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In this work, an electro-optic (EO) ring resonator modulator was designed and fabricated in a waveguide consisting of a titanium dioxide (TiO2) core, silicon dioxide (SiO2) buffer layer, EO polymer claddings, and electrodes. By optimizing the thickness of the TiO2 and SiO2 layers, the modulator could satisfy the single-mode requirement; furthermore 52.5% TM mode was confined in the active EO polymer layers. The designed modulator could also pole the EO polymer effectively regardless of its resistivity. Therefore, the EO modulator was observed to show a high resonance wavelength shift of 2.25 × 10−2 nm/V. The intensity modulation at 1550 nm showed a Vp-p = 1.9 V for a 3dB distinction ratio.
© 2014 Optical Society of America
1. Introduction
Inside optical transmission systems, electro-optic (EO) modulators are one of the fundamental building blocks [1]. Among the materials used to fabricate modulators, EO polymers have recently demonstrated powerful advantages over rival materials. Firstly, their EO coefficient (r33) can reach up to 300 pm/V in a thin-film and 100 pm/V in-device, which are much higher values than those of inorganic crystals such as lithium niobate [2, 3]. Secondly, the small microwave and optical velocity mismatch allows an RF device design to be utilized in high bandwidth applications. For instance, EO polymer modulators already been demonstrated with a high bandwidth >100 GHz [4, 5]. Finally, EO polymers can be processed to facilitate integration with other devices such as semiconductor light sources and detectors, low voltage CMOS drivers, and silicon based waveguides [5, 6].
Ring resonator modulators have many desirable characteristics, such as a compact footprint, low drive-voltage, and high wavelength tunability. EO polymer ring resonator modulators have been fabricated by using a traditional three-layer structure waveguide, i.e. EO core and top-bottom claddings. Such an EO polymer waveguide can be prepared by multi-step spin-coating. Finally vertical micro-strip line traveling-wave electrodes can be arranged to control the light signal by RF [7, 8]. Recently, many classes of EO polymers have been exploited in modulators to realize a very low driving voltage and high bandwidth applications [3, 9]. However, it remains a challenge to successfully utilize EO polymers as it is rather difficult to obtain a sufficiently high EO activity in device due to electrical resistivity issues in spite of the materials expected performance [8, 10, 11]. Such a difficulty is attributed to the EO polymer’s low resistivity as a result of highly conjugated π-electron chromophores and by using a high loading concentration.
Recently, we designed and fabricated a TiO2/EO polymer hybrid waveguide as a phase modulator and obtained an in-device r33 of 100 pm/V [12]. Such a high EO activity can be potentially attributed to the EO polymer which has a high r33 property and the enlarged poling efficiency in the waveguide. Furthermore, the TiO2 core and EO polymer have a large refractive index contrast, so that the waveguide device can be miniaturized in size by utilizing large bending curve and circle designs. In this work, we have successfully designed a hybrid ring resonator modulator with a optimized mode confinement in the EO polymer layers and maximized poling efficiency. We have demonstrated a single mode waveguide with a 3 dB/cm propagation loss. This ring resonator modulator exhibited a DC voltage resonance tunability of 2.25 × 10−2 nm/V.
2. Design of the TiO2/EO polymer ring resonator
A schematic diagram of the ring resonator is shown in Fig. 1(a).The TiO2 section is in the form of a ridge with a total thickness of 0.3 μm, where the slab is 0.15 μm and rib width is 2 μm. The top and bottom EO polymer layers have a thickness of 2.5 and 1.5 μm, respectively. For the theoretical optimization to yield a single-mode waveguide, we inserted a thin sol-gel SiO2 buffer layer between the TiO2 and a bottom EO layer with a thickness (dSiO2) of 0.05-0.5 μm. The refractive indices of the EO polymer, TiO2, and sol-gel SiO2 at 1550 nm were 1.66, 2.30, and 1.44, respectively. Figure 1(b) shows the typical TM0 modal pattern across the waveguide obtained by using a BPM calculation. It can be observed that there is a clear mode boundary at the core interface and the optical mode extends deeply into the top and bottom cladding layers. Clearly, the ideal is dSiO2 = 0, in order to obtain the highest confinement of light in the EO polymer layers.
Fig. 1 (a) Designed TiO2/EO polymer ring resonator waveguide, and (b) cross-section of the TM0 mode of the waveguide by using sol-gel SiO2 buffer layer with dSiO2 = 0.2 µm.
