We designed a 20 GHz Mach Zehnder interferometric EO-modulator based on a new developed polyesterimide. Measurements show a V/sub /spl pi// of 7.5 V, an insertion loss of 11 dB and an extinction ratio exceeding 20 dB for an interaction length of 2 cm

Chan, Y.P.

Collins, J.V.

Gilde, M.J.

Kershaw, S.V.

Lambeck, Paul

Lecomte, J.P.

Meyrueix, R.

Tapolsky, G.

van Schoot, J.B.P.

English

Lambeck, P.V.

University of Twente Research Information

WeP. 18 22nd Eur.. Jpean Conference on Optical Communication - ECOr3'96, Oslo 3. 281 DESIGN AND REALISATION OF A MZI TYPE POLYMER BASED HIGH SPEED EO-MODULATOR J.B.P. van Schoott, P,V. Lambeckt, M.J. Gildet, Y.P, Chad,  J.P. Lecomtet, G. Tapolskyt, R. MeyrueixS, S.V. Kershaw" and J.V. Collins" f MESA Research Institute, Lightwave Devices Group, University of Twente, PO-box 21 7, 7500 AE Enschede, The Netherlands. * X-31-53-4892746. Fax: X-31-53-4309547. * BT Laboratories, Martlesham Heath, Ipswich, UK. zr X-44-1473-643858, Fax: X-44- f Flame1 Technologies, Parc Club du Moulin d Vent, 33, av. Dr. Georges Lkvy, 69693 1473-646534. Venissieux, France. * X-33 - 78- 776804, Fax: X-33 - 78-0075 78. Abstract We designed a 20GHz EO-modulator based on a new developed polyesterimide. Measurements show a VX of 7SV,  an insertion loss of l l d B  and an extinction ratio exceeding 20dB for an interaction length of 2cm. Introduction In electro-optical modulators, polymers are important because of their low index of refraction and dielectric constant allowing for bandwidths exceeding those of LiNb03-based devices. In this device we applied the electro-optical polyesterimide EOP041 as developed by Flame1 Technologies. Each monomer bears the dicyanomethylene isophorone diethanoltoluidine (DCIE) NLO chromophore3. The polymer chains are terminated with two bismaleimides end groups4. The polymer is thermally cross linked via the bismaleimides terminal groups. The r33 coefficient is around 15 pm/V and the decay is about 7% over 3 months at 85°C under atmospheric conditions. This large stability is due to the high glass transition temperature of 180°C. EOP04 can be photo bleached allowing for defining channel wave guides in a relatively easy way. The refractive index of the poled material is 1.68 @ 1300 nm for TM polarised light in the unbleached state and 1.6 1 in the completely bleached state. The bulk transparency is 0.5 dB/cm at 1300 nm and 0.7 dB/cm at 1550 nm. The modulator is based on a double Y-branch Mach-Zehnder Interferometer. The electrodes are co-planar travelling wave electrodes allowing for a band width of 20 GHz. The electrical field and the optical field are not velocity matched. figure I Schematic view of device. 3. 282 22nd European Conference on Optical Communication - ECOC’96, Oslo WeP. 18 Design The available polymers are CLP332 (a cladding polymer from Flame1 Technologies, n = 1.615 @ 1300nm), PC (a cladding polymer from Akzo-Nobel, n = 1.55 @ 1300 nm) and the electro-optical polymer EOP04. The design of the slab structure is a trade-off between coupling losses to the fibres and the wish of a low Vn: i.e. the voltage needed to switch from the “on” to the “off’ state. Low coupling losses require a thick slab while a thin slab increases the drive field over the electro-optical layer, thus decreasing the V,. An optimum layer stack is found with a 4-layer structure as shown in figure 2. The low index contrast between layer 2 and 3 gives rise to a large field diameter while the high index contrast with the two PC layers reduces the two evanescent fields effectively thus reducing the total thickness. A 5-layer structure (also a CLP33 layer in between the EO-layer and the upper PC layer) will improve the structure even more, however, the CLP33 layer is not transparent for UV light which makes bleaching of the wave guides impossible. A general treatment of the design process will be given in a separate paper6. We show however the considerations that determined the thickness of electro-optical layer. For a low V,, a large fraction of the propagating mode must be located in the