A Novel High-Speed Polymeric EO Modulator Based on a ...
B E E PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 10, OCTOBER 2005
A Novel High-Speed Polymeric EO Modulator Based on a Combination of a Microring Resonator' and an MZI A. Leinse, M. B. J. Diemeer, A. Rousseau, and A. Driessen
Abstract-A Mach-Zehnder interferometer with an electrooptic polymer mircroring resonator adjacent to one of its branches is realized in a polymer layer stack. The microresonator is defined by reactive ion etching in the nonlinear PMMA-DRl polymer and waveguide definition is done without etching, by using a negative photoresist (SUS)as waveguide layer. Electrooptic coefficients of 10 pmN and modulation frequencies of 1 GHz were measured.
Fig. 1. Schematic view of an MR with two port waveguides.
Index Terms-Electmoptic modulation, integrated optics, microresonators, optical polymers.
W
ITH THE increasing penetration of optical techniques from long-haul point-to-point connections into metropolitan and even access networks, the role of integrated optics becomes more and more important. This evolution is mainly pushed by the rapidly increasing bandwidth need of the user leading eventually to fiber to the home. Integrated optical devices have to compete with bulk optic solutions currently used in long-haul connections that are optimized for performance. In metro or access optical networks, however, low-cost is the most important issue, as devices are shared by a very restricted number of users. Besides, in these networks the large number of optical nodes demands devices with a high functional complexity. Promising elementary building blocks for the desired integrated optical devices are ultracompact optical mircroring resonators (MR) [I], [2]. These MRs can be used, for instance, for passive wavelength slicing, wavelength (de)-multiplexing, routing, and, as will be shown below, for high-speed modulation. 11. DESIGN AND REALIZATION A basic design of an MR consists of a circular waveguide, which couples to two straight port waveguides (Fig. 1; see, for example, [3]-[5]). By changing the optical path-length of a single round-trip, the resonance condition of the MR can be changed and an amplitude Manuscript received February 23, 2005; revised May 30, 2005. This work was supported by the EC funded IST-project NAIS, Next-Generation Active Integrated-Optic Subsystems (IST-2000-20\8018), and by the MESA+ strategic research orientation TeraHertz. A. Leinse, M. B. J. Diemeer, and A. Driessen are with the Integrated Optical Microsystems Group, University of Twente, Enschede 7500 AE, The Netherlands (e-mail: [email protected]; [email protected]; [email protected]). A. Rousseau is with Laboratoire des Materiaux Macromoleculaires, Villeurbanne 6962 1, France (e-mail: alain.rousseau@insa-lyon fr). Digital Object Identifier 10.1109iLPT.2005.854354
Fig. 2. Output power and phase response of a slightly lossy MR with a single port waveguide at the through port around a resonance wavelength.
and phase change can be generated in both the through and the drop port. By sandwiching the MR between electrodes and applying an electric radio-frequency (RF)-field, high-speed electrooptical modulation becomes possible. Because of their compatibility with metal electrodes and high electrooptic coefficients, polymers are very suitable for this type of devices. The properties of the entire device strongly depend on the amount of light coupled from both waveguides to the MR and vice versa. This coupling is determined by the position of the MR relative to the waveguides, making alignment a critical issue. By reducing the number of port waveguides to a single one, the spectral behavior of the MR is less sensitive to changes in this single coupling constant (as was discussed by [6]). In this case, the amplitude spectrum of a lossless MR is essentially flat because no power can be coupled to a nonexisting drop port. For an electrooptic modulator, the phase change induced by the MR can be utilized instead of the amplitude change, because in an ideal lossless ring the phase at the through port changes from zero to 27~at resonance. Fig. 2 shows schematically the output spectrum and the phase response of a slightly lossy MR around a resonance wavelength (A,) [5]. This phase response can conveniently be converted to an intensity modulation by combination with'a Mach-Zehnder interferometer (MZI) [5], [7], [8]. When switching between the on-
1041-1 135/$20.00 O 2005 IEEE
I
2075
LElNSE er 01.: NOVEL HIGH-SPEED POLYMERlCLO MODULATOR BASED ON A COMRINATION
' I
e.
