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Abstract
Efficient electro-optic (EO) modulation can be generated in the hybrid silicon modulator with EO polymer in the form of an in-plane coplanar waveguide and electrode structure. Strong confinement of the optical field in the hybrid structure is critical to performing efficient electric poling and modulation of the EO polymer. The waveguide consists of silica-based side claddings and an EO core for increasing the integral of the optical field and the overlap interaction between the optical field and the modulated electric field within the EO polymer. We discuss in detail the volume resistivity dependence of the efficiency of electric poling and modulation for various side-cladding materials. In a Mach-Zehnder interferometer modulator, the measured half-wave-voltage length product (VπL) is 1.9 V·cm at an optical communication wavelength of 1,550 nm under the TE optical mode operation. The high-speed signaling of the device is demonstrated by generating on-off-keying transmission at signal rates up to 52 Gbit/s with a Q factor of 6.1 at a drive voltage of 2.0 Vpp.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the explosive growth of data traffic driven by bandwidth-consuming applications such as data-center-based cloud services and streaming media, the optical communication network faces unprecedented challenges [1–3]. In order to meet the large bandwidth requirements of interconnect opto-electric technologies, high-speed, small-footprint, and efficient optical modulators are essential for enhancing data transmission efficiency and decreasing power consumption [4–6]. The past decades have seen significant advances in silicon photonics platforms. By leveraging the mature complementary metal-oxide-semiconductor (CMOS) fabrication technologies, the silicon photonic devices could be manufactured at a low cost and large scale [7]. Silicon-based modulators have been developed on the basis of the concept of carrier injection or depletion using silicon pn-junctions and pin-junctions [8]. Other semiconductor modulators based on the Franz-Keldysh effect were also demonstrated to be compatible with the CMOS technology. Another widely adopted concept is heterogeneous or hybrid integration, which could integrate different material platforms and preferentially exploit their individual strengths. Among these materials, which include thin lithium niobate [9], barium titanate [10], and lead zirconium titanate [11], electro-optical (EO) polymers inherently allow the simplest fabrication method involving coating of the waveguide used for the modulator application [12]. In contrast to the birefringence of ferroelectric materials induced by the dipolar distortion in a crystal lattice [13], the Pockels effect of EO polymer is attributed to π-electron delocalization, which supports an ultrafast electric response with a high EO coefficient. Thus, the silicon-polymer hybrid (SPH) scheme provides alternative concepts for optical modulators, promising an impressive performance featuring high-speed modulation and high-yield fabrication while at the same time retaining the benefits from the mature silicon photonics technology.
A variety of SPH modulators have been demonstrated in ultra-high-speed modulation, low-driving voltage, and simple fabrication. The flexible and simple solution-based process enabled EO activity on various types of silicon waveguides using slot, strip, ridge, and photonic crystals [4,6,11,14–17]. A half-wave-voltage length product (VπL) as small as 0.35 V·mm [18] and a modulation frequency of up to 100 GHz [19] have been obtained using a silicon slot and organic hybrid waveguide structure. Nevertheless, very few EO polymer modulators based on the simplest and most common in-plane coplanar waveguide (CPW) structure have exhibited efficient modulation capabilities in terms of small-voltage driving and high-speed signaling. Previously reported CPW devices were comparable to inorganic modulators with VπL values of 7.0 V·cm [20], 7.7 V·cm [21], 16 V·cm [22], but fell short of more recent demonstrations using slot or photonic crystal waveguide devices. Generally, the modulation efficiency of the EO modulator is positively correlated with the Pockels coefficient of EO material and the overlapping degree between the optical and the electric fields. The highly ordered chromophore alignment in the electrically poled EO polymer needs to be processed in the modulator [23]. In simplified EO polymer modulators, electric poling is typically driven by the device modulation electrodes. Therefore, the challenge of efficient EO modulation is to achieve both the highest poling efficiency and modulation efficiency. To this end, an efficient CPW modulator can be obtained by optimizing the overlap factor between the optical mode and the modulation electric field within the EO polymer. Electric poling is the process that activates the EO polymer through the electrodes. We have previously demonstrated the SPH traveling-wave modulator consisting of a Mach-Zehnder interferometer (MZI) silicon waveguide with EO polymer cladding and electrodes [24]. A practical design of a 50-nm-thick silicon core covered with a 1.0-μm-thick EO polymer layer taking the form of a TM optical single mode performs the EO modulation. More recently, the modulation performance is characterized by a VπL of 1.4 V·cm, an in-device EO coefficient of 220 pm/V, a 70 GHz bandwidth, and 100 Gbaud signaling [6]. Nevertheless, this SPH modulator does not have an in-plane CPW scheme. The push-pull operation in the MZI requires electric dual-driving, increasing the complexity to perform the best modulation.
