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A variable optical attenuator based on optofluidic technology

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2008 J. Micromech. Microeng. 18 115016

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IOP PUBLISHING JOURNAL OFMICROMECHANICS ANDMICROENGINEERING

J. Micromech. Microeng.18(2008) 115016 (5pp) doi:10.1088/0960-1317/18/11/115016

A variable optical attenuator based on

optofluidic technologyYu Hongbin, Zhou Guangya, Chau Fook Siong and Lee Feiwen

Micro/Nano Systems Initiative Technology, Department of Mechanical Engineering,National University of Singapore, Singapore 117576

Received 3 June 2008, in final form 16 July 2008

Published 7 October 2008

Online atstacks.iop.org/JMM/18/115016

Abstract

A novel variable optical attenuator based on optofluidic technology has been fabricated and

demonstrated. Light attenuation is caused by the optical absorbing capability of liquid and themodulation can be achieved by changing the liquid dimension in the optical transmission path.

In our design, this is done by the deformation of an air-controlled membrane and its

subsequent contact with a rigid plate. From the results, it can be seen that nearly 38 dB

dynamic range and 0.479 dB insertion loss can be achieved and the polarization-dependent

loss (PDL) is also demonstrated to be less than 0.4 dB in the whole working range.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

The variable optical attenuator (VOA) is an optical componentcapable of changing the intensity of the transmitted light. It is

one of the most commonly used optical components and has

wide applications especially in fiber-optic communications.

Recently, VOAs based on microelectromechanical systems

(MEMS) technology have received much interest due to

advantages of large dynamic range, small footprint and

easy integration [1, 2]. Many novel structures successfully

demonstrated include those in which the modulation of

transmission intensity is achieved either by moving a blade in

or out of the optical path between the emitter and the receiver

so as to change the beam transmission width or by rotating a

mirror to changethe directionof thereflectedlight [3, 4]. Sincemost of the devices are fabricated by surface micromachining

or bulk micromachining processes developed from mature

silicon-based microelectronics technology, highly specialized

equipments, such as deep reactive ion etcher (DRIE), low-

pressure chemical vapor deposition (LPCVD) and thermal

oxidation furnace, are required. In addition, multifarious

control parameters associated with these facilities must be

carefully chosen and controlled to obtain the designed

performance. This will largely complicate the fabrication of

prototype, resulting in increased cost and development time.

With the advent of optofluidic technology, which

integrates optics and microfluidics, new possibilities for light

manipulation have opened up. Due to the potential gains oflow fabrication cost, high fabrication yield, short fabrication

period, prominent tunability, reconfigurability and adaptability

advantages as described in the literature [5, 6], optofluidic

technology has gained much attention from researchers aroundthe world. Many devices have been successfully developed for

microfluidic dye lasers [7, 8], optical sensing [9], biological

detection [10] and active light modulation applications [11].

Some new types of VOAs were also demonstrated. In these

cases, the transmission magnitude is attenuated by using a

liquid with refractive index higher than that of the waveguide

core to directly interact with the core region, thus breaking the

total internal reflection (TIR) condition [12,13,14]. Although

those VOAs demonstrate good performance, the need for the

bared fiber core, long period grating fabricated into the fiber

core and/or having additional waveguide fabrication steps,

makes the fabrication process cumbersome.In this paper, a novel VOA design is presented. It is

a transmission-type attenuator and the working principle is

based on the modulation of both the thickness of a light-

absorbing liquid and a light-transmitting aperture. Like most

devices based on optofluidic technology, it can be fabricated

with several lithography and bonding steps and the only

materials involved are polydimethylsiloxane (PDMS) and

SU-8 for the structure and the mold, respectively, This design,

described in more detail in the following sections, facilitates

the development of new devices, which usually includes

prototype fabrication, testing and design optimization. A

simple fabrication technique also improves the possibility

of high fabrication yield and reproducibility. From themeasurement results obtained and presented here, it can be

0960-1317/08/115016+05$30.00 1 2008 IOP Publishing Ltd Printed in the UK
http://dx.doi.org/10.1088/0960-1317/18/11/115016http://stacks.iop.org/JMM/18/115016http://stacks.iop.org/JMM/18/115016http://dx.doi.org/10.1088/0960-1317/18/11/115016


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J. Micromech. Microeng.18(2008) 115016 H Yuet al

Cavity Microchannel

Membrane

Liquid accommodating cavity

Accesses for capillary tube

Part 1

Part 2

Part 3

Figure 1. Schematic of the device.

seen that the resulting performance is comparable to those

described in other previous works. All of these characteristicsmake it a potential candidate in applications such as fiber to

the home (FTTH) and bio-related areas.

