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Abstract
Electro-optic (EO) modulators are typically made of inorganic materials such as lithium niobate; the replacement of these modulators with organic EO materials is a promising alternative due to their lower half-wave voltage (Vπ), ease of handling, and relatively low cost. We propose the design and fabrication of a push-pull polymer electro-optic modulator with voltage-length parameters (VπL) of 1.28 V·cm. The device uses a Mach–Zehnder structure and is made of a second-order nonlinear optical host-guest polymer composed of a CLD-1 chromophore and PMMA polymer. The experimental results show that the loss is 1.7 dB, Vπ drops to 1.6 V, and the modulation depth is 0.637 dB at 1550 nm. The results of a preliminary study show that the device is capable of efficiently detecting electrocardiogram (ECG) signals with performance on par with that of commercial ECG devices.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Electro-optic (EO) modulators are among the key components in optical fiber communication systems. With research development, electrocardiogram (ECG) methods and other weak signal monitoring techniques have high requirements for device performance that include low half-wave voltages (Vπ). The LiNbO3 EO modulator is the most widely used commercial EO modulator at present, but the EO coefficient cannot be further improved due to the limitation of the lithium niobate crystals [1]. Compared with traditional EO materials, nonlinear polymer materials formed by doping chromophores into organic polymers have the advantages of a high EO coefficient, a fast response speed, easy integration, strong tunability and low cost. For EO modulators, the proposed EO coefficients of several EO polymers greatly exceed those found in traditional single crystals (e.g., LiNbO3) [2–4]. In recent years, with the development of chromophore interaction theory and material synthesis, many novel second-order nonlinear optical (NLO) chromophores have been proposed; these greatly improve the EO coefficient of EO polymers [5–9]. Highly active chromophores have large dipole moments and high first-order hyperpolarizability (β) [10]. The EO activity of the EO polymer is proportional to the molecular β, and many chromophores exhibiting exceptional β values have been synthesized [11,12]; in poling-induced arrangements, the attenuating effect of the electrostatic interactions between chromophores [13] prevents the attainment of a higher EO coefficient from fairly strong chromophores with the increase in the chromophore load density. Chromophores such as CLD-1 have been proposed; these chromophores can suppress the dipole–dipole interactions among chromophores by modifying the shape of chromophores and minimizing the spatially anisotropic intermolecular electrostatic interactions [14]. Y. Shi et al. prepared an EO polymer doped with chromophore CLD-1, and its EO coefficient reached 85 pm/V [15]. T. P. George added the chromophore AJL8 to APC, and its EO coefficient reached 94 pm/V [16]. The high EO coefficient of the material greatly reduces Vπ and improves the modulation efficiency of the device.
We propose a host-guest nonlinear polymer EO modulator using a polyene bridge second-order nonlinear chromophore CLD-1 as a guest in a polymethyl methacrylate (PMMA) host. These large groups prevent the chromophores from becoming too close to each other, resulting in centrally symmetric aggregates. Even in the host and guest polymer system, a high chromophore concentration and a higher electro-optical coefficient are obtained. The CLD-1/PMMA EO Mach-Zehnder modulator (MZM) has excellent performance with a loss of 1.7 dB at 1550 nm, a Vπ of 1.6 V and a modulation depth of 0.637 dB.
2. Design and characterization of both materials and devices
2.1 Preparation of CLD-1/PMMA EO materials
The material of the device plays a vital role in the improvement of the performance; we chose to use CLD-1 as the chromophore to improve the EO coefficient of the device. The basic structure of the CLD chromophore is obtained by replacing the thiophene unit in FTC with a diene [17]. FTC was the first chromophore that exhibited a high nonlinearity induced by the efficient electron acceptor 2-(3-cyano-4,5,5-trimethyl-5H-furan-2-ylidene)-malononitrile. The three cyano groups in the acceptor are aligned along the charge-transfer direction of the chromophore; thus, they can pull the electron more efficiently to provide a relatively large dipole moment (12-15 Debye). The tert-butyldimethylsilyl (TBDMS) groups were attached to the p-conjugation system to improve chromophore solubility and reduce dipole–dipole interactions between chromophores. The two butyl groups on the thiophene increase the solubility and reduce the interchromophore electrostatic interaction, which can lead to aggregates. The molecular structures of FTC and CLD-1 are shown in Fig. 1. The two bulky t-butyldimethylsilyl groups used in CLD-1 enhance the molecular cross-section so that aggregates are less likely to form. The material selection reduces the interchromophore interaction during the poling process to increase the poling efficiency and reduces the Vπ of the MZM.
Fig. 1. The principle and process of EO properties for NLO polymer materials. (a) Chemical structures of the high-β chromophores FTC. (b) Chemical structures of the phenyltetraene bridged CLD chromophores. (c) Schematic diagram of the poling process. (d) Typical temperature and voltage profiles of the poling process.