electro-optical layer while the total thickness of the polymer stack must be kept as small as possible. These two requirements can be combined in one parameter S which can be expressed as: aNefh*2 tl + t2 +t3 +t4 S =  where we divide the sensitivity of the propagation constant for an applied electrical field through the total thickness of the slab. The differences between the dielectric constants of the four layers have been neglected. This S parameter is drawn as a function of thickness of the electro-optical layer in figure 3 together with the modal overlap with a lensed fibre (l/e field diameter of 4.5 pm) and a cleaved fibre (l/e field diameter of 10 pm). The thickness of the CLP33 layer is taken lpm and the thickness of the PC layers are chosen in such a way that the simulated attenuation due to each electrode is 0.1 dB/cm. The structure becomes bi-modal if the thickness of the electro-optical layers exceeds 1.7pm. From figure 3 it is clear that an optimum is found for a 1.7pm thick electro-optic layer together with coupling to lensed fibres and a relatively large bleached index contrast. 1”’ 0.18, 11.0 0.16 c CLP33 Its = 1.Opm 0.08‘ I I I I ‘0.5 0.6 0.8 1.0 1.2 1.4 1.6 1.8 t2 [pml figure 2 The optimised four layer structure together with the TW-electrodes on top of the device and the floating electrode below the stack. figure 3 The S-parameter and overlap as used in the optimisation process of the slab as drawn in figure 2. Solid line: S-parameter, dotted line: overlap for  AN=0.004, dotted line: overlap for AlV=0.005. WeP. 18 22nd European Conference on Optical Communication - ECOC'96, Oslo 3. 283 The optimised structure is drawn in figure 2 where we have increased the thickness of the PC layers a little in order to prevent unwanted attenuation at the electrodes. In this figure, also the coplanar TW-electrode is shown, together with a bottom electrode. This floating electrode prevents penetrating of the electrical field in the silicon substrate at high frequencies, thus increasing this field in the vertical direction. The electrode width and spacing are chosen 20 and 7pm respectively for a 50Q impedance transmission line. The wave guides are bleached in such a way that the horizontal overlap with the fibre profile is optimised while the channels are still mono-modal7. Realisation The slabs are produced by spin coating the four layers on top of Cr/Au or Al-coated silicon wafers. Each layer was cured thermally before the next layer was coated in order to prevent degradation of the already coated surface by the solvent. A Ti-layer which has been patterned with Reactive Ion Etching (RIE) is used as a bleaching mask for the channel wave guide definitions. After bleaching, the Ti-layer is removed and a gold poling electrode is evaporated on top of the sample. The sample is poled during 20 minutes at 500V near the glass transition temperature of the electro-optical layer. The poling field is switched off when the sample is cooled down well below the glass transition temperature of the electro-optical layer. After poling, this electrode has been used for defining the drive electrodes either by a wet etch, which will produce thin and lossy electrodes, or as a seed for the electroplating of the travelling wave electrodes up to a thickness of 5pm giving us the desired 20GHz bandwidth. Finally, the devices are diced out and polished. Up to now, only devices with thin electrodes have been measured. Results of several devices are given in table 1. The attenuation values are measured with the Fabry-Perot method5 for straight channel wave guides. By comparison the insertion loss of a modulator and a straight channel wave guide we can conclude that the Y-branches do not contribute to the insertion loss. Furthermore, the losses caused by the imperfect lensed fibres are 1.7 dB, the overlap losses are 1.9 dB per end face and the losses due to the residual roughness of the end faces 0.5 dB per end face. A typical response is given in figure 4. The frequency response is completely flat up to 5 MHz which was limited by our modulator's band width. 