-561
ii:ct,cr;es
P~.'o\~A-DRI
SU8 ridge
1540
. . . . : . . . . : . 1545
,550f
. . . : - . . . I 1555
modulation wavelength
1560
wavelength (nm)
Rc.4. Measured spcclnltn (TE) of the MZI 1
rlrree~d~~~~cnsional view: ( b l crass Fig. 3, hf%l + ring d c v ~ c c :i;,,sclic~l~alir section through MR and w;t$,r:o~des.
and off-resonance conditions, the d i k r e n c e between the two branches of the MZI can switch from ?i to zero. Such a combination between an MR and MZT, as is shown in Fig. 3, has been realized. The functionality ofpolymelic electrooptic MRs was demonstrated [41 in a device, in which both the waveguide and the MR definition were done by reactive ion etching (RIE). This etching will induce losses due to roughness in both the MR and the waveguides. Fabricating the waveguide by an etch-free lithographic process will reduce this problem. The port waveguides are fabricated by photodefinition with the aid of a negative photoresist (SUX, 7 1 = 1.57 at 1550 nm) in a two-layer lithographic process. The first layer defines the 0.7-i~m-thickslab of the wavezuide, of which the part under the MR is removed in order to prevent coupling from the MR to the slab [3]. The second layer is a thin layer of 300 nm in which 2-pm-wide waveguides are defined lithographically. Between the waveguides and the MR. a I-[cm-thick layer of a methylsilicone based 1-esin (n = 1.4 at 1550 nm; commercially available under the name PS233 Glassclad by UnitedChelnicalTechnologies) is used. This same material is also used for the 4-jim cladding layer over the MR. For the MR (with a height of 0.8 and a radius of 150 j ~ r n ) . a nonlinear polymer PMMA-DRI ( 7 r = 1G at 1550 nm; syothesized by the Ecole Nationale Supelleure de Chimie de Montpellier) is used, which is patterned by standard lithography followed by RIE. MR-losses are reduced by heating the polymer close to its glass transition lemperature, causing a reflow (smoothening) of the sidewalls 191. The MZJ is balanced by applying a heater over one of the branches, as is theoretically discussed in [8]. The spectral behavior of thedevice is measured by coupling-in light with a butt coupled fibei-setup. The spectrum of the transverse-electric (TE) mode shows rnultimodal behavior, as two resonance modes can clearly be seen in Fig. 4. The presence of a second mode should not cause problems for the perforn~anceof the inodulator, because only the steep flank of the zeroth-order
+
ring device. The vertical line indicates the wsvelengtl~of the CW tunahle laser chosen for the modulation measuremeno.
liiode is used for modulation at a fixed wavelength. Applying a voltage to the electrodes above and below the MR results in an electric field that changes the refractive index of the polymer. While applying a modulating voltage, the amount of optical modulation at the output port is measured. With the slope of the spectrum at the modulation wavelength known, the induced wavelength shift of the spectlum can be calculated for a certain modulation lipple. 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 electrooptic coefficient is determined. The r33 value found for this device is approximately 10 p i N , which is in accordance to the values found in literature 1101. Because the benefit of using the MR with the MZI is the easier fabrication, the electrode definition should also not be a critical fabrication step. It is, therefore, preferred to use the electrodes as lumped elements instead of RF designed traveling wave electrodes. A rule of thumb in RF design states that an electrode can he used as a lumped element as long as its length is smaller than 10% of the wavelength of the electrical driving field. For electrode structures with a length of 1 cm in t~ material with a dielectric constant of three, this corresponds to an electrical Requency as high as 2 GHz. If the electrode size can be reduced to the size of the MR. the tnodulation frequency can even be 60 GHz without any special RF design of the electrode. The frequency ofthe applied ~nodulationvoltageis swept by a network component analyzer (NCA) and the modulation depth as a function of the electrical frequency is determined. These measurements were done with and without an electrical aniplifier 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 Fig. 5 . The optical detector had a spectral response which started to decay around I GHz. The measured device l-esponse was corrected for this and the resulting frequency response is shown in Fig. 6. In Fig. 6. four different traces can be distinguished. These four traces are as follows: I ) measured frequency response of the device without electrical amplification and detector correction; 2) measured frequency response of the device with electrical an~plificationand without detector correction;
IEEE PHOTONICS lZCHNOLOGY LETTERS. VOL. 17, NO. 10, OCTOBER 2005
2076
1,1111,1111111,1111>,,,,,,t,,l,,11111111,,,,,~
laser
Fig. 5. Schematic representation of the used measurement setup.