However, dSiO2 = 0 is contradictory to the single mode requirement in the hybrid waveguide because there also exists a TM1 mode. Figure 2(a) shows the calculated optical loss of TM0 and TM1 modes at the various thicknesses of the SiO2 buffer layer. In the TM0 mode, the Au electrode absorption loss is small (1 dB/cm). While the TM1 mode indicates a loss of 4.3 dB/cm for dSiO2 = 0, which increases to about 18 dB/cm when dSiO2>0.3 μm. The light confinement of the mode in the EO polymer layers is the important parameter to enable the highest EO activity in the hybrid waveguide. Figure 2(b) shows the change of the confinement factor (Γ) in the EO layers for the various dSiO2. The top EO layer has a constant Γ of about 47.5%. While, Γ attributed from the bottom EO layer significantly decreases by increasing the SiO2 thickness. By taking into account such a tradeoff between Γ and loss, we chose dSiO2 = 0.2 μm to diminish the TM1 mode and obtain Γ = 5% from the bottom EO layer. As the result, the hybrid waveguide allows only one TM0 mode, and the mode contribution to the EO activity is totally Γ = 52.5%. This Γ is almost 6% higher than that in our previous work [12]. Due to the excellent sensitivity of the ring resonator, this higher Γ will result in a large tunability and low Vp-p for the modulator.
Fig. 2 (a) Calculated mode loss in TM1 and TM0 and (b) confinement factor from the top and bottom EO polymer layer at various thicknesses of sol-gel SiO2 layer.
In an EO ring modulator, the shift of the resonant wavelength (Δλi) is given by the following equation
where λi is one resonance peak wavelength, neff the effective refractive index, and Δneff is the effective refractive index change. In the designed hybrid waveguide, Δneff is induced by the refractive index change of the EO polymer (ΔnEO), which can be expressed aswhere nEO is the refractive index of the EO polymer, V the applied voltage, and d is the inter-electrode gap. By using the above two equations, the tunability can be predicted as shown in Fig. 3.In the calculation, we used λi = 1550 nm and r33 = 100 pm/V. From the slope of the calculated Δλi vs. V, the wavelength shift against the applied electric field was found to be 2.29 × 10−2 nm/V.Fig. 3 Calculated shift of the resonant wavelength of the ring resonator at various applied DC voltages.
Poling is a process to induce the Pockels effect in the EO polymer by introducing the noncentrosymetry of the chromophore ordering. This process involves heating of the waveguide to near the polymer glass transition temperature in the presence of a strong electric field. In the TiO2/EO polymer hybrid modulator, the poling voltage loading on EO polymer (VEO) can be expressed as
where Vtotal is the total applied poling voltage, ρEO and ρSiO2 are the electrical resistivity of the EO polymer and the sol-gel SiO2, and dEO is the thickness of EO polymer layer. Since the resistivity of TiO2 is only ~105 Ωm, its effect is negligible. Figure 4 shows the dependence of the poling voltage efficiency (VEO/Vtotal) on the various ρEO by assuming two kinds of waveguides with a 1.0 µm-thick or 0.2 µm-thick SiO2 buffer layer. The measured electrical resistivity of the 1 µm-thick sol-gel SiO2 film was 3.0 × 107 Ωm, and that of the 0.2 µm-thick was 1.8 × 105 Ωm. Here, we used a ρEO of between 105 and 108 Ωm, which have been recently reported in the references for high EO active polymers [12, 13]. In Fig. 4, a hybrid waveguide with a 0.2 µm-thick sol-gel SiO2 film can ensure that the whole voltage is applied on the EO polymer layer. However, the waveguide using a thicker sol-gel SiO2 film only allows VEO/Vtotal = 50% for the EO polymer when ρEO > 107 Ωm. As a result, the designed TiO2/EO polymer waveguide in this work is capable of efficiently poling an EO polymer layer regardless of its conductivity.Fig. 4 Dependence of the poling voltage efficiency on the EO polymer electrical resistivity with 0.3 µm and 1 µm sol-gel SiO2 as cladding layers.
3. Experiments and results
The EO polymer was synthesized according to procedures described in the literature [13], and its structure is shown in Fig. 5(a).According to the designed structure, a TiO2/EO polymer hybrid waveguide was fabricated. Firstly, the bottom EO polymer film was spin-coated onto the Au/SiO2/Si substrate, and its thickness was 1.5 μm after removing the residual solvent by heating at 120°C for 24 hours under vacuum. A solution of sol-gel SiO2 was prepared by mixing ethylsilicate and triethoxythysilane with a mass ratio of 30/70. Acetic acid was used as the catalyst, and a water/ethanol mixture the solvent. The solution was spin-coated onto the EO polymer and baked at 120 °C to form a 0.2 μm-thick film. This thin film is a buffer layer to avoid any damage during RF sputtering. Subsequently, the TiO2 layer obtained by RF-sputtering was patterned by electron-beam lithography, followed by reactive ion etching using CHF3 gas. The radius of the ring was 200 μm with a bus-ring gap of 0.2 μm. Figures 5(b) and 5(c) show a top view of the ring resonator measured using a scanning electron microscope (SEM). Finally, the top EO polymer was spin-coated and the Au electrode was deposited onto the ring area.