In this study, we demonstrate a mode-confined SPH modulator with an in-plane CPW and electrodes configuration. The phase shifter part of the SPH modulator consists of a 50-nm-thick silicon core, EO polymer, silica-based side claddings, and an electrode. The significance of the fabricated device is the increasing overlap factor between the optical field and the EO polymer whilst retaining a low optical loss. Due to the refractive index difference between the EO polymer and side-cladding material, the optical mode is highly confined by the EO polymer while the CPW electrode gap is reduced to as small as 4.0 μm. The performance of the SPH modulator was optimized for the best overlap factor and measured by a simple push-pull electric drive using in-plane ground-signal-ground (GSG) electrodes. The measured VπL, as small as 1.9 V·cm at a wavelength of 1,550 nm, is the result of efficient electric poling and modulation, which are significantly improved compared to previous CPW EO polymer modulators. We also demonstrate the feasibility of high-speed on-off keying (OOK) transmission at data rates of up to 52 Gbit/s.
2. Modulator design and measurement
2.1 Waveguide design
Figure 1(a) is a schematic of the cross-section of the phase shifter part of the designed SPH modulator. The 50-nm-thick and 1.0-μm-wide silicon is chosen as the waveguide core. The coplanar electrodes are set at a distance of 1.5 μm to the core, leading to an electrode gap of 4.0 μm. An EO polymer (whose molecular formula is shown in Fig. 1(b)) with a thickness of 1.2 μm was used as the top cladding. Because of the limited confinement effect of such an ultrathin silicon core, the optical electric field in silicon penetrates deep into the EO polymer. Therefore, a large optical modal overlap could be formed in the EO polymer, which is critical for increasing the EO activity. In such a hybrid structure, the optical modal confinement factor can be defined as the ratio of the partial power distributed in the EO polymer to the total power in the entire waveguide. Generally, in the design of CPW modulators, in-plane electrodes along the waveguide should be placed at a sufficiently large distance from the core in order to avoid the metal absorption loss. Thus, there is an apparent trade-off between the electric-driven modulation efficiency and optical loss. Considering the excess expansion of the optical field and its overlap with the electrodes, it is of great significance to form a strong modal confinement around the core waveguide. To this end, silica-based side claddings are introduced between the silicon core and electrode, as shown in Fig. 1(a). Since the refractive index of the silica-based side cladding is smaller than those of the EO polymer and silicon, the optical mode is discontinuous across the waveguide, while the modal intensity is mainly concentrated around the core of silicon and EO polymer. Figure 1(c) and (d) are the optical modal distributions calculated for the hybrid waveguides without and with side claddings, respectively. Simulation results are obtained using the 3D beam propagation method (BPM) with a commercial software package (BeamPROPTM, RSoft, Synopsys Inc.) and refractive indices of 3.48, 1.68, and 1.45 for silicon, EO polymer, and sol-gel trimethoxy-silane (TMOS) derivatives, respectively. As expected for the hybrid waveguide with side claddings, the optical field is sufficiently isolated from the electrodes and its intensity is highly concentrated around the EO polymer. However, the waveguide without side claddings inevitably shows an overlap of the optical field with the electrodes. Further details of the side cladding can be discussed in terms of the optimum confinement factor and propagation loss. Figure 1(e) plots the variation of the confinement factor and the propagation loss in the phase shifter part of the SPH modulator, observed when the width of the side cladding is varied from 0 to 1.5 μm. The absorption loss is deduced from the imaginary part of the effective refractive index. The confinement factor gradually decreases from 56.1% to 53.5% when the width of the side cladding changes from 0 to 1.2 μm. The propagation loss is more sensitive to the change in the side claddings. We observe a significant reduction in propagation loss when side claddings with a width larger than 0.4 μm are introduced. To minimize the waveguide loss, we chose a 1.2-μm-wide side-cladding for device fabrication, which corresponds to a calculated confinement factor of 53.5% with a calculated propagation loss of 0.022 dB/cm. In the designed CPW and electrode, highly overlap integral between the optical mode and electric RF mode can be expected to perform efficient modulation. The electric-field profile of the RF mode is calculated using an eigenmode solver (CST Microwave Studio) and shown in Fig. 1(f). The applied electric signal provides a uniform RF mode and completely overlaps with the optical mode around the core of silicon and EO polymer.
