thosethat use the thermo-optic effect of the waveguide. But inthese non-mechanical types of VOA the settings for attenuation are wavelength-dependent. Mechanical VOAs, on
the other hand, use a motor or similar means to move an
optical filter having an optical attenuation gradient making
it possible to realize a VOA with outstanding wavelength
flatness by selecting an optical filter with a low wavelength
dependence.
The factors described above have led to a burgeoning
demand for VOAs in recent years. This paper reports on
the development of a mechanical VOA, a type selected
because of its superior wavelength flatness and because
it has a proven record in installations with EDFAs.

2.

DEVELOPMENTAL TARGET

Table 1 shows the target specifications for the variable
optical attenuator. In consideration of the fact that thewavelength range of EDFAs extends from the C-band(1525-1560 nm) to the L-band (1575-1610 nm), it was
decided to make the VOA applicable to both.
In setting the specifications as shown in Table 1 we tookaccount of the needs of EDFA designers and developers,
specifically as related to the points detailed below. Amongexisting VOAs, one non-mechanical type was outstandingin terms of optical input power, but somewhat inferior as towavelength flatness, whereas a mechanical VOA wasgood in terms of flatness but had an optical input power of
only about 50 mW. The target therefore was to realize a
VOA that would combine good flatness with an optical
input power of 200 mW.

15

The term "repeatability" in Table 1 denotes the setting
accuracy for attenuation when resetting the voltage
detected by the potentiometer or the number of pulses
applied to the stepping motor, and "backlash" denotes theattenuation setting accuracy under the above conditionswhen settings have been made including reversing thedirection of motor operation.

3.

STRUCTURE OF THE VOA

Let us describe the structure of the VOA that was developed through this research by following along the lightpath. The input and output fibers are single-mode fibers(SMFs). The ferrules at the fiber ends are angled-polished
at 6° and an anti-reflection (AR) coating is applied. Beamdiameter is expanded by a lens and the distance between
the lens and the ferrule is adjusted so that the coupling
efficiency is maximized at the operating distance. Noadhesive was used in the light path of the VOA, and thiswas essential to improving optical input power performance. The light beam from the input lens was reflected
180° by a prism, in order that the input and output fibers
would be arranged on the same side of the VOA module,with consequent savings of space. From the prism thelight beam passes through an optical filter (attenuator element) having an optical density gradient in the direction offilter travel, and is then focused by a lens at the output
SMF.
Among attenuator elements, filters with a low level ofwavelength dependence are referred to as neutral density(ND) filters. The ND filter is connected by means of a plastic nut to the stepping motor, and regulates attenuation by
a reciprocating movement in the direction of the density

Table 1

gradient, driven by a tap formed on the nut and by a screw
rotated by the stepping motor. To suppress return loss, it
is mounted at an angle to the light path.
A potentiometer is provided to detect attenuation. A
brush mounted on the nut detects resistance values by
establishing contact with a resistance film. These values
yield the filter position and thereby the corresponding
attenuation. Settings for the attenuation of the VOA
installed in the EDFA are controlled by feedback fromEDFA output based on the pre-measured relationshipbetween potentiometer resistance and attenuation.
External interfaces for power supply to the stepping
motor and potentiometer, and for reading of the resistancevalues are provided via a flexible circuit board by meansof pin electrodes formed on the bottom panel of the VOA.
To maintain the performance of moving components
such as the stepping motor and brush of the potentiometer and electrical contacts on the circuit board, the VOA
module package is filled with nitrogen gas and hermetically sealed by welding.

4.

PROTOTYPE

Photo 1 shows the VOA developed in this work. The target dimensions of 30 mm width, 50 mm length (30 mmlength for the fiber-protective sleeve) and 11 mm height
were achieved.
4.1 Optical Characteristics
4.1.1 Insertion loss
Figure 1 is a histogram of VOA insertion loss. Using a
1.55-µm light source, average insertion loss is 0.28 dBwith a standard deviation of 0.09 dB. This corresponds toVOA target specifications

Optical characteristics
Wavelength range
Insertion loss
Return loss
Wavelength flatness

1525-1610 nm<0.6 dB>45 dB
0.2 dB or 1% of attenuation,
whichever is greater

Polarization-dependent loss (PDL)<0.15 dB

Temperature dependence<0.15 dB

Optical input power

200 mW

Photo 120

Drive characteristics

Motor operating voltage

16152-phase stepping motor
3.3 V

Attenuation detector

Potentiometer

Resolution of attenuation<0.1 dB/stepRepeatability<0.1 dBBacklash<0.2 dB

Size (W x L x T)

