A Mach-Zehnder interferometer with an polymer electro-optic micro-ring resonator on one of its branches is realized in a polymer layerstack and characterized. Electro-optic coefficients of 10 pm/V and modulation frequencies of 1 GHz were measured

Diemeer, M.B.J.

Driessen, A.

Leinse, A.

Rousseau, A.

English

Leinse, Arne

Diemeer, Mart

NARCIS 

High speed polymer E-O modulator consisting of a MZI with a microring resonator

University of Twente Research Information

12th EUROPEAN CONFERENCE ON INTEGRATED OPTICS --.-.~~--... 2 -  !I7-.V..T-.# . ....-7,-";p,.m-7,*T*7-+T , 7 - - s  - HIGH SPEED POLYMER E - 0  MODULATOR CONSISTING OF A MZI WITH A MICRO RING RESONATOR A. ~ e i n s e ' ,  M.B.J. Diemeer', A. ~ o u s s e a u ~ ,  A. Driessen' ' Integrated Optical Micro Systems Group, MESA+ Research Institute, University of Twente, P.O. Box 21 7, 7500 AE, Enschede, The Netherlands 2 Ecole Nationale Superieure de  Chimie de  Montpellier, 8, Rue de  l4cole normale F - 34296 Montpellier Cedex 5 A.Leinse@ewi.utwente.nl SUMMARY An integrated optical electro-optic modulator consisting of a Mach Zehnder Interferometer with a micro ring resonator has been realized and characterized. The response measurements exhibit a 3dB roll-off at 0.5 GHz.. KEYWORDS polymer, electro optic, microring resonator, Mach Zehnder Interferometer ABSTRACT A Mach Zehnder interferometer with an polymer electro optic micro-ring resonator on one of its branches is realized in a polymer layerstack and characterized. Electro optic coefficients of 10 pmN and modulation frequencies of lGHz were measured. lNTRODUCTION The role of integrated optics becomes more and more important in data communication. In order to let integrated optical components replace its electrical equivalents, active optical components are needed. A very important component is the electro-optic modulator, which is used to convert data-streams from the electrical to the optical domain. This paper deals with the realization and characterization of such a modulator that is based on the resonance principle of a micro-ring resonator (MR) and the phase response of a Mach Zehnder Interferometer (MZI). DESIGN AND REALIZATION A MR can be realized by fabricating a circular waveguide, which couples vertically to two straight port waveguides (figure I), see, for example [1][2][3]. By changing the optical path within the ring its resonance wavelengths can be shifted and both in the through and drop port an amplitude as well as ohase modulation can be eenerated 111. This chanee in the rine - L .  - - can be induced by temperature (placing a heater above the MR) or by an electric field (sandwiching the MR between two electrodes). Because of their high thermo- and electro-optic coefficients, polymers are very suitable for this type of devices. One of the problems in realizing such a device in a vertical coupling arrangement is the position of the MR relative to the two port waveguides. This position determines the amount I : Sc'lci'li"'c 'ie'v a MR of light that couples from the waveguides to the ring (and vice versa) at with two port waveguides both the through and drop port side (the coupling constants). These coupling constants are critical paranieters determining the spectral behaviour. Coupling the MR to a single waveguide simplifies the fabrication process because there is only one coupling constant involved. In addition, the spectral behaviour of the MR is less sensitive to changes in this single coupling constant.. In an ideal lossless ring the through port spectrum has no resonance dips because no light is coupled to the drop port waveguide. In order to use a MR coupled to a single waveguide, the phase Thursday 7th April 12th EUROPEAN CONFERENCE ON INTEGRATED OPTICS . . ... -, ="7.< wp .:.;-*.v"-.'>=.? .... m---..-*;.---- : .. .. , , . .  ~~ . . ~~ change around a resonance wavelength can be used. Fig. 2 shows schematically the output spectrum and the phase response of a slightly lossy MR around a resonance wavelength (h,) 121. This phase response can conveniently be converted to an intensity modulation by combination with an MZI. When switching between the on-resonance and off-resonance condition, the difference between the two branches of the MZI can switch from n to 0. This combination between a MR and MZI is shown (including a cross section through MR and waveguides) in figure 3. The passive behaviour of such is device is described by [4]. Figure 2:'Output power and phase response of the through port waveguide around a resonance wavelength eguides ( 2 )  (h i  Figure 3: A MZIiring device: (a) 3D image(b) cross section through MR and waveguides The complete device is fabricated in a polymer layerstack, which can be deposited by spin-coating. The material used for the 2 pm wide ridge waveguides is the negative photoresist SU8. The ridges have a height of approximately 300 nm. The ridge waveguides can be defined in a two step lithographic process without any additional etching-step. The MR (with a radius of 150 pm) is fabricated in the nonlinear polymer PMMA-DRI (synthesized by Ecole Nationale Superieure de Chimie de Montpellier within the IST project NAIS). The MR is defined by lithography followed by reactive ion etching. Both, the MR and the ridge waveguides, are surrounded by a methyl-silicone resin (sold under the name PS233 Glassclad by United Chemical Technologies). The splitter and combiner in the MZI are fabricated by making two y-junctions in the single mode waveguide. The fabrication process is shown in figure 4 in some detail. s,-J Si - Evaporsc gold -Id -Lithograph) etch gold 1 -Spin coal. soft bake SU8 (ii8urlod <ilnrrrlsd Glarscl:ld 1 Sild (il%\dd - ILilhagmphy. sI hardhake SLIS 1 -Spin coal. hake -Spin coal. son hake i3,M,4.1>Ri 1 lllin SU8 layer - Spuner gold 1 Figure 4: Fabrication process for vertical stacked MR and MZI Thursday 7th April 12th .EUROPEAN CONFERENCE ON INTEGRATED OPTICS ~ ; ~ s - - ~ - B - ? - a X - a ? - . a , n . :  -1 , ' m ~ I . - . ~ l . . p  . ..., ; -  ~ " ., . , . , ~<... ,.~ . .  ~ . ~~ ~ The part of the SU8 layer under the MR is removed lithographically in order to prevent coupling of light from the MR to this layer. Coupling to this layer would induce additional optical loss in the MR. Fabricating a balanced MZ1 is relatively easy when applying a heater over one of the branches. This branch can be tuned, such that both branches have the desired optical path length difference. CHARACTERIZATION While coupling in light at the input waveguide, the spectrum can A4 . . . be measured at the output waveguide. For the TE mode this resonator modes, see figure 5. When uslng the MR as a modulator, this does not matter 1 ' because a single input wavelength (on a flank of one of the steep JB . . . . .  . . . . .  1MO 4545 4 & O f " ' , '  1555 " ' I  dips) is chosen as a modulation 1560 ,nm, modulanon w=vsl*n@h wavelength. With an electrode above and below the MR an Figure 5: Measured Spectrum (TE) of the MR-MZI device. The vertical electric field is applied over the line is the wavelength of the CW tunable laser at which modulation was MR changing its refractive index. measured Only the shift in the position of this single dip is of influence on the output. While applying a modulating voltage between top and bottom electrode, the amount of optical modulation is measured. Because the slope of the spectrum around the modulation wavelength is known, the induced wavelength shift of the spectrum can be calculated for a certain modulation ripple. From this wavelength shift, the change in the refractive index of the MR-material can be calculated and with the electric field applied known, the electro optic coefficient is determined. The rj3 value found for this device is approximately 10 pmN, which is in accordance to the values found in literature [ S ] .  The electrode structure used is a lumped element structure because the length of the electrodes above such a MR can be small. A rule ofthumb in RF design states that an electrode can be used as a lumped element as long as its length is smaller than 10% of the wavelength of the electrical driving field. Even for electrode structures of 1 cm in length this corresponds to an electrical frequency as high as 2 GHz. The electrode size can even be reduced to the size of the MR, making frequencies of 60 GHz possible without special RF electrode design. The frequency of the applied modulation voltage is changed by a network component analyzer (NCA) and the modulation depth as a hnction of the electrical frequency is determined. These measurements were done with and without an electrical amplifier between the NCA and the electrode. This amplifier is used to increase the limited voltage applied by the NCA. The setup is schematically