2. Device design

Similar to most devices using optofluidic technology, the

main body of the device is made from polydimethylsiloxane

(PDMS), and consists of three functional parts as shown

in figure 1: (1) a deformable part, in which structures

commonly used in liquid lens constitution, i.e. the deformable

membrane, cavity and microchannel are fabricated into the

lowest substrate, (2) a thicker PDMS substrate to act as a rigidupper cap and (3) a cavity sandwiched between parts 1 and 2

used to accommodate a light-absorbing liquid.

To start with, air is first injected into the lens cavity via the

microchannel to deform the membrane into a symmetrically

convex shape, given by Poissons equation [15],

2w(r, ) = P /T , (1)

where w(r,) is the membrane shape function, P is the air

pressure andTis the membrane tension.

The solution of equation (1) is

w(r,) =P a2

4T1 r

a2 , (2)

wherea is the radius of the membrane.

As more air is injected, the light-absorbing liquid solution

which fills the cavity space will be gradually displaced by the

swelling membrane structure and squeezed out through the

plastic tube connected to the cavity. When the deformation

front of the membrane touches the bottom surface of the upper

substrate, thesituation changes from that of free bending to one

of combined bending and contact. From the classical Johnson,

Kendall and Roberts (JKR) theory, the contact surface between

the membrane and the upper substrate forms a perfect circle

and the free energy of the whole system consists of three parts:

(1) the elastic strain energy stored in the membrane, (2) theenergy of adhesion caused by contact and (3) the mechanical

(a) (b)

(c) (d)

(e) (f)

(g) (h)

(i) (j )

Figure 2. Process flow: (a) pattern for the microchannel, (b) patternfor the cavity, (c) PDMS coating, (d) peel-off from mold, (e) PDMScoating for the membrane, (f) oxygen-plasma-activated bonding, (g)peel-off to form part 1, (h) bond to the second PDMS substrate, (i)seal with the upper plate and (j) make accesses to each part.

Table 1. Specific dimensions of the design.

Part Size

Membrane Diameter: 10 mm; thickness: 30mMicrochannel Width: 500m; depth: 80mAir cavity Depth: 180mLiquid cavity 15 15 2 mm3

Upper plate Thickness: 4 mm

energy provided by the external air injection system. At

equilibrium, the free energy should be minimized. As a result,

the radius r of the contact area can be determined through

performing partial differentiation of the free energy. The

detailed analysis of deformation is relatively complicated andbeyond the scope of this paper; more information can be found

in[1618].

The detailed design dimensions of the device

demonstrated in this paper are shown in table 1.

3. The fabrication process

The specific process flow is shown in figure 2. First,

an SU-8 layer of 80 m thickness is spun onto a 4 inch

silicon wafer. After performing standard photolithography

procedures including soft bake, exposure, develop and hard

bake, the molds for the cavity andmicrochannel components inthe lens structure are patterned into the SU-8 layer. A second

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(a) (b)

Air inlet

Liquid outlet

Figure 3. Fabricated device working at two different states: ( a)small membrane deformation (before contact with the upper plate)and (b) large deformation (after contact with the upper plate).

Paddle polarization

controller

Beam splitterDevice

Syringe pumping system

Tunable laserPower meter

Fiber coupler

Figure 4. Schematic of the test setup.

100 m thick SU-8 layer is added to increase the height of

the cavity via the same processes as those of the first layer.