30 x 50 x 11 mm

Package

Hermetically sealedFrequency

Motor typeAppearance of the VOA

1050~0.2~0.4~0.6~0.8~1.0

Insertion loss (dB)Figure 1

Histogram of VOA insertion loss

Furukawa Review, No. 202001

the loss between collimators, and the loss in the prism(including AR coating) and that part of the ND filter that is
not coated with metallic film. It can be seen that theresults obtained from the prototype fully satisfy the developmental target of 0.6 dB.
4.1.2 Return lossFigure 2 shows the dependence of return loss on motor
stepping count at the input and output ports measured fora 1.55-µm light source. This result shows the return loss
measured for the entire attenuation range--that is to say
for attenuation loss measured while changing motor stepping count. It can be seen that the developmental target of
45 dB or more was attained for the whole of the attenuation range.
Figure 3 shows a histogram of VOA return loss for theinput and output ports. In Figure 3 the values taken for
return loss for the VOA module are the worst values in thewhole attenuation range. The values obtained for average
return loss were 48.3 dB at the input port with a standard
deviation of 2.2 dB, and 49.1 dB at the output port with astandard deviation of 1.8 dB.
4.1.3 Wavelength characteristics
Figure 4 shows the wavelength dependence of the attenuation spectra. This represents the developmental target. It
was confirmed that flatness (maximum attenuation minus
minimum) was held to 0.2 dB or less for all ranges ofattenuation in the C or L bands, and to 0.3 dB or less forthe range covering both C and L bands. Neither were any

periodic ripples observed in these spectra, clearly showingthat no multiple interference occurred within the ND filteror other optical components. Specifically, defining a ripple
as the difference between maximum and minimum attenuation in any 2-nm band at wavelengths of from 1520 to1620 nm, the average ripple value was 0.032 dB.
4.1.4 Polarization-dependent loss
Figure 5 shows the dependence of polarization-dependent
loss (PDL) on motor stepping count. It can be seen thatthe developmental target of 0.15 dB or less was achieved
for the whole of the attenuation range (corresponding tomotor stepping count). The wavelength-dependence of
PDL was also measured for the C and L bands (no graphshown), and it was confirmed that there was no significant
wavelength dependence, and the target value of 0.15 dB
was satisfied.
4.1.5 Temperature dependence
Figure 6 is a histogram of the temperature dependence ofinsertion loss from -5 to +70°C. The average value was
0.10 dB with a standard deviation of 0.05 dB. It can beseen that the target of 0.15 dB or less was comfortablyachieved. Temperature dependence is believed to beimpacted by the method by which the prism is immobilized, and the good results obtained are attributed to thecare taken regarding the amount of epoxy adhesive used,
and the position and area to which the adhesive wasapplied.

055

Attenuation (dB)

Return loss (dB)

60

50

45
Input port

100

200-10-15

Output port-20
1520

40
0-5300

400

500

600

700

1540

1560

1600

1620

Wavelength (nm)Motor stepping countFigure 2

1580

Dependence of return loss on motor stepping countFigure 420

Wavelength dependence of attenuation spectra

0.30
Input port

Output port

0.25
0.20

PDL

Frequency

15100.15
0.10

50.05
00.00~40~45~50~55~60

0Return loss (dB)Figure 3

Histogram of VOA return loss

100200300

400

500

600

700

Motor stepping countFigure 5

Dependence of PDL on motor stepping count

1770

14

ND filter

12

Temperature (°C)

10Frequency

VOA Module60

864504030

220

0~0.05~0.10~0.15~0.200~0.25

510Figure 8

Histogram of temperature dependence of insertion
loss200 mW

Temperature of ND filter and VOA module

50 mW

0.40
0.20
0.00
-0.20

15105-0.40
-0.60
0100

200

300

400

05000.0Time (hr)Results of high-power optical input power test ofVOA

4.1.6 Input power
Figure 7 shows the results of a high-power optical input
power test, in which changes in the attenuation of theVOA were measured as input power changed. As the figure shows, a steady drop in attenuation is seen when optical input power is 500 mW. This is thought to be due tothe ND filter's losing its attenuation film function as aresult of oxidation of the metals of which it is composed.
The reason that the ND filter film becomes oxidized isthought to be absorption of the optical input power by theND filter film resulting in a rise in film temperature. Thus toguarantee use at the target of 200 mW, we present inFigure 8 the results of measurements of ND filter andVOA module temperature at an optical input power of 300mW. It can be seen that ND filter temperature was approximately 60°C and module temperature was about 40°C. It
is therefore hypothesized that ND filter film oxidation at500 mW input is promoted by the rise in temperature dueto the absorption of the optical input power by the ND filter
film and its conversion into heat. From Figure 7, however,it can be seen that, despite the fact that at the target inputof 200 mW there was some small increase in attenuation
at the early stages of irradiation, which may be attributable
to the temperature characteristics of the module itself, theoxidation of the ND filter film progressed extremely slowly
so that no change over time in attenuation was measured.
Further the temperature of the VOA module only rose to1825

20500 mW

Attenuation (dB)Change in attenuation (dB)

