ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

Polymer Thermooptic Switch for C-Band Based onMultimode Interference Mach-Zehnder Interferometer

Chong Siew Kuang and Sahbudin Shaari, Member, IEEEPhotonics Technology Laboratory

Institute of Micro Engineering and Nanoelectronics (IMEN)Universiti Kebangsaan Malaysia

43600 UKM Bangi, Selangor, MALAYSIAEmail: skchong_)eng.ukm.my

Abstract This paper presents thermoopticswitch based on multimode interferenceMach-Zehnder interferometer. The design ofMach-Zehnder interferometer is verified byusing Beam Propagation Method. The switchhas 0.99 dB of insertion loss, 38.15 dB ofcrosstalk and 0.02 dB of polarizationdependent loss at cross state; 0.99 dB ofinsertion loss, 29.92 dB of crosstalk and 0.01dB of polarization dependent loss at bar state.The extinction ratio of the switch is 38.15 dBon path 1 and 29.92 dB on path 2.

I. INTRODUCTION

THE rapid and global spread oftelecommunication has accelerated the growth inoptical communications network. Opticalwavelength-division-multiplexing (WDM) andoptical access networks for fiber-to-the-home(FTTH) have been developed to enlarge thenetwork service capacity and increase itsflexibility [1]. Such systems transport tens tohundreds of wavelengths per fiber with eachwavelength modulated at 10 Gb/s or more.Unforunately, electronic switching seem unableto keep up with increasing of network capacity.In this context, all-optical switching fabrics willbe a significant breakthrough in order to relievethe capacity bottleneck of electronic-switchednetworks. The main attraction of opticalswitching is that it enables routing of opticaldata signals without the need for conversion toelectrical signals.Optical switches are becoming increasingly

important in optical path cross-connects,protection switching, optical add/drop

multiplexing, optical signal monitoring andnetwork provisioning [2]. Up to now, differentswitch types have been fabricated, includingdirectional coupler, MZI and Y-branch switch.Directional coupler switch is highly sensitive todimension variations during fabrication and thepolarization of the optical field [3]. Y-branchswitch is polarization independent over a widerange of wavelength. However, even withoptimal design, the device is > 2 mm in lengthfor acceptable operating voltages or currents andrequires high electrical power to achieveadequate crosstalk suppression [3]. MZI arepreferred in which, by using strained quantum-well materials, fully polarization independentswitching with good extinction ratio can beachieved [3]. On the other hand, compactness,suitability for integration, polarization andwavelength-insensitive characteristics andfabrication tolerances shown by MMI coupler,make this device ideal candidate for integrationinto MZI structure. Many optical devices usingMMI effect have been reported so far, such asphase diversity networks, Mach-Zehnderinterferometer (MZI) switches and modulators,balanced coherent receivers and ring laser [4].This paper describes MMI MZI thermooptic

(TO) switch for WDM for C-band. UV curablefluorinated acrylic polymer (ZPU12-RI) whichare supplied by Zen Photonics are chosen as thecore and cladding materials because of their lowpropagation loss, high environmental stability,precise control of refractive index, good andeasy ability to process, excellent adhesionproperties, and easy control of film thickness.On the other hand, Au will be used as the thinfilm heater. TO polymer waveguide switches are

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

advantageous for low-speed applications asbypass or protection switches. First, large opticaldevices can be made more easily becausepolymer waveguides are easier to fabricate thansilica-based waveguides. Second, the TO effectof polymers is ten times larger than that of silica[5]. This means that the refractive indexes ofpolymers are highly dependent on thetemperature. Therefore, using polymers insteadof silica can significantly reduce the electricalpower needed for switching. (Conventionalsilica TO switches require a large amount ofelectrical power, 400 to 500 mW) [6].

II. THEORETICAL CONSIDERATION

The operation of optical MMI devices is basedon the self-imaging principle. Self-imaging ofperiodic objects illuminated by coherent lightwas first described more than 150 years ago.Self-focusing (graded index) waveguides canalso produce periodic real images of an object.However, the possibility of achieving self-imaging in uniform index slab waveguides wasfirst suggested by Bryngdah and explained inmore detail by Ulrich [4]. The principle of self-imaging can be stated as follow: Self-imaging isa property of multimode waveguides by whichan input field profile is reproduced in single ormultiple images at periodic intervals along thepropagation direction ofthe guide [4].