( F : finesse of the MR; R: radius; n,: group index of the mode in the MR; c : speed of light in vacuum). With realistic values of R and 71, (1 50 fim and 1.5) and I -GHz modulation speed (rCav < 1 ns), the finesse can be as high as 1300 before this effect would he dominant. As can be seenfromFig. 4, the finesse in our ring is two orders of magnitude lower (about ten).
-60
111. CONCLUSION
U
-
+
-70
The realized polymeric electrooptic MZI MR demonstrates a large potential for high-speed modulation. Modulation frequencies well above 1 GHz could he possible in the future if the material stack is slightly altered. Changing the claddinglayer might reduce the rolloff above 500 MHz. In addition, as stand-alone phase modulator or two-port device electrooptic MRs could serve as ultracompact modulators and switches in complex integrated optic structures.
.-rn
-,W
-5
-80 -90
m
-100
a
-110 .t:
$
-120 -130 1.Et06
1E+07 Frequency (Hz)
IEtO8
1E+09
Fig. 6 . Measured frequency specuutn ofthr MR MZI devicz.
3) measured frequency response of the device with electrical amplification and with detector correction; 4) noise signal of the detector with amplifier (with the laser power off). Traces 2 and 3 both show a ripple with a periodicity of 50 MHz, which is probably caused by the amplifier which senses reflections from the electrodes. This ripple is not caused by the device because in line 1 the frequency response is flat. Modulation frequencies up to I GHz can be measured. With this performance, data rates exceeding 1 Gbls could be transmitted because with some specific modulation techniques (like for instance quadrature amplitude modulation), data rates of 2-3 Gbls could be possible. The rolloff above 500 MHz is probably caused by the layerstack. The effective current through this layerstack is dependent on l/wC in which w is the electrical angular frequency and (7 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 a lower electrical field.
(41
1.51
161 171 181
191
1101
[ I I]
A. Driessen el 01.. "Microresonaton as building blocks fur VLSl phatonics:' in AIP Conf P m c , vol. 709,2004, pp. 1-18, B. E. Little, S. T. Chu, W. Pan, and Y. Kukubun, "Microl.ing resonator arrays for VLSl photonics," IEEEPhoron Tc.cl~,rol.Len,vol. 12, no. 3. pp. 323-325. Mat 2000. A. Leinse, A. Driessen, and M. B. I. Dienteer. "Electrooptical modulalion i n n polymer ring resonator," in AIP Co,!f P!',nf,vol. 709,2004, pp. 4.-7 .1 4.1. 1. . P. Rahiei, W H. Steier, C. Zhang. and L. R. Dalton, "Polymer microring lilten and modulators:' J. Lighiw Techno/.. vol. 20. no. I I . pp. 1968-1975, Nov. 2002. S. Blair, J. E. Heebner. and R. W. Boyd, "Beyond the absorption limited nonlinear phase shift with inicmring resonators," Opt. Leri.. vol. 27. no. 5, pp. 357-359, Mar. 2002. B. E. Liltle, S. T Chu. and H. A. Haus, "Track changing by use of lbe phase response of microspheres and resonators," Opr. Lor., vol. 23, no. 12, pp. 894-896. 1998. I? P. Absil, J. V Hryl~iewicz,B. E. Little, R. A. Wilson. L. G. Joneclar, and P. T. Ha, "Compact microring notch filters.'' lEEE Plzoron. Techno/. Len., vol. 12, no. 4. pp. 398400, Apr 2000. J. Yang. F. Wnnp. X. Jimg, H. Qu, M. Wane. and Y Wmg. "Influenceof loss on linearity of microring-assisted Mach-Zehnder modulator." Opi. Expmr.~,vol. 12, no. 18, pp. 4178-lI8X. 2004. A. Leinse. M. B. I. Diemeer. and A. Drirssen, "Scattering lossreduction in polymer waveguides by reflowing." EIrcr,n. L~rt..vol. 40. no. 16. pp. 992-993, Aug. 2004. F. Michelotti, A. Beiardini. M. C. Larciprete, M. Benolotti,A. Rousseau. A. Katsimihety, G . Schoer. and J. Muellcr. "Measurement of the electrooptic properties of poled polymers al A = 1.35 prn by means of sandwich slructures with zinc oxide transparentelectrode."Appl. Phvs. Leir.. vol. 83. no. 22, pp. 44774479, 2003. A. Leinse, "Polymeric microring resonator based electro-optic modulatm," Ph.D. thesis, Inregrated Optical Micro Systems Group. Univ of Twente, The Netherlands, 2005.