0.60

Figure 720Time (hr)Temperature dependence of insertion loss (dB)Figure 615

1.0

2.0

3.0

4.0

Potentiometer voltage (V)Figure 9Relationship between potentiometer voltage andattenuation

somewhat less than 40°C, confirming that operation underconditions of actual installation would present no problem.
4.2 Operating Characteristics
4.2.1 Attenuation setting accuracy
Figure 9 shows the relationship between potentiometer
voltage and attenuation (Note that the attenuation range
for the ND filter installed in this VOA was 0 - 20 dB.)
Attenuation does show some degree of departure from linearity in the vicinity of 5 dB, but when a single pulse wasapplied to the stepping motor in a direction such thatattenuation was increased or decreased, the direction ofincrease or decrease in the attenuation and the potentiometer voltage detected were in agreement, confirmingthat the VOA was functioning normally. The amount ofchange in attenuation for each pulse applied to the stepping motor averaged 0.040 dB/step, comfortably realizing
the target value of 0.1 dB/step.
4.2.2 Repeatability and backlash
The next item evaluated was the repeatability and backlash when a series of 100 pulses was applied to the stepping motor in both directions. The histogram in Figure 10
shows the results. Repeatability was 0.011 dB with a standard deviation of 0.010 dB, and backlash was 0.041 dBwith a standard deviation of 0.021 dB, achieving the target
values in both cases.

Furukawa Review, No. 202001

150.08
Repeatability

Insertion loss (dB)

BacklashFrequency

1050~0.02

~0.04

~0.06

~0.08

Potentiometer voltage (%)

0.08

Insertion loss (dB)

-0.04

0.40

Figure 10 Histogram of repeatability and backlash

0.06
0.04

0.20

0.00

-0.20

-0.40

0.02

ABCDE

A: Temperature-humidity aging
B: Temperature-humidity cycling
C: High temperature storage (dry)
D: Low temperature storage
E: High temperature storage (damp)

0.00
-0.02
0.15

Potentiometer voltage (%)

0.00

-0.08

~0.10

Repeatability/backlash (dB)Figure 12 Results of Bellcore environmental tests

0.10

0.05

0.00

-0.05

Vibration test

Mechanical shock test

Figure 11 Results of Bellcore mechanical tests

5.

0.04

RELIABILITY TESTING

5.1 Bellcore Test
The following presents the results of Bellcore tests (GR1209 and 1221) conducted to test the reliability of theVOA module developed in this report.
5.1.1 Mechanical tests
Figure 11 shows the results of mechanical tests carriedout on three modules each--for vibration (Bellcore 1209:10-55 Hz, amplitude 1.52 mm, 2 hours); and for mechanical shock (Bellcore 1221: 6-ft drop, 8 cycles x 5 times)--in
terms of before-and-after change in insertion loss andchange in voltage detected by the potentiometer (as apercentage of maximum voltage) when attenuation wasset to insertion loss and maximum attenuation.
After each of these mechanical tests there was virtually
no change, either in insertion loss or potentiometer voltage, and the units tested were considered to have passedthe Bellcore tests.

5.1.2 Environmental tests
Figure 12 shows the results of tests for environmental
temperature and humidity that were carried out on threemodules each--for temperature-humidity aging (Bellcore1209: 85°C, 85% RH, 14 days); for temperature-humidity
cycling (Bellcore 1209: -40 to +75°C, 10-80% RH, 42cycles, 14 days); for high temperature storage (damp)(85°C, 85% RH, 2000 hours); for high temperature storage (dry) (85°C, 2000 hours); and for low temperaturestorage (-40°C, 2000 hours) (two modules only)--in termsof before-and-after change in insertion loss and change involtage detected by the potentiometer (as a percentage ofmaximum voltage) when attenuation was set to insertion
loss and maximum attenuation. The results given for thehigh-temperature storage test and low-temperature storage tests are interim results at the 1000-hour stage, andthe tests continue.
After each of these environmental tests there was virtually no change, either in insertion loss or potentiometer
voltage, and the units tested were considered to havepassed the Bellcore tests.

6.

CONCLUSION

A variable optical attenuator (VOA) has been developedthat is primarily installed within an optical amplifier and iscapable of controlling attenuation by means of external
electrical signals that adjust dynamic variations in opticaltransmission path loss, offers superior wavelength flat-19

ness and is capable of accepting high levels of power. It
was confirmed that the following target values were satisfied:
Wavelength range
1525-1610 nm
Insertion loss
<0.6 dB
Return loss
>45 dB
Wavelength flatness<0.2 dB (C-band, L-band)<0.3 dB (C+L bands)
PDL<0.15 dB
Temperature dependence<0.15 dB
Optical input power
200 mW
Resolution of attenuation<0.1 dB/step
Repeatability<0.1 dB
Backlash<0.2 dB
Size (W x L x T)
30 x 50 x 11 mm
Bellcore tests were carried out to evaluate VOA module
reliability, and satisfactory results were confirmed. Testsfor high-temperature storage and low-temperature storage
are continuing on the way to the 2000-hr mark.

REFERENCES
1) Mizuno et al.: Furukawa Electric Review, No.105, p36, 2000. (inJapanese)
2) S.Kinoshita, Y.Sugaya, H.Onaka, M.Takeda, C.Ohshima, andT.Chikama: Optical Amplifiers and Their Applications, (1997), 49.
3) "Trends in Variable Optical Attenuator Products", OPTOCOM,May, 2000, p64. (in Japanese)

Manuscript received on November 22, 2000.

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Furukawa Review, No. 202001