Both layers are then combined to act as the master mold for

the subsequent PDMS casting process, in which a 3 mm thick

layer of PDMS is evenly poured onto the mold and then peeledoff. Oxygen-plasma-activated bonding is needed to bond the

PDMS substrate onto a 30m thick PDMS membrane, which

had already been spun onto another silicon wafer. Another

PDMS substrate with a through square opening punched into

it is used to define part 3 (as mentioned above), while a third

PDMS substrate is kept intact to serve as the rigid upper plate.

Finally, all the three parts are adhesively bonded followed by

manual drilling of holes to create access. Figure3 shows the

fabricated completed working device at two different states of

membrane deformation. (A red dye solution is used to fill the

liquid cavity for visualization purposes.)

4. Results

To assess the devices performance, it is inserted into the

optical path of a laser beam. The intensity indicates that the

device transmitted for different device working stages (i.e.

the air pressure applied) is recorded and the attenuation

magnitude computed. The results are shown in figure 4.

From theoretical analysis, the maximum pressure required to

be applied on the membrane during device actuation is only

6.25 Pa. To obtain sufficient resolution over this relatively

small range, a programmable fluid injection system with 1 l

resolution at a constant velocity as low as 100 l min1 is used

to control the volume of air pumped into the lens cavity. It isobvious that the chamber pressure will increase with increasing

0 200 400 600 800

-40

-35

-30

-25

-20

-15

-10

-5

0

Attenuation(dB)

Air volume injected (l)250

Figure 5. Attenuation as a function of the air volume injected.

(

(a) (b)

Incident light

Figure 6. Schematic of the device operation: (a) modulation of theliquid layer thickness in the vertical direction and (b) modulationrealized in the lateral direction.

air volume introduced into a hermetic space. As a result, themembrane can be deformed to different extents accordingly.

Since it is difficult to accurately measure the chamber pressure

in situ, the resulting attenuation, which is converted from therecorded light intensity, is expressed as a function of the air

volume rather than the air pressure in figure5.

From figure5, which is for an optical input signal with

a wavelength of 1550 nm, it can be seen that a minimumattenuation of38 dB can be achieved at the initial stage. This

is because the membrane is flat as no air is introduced into the

lens cavity and no light-absorbing liquid has been squeezedout. As a result, thelight-absorption lengthby theliquid is at its

peak value, thus transmitting minimal light through the device.

With the injection of air, the membrane is deformed upwardand this causes more liquid to be removed gradually from the

optical transmission path as shown in figure 6(a). During

this stage, until the air volume gets around 250 l, a linear

modulation of attenuation with 0.1226 dB l1 rate can beachieved. After this point, the attenuation modulation deviates

from linearity and its rate decreases with further introduction

of air. This turning point corresponds to the time when the

membrane makes contact with the upper plate as mentionedabove. At this stage, as shown in figure6(b), since there is no

liquid remaining in the contact region between the membrane

and the upper plate, the optical input signal can only be partlyattenuated by the absorption of light by the liquid around this

area. As a result, the intensity of transmission is modulatedmainly by controlling the liquid dimension perpendicular to

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0 -5 -10 -15 -20 -25 -30 -350.0

0.1

0.2

0.3

0.4

Polarizationdepe

ndentloss(dB)

Attenuation (dB)

Figure 7. Measurement of PDL.

1520 1540 1560 1580 1600 1620

-36

-32

-28

-24

-20

-16

-12

-8

-4

0

Attenuation

(dB)

Wavelength (nm)

Figure 8. Measured WDL for the fabricated device.

the transmitting direction rather than in the parallel direction

as in the previous stage. The maximum attenuation is limitedto 0.479 dB because of the absorption of PDMS at this

spectrum and the background noise floor of the detector.