A Novel High-Speed Polymeric EO Modulator Based on a Combination of a Microring Resonator' and an MZI A. Leinse, M. B. J. Diemeer, A. Rousseau, and A. Driessen
Abstract-A Mach-Zehnder interferometer with an electrooptic polymer mircroring resonator adjacent to one of its branches is realized in a polymer layer stack. The microresonator is defined by reactive ion etching in the nonlinear PMMA-DRl polymer and waveguide definition is done without etching, by using a negative photoresist (SUS)as waveguide layer. Electrooptic coefficients of 10 pmN and modulation frequencies of 1 GHz were measured.
Fig. 1. Schematic view of an MR with two port waveguides.
Index Terms-Electmoptic modulation, integrated optics, microresonators, optical polymers.
W
ITH THE increasing penetration of optical techniques from long-haul point-to-point connections into metropolitan and even access networks, the role of integrated optics becomes more and more important. This evolution is mainly pushed by the rapidly increasing bandwidth need of the user leading eventually to fiber to the home. Integrated optical devices have to compete with bulk optic solutions currently used in long-haul connections that are optimized for performance. In metro or access optical networks, however, low-cost is the most important issue, as devices are shared by a very restricted number of users. Besides, in these networks the large number of optical nodes demands devices with a high functional complexity. Promising elementary building blocks for the desired integrated optical devices are ultracompact optical mircroring resonators (MR) [I], [2]. These MRs can be used, for instance, for passive wavelength slicing, wavelength (de)-multiplexing, routing, and, as will be shown below, for high-speed modulation. 11. DESIGN AND REALIZATION A basic design of an MR consists of a circular waveguide, which couples to two straight port waveguides (Fig. 1; see, for example, [3]-[5]). By changing the optical path-length of a single round-trip, the resonance condition of the MR can be changed and an amplitude Manuscript received February 23, 2005; revised May 30, 2005. This work was supported by the EC funded IST-project NAIS, Next-Generation Active Integrated-Optic Subsystems (IST-2000-20\8018), and by the MESA+ strategic research orientation TeraHertz. A. Leinse, M. B. J. Diemeer, and A. Driessen are with the Integrated Optical Microsystems Group, University of Twente, Enschede 7500 AE, The Netherlands (e-mail: [email protected]; [email protected]; [email protected]). A. Rousseau is with Laboratoire des Materiaux Macromoleculaires, Villeurbanne 6962 1, France (e-mail: alain.rousseau@insa-lyon fr). Digital Object Identifier 10.1109iLPT.2005.854354
Fig. 2. Output power and phase response of a slightly lossy MR with a single port waveguide at the through port around a resonance wavelength.
and phase change can be generated in both the through and the drop port. By sandwiching the MR between electrodes and applying an electric radio-frequency (RF)-field, high-speed electrooptical modulation becomes possible. Because of their compatibility with metal electrodes and high electrooptic coefficients, polymers are very suitable for this type of devices. The properties of the entire device strongly depend on the amount of light coupled from both waveguides to the MR and vice versa. This coupling is determined by the position of the MR relative to the waveguides, making alignment a critical issue. By reducing the number of port waveguides to a single one, the spectral behavior of the MR is less sensitive to changes in this single coupling constant (as was discussed by [6]). In this case, the amplitude spectrum of a lossless MR is essentially flat because no power can be coupled to a nonexisting drop port. For an electrooptic modulator, the phase change induced by the MR can be utilized instead of the amplitude change, because in an ideal lossless ring the phase at the through port changes from zero to 27~at resonance. Fig. 2 shows schematically the output spectrum and the phase response of a slightly lossy MR around a resonance wavelength (A,) [5]. This phase response can conveniently be converted to an intensity modulation by combination with'a Mach-Zehnder interferometer (MZI) [5], [7], [8]. When switching between the on-
1041-1 135/$20.00 O 2005 IEEE
I
2075
LElNSE er 01.: NOVEL HIGH-SPEED POLYMERlCLO MODULATOR BASED ON A COMRINATION
' I
e.