Fig. 5 (a) EO polymer structure, (b) SEM image of the TiO2 ring resonator, and (c) zoomed in bus-ring section.
To realize an EO ring resonator modulator, the EO polymer was firstly poled. The poling field was set to 110 V/µm at an initial temperature of 26°C. The poling temperature was increased to 141°C over 5 minutes, and the sample was maintained at 141°C for 2 minutes, and then cooled to room temperature rapidly. After poling, the propagation loss was measured as 3 dB/cm. We found little change in the loss before and after poling. However, the propagation loss is higher than the calculated TM0 mode loss in Fig. 2(a), which is caused by EO polymer material absorption and some part of light in TM1 mode. To realize a low coupling loss for the waveguide, some optimized waveguide structure should be designed in future, such as by using taper input-output sections [1]. The transmission spectra of the ring resonator were measured by an end-fire coupling system. Light from a tunable laser (λ = 1545-1555 nm) was coupled into the waveguide through a polarization-maintaining fiber. A polarizer located between the laser and the fiber was used to ensure the input light as TM polarization. The output light from the waveguide was collected by another fiber and then detected by a photo-detector.
To characterize the modulator property, transmission spectra with a DC bias voltage from −10 V to + 10 V were measured by using a digital function generator. Figure 6(a) shows the measured transmission spectrum of the ring resonator. The resonance peak near the wavelength of 1550 nm indicates a Q value of 1.1 × 104 and an extinction depth of 10 dB. The spectral shift of the resonance under DC bias voltage is shown in Fig. 6(b). When a positive bias voltage was applied to the ring resonator, the resonance shifted to a shorter wavelength by increasing the voltage. While it shifts to the longer wavelength with negative bias voltage. During the spectral shift by applying DC bias voltages, little changes were found in the Q and extinction depth of the EO polymer ring modulator, which is a clear advantage over some silicon ring resonator modulators [14, 15]. From Fig. 6(b), the tunability of the modulator was estimated to be 2.25 × 10−2 nm/V. This measured tunability is identical to the expected property as shown in Fig. 3. Such electric field-induced tunability is the highest level so far for an EO polymer modulator based on the ring resonator previously reported [6, 10, 11]. Consequently, we obtained an in-device r33 of the TiO2/EO polymer hybrid waveguide of ~96 pm/V at 1550 nm.
Fig. 6 (a) Transmission spectrum of the TiO2 / EO polymer ring resonator, (b) the spectral shift in resonance with applied voltages between ± 10V.
The low-frequency AC response of the ring resonator was measured by applying a field at a frequency of 100 kHz. In this measurement the laser wavelength was scanned until the maximum extinction depth was observed and then fixed at that point. Figure 7 shows the clear sin wave form response of the output light intensity with an extinction ratio of 3 dB using VP-P = 1.9 V. The highest modulation bandwidth fM in a ring resonator modulator can be expressed as:
where fRC and fτ are the bandwidth of the capacitive electrode and the optical mode, respectively [16]. If a traveling wave electrode is utilized, the fM is mainly determined by the fτ which can be obtained byHere τ is the photon lifetime and ω0 is the circular frequency of one resonance. By using the measured Q of 1.1 × 104 and resonance wavelength of 1550nm, fM for our device can be estimated to be around 17 GHz for future high-bandwidth applications.4. Conclusions
In this work, we fabricated a TiO2/EO polymer ring resonator modulator. The propagating light field in the core extended into the cladding layers, so that the ring worked effectively as an EO modulator. By using an EO polymer in both the top and bottom claddings, the confinement factor was optimized to yield the highest EO activity. 100% of the electric field could be applied to the EO polymer layers during the poling regardless of the materials resistivity. As a result, the ring modulator had a resonance tunability of 2.25 × 10−2 nm/V at 1550 nm and a Vp-p of 1.9 V at 100 kHz.
Acknowledgments
This work was supported by the Cooperative Research Program of “Network Joint Research Center for Materials and Devices” of the Ministry of Education, Culture, Sports, and Science and Technology, Japan and JSPS KAKENHI Grant Number 26289108. This work was also partially executed under the Communicated Research of National Institute of Information and Communications Technology.
