A titanium dioxide (TiO2) / electro-optic (EO) polymer hybrid waveguide modulator was designed and fabricated. This modulator possessed a significant advantage for realizing high poling efficiency regardless of the EO polymer resistivity. The in-device EO coefficient was measured to be 100 pm/V, which was 32% higher than that of the thin polymer film. As a result, the phase modulator displayed a VπL figure of merit of 3.5 V∙cm at 1550 nm, which can be reduced further in a push-pull Mach-Zehnder interferometer structure. Temporal stability test of the modulator at 85°C indicated only 8% change of Vπ over 500 hours. The propagation loss in the waveguide was measured as ~3 dB/cm.
© 2016 Optical Society of America
Over the past decade, the requirement for telecommunications has exponentially grown, which also increased the demand for high speed communication technologies. In this scenario, optical networks play a key role as the primary carriers of information and are continuously rising in importance due to their increasing presence in last mile connections and fiber-to-the-home links. Inside optical transmission systems, EO modulators are one of the vital building blocks . Among the different types of materials used to construct modulators, EO polymers offer intrinsic advantages such as a large EO coefficient (r33), high bandwidth, low dielectric constant and loss, and excellent compatibility with other materials and substrates . Therefore, recent progress in the development of EO polymers opens up the possibility that EO modulators can be fabricated with a low driving voltage and a high bandwidth [3, 4].
For a conventional modulator, it is usually constituted with a three-layer structure, i.e. cladding / EO polymer core / cladding. To create EO effect, EO polymer core must be poled with an enough high DC voltage at or near its glass transmission temperature. To drop sufficient DC poling voltage on the EO core, it is required that the resistivity of EO polymer must be higher than that of cladding. However, the latest EO polymer with high r33 can have a resistivity as low as ~106 Ωm due to the use of highly conjugated π-electron chromophores with the maximum loading in the polymer host [5–7]. The low resistive EO polymer results in the difficulty in selecting cladding materials whose resistivities are generally 107 –109 Ωm . In addition, long-term thermal stability of EO polymer with high r33 is important for actual device operations.
In our previous work, we have shown the promising properties of TiO2 applied in the EO polymer modulator . However, that modulator will encounter some limitations when using some high r33 EO polymers which is loaded very high chromophore density. In that case, most of poling electric field drops on the relatively high resistive SiO2 cladding (107 Ωm), which is similar to the conventional modulators mentioned above. In this work, we show a novel TiO2 / EO polymer hybrid waveguide modulator. Even using a low resistive EO polymer, the modulator demonstrates an in-device r33 32% higher than that in thin film. The measured VπL figure of merit of the phase modulator is found to be 3.5 V∙cm at the wavelength of 1550 nm. The experimental results also show an 8% increase of Vπ at 85 °C over 500 hours.
2. Experiments and results
2.1 Waveguide design
Figure 1(a) shows the designed structure of the waveguide modulator. The thickness of EO polymer is 2.5 μm for the top layer and 1.3 μm for the bottom layer. The TiO2 section is a ridge architecture with a total thickness of 0.3 μm, a slab thickness of 0.15 μm, and a rib width of 2 μm. The thickness of the sol-gel SiO2 dSiO2 is 0.3 μm. Figure 1(b) shows the TM0 mode pattern of the waveguide with dSiO2 = 0.3 µm. The refractive indices of the EO polymer, TiO2, and SiO2 are 1.66, 2.30, and 1.44 at a wavelength of 1550 nm, respectively. It was observed that more of the light in the waveguide extends deep into the top EO polymer layer. According to the calculation, ∼47.5% of the light is confined in the top EO layer, and ∼1.5% of the light in the bottom layer. Almost half of the light confined in the top EO polymer is induced by the high-low refractive index contrast between TiO2 and the EO polymer, which is quite similar to the phenomenon in slot waveguides [8, 9].Though the ∼1.5% has a little effect on a phase modulator waveguide, it will significantly affect in some optical devices such as ring-resonator or photonic crystal modulators.
Clearly, the ideal is dSiO2 = 0 μm as shown in Fig. 1(c) to realize: (1) all of poling voltage drops on EO polymer layers regardless of its resistivity; (2) the propagated light is extended in both the top and bottom EO polymer layers. However, in a modulator, the largest on-off ratio is realized in a single-mode waveguide. Unfortunately, the modal calculation indicates that dSiO2 = 0 μm conflicts with the request of single-mode operation of a modulator. Figure 1 (d) shows the TM0 and TM1 mode existing when dSiO2 = 0 μm. As a result, the sol-gel SiO2 layer between bottom EO polymer and TiO2 acts as a cladding for realizing single mode and also a buffer for the fabrication (explained below).