Another important performance indicator, VOA-polarization-dependent loss (PDL), is provided in figure 7,

in which the polarization of the transmitted light is changed

by means of a paddle polarization controller; the maximum

output power difference is chosen for demonstration. It isobvious that our device exhibits good PDL performance with

values of less than 0.4 dB in the whole working range.The wavelength-dependent loss (WDL) is studied by

using a tunable laser to scan in the spectrum region, from

1520 nm to 1620 nm. Figure8shows that the WDL increases

with theattenuationand hasa maximum of 1.8dB. As analyzedabove, the optical input signal is attenuated by absorption by

the liquid located in the transmission path. The liquid adopted

in this paper is deionized water and it has different absorption

coefficients in this spectrum. The thicker the water layerin the cavitywhich corresponds to higher attenuationthe

larger the difference in attenuation. This can be improved by

using other liquid-light absorbers with a smaller absorptiondifference.

All of the results presented above are taken from theaverage data of 10 measurements performed at different

-40 -35 -30 -25 -20 -15 -10 -5 00.000

0.005

0.010

0.015

0.020

RMSerror(dB)

Attenuation (dB)

Figure 9. Results of optical repeatability with respect to opticalattenuation at 1550 nm wavelength.

dates. The root-mean-square (RMS) error of the measured

attenuation for an incidence beam at 1550 nm wavelength,

as shown in figure9, is chosen to demonstrate the stability

of the device. It is clear that the device shows good optical

repeatability across the entire working range with the RMS

error remaining less than 0.02 dB.

5. Conclusion

In summary, a novel variable optical attenuator design based

on optofluidic technology has been successfully fabricated and

demonstrated. The modulation of attenuation is achieved by

changing the optical path length through a light-absorbing

liquid brought about by the deformation of a membrane

actuated by theair pressure. Results show that a dynamic range

of38 dBand aninsertion loss of 0.479 dB can beachieved. A

PDL value of less than 0.4 dB is realized in the whole working

range. The extent of WDL found to be 1.8 dB. It is to be

noted that in order to facilitate the verification of the feasibility

of this design concept, it is designed such that an additional

pneumatic control system is needed to actuate the device.

Based on the experimental results obtained, it is envisaged

that a device integrated with the actuator, such as a thermal

actuator, will be eventually developed. At the same time, with

the development of pressure sensors such as those described in

[19,20], it is also possible to integrate them into the device to

provide more accurate in situ control over the device operation.Consequently, it can be expected that the cost of the whole

device will be brought down by adopting this design concept.

Another point that needs to be made is that in addition to ease

of fabrication, potential low cost and comparable performance

with MEMS counterparts, it is very easy to design andfabricate

devices with different performances to meet the requirements

of different applications by simply changing the component

dimensions.

Acknowledgment

Financial support by the Ministry of Education (MOE)Singapore AcRF Tier 1 funding under grant no. R-265-

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000-235-112 and R-265-000-211-112/133 is gratefully

acknowledged.

References

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[3] Zhang X M, Liu A Q, Cai H, Yu A B and Lu C 2007Retro-axial VOA using parabolic mirror pairIEEE Photon.Technol. Lett.19 6924

[4] Syms R R A, Zou H, Stagg J and Veladi H 2004 Sliding-bladeMEMS iris and variable optical attenuatorJ. Micromech.Microeng.14 170010

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[11] Erickson D, Rockwood T, Emery T, Scherer A and Psaltis D2006 Nanofluidic tuning of photonic crystal circuits Opt.Lett.31 5961

[12] Cattaneo F, Baldwin K, Yang S, Krupenkine T,Ramachandran S and Rogers J A 2003 Digitally tunablemicrofluidic optical fiber devicesJ. Microelectromech. Syst.1290712

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[14] Zhu L, Huang Y and Yariv A 2006 Integrated microfluidicvariable optical attenuatorOpt. Express13 991621

[15] Dayton D C, Mansell J D, Gonglewski J D and Restaino S R2001 Novel micromachined membrane mirrorcharacterization and closed-loop demonstration Opt.Commun.200 99105

[16] Pedersen P 2006 A direct analysis of elastic contact usingsuper elementsComput. Mech.37 22131

[17] Shanahan M E R 1997 A novel test for the appraisal ofsolid/solid interfacial interactionsJ. Adhes.63 1529[18] Dalrymple A J 1999 The effect of adhesion on the contact of

an elastica with a rigid surfaceMaster ThesisVirginiaPolytechnic Institute and State University

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