References and links
1. G. T. Reed, G. Mashanovich, F. T. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4(8), 518–526 (2010). [CrossRef]
2. A. Jen, J. Luo, T.-D. Kim, B. Chen, S.-H. Jang, J.-W. Kang, N. M. Tucker, S. Hau, Y. Tian, J.-W. Ka, M. Haller, Y. Liao, B. Robinson, L. Dalton, and W. Herman, “Exceptional electro-optic properties through molecular design and controlled self-assembly,” Proc. SPIE 5935, 593506 (2005). [CrossRef]
3. J. Luo and A. K.-Y. Jen, “Highly efficient organic electrooptic materials and their hybrid systems for advanced photonic devices,” IEEE J. Sel. Top. Quantum Electron. 19(6), 3401012 (2013). [CrossRef]
4. D. Chen, H. R. Fetterman, A. Chen, W. H. Steier, L. R. Dalton, W. Wang, and Y. Shi, “Demonstration of 110 GHz electro-optic polymer modulators,” Appl. Phys. Lett. 70(25), 3335–3337 (1997). [CrossRef]
5. M. Lee, H. E. Katz, C. Erben, D. M. Gill, P. Gopalan, J. D. Heber, and D. J. McGee, “Broadband modulation of light by using an electro-optic polymer,” Science 298(5597), 1401–1403 (2002). [CrossRef] [PubMed]
6. B. A. Block, T. R. Younkin, P. S. Davids, M. R. Reshotko, P. Chang, B. M. Polishak, S. Huang, J. Luo, and A. K. Y. Jen, “Electro-optic polymer cladding ring resonator modulators,” Opt. Express 16(22), 18326–18333 (2008). [CrossRef] [PubMed]
7. B. Bortnik, Y.-C. Hung, H. Tazawa, B.-J. Seo, J. Luo, A. K.-Y. Jen, W. H. Steier, and H. R. Fetterman, “Electrooptic polymer ring resonator modulation up to 165 GHz,” IEEE J. Sel. Top. Quantum Electron. 13(1), 104–110 (2007). [CrossRef]
8. H. Tazawa, Y. Kuo, I. Dunayevskiy, J. Luo, A. K. Y. Jen, H. Fetterman, and W. Steier, “Ring resonator based electrooptic polymer traveling-wave modulator,” J. Lightwave Technol. 24(9), 3514–3519 (2006). [CrossRef]
9. J. G. Grote, J. S. Zetts, R. L. Nelson, and F. K. Hopkins, “Effect of conductivity and dielectric constant on the modulation voltage for optoelectronic devices based on nonlinear optical polymers,” Opt. Eng. 40(11), 2464–2473 (2001). [CrossRef]
10. P. Rabiei, W. H. Steier, C. Zhang, and L. R. Dalton, “Polymer micro-ring filters and modulators,” J. Lightwave Technol. 20(11), 1968–1975 (2002). [CrossRef]
11. B. J. Seo, S. Kim, H. Fetterman, W. Steier, D. Jin, and R. Dinu, “Design of ring resonators using electro-optic polymer waveguides,” J. Phys. Chem. C 112(21), 7953–7958 (2008). [CrossRef]
12. F. Qiu, A. M. Spring, F. Yu, I. Aoki, A. Otomo, and S. Yokoyama, “Thin TiO2 core and electro-optic polymer cladding waveguide modulators,” Appl. Phys. Lett. 102(23), 233504 (2013). [CrossRef]
13. X. Piao, Z. Zhang, Y. Mori, M. Koishi, A. Nakaya, S. Inoue, I. Aoki, A. Otomo, and S. Yokoyama, “Nonlinear optical side-chain polymers postfunctionalized with high-β chromophores exhibiting large electro-optic property,” J. Polym. Sci. Part A 49(1), 47–54 (2011). [CrossRef]
14. T. Baba, S. Akiyama, M. Imai, N. Hirayama, H. Takahashi, Y. Noguchi, T. Horikawa, and T. Usuki, “50-Gb/s ring-resonator-based silicon modulator,” Opt. Express 21(10), 11869–11876 (2013). [CrossRef] [PubMed]
15. D. Marris-Morini, C. Baudot, J.-M. Fédéli, G. Rasigade, N. Vulliet, A. Souhaité, M. Ziebell, P. Rivallin, S. Olivier, P. Crozat, X. Le Roux, D. Bouville, S. Menezo, F. Bœuf, and L. Vivien, “Low loss 40 Gbit/s silicon modulator based on interleaved junctions and fabricated on 300 mm SOI wafers,” Opt. Express 21(19), 22471–22475 (2013). [CrossRef] [PubMed]
16. J. M. Brosi, C. Koos, L. C. Andreani, M. Waldow, J. Leuthold, and W. Freude, “High-speed low-voltage electro-optic modulator with a polymer-infiltrated silicon photonic crystal waveguide,” Opt. Express 16(6), 4177–4191 (2008). [CrossRef] [PubMed]