For MZI TO switch, it consists of a 3-dBcoupler that splits the signal into two beams,which then travel through two distinct arms ofsame length, and of a second 3-dB coupler,which merges and finally splits the signal again[2]. Heating one arm of the interferometercauses its refractive index to change, whichleads to the guided wave being phase shifted [7].The configuration of the 2x2 MZI switch is

shown in Figure 1, where the input signal issplited by a 2x2 MMI splitter, fed intoindividually addressable waveguide phaseshifters, and recombined in the 2x2 MMIcoupler. Both the MMI couplers can support 2modes. When a Gaussian field is launched intothe MMI coupler, 2-fold images and singleimages are reproduced at periodic intervalsalong the propagation direction of the guide.Then, the guided modes split equally betweenthe two arms. By controlling N phase shifters,

either electrooptic (EO) or TO, the input fieldcan be switched to any one of the N outputwaveguides. However, high-driving voltage isthe major drawback of EO devices, therefore TOphase shifter is chosen in this project. Hardeffort has been focused on applying polymeroptical waveguides to economical and practicaloptical devices, and to interconnections inoptical communication systems. This is becausepolymer optical waveguides can be fabricated byspin coating more easily than inorganic materialwaveguides can be fabricated.

Arm I Path 1

Arm 2 Path 22x2MIMMsplitter

Phase shifter 2x2 MMIcombiner

Fig. 1 Schematic layouts of a I x2 MZI, comprisingMMI couplers as splitting / combiningelements and waveguide phase shifters.

Au

core

-4111 | cladding

* Si substrateFig. 2 Vertical cross section of 2 x 2 NMMI MZI TO

switch.

Figure 2 shows the vertical cross section of theproposed 2 x 2 MMI MZI TO switch, with Auplaced on top of the waveguide. The separationbetween the two waveguide arms is 250p m.The Au thin film heater is 5000p m long and7 p m. The index contrast between the claddingand the core is 0.006, so it will reduce theelectrical power consumptions of the switch.The two output ports are 250 p m apart betweenthem for practical application. The size of theburied square core polymer waveguide is7x7piM2. Because the core size is compatiblewith single mode fiber, therefore connection loss

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

between the waveguide and single mode fiber islow.

In TO switch, the output power is in the path 2(cross state) when no heating power is applied. Itcan be switched to the path I (bar state) byadjusting the heating power applied to one of theTO phase shifter. When the electrical power isapplied to heater 2, the refractive index of arm 2decreases, and the input optical power directedto path 1. Similarly, when the electrical power ofarm 2 is increased, the optical power switchesback to path 2. The results are obtained by usingBeam Propagation Method.

III. RESULT

The thermooptic coefficient of fluorinatedacrylic polymer is -1.425x 10-4/oC. Fig. 3 showsthe 2-D temperature distribution across theburied waveguide cross section induced by a 50,um wide Au thin film heater positionedsymmetrically with temperature change of 2.8°C. The position of center of the core is at x = 0and y = 0. The total thickness of the polymer is37 Htm , where thickness of the upper-cladding,core and lower-cladding are 15 ,um, 7 ,um and 15pim respectively. The core and cladding layersare assumed to have the same thermalconductivity, which is 0.2 W/m0C because theyare made of the same material. The Si substrateis regarded as a perfect heat sink while thethickness of the thin film heater is alsonegligible. Furthermore, the heat flow into theair is also negligible. The result indicates that theheat is transferred quadratically from top tobottom of the waveguide. The temperature nearthe top surface is higher and reduced slowly tothe bottom surface.

Fig. 4 also shows the temperature distribution,but is for heater width of 7 ,um. It can be seenthat heater width of 50 ,um has a widertemperature distribution compare to heater widthof 7 jim. However, heater width of 50 jimrequire higher power to achieve the sameinduced temperature by heater width of 7 ptm,therefore heater width of 7 jim is better.

2.8 'C

"Z

20

10

0

-10

0X (pLM)

IC

Fig. 3 Temperature distribution in buried waveguidewith heater width of 50 ,um and temperaturechange of 2.8 'C.

20.2.8 'C

.Ii10 -

0

-10

ii0 IC

X (pm)Fig. 4 Temperature distribution in buried waveguidewith heater width of 7 pim and temperature change of2.8 °C.

0)

(U

C.

0)

0U

+-b

4.54.03.53.02.52.01.51.00.50.0

-15 -10 -5 0 5 10 15

Position along axis-x

Fig. 5 Temperature profile due to temperature changeof the electrode heater.

Fig. 5 shows the temperature profile of thecore layer due to different temperature change ofheater. It indicates that temperature of center of

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

the core is about 39.6 % of heater's temperature.On the other hand, Fig. 6 shows effective indexchange at the core layer due to temperaturevariation of heater. It can be seen that theeffective index change is in the order of 10-4.