shown in figure 6. Network component laser - Figure 6:  Schematic representation of the used measurement setup The detector used had a spectral response which started to decay around 1 GHz. This spectral response was measured and the measured device response was corrected for this response. The different measurements done are shown in figure 7. i~ EC10'05 . i iI"tuiow~m(onlaim~~nlngr~dOm Thursday 7th April In figure 7 four lines are shown. These lines are: 1. Measured frequency response of the 8 device without electrical amplification and detector correction 2. IbIeanUed frequency response of the device with electrical amplification and without detector correction 3. Measured fkquency response of the I-,,, ~. ... .................. ~~ . .  device with electrical amplification and with detector correction 4. The noise signal of the detector (with the laser power off) I.E+W 1 .€a7  1 .€+on 1 .E+OS Frequency (Hz) In lines 2 and 3 a modulation can be seen Figure 7: Measured frequency spectnun of the MR MZI device with a periodicity of 50 MHz. This is probably caused by the amplifier which gets reflections from the electrodes back. This modulation is not caused by the device because in line 1 the frequency response is flat. It can be seen that modulation frequencies up to 1 GHz can be measured. With this performance, data-rates exceeding 1 Gbit/s can be transmitted. With some specific modulation techniques (like for instance quadrature amplitude modulation), data-rates of 2 to 3 Gbit/s will be possible. The roll-off above 500 MHz is caused by the layerstack. The effective current through this layerstack is dependent on l/oC in which o is the electrical frequency and C is the capacitance of the layerstack. Increasing the frequency results in an increase in the current through the layerstack. Because the NCA drives the electrodes with a constant modulation power, a higher current also means a lower voltage over the electrodes and therefore the electrical field decreases. The roll off is certainly not caused by bandwidth limitations of the MR, because the characteristic time for a MR to build up the optical field is equal to: - FRn, =cm - C (1) In which F is the finesse of the MR. R the radius, n, the group index of the mode in the MR and c the speed of light in vacuum. With realistic values of R and n, (150 pm and 1.5) it can be seen that for 1 GHz modulation (r,, < 1 ns) the finesse can be as high as 1300 before this effect would be dominant. As can be seen from figure 5 the finesse in our ring is much lower (about 10). CONCLUSION A polymer electro optic MR resonator has been fabricated and characterized. By coupling the MR with one of the branches of the MZI a modulator with larger fabrication tolerances than a standard MR is realized. With a NCA the frequency behaviour of the modulator was measured up to lGHz with an electro optic coefficient (r33) of 10 pmN. REFERENCES I) Rabiei P, Steier WH, Cheng Zhang, Dalton LR. 'Polymer Micro-Ring filters and modulators', Journal of lightwave technology, vo1.20, nO1 1, p1968-1975, November 2002. 2) Blair S, Heebner JE, Boyd RW. 'Beyond the absorption limited nonlinear phase shift with microring resonators', Optics Letters, vol27, n05, p357-359, March 2002 3) A. Leinse, A. Driessen, M.B.J. Diemeer, 'Electro optical modulation in a polymer ring resonator', 39th Course Microringresonators as building blocks for VLSI photonics, International School of Quantum Electronics , October 2003, Erice, Sicily 4) Absil PP; Hryniewicz JV; Little BE; Wilson RA; Joneckis LG; Ho PT, 'Compact microring notch filters', IEEE Photonics Technology Letters, vol 12, no 4, p 398-400, April 2000 5) Nagtegaele P, Brasselet E, Zyss J,. 'Anisotropy and dispersion of a Pockels tensor: a benchmark for electro-optic organic thin-film assessment', Journal of the Optical Society of America B , vol20, no 9, Sept. 2003, p 1932-6 AKNOWLEDGEMENT The work presented was done within the fkme work of the IST project NAIS and the MESA+ Strategic Research Orientation "TeraHertz Signal Processing" Thursday 7th April 

Beyond the absorption limited nonlinear phase shift with microring resonators',

Polymer Micro-Ring filters and modulators',

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High speed polymer E-O modulator consisting of a MZI with a microring resonator

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