-561
ii:ct,cr;es
P~.'o\~A-DRI
SU8 ridge
1540
. . . . : . . . . : . 1545
,550f
. . . : - . . . I 1555
modulation wavelength
1560
wavelength (nm)
Rc.4. Measured spcclnltn (TE) of the MZI 1
rlrree~d~~~~cnsional view: ( b l crass Fig. 3, hf%l + ring d c v ~ c c :i;,,sclic~l~alir section through MR and w;t$,r:o~des.
and off-resonance conditions, the d i k r e n c e between the two branches of the MZI can switch from ?i to zero. Such a combination between an MR and MZT, as is shown in Fig. 3, has been realized. The functionality ofpolymelic electrooptic MRs was demonstrated [41 in a device, in which both the waveguide and the MR definition were done by reactive ion etching (RIE). This etching will induce losses due to roughness in both the MR and the waveguides. Fabricating the waveguide by an etch-free lithographic process will reduce this problem. The port waveguides are fabricated by photodefinition with the aid of a negative photoresist (SUX, 7 1 = 1.57 at 1550 nm) in a two-layer lithographic process. The first layer defines the 0.7-i~m-thickslab of the wavezuide, of which the part under the MR is removed in order to prevent coupling from the MR to the slab [3]. The second layer is a thin layer of 300 nm in which 2-pm-wide waveguides are defined lithographically. Between the waveguides and the MR. a I-[cm-thick layer of a methylsilicone based 1-esin (n = 1.4 at 1550 nm; commercially available under the name PS233 Glassclad by UnitedChelnicalTechnologies) is used. This same material is also used for the 4-jim cladding layer over the MR. For the MR (with a height of 0.8 and a radius of 150 j ~ r n ) . a nonlinear polymer PMMA-DRI ( 7 r = 1G at 1550 nm; syothesized by the Ecole Nationale Supelleure de Chimie de Montpellier) is used, which is patterned by standard lithography followed by RIE. MR-losses are reduced by heating the polymer close to its glass transition lemperature, causing a reflow (smoothening) of the sidewalls 191. The MZJ is balanced by applying a heater over one of the branches, as is theoretically discussed in [8]. The spectral behavior of thedevice is measured by coupling-in light with a butt coupled fibei-setup. The spectrum of the transverse-electric (TE) mode shows rnultimodal behavior, as two resonance modes can clearly be seen in Fig. 4. The presence of a second mode should not cause problems for the perforn~anceof the inodulator, because only the steep flank of the zeroth-order
+
ring device. The vertical line indicates the wsvelengtl~of the CW tunahle laser chosen for the modulation measuremeno.
liiode is used for modulation at a fixed wavelength. Applying a voltage to the electrodes above and below the MR results in an electric field that changes the refractive index of the polymer. While applying a modulating voltage, the amount of optical modulation at the output port is measured. With the slope of the spectrum at the modulation wavelength known, the induced wavelength shift of the spectlum can be calculated for a certain modulation lipple. 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 electrooptic coefficient is determined. The r33 value found for this device is approximately 10 p i N , which is in accordance to the values found in literature 1101. Because the benefit of using the MR with the MZI is the easier fabrication, the electrode definition should also not be a critical fabrication step. It is, therefore, preferred to use the electrodes as lumped elements instead of RF designed traveling wave electrodes. A rule of thumb in RF design states that an electrode can he used as a lumped element as long as its length is smaller than 10% of the wavelength of the electrical driving field. For electrode structures with a length of 1 cm in t~ material with a dielectric constant of three, this corresponds to an electrical Requency as high as 2 GHz. If the electrode size can be reduced to the size of the MR. the tnodulation frequency can even be 60 GHz without any special RF design of the electrode. The frequency ofthe applied ~nodulationvoltageis swept by a network component analyzer (NCA) and the modulation depth as a function of the electrical frequency is determined. These measurements were done with and without an electrical aniplifier 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 Fig. 5 . The optical detector had a spectral response which started to decay around I GHz. The measured device l-esponse was corrected for this and the resulting frequency response is shown in Fig. 6. In Fig. 6. four different traces can be distinguished. These four traces are as follows: I ) measured frequency response of the device without electrical amplification and detector correction; 2) measured frequency response of the device with electrical an~plificationand without detector correction;
IEEE PHOTONICS lZCHNOLOGY LETTERS. VOL. 17, NO. 10, OCTOBER 2005
2076
1,1111,1111111,1111>,,,,,,t,,l,,11111111,,,,,~
laser
Fig. 5. Schematic representation of the used measurement setup.
