active polymer glass hybrid waveguides
TRANSCRIPT
Ž .Materials Science and Engineering C 8–9 1999 401–405www.elsevier.comrlocatermsec
Active polymer glass hybrid waveguides
Burkhard Elling ), Rudi DanzFraunhofer-Institut fur Angewandte Polymerforschung, Kantstrasse 55, D-14513 Teltow, Germany¨
Abstract
A hybrid bidirectional photonic device is realized with a Mach–Zehnder interferometer structure in a passive glass waveguide byarrangement of polymer–metal layer system as active cover. The hybrid waveguide takes the advantage of the good optical properties inglass waveguides together with the active electrical and nonlinear optical properties of special polymers. The combination of modifiedlayer structures allows very different applications. The described thermally tuned interferometer can used as an electrically controlledvariable attenuator or pyroelectric sensor. q 1999 Elsevier Science S.A. All rights reserved.
Keywords: Polymer glass hybrid; Waveguides; Optical properties
1. Introduction
For the communication and sensor techniques there areseveral passive components based on glass waveguides
w xavailable 1 . In the field of high transparency, high-speedand active photonic devices such as modulators or switchesthere is an increased interest in new developments. Theneed for active devices leads to polymer waveguides asoptical linear and nonlinear active materials. In order tocreate an active polymer waveguide, both it is necessary tostructurize and also to orient the active polymer groups in
w xan electric field 2–4 . This preparation in generally is veryw xdifficult and leads to an increase in waveguiding loss 5 .
Another way for new devices is the combination of passiveglass waveguides corresponding to their good and stableoptical properties with active polymeric materials to builda hybrid waveguide. In dependence on the hybrid layersystem, is it possible to realize different types of modula-tors and sensors. The functionality of such hybrid systemsis highly complex and demonstrated in Fig. 1.
2. Experimental
A vast variety of different waveguide polymer hybridsystems has been realized. The glass waveguides with
) Corresponding author. Tel.: q49-3328-46328; fax: q49-3328-46317;E-mail: [email protected]
different channel structures were manufactured by theintegrated optics company IOT, Waghausel, Germany.¨
Ž . w xThe polymers poly methylmethacrylate PMMA andŽ . w Ž .xpoly vinylidenefluoride-trifluoroethylene P VDF-TrFE
were separately dissolved in ethylacetoacetate and meth-ylethylketone, respectively and then mixed. To preparethin layers the polymer composites were deposited byspin-coating on the waveguide glass and subsequentlyannealed to remove the solvent and fix the morphologicalstructure. All processes were done in a clean room toprevent layer defects. In dependence on the device func-tion thickness monitored silver electrodes were depositedin a sputter process. With the BPM-simulator ‘‘CAOS’’the best cover index fit for an optimal evanescent fieldinteraction between the waveguide and the cover system iscalculated to be n s1.472 and realized in an 80%c
Ž .PMMAr20% P VDF-TrFE composit. This is the firstcover layer on the top of the glass waveguide which actsalso as a buffer to the electrode layer. For the considera-tion of metal layers in the mode field simulation a very
w xdifficult numerical problem is to solve 6 . This is notpossible with the BPM software.
We built different layer systems on Mach–Zehnderinterferometer glass waveguides.
In the Fig. 2 is shown the calculated mode intensity inthe BGG31-based glass hybrid waveguide for differentcover layers without internal attenuation. All refractiveindices are assumed to be real. The evanescent field de-pends strongly on the cover index. The interaction of themodal field with the cover layer increases with higher
0928-4931r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0928-4931 99 00073-9
( )B. Elling, R. DanzrMaterials Science and Engineering C 8–9 1999 401–405402
Fig. 1. Polymer glass waveguide systems.
cover index but also the attenuation in the glass wave-guide. Therefore, it is necessary to optimize the coverindex respective to a high interferometer output.
The output intensity of a Mach–Zehnder interferometercan be described by
Is0.5 I q I q2 I I cos f qDf 1Ž . Ž .(1 2 1 2 0
The intensities I and I result from the input intensity to1 2
the interferometer and are more or less identical in depen-dence on the optical symmetry in the 3 dB-coupler of the
interferometer which causes also the static phase differ-ence f and therefore the linearity of the output. The0
relative phase shift Df can be calculated by
2pDn LeDfs 2Ž .
l
where Dn is the effective index change of the guidede
mode and L, l are the interaction length and wavelength,respectively. For a full switching cycle, the Mach–Zehnderinterferometer requires a relative phase shift between the
Fig. 2. Calculated mode intensity distribution in the hybrid waveguide at 632.8 nm.
( )B. Elling, R. DanzrMaterials Science and Engineering C 8–9 1999 401–405 403
Fig. 3. Sensitivity of effective mode index to change in cover index.
two interferometer arms of Dfs180. In the experimentalsetup with Ls10 mm and ls0.633 mm there is, there-fore a change in the effective mode index of Dn s3.165e
=10y5 necessary and realized with a change in coverindex.
In Fig. 3, the calculated effective mode index for theused waveguide is given. Furthermore, the sensitivity tochanges in the cover index realized by optical, thermal oracoustical influences are shown in this figure. A cover
Ž .index for the PMMArP VDF-TrFE composite of n sc
Fig. 4. Experimental setup for the dielectric–optical modulation.
( )B. Elling, R. DanzrMaterials Science and Engineering C 8–9 1999 401–405404
Fig. 5. Intensity modulation in Mach–Zehnder hybrid by dielectric heating.
1.472 results in high sensitivity for the cover index change.In the present arrangement, the Mach–Zehnder waveguide
Ž .is coated firstly with a 80r20 PMMArP VDF-TrFE layerand secondly separated by an 80 nm silver electrode with a
Ž .20r80 PMMArP VDF-TrFE layer. Finally, a 100-nmsilver electrode is deposited by sputtering.
In order to realize high nonlinear effects in polymers,generally, it is necessary to polarize the polymer layerbetween the electrodes. However, this is not the case fordielectric heating and some other effects. A linearly polar-
Ž .ized HeNe-laser ls0.633 mm is coupled by microscopeobjective into and out of the waveguide. The transmittedintensity is detected by a photodiode and oscilloscopicallydisplayed. The dielectric excitation is carried out by a lowfrequency amplitude modulated high frequency. The exper-imental setup for the dielectric–optical modulation is shownin Fig. 4.
3. Results
In Fig. 5, the output intensity of a dielectric heatedMach–Zehnder hybrid waveguide is shown. With a 10 mmlong phase shifter electrode at the top of the secondarycover layer a 10 dB extinction ratio was achieved. The halfwave voltage is there in the order of 1 V. In order toachieve a higher degree of modulation, it is necessary tofurther optimize the interferometer symmetry.
In Fig. 6 are shown the switching oscilloscope tracesfor the driver and output signals of the hybrid dielectricmodulated Mach–Zehnder interferometer.
An other possibility for strong modulation of the guidedmode intensity comes with the excitation of surface plas-
mons in a special hybrid construction with respect to thethickness or refractive index of the different polymer andmetal layers. Modifications of these hybrid systems canalso used in a bidirectional manner in the field of inte-grated optical sensors, because the excitation of plasmonsis very wavelength sensitive. Furthermore acts the poledŽ .P VDF-TrFE composite as a pyroelectric sensor element.
Fig. 6. Switching response curves for dielectric-modulated Mach–Zehnderinterferometer.
( )B. Elling, R. DanzrMaterials Science and Engineering C 8–9 1999 401–405 405
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