Fig. 1. EO polymer modulator. (a) Schematic and cross-section of designed SPH modulator (not to scale). (b) Molecular structure of EO polymer. (c) and (d) Fundamental TE optical modes in phase shifters without and with silica side claddings, respectively. (e) Electric-field profile of the RF mode in phase shifters with silica side claddings. (f) Calculated modal confinement factor and propagation loss of SPH waveguides with silica side claddings of varying width
2.2 Poling efficiency optimization
The chromophore molecules in the initially deposited EO polymer exhibit a random alignment, making the macroscopic Pockels coefficient zero. Thus, the fabrication of the EO polymer modulator is completed by the electric poling process. Generally, applying a relatively high electric field to the EO polymer at a temperature around its glass transition temperature (Tg) is preferred to induce high molecular ordering and obtain large EO activity in the modulator. The designed SPH modulator can be modeled as a multilayer dielectric structure so that the applied DC voltage across the electrodes could be divided into two parts, corresponding to the distributed voltages in the EO polymer and side claddings. To increase the poling efficiency, a sufficiently high DC electrical field has to be applied to the EO polymer layer. Therefore, material selection for the side cladding, particularly in terms of its electric resistivity, is key in experimental design. Side cladding with excessively high volume resistivity would result in poling difficulty and thus deteriorate the modulation of the device.
In this study, we discuss spin-on silica-based materials for the fabrication of side claddings. These can be prepared from hydrogen silsesquioxane (HSQ) or TMOS derivatives. First, the electrical volume resistivity of the EO polymer was measured at different temperatures for a comparison with silica-based materials. The resistivity was measured for a roughly 1.0-μm-thick single layer of each material. The films between planar electrodes were subjected to a DC electric field of 50 V/μm and heated from room temperature to 175°C. As shown in Fig. 2, the resistivity of the EO polymer decreased as the temperature increased up to 155°C and varied rapidly at higher temperatures. The observed transition temperature was identical to the glass transition temperature of the EO polymer (Tg = 166°C). The spin-on silica material prepared from HSQ exhibited extremely high resistivities over the entire temperature range. The measured values were two orders of magnitude larger, and almost identical to the values measured for a SiO2 film prepared by CVD deposition. Thus, HSQ made it difficult to drop sufficient poling voltage across the EO layer of the SPH modulator. The TMOS derivatives are an organic-inorganic hybrid, and are suitable for the side claddings according to the measured volume resistivities shown in Fig. 2. Over the entire temperature range, the volume resistivities of the TMOS are lower than the values for the EO polymer. More importantly, the resistivity of the TMOS drops below that of the EO polymer at poling temperature above 160°C, by about two orders of magnitude. A high poling efficiency would be expected for the EO polymer in an SPH modulator. The relatively small resistivity of organic-inorganic TMOS can be explained by the proton conduction facilitated by free water [25]. Considering that proton conduction is a thermal excitation process, the activation energy of proton conduction will decrease with increasing temperature, leading to an increase in conductivity.
Fig. 2. Temperature dependence of volume resistivities for various silica-based materials and EO polymer. The yellow shadow corresponds to the optimal poling temperature range for the EO polymer with a Tg of 166°C.
Based on the MZI waveguide configuration, the coplanar GSG electric transmission line is designed for both modulation and poling. For the modulation, an electrical signal is connected to the central electrode while the two side electrodes are grounded. For electric poling, positive and negative voltages are applied to the two side electrodes, while the central is grounded. The computed electric potential and electric field across the waveguide are discussed using a finite element method based MATLAB toolbox. In the calculations, we used measured electrical volume resistivities of 109Ω·m and 107 Ω·m for, respectively, the EO polymer and the TMOS side claddings, and applied a DC poling voltage of 400 V onto the electrodes. For comparison, we also made calculations using the HSQ side cladding with a resistivity of 1011 Ω·m. The disturbance caused by the upper air and bottom silicon (p-doped, 1×1015 cm-3) substrate was also considered. The electric potential and electric field calculations are performed to clarify the influence of the claddings’ resistivity during the poling process. As demonstrated in Fig. 3(a) and (c), an uneven potential distribution occurred owing to the space-charge effect of the silicon substrate and the discontinuous dielectric constant distribution [26]. From Fig. 3(b), the applied electric field in the EO polymer sandwiched by the TMOS side claddings can be estimated to be about 133.9 V/μm. The calculated electric filed is the weighted average number relevant to the optical modal distribution as discussed in Fig. 1(d). The optical mode has the highest field intensity at the center above the Si core, where the electric field has its intensity with uniform coverage. When HSQ forms the side claddings, the electric field in the EO polymer is reduced to only 1.9 V/μm, which is almost 70 times smaller than the value obtained for the TMOS side claddings. This result is in an agreement with the multilayer distribution model mentioned above, and demonstrates that proper material selection for the side claddings can improve the poling efficiency.