( F : finesse of the MR; R: radius; n,: group index of the mode in the MR; c : speed of light in vacuum). With realistic values of R and 71, (1 50 fim and 1.5) and I -GHz modulation speed (rCav < 1 ns), the finesse can be as high as 1300 before this effect would he dominant. As can be seenfromFig. 4, the finesse in our ring is two orders of magnitude lower (about ten).
-60
111. CONCLUSION
U
-
+
-70
The realized polymeric electrooptic MZI MR demonstrates a large potential for high-speed modulation. Modulation frequencies well above 1 GHz could he possible in the future if the material stack is slightly altered. Changing the claddinglayer might reduce the rolloff above 500 MHz. In addition, as stand-alone phase modulator or two-port device electrooptic MRs could serve as ultracompact modulators and switches in complex integrated optic structures.
.-rn
-,W
-5
-80 -90
m
-100
a
-110 .t:
$
-120 -130 1.Et06
1E+07 Frequency (Hz)
IEtO8
1E+09
Fig. 6 . Measured frequency specuutn ofthr MR MZI devicz.
3) measured frequency response of the device with electrical amplification and with detector correction; 4) noise signal of the detector with amplifier (with the laser power off). Traces 2 and 3 both show a ripple with a periodicity of 50 MHz, which is probably caused by the amplifier which senses reflections from the electrodes. This ripple is not caused by the device because in line 1 the frequency response is flat. Modulation frequencies up to I GHz can be measured. With this performance, data rates exceeding 1 Gbls could be transmitted because with some specific modulation techniques (like for instance quadrature amplitude modulation), data rates of 2-3 Gbls could be possible. The rolloff above 500 MHz is probably caused by the layerstack. The effective current through this layerstack is dependent on l/wC in which w is the electrical angular frequency and (7 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 a lower electrical field.
(41
1.51
161 171 181
191
1101
[ I I]
A. Driessen el 01.. "Microresonaton as building blocks fur VLSl phatonics:' in AIP Conf P m c , vol. 709,2004, pp. 1-18, B. E. Little, S. T. Chu, W. Pan, and Y. Kukubun, "Microl.ing resonator arrays for VLSl photonics," IEEEPhoron Tc.cl~,rol.Len,vol. 12, no. 3. pp. 323-325. Mat 2000. A. Leinse, A. Driessen, and M. B. I. Dienteer. "Electrooptical modulalion i n n polymer ring resonator," in AIP Co,!f P!',nf,vol. 709,2004, pp. 4.-7 .1 4.1. 1. . P. Rahiei, W H. Steier, C. Zhang. and L. R. Dalton, "Polymer microring lilten and modulators:' J. Lighiw Techno/.. vol. 20. no. I I . pp. 1968-1975, Nov. 2002. S. Blair, J. E. Heebner. and R. W. Boyd, "Beyond the absorption limited nonlinear phase shift with inicmring resonators," Opt. Leri.. vol. 27. no. 5, pp. 357-359, Mar. 2002. B. E. Liltle, S. T Chu. and H. A. Haus, "Track changing by use of lbe phase response of microspheres and resonators," Opr. Lor., vol. 23, no. 12, pp. 894-896. 1998. I? P. Absil, J. V Hryl~iewicz,B. E. Little, R. A. Wilson. L. G. Joneclar, and P. T. Ha, "Compact microring notch filters.'' lEEE Plzoron. Techno/. Len., vol. 12, no. 4. pp. 398400, Apr 2000. J. Yang. F. Wnnp. X. Jimg, H. Qu, M. Wane. and Y Wmg. "Influenceof loss on linearity of microring-assisted Mach-Zehnder modulator." Opi. Expmr.~,vol. 12, no. 18, pp. 4178-lI8X. 2004. A. Leinse. M. B. I. Diemeer. and A. Drirssen, "Scattering lossreduction in polymer waveguides by reflowing." EIrcr,n. L~rt..vol. 40. no. 16. pp. 992-993, Aug. 2004. F. Michelotti, A. Beiardini. M. C. Larciprete, M. Benolotti,A. Rousseau. A. Katsimihety, G . Schoer. and J. Muellcr. "Measurement of the electrooptic properties of poled polymers al A = 1.35 prn by means of sandwich slructures with zinc oxide transparentelectrode."Appl. Phvs. Leir.. vol. 83. no. 22, pp. 44774479, 2003. A. Leinse, "Polymeric microring resonator based electro-optic modulatm," Ph.D. thesis, Inregrated Optical Micro Systems Group. Univ of Twente, The Netherlands, 2005.