2.2 Fabrication and measurement
Based on the above calculation, a waveguide was fabricated by the following steps. The EO polymer used in this study was synthesized according to our previous methods , and its structure is shown in Fig. 1(b). The loading density of the chromophore in this EO polymer is 40 wt%. The thickness of the EO polymer film was 1.5 μm after spin-coating from a cyclopentanone solution onto the Au/SiO2/Si substrate. Removal of the residual solvent was completed by heating at 120°C for 24 hours under vacuum. A solution of sol-gel SiO2 was prepared by mixing triethoxythysilane and ethylsilicate with a mass ratio of 70/30. 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 0.3 μm-thick film. This thin film is a buffer layer to avoid any damage on the bottom EO polymer during RF sputtering TiO2. The TiO2 was deposited by RF magnetron sputtering for a TiO2 target with Ar/O2 gas (95:5). After preparation of the ridge structure on the TiO2 by photolithography and reactive ion etching , the EO layer was spin-coated again, and the top Au electrode was formed. The top-view of the modulator is shown in Fig. 2.
To realize a modulator, we firstly poled the EO polymer. In this study we used the thin sol-gel SiO2 layer, therefore determining the resistivities of the sol-gel SiO2 and EO polymer is important to achieve a high poling efficiency. The resistivity of the EO polymer was measured to be ~106 Ωm at the various film thicknesses as shown in Fig. 3(a). In our experiment, the resistivity of the sol-gel SiO2 film thicker than 1 µm is in the order of 107 Ωm. It is known that the sol-gel derived SiO2 film dried in air becomes rather porous containing a large amount of internal pores and cracks . We found that the resistivity of the sol-gel SiO2 film decreases with the thickness thinner than 1.0 µm as shown in Fig. 2. The measured resistivity of the 0.3 µm-thick film was 105 Ωm. Finally, the sol-gel SiO2 and TiO2 are both with a resistivity of 105 Ωm, so all the poling voltage can drop on the EO polymer regardless of its resistivity. 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 kept at 141°C for 2 minutes, and then cooled to room temperature rapidly. After poling, the waveguide propagation loss was measured by using the cut-back method. Figure 3(b) shows the measured instertion loss with the different waveguide lengths. The propagation loss of the waveguide is obtained as ~3.0 dB/cm. This propagation loss was almost the same as that of the un-poled sample.
To determine the Vπ, the waveguide was observed as an intensity modulator using a cross-polarization setup. Input light at 1550 nm with a + 45° linear polarization was coupled into the modulator through a polarization maintaining fiber. Output light passing a −45° polarizer was collected by a photo-detector. The output light was measured with an applied triangular voltage waveform at a frequency of 1 kHz as shown in Fig. 4. From a clear modulation output function of the waveguide, a Vπ of 5.0 V was obtained. Considering the electrode length of 0.7 cm, the VπL figure of merit can be calculated as 3.5 V∙cm. Since the modulator in this work was measured with 45° linear polarization light, the VπL can be reduced to ~1.2 V∙cm a push-pull MZI structure .
By using the measured VπL and the waveguide structure in Fig. 1, the in-device r33 was estimated to be 100 pm/V. In the reference experiment, r33 of the same EO polymer in film was measured on ITO glass by Teng-Man method, and the measured value is 90 pm/V at the wavelength of 1310 nm . Considering the modulator working at 1550 nm, the measured in-device r33 was 32% larger than the r33 in the film. This enhancement could be due to the higher poling efficiency induced by the field distribution flattening effect on the TiO2 and SiO2 layers .
The EO polymer used in this study showed the glass transition temperature (Tg) of 140°C. Although such high Tg property is favorable to keep the chromophore alignment stability, a certain reduction of the EO activity is inevitable accompanied in the long-term application. In Fig. 5, long-term temporal stability of the fabricated modulator was recorded by heating the poled waveguide at 85 °C and measuring the Vπ at various times. Results were compared with the reference modulator kept at room temperature. The EO activity showed an initial fast decay during the first 30 hours, whereas the Vπ value could be maintained at 85°C for over 500 hours. Measured Vπ of 5.4 V was only a 8% increase of the original Vπ.
We have demonstrated a TiO2 / EO polymer hybrid modulator. The EO polymer is used as the claddings and directly faced to the electrodes. All the poling electric field dropped on the EO polymer layer, so that a high r33 was achieved. The fabricated modulator showed a low VπL of 3.5 V∙cm, which can be reduced further in a push-pull MZI waveguide modulator. Due to electric resistivity independent of the EO polymer, our simple design but effective modulator should lend itself to a large range of EO polymeric materials.
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 No. 26289108 and 266220712.
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