2.3 Device fabrication and measurement
The fabrication of the SPH modulator started with a silicon-on-insulator (SOI) wafer having a 220-nm top silicon layer and 2.0-μm buried oxide layer. The silicon layer was thinned down to 50 nm using thermal oxidation followed by wet etching. The silicon waveguides were patterned into an MZI utilizing an electron-beam lithography system (ELS-G100, Elionix) and an inductively coupled plasma etching system (RIE-400iPB, SAMCO). The waveguide widths in the phase shifter part and other passive components are 1.0 μm and 4.0 μm, respectively. The coplanar GSG electrodes, made of 400-nm-thick gold, were fabricated via a standard lift-off process using a polymethyl methacrylate (PMMA) resist and a vacuum-deposited gold layer. The sol-gel solution of TMOS derivatives was prepared as described in [27] and used to prepare side claddings via a second lift-off process using a PMMA mask as a forming mold. The EO polymer used in this study exhibits a high EO coefficient and thermal stability because of its high-temperature glass transition properties (Tg = 166°C) [28]. The 15 wt% EO polymer in cyclopentane was spin-coated onto the MZI waveguide to form a 1.2-μm-thick layer after drying at 95°C for 24 h. Finally, the EO polymer above the contact electrode pads was removed by oxygen plasma etching using a metal mask, and then the electric probes could easily contact with the electrodes. To activate the SPH for the EO modulation, an electrostatic field of 100 V/μm was applied through the ground and signal electrodes. Molecular ordering of the EO polymer was induced by increasing the temperature to around 160°C, and then locked by cooling to room temperature.
Fig. 3. The simulated electrical potential and electric field distribution during the poling process of the SPH modulator using side claddings with TMOS (a, b) and HSQ (c, d).
Figure 4 shows the false-colored tilted cross-sectional scanning electron microscope (SEM, JSM-7000F, JEOL) image of the fabricated phase shifter. Length measurement analysis indicated that the fabricated waveguide was well matched to the design. After spin-coating the EO polymer, the insertion loss of the SPH at a wavelength of 1,550 nm was measured to be about 7 dB, which consists of 4 dB loss of the phase shifter part and 3 dB loss of the other passive waveguide components including transport waveguides, spot-size size converter, and two Y-junctions. From the length of the phase shifter (8.0 mm), the propagation loss (α) is calculated to be 0.5 dB/mm. The total fiber-to-fiber insertion loss added another coupling loss of about 6 dB. Generally, the optical loss of the silicon waveguide can be described by the attenuation coefficient and the roughness of the sidewall derived by Payne and Lacey [29]. Considering the 50-nm-thick silicon waveguide in the SPH, the scattering loss caused by the sidewall roughness can be regarded as negligible. Here, the optical propagation loss of the SPH waveguide is mainly dominated by the material absorption loss of the EO polymer (∼0.4 dB/mm).
Fig. 4. False-colored SEM image of the phase shifter in the SPH modulator. The phase shifter arm of MZI comprises a 50-nm-thick silicon core (blue) and coplanar ground-signal-ground (GSG) electrodes made of gold (yellow). TMOS claddings (green) with a height of 0.8 μm and a width of 1.2 μm was deposited between the silicon waveguide and gold electrodes.
To determine the modulation efficiency, we measured the Vπ of the SPH modulator driven by a 1.0 kHz triangular electrical signal. As depicted in Fig. 5, the modulated signal (green curve) was detected using a photodetector and recorded together with the electrical signal (blue curve) of an oscilloscope. The measured Vπ was 2.4 V at a wavelength of 1,550 nm, corresponding to a VπL of 1.9 V·cm. By using the measured propagation loss of 0.5 dB/mm for the phase shifter, we obtained the loss efficiency product (α·VπL) of 9.5 V·dB. The measured VπL of the SPH is comparable or slightly larger than the value measured in our previous device (1.4 V·cm) in Ref. [6]. Considering the in-plane CPW scheme of the SPH modulator, the single-drive push-pull operation of this device could highlight the modulation efficiency and high-speed signaling with the small driving voltage. We also tested CPW modulator having the same waveguide and electrode structure but without side claddings. After electric poling, we measured a Vπ of 4.1 V and an insertion loss of 17 dB. These modulation and optical loss properties are clearly worse. As expected from the optical mode and electric field simulations, the side claddings prepared in the SPH modulator are key to significantly increasing the efficiency of the electric poling and modulation.