10 I 1 5 0 -5 - 10 0 5 10 15 Time [ms] 0.6 0.4 0.2 ]O.O figure 4 Design parameters of the modulator. The electrodes are tapered in order to get accessible bond pads. figure 5 Response of UT-F9lM12. V K  is equal to 4.2V, the modulation frequency is 100 Hz. 3. 284 22nd European Conference on Optical Communication - ECOC'96, Oslo WeP. 18 Device UT-F9/M13 UT-F9/M12 kv W prop. loss IL ER Vz Vn*Leff r33 ["I ["I [pml rdB/cml [dBl [dBl [vl [V-ml [pm/vl 52 39.5 4 1.5 14.3 11.4 4.2 0.170 12.3" 52 39.5 3 20.5 24.2 4.2 0.170 12.3* I UT-L6/M15 I 52 39.5 4 22.7 21.7 4.2 0.170 14.1 table I Results as obtained with the realised devices. The devices marked with an asterix are poled in a way different from the other devices, so the '33 values as mentioned here are not relevant. 1L: insertion loss (fibre to fibre), ER: extinction ratio. Device UT-F9lM12 is not bleached long enough which causes increased coupling losses, device UT-L6lM15 is equipped with MMI splitters . Conclusions We have designed and realised an electro-optical modulator that shows a very good low frequency state behaviour. The insertion loss is below 11 dB for Vz = 8 V which has up to our knowledge never been reported before in literature for polymeric electro-optic modulators. By improving the design of the structure and using better quality lensed fibres, the insertion loss can be kept below 8 dB. Extinction ratio values are typical better than 20 dB, 33 dB has been measured. The frequency response is completely flat up to 5 MHz with thin electrodes. Stability measurements on these polymers show that the electro-optic coefficient of the EOP04 polymer is very stable in time. We are now processing the high speed modification. The designed bandwidth is larger than 20 GHz. In this design no velocity matching has been taken into account. With velocity matching, a band width around 100 GHz can be achievedg. In this article we have shown that polymeric electro-optical modulators are becoming competitive with it's LiNbO3 counterpart, and the limits are not yet reached. References 1 CLP33 French patent FR 93 12783 2 EOP04 French patent FR 93 08027 3 R. Meyrueix, G .  Tapolsky, Y.P. Chan and J.P. Lecomte, Orientation of chromophores within NLO- functionalised polymers: comparison between theory and experiment, Nonlinear Optics, 1995,9,293-303 4 R. Meyrueix et al., Functionalised polyestherimides for non-linear optics, to be published 5 D.F. Clark and M.S. Iqbal, Simple extension to the Fabry-Perot technique for accurate measurement losses in semiconductor waveguides, Optics Letters, 1990,1291-1293 6 J.B.P. van Schoot, et al., to be published 7 J.B.P. van Schoot and R. Meyrueix, 2D index profiles in photo bleached polymers: theory and experiments, to be published 8 L.R. Dalton, L.P. Yu, M. Chen, L.S. Sapochak and C. Xu, Recent advances in the synthesis and characterization of nonlinear optical materials: second-order materials, Synthetic Metals, 54, 1990,155-160 Acknowledgement This work has been carried out during the European EUREKA project Polymer based Electro- Optical Components for Optical Communications (PEOCOC, EU868). We would like to thank the French Ministry for Research and Education, the Dutch Ministry of Economic Affairs and the Royal PTT Netherlands N.V. for partially funding the project. We are also grateful to AKZO-Nobel for permitting us to apply their cladding polymer PC. 

Design and realisation of a MZI type polymer based high speed EO-modulator

2D index profiles in photo bleached polymers: theory and experiments,

Functionalised polyestherimides for non-linear optics,

Orientation of chromophores within NLOfunctionalised polymers: comparison between theory and experiment, Nonlinear Optics,

Recent advances in the synthesis and characterization of nonlinear optical materials: second-order materials,

Simple extension to the Fabry-Perot technique for accurate measurement losses in semiconductor waveguides, Optics Letters,

Design and realisation of a MZI type polymer based high speed EO-modulator

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