0

Ce

a)

091

0.0006

a)

C)x(a"0

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1.0 -

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-' 0.0002

0.0001

1.55 1.60Wavelength (u m)

A Path I (TE) U Path 2 (TM)XlC Path I (TE) X Path 2 (TM)

Fig. 8 Ouputapplied.

power when no electrical power is

0-15 -10 -5 0 5 10 15

Position along x-axis (um)

0

-a)

03ce4bP0

1.0

0.8

0.6

0.4

0.2

0.0 IFig. 6 Effective index profile due to temperaturechange of the electrode heater.

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Fig. 7 OutIincre

1.50 1.55 1.60

Wavelength (u m)

A Path I (TE) U Path 2 (TE)9Path I (TM) XPath2(TM)

f1l{ 1\j --- l ti} Fig. 9 Ouput power when temperature of electrode

Fig. 7 shows the output power from path I andpath 2 due to temperature variation of heater.

, Ifff\ l ffi ffi X This indicates that the switch has multiple--------'-' switching states. The first to the fourth switching

states are 0 0C, 2.8 C, 5.6 °C and 8.4 °Crespectively. It can be concluded that

_T _ thermooptic effect enables even passive0 1 2 3 4 5 6 7 8 9 10 waveguides with optical switching and tuning

Induced temperature (C) functions. Fig. 8 and Fig. 9 show output powerfrom path I and path 2 for C-band due to

l- Path 1 a ee Path 2 1 temperature change of 0 0C and 2.8 'Crespectively. In addition, Table I summaries allthe results based on the characteristic shown by

put power when temperature of heater is TO switch. MMI couplers are ideal candidate for,ased from 0°C to 10°C. use in MZI TO switch because of their good

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

balancing, stable relative phases and polarizationinsensitivity.

TABLE ICharacteristic of MMI MZI TO switch

Switching statesCross Bar

Operating window (nm) 1550Insertion loss (dB) 0.99 0.99Crosstalk (dB) 38.15 29.92Polarization dependentLoss (dB) 0.02 0.01Extinction ratio (dB) Path 1 Path 2

I __________________ 38.15 29.92

IV. CONCLUSION

We have demonstrated a polymer waveguide TOswitch for C-band. The switch is composed ofMMI MZI with Au thin film heaters as phaseshifters. For cross switching state, the switch has0.99 dB of insertion loss, 38.15 dB of crosstalkand 0.02 dB of polarization dependent loss. Forbar switching state, the switch has 0.99 dB ofinsertion loss, 29.92 dB of crosstalk and 0.01 dBof polarization dependent loss. The extinctionratio of the switch is 38.15 dB on path 1 and29.92 dB on path 2. These results can be use as abenchmark for fabrication in the future.

ACKNOWLEDGMENT

Environment for supporting this work underNational Photonics Top Down Research ProjectIRPA 020202T001.

REFERENCES

[1] A. Himeno, K. Kato and T. Miya, "Silica-based planarlightwave circuits," J. of Selected Topics in QuantumElectronics, vol. 4 , no. 6, pp. 913-924, Nov/Dec1998.

[2] G. I. Papadimitriou, C. Papazoglou and A. S.Pomportsis, "Optical switching: switch fabrics,techniques, and architectures," J. Lightwave Technol.,vol. 21, no.2, pp. 384-405, Feb. 2003.

[3] M. P. Earnshaw and D. W. E. Allsopp, "Semiconductorspace switches based on multimode interferencecouplers," J. Lightwave Technol., vol. 20, no.4, pp.643-650, Apr. 2002.

[4] L. B. Soldano and E. C. M. Pennings, "Optical multi-mode interference devices based on self-imaging:principle and applications," J. Lightwave Technol.,vol. 13, no.4, pp. 615-627, Apr. 1995.

[5] Z. Xu, Q. Xie, Z.Tan, Q. Wu, and Y. Chen, "Heat-resistant optical waveguides using new silicone-basedpolymers," First Joint Symp. Opt-. MicroelectronicDev. Circuits, Nanjing, China, Apr. 2000, pp. 138-141.

[61 J. Kobayashi, T. Matsuura, Y. Hida, S. Sasaki, and T.Maruno, "Fluorinated polyimide waveguides with lowpolarization-dependent loss and their applications tothermooptic switches," J. Lightwave Technol., vol. 16,no. 6, pp. 1024-1029, June 1998.

[71 Y. Hida, H. Onose and S. Imamura, "Polymerwaveguide thermooptic switch with low electric powerconsumption at 1.3 /U m," IEEE Photon. Technol.Lett., vol. 5, no. 7, pp. 782-784, July 1993.

The author would like to thank the MalaysianMinistry of Science, Technology and the

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