Fig. 5. Measurement of the Vπ-voltage of the fabricated SPH modulator showing the intensity modulated signal (green) driven by a 1 kHz triangular electrical signal (bule).
High-speed signal generation of the fabricated SPH modulator was evaluated by the experimental setup, as illustrated in Fig. 6(a). An electrical signal with a pseudo-random binary sequences (PRBS) length of 211-1 was generated by an arbitrary waveform generator (AWG, M8196A, Keysight) and then linearly amplified by an RF amplifier (S804B, SHF). Subsequently, the driving signal was fed into the GSG coplanar transmission line electrodes via a microprobe (ACP-65-GSG-150, FormFactor). The other end of the electrode was terminated by a 50 Ω impedance load using another microprobe to avoid signal reflections. The modulated optical signal after the SPH was detected by a 70 GHz photodiode (XPDV3120R, Finisar). An erbium-doped fiber amplifier (EDFA) was used in front of the photodetector to improve the power budget of the received signal. An optical band-pass filter (BPF) was used to eliminate the EDFA noise. Finally, the received signal was analyzed by a digital communications analyzer (DCA-X 86100D, Keysight) or a real-time oscilloscope (DPO77002SX, Tektronix). Figure 6(b) and (c) show the measured OOK eye patterns at bit rates of 36 and 52 Gbit/s, respectively. With a peak-to-peak driving voltage of 2.0 Vpp, clear eye openings could be achieved. The measured signal can be quantified by the Q factor, which is a figure of merit that quantifies the modulation required in the high-quality transmission. The measured Q factors are 8.2 and 6.1 at 36 and 52 Gbit/s, respectively, demonstrating the feasibility of the modulator at high-speed data rates. Bit error rate (BER) analysis is a more direct signal quality metric used for high-speed data transmission. BER analysis was carried out with offline post-processing that included timing recovery, deed-forward equalization, and error counting. The measured BER at 36 Gbit/s OOK is 5.5×10−5, which is well below the 7% overhead hard-decision forward error correction threshold (HD-FEC, 3.8×10−3). The BER increases to 1.2×10−3 as the bit rate is enhanced to 52 Gbit/s. The signal quality change observed in the Q factor and the BER at higher data rate is attributed to the fundamental limitation of the fabricated traveling-wave modulator, which is particularly attributed to the imperfection of the electrode. The EO polymer modulator is very suitable to reduce the velocity mismatch between RF and light waves. Note that in the current SPH device, we can address the technical issue in the fabrication to perform a fully designed electrode structure as demonstrated in our previous device [6]. Further improvement of the lithography and metal deposition techniques for the SPH would realize the modulation with larger bandwidth and higher speed operation.
Fig. 6. High-speed signal generation of SPH modulator. (a) Experimental setup for signal generation and detection. AWG arbitrary waveform generator, AMP amplifier, PC polarization controller, EDFA erbium-doped fiber amplifier, VOA variable optical attenuator, BPF optical band-pass filter. (b) and (c) Detected eye diagrams for data rates 36 and 52 Gbit/s, respectively
3. Conclusion
In summary, we designed and experimentally demonstrated an efficient SPH modulator with simple in-plane CPW structure. The EO polymer waveguide consists of an ultra-thin silicon core and side claddings, which could tightly confine the optical field in the waveguide and increase the overlap factor of the optical mode and modulation electric field within the EO polymer. After the selection of a suitable side cladding material, the modulation electric field fell almost entirely on the EO polymer to achieve a high modulation efficiency. The device featured a relatively small VπL (1.9 V·cm) compared to previous EO polymer MZI modulators. Furthermore, the optical propagation loss in the phase shifter part of the device was only 0.5 dB/mm. As the first-generation proof-of-concept CPW SPH modulator, signal generation of OOK transmission was successfully demonstrated at data rates up to 52 Gbit/s, which leaves ample room for increasing the transmission bandwidth and reducing the driving voltage.
Funding
Japan Society for the Promotion of Science (JP19H00770); Core Research for Evolutional Science and Technology (JPMJCR1674); Strategic International Collaborative Research Program (JPMJSC1807); National Institute of Information and Communications Technology (0210102).
Acknowledgments
We wish to acknowledge the Cooperative Research Program of “Network Joint Research Center for Materials and Devices” and “Dynamic Alliance for Open Innovation Bridging Human, Environment, and Materials” of the Ministry of Education, Culture, Sports, Science and Technology of Japan.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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