CLD-1 particles were dissolved with 1,2-dichloropropane and mixed with a PMMA photoresist using ultrasonic agitation. The optimal parameters for the spin coating of CLD-1/PMMA were as follows: a spinning speed of 1000 rpm and spinning time of 30 s; the formed polymer film showed good surface morphology with a thickness of 1 µm. The chromophore molecules in the doped nonlinear polymer are irregularly arranged, but the electro-optic effect was not observed. Therefore, the electro-optical effect of the material was only revealed by breaking the central symmetry of the material using a polarization process. We used a homemade polarization box to treat the polymer film; this box mainly included a high voltage power supply and vacuum oven. The positive pole of the polarization box was connected to a needle-like electrode, and the negative pole was connected to indium tin oxide (ITO) conductive glass to provide a polarization electric field. When the electrode was polarized in air, the dissolved oxygen reacted with the chromophore in the polymer under high temperature and a high electric field and resulted in loss of the waveguide. Therefore, at high polarization temperatures, a time of approximately 30 minutes at 60°C was required to purge dissolved oxygen from the polymer and prevent degradation caused by the oxygen current. The CLD-1 molecule was oriented using a high-temperature polarization electric field, and the directionally polarized CLD-1 molecule was fixed in the PMMA network to form a stable polarization polymer with a high electro-optical coefficient. The EO coefficient of the polymer film at 1550 nm was 70.5 pm/V at a poling temperature (Tp) of 85°C and the poling voltage (Vp) was 1800V when CLD-1 chromophobe was 30% doped into the PMMA matrix. The molecular structure and polarization process of chromophores are shown in Fig. 1.
2.2 MZM device design and characterization
The structural design of the device was carried out according to the refractive index of the selected material (PDMS refractive index 1.41, CLD-1/PMMA refractive index 1.488). The MZM consisted of a 1 × 2 beam splitter, 2 × 1 beam combiner, S-shaped bent waveguide and phase shifter. Figure 2(a) shows the structure of MZM. A 1 × 2 multimode interference (MMI) coupler and a 2 × 1 MMI coupler were placed at the input and output ends of the MZM as a beam splitter and combiner, respectively. We designed the MMI based on the principle of self-imaging in an optical multimode waveguide so that the input light was evenly divided into two beams with equal intensity and phase. In the 1 × 2 coupler, the size of the flat waveguide was 16 × 150 µm. The input/output waveguide consisted of a tapered waveguide with a length of 20 µm, a width of 3.6 µm and a width of 1.5 µm connected to the flat waveguide end. Figure 2(b) shows the structural parameters of the 1 × 2 MMI coupler. Figure 2(c) and Fig. 2(d) show the optical field diagram of the 1 × 2 MMI coupler and the 2 × 1 MMI coupler (simulated by COMSOL Multiphysics 6.0); these figures show that the input of the 1 × 2 coupler is optically coupled into the output waveguide and the 2 × 1 coupler is symmetrical to the 1 × 2 coupler.
Fig. 2. Overall structure and partial optical field of the EO MZM. (a) Device structure of the EO MZM. (b) Structural parameters of the 1 × 2 MMI coupler. (c) Optical field diagram of the 1 × 2 MMI coupler. (d) Optical field diagram of the 2 × 1 MMI coupler.
The size of the S-shaped curved waveguide was also considered to reduce the loss of the designed MZM. Considering the coupling between the front end and the 1 × 2 MMI coupler, a rectangular waveguide with a width of 1.5 µm and a height of 1 µm was used to design the curved waveguide in the MZM. Therefore, the curvature radius and bending angle were two important factors in the design of the branch waveguides with a fixed width and height. After simulated by Rsoft, when width of S-shaped bent waveguide was 2 µm, the radius of curvature was 1750 µm and the branch angle was 0.08 rad, the loss of the S-shaped curved waveguide composed of two identical arcs was the lowest. The optical field and structural parameters of the S-shaped curved waveguide are shown in Fig. 3(a) and Fig. 3(b).
Fig. 3. Structural parameters and optical field of a part of the EO MZM. (a) Structural parameters of the curved waveguide. (b) Optical field diagram of the curved waveguide. (c) Optical field diagram of the phase shifter. (d) Structure of CPW for the EO MZM modulator.
The phase shifter of the MZM determines the modulation efficiency of photoelectric conversion of the device, and its modulation electrodes are commonly microstrip line (MSL) and coplanar waveguide (CPW) electrodes. MSL electrodes can provide higher electric field utilization but can only achieve single-arm modulation due to process limitations. The electric field generated by the CPW electrode is relatively weak, and the electric field utilization efficiency is not high; the push-pull structure can reduce Vπ by approximately half. A CPW electrode with a push-off structure and length of 0.8 cm was used. We used Ansys HFSS to simulate the impedance of the electrodes. When impedance is determined to be approximately 50 Ω, the electrode parameters of the device are electrode width = 35 µm, electrode spacing = 2.3 µm, and electrode thickness = 200 nm. In addition, the electrode transition region scheme proposed in the literature is used to smoothly transition the electrode size to the pad size to ensure the matching of characteristic impedance and the continuity of transmission, as shown in Fig. 4. The optical field diagram and electrode structure of the phase shifter are shown in Fig. 3(c) and Fig. 3(d). Simulation results showed that the half-wave voltage (Vπ) of the MZM was 1.6 V, and the loss of the MZM was 0.9 dB.
Fig. 4. Schematic diagram of electrode structure. (a) Schematic diagram of electrode cross-section (b) Schematic diagram of electrode pattern.
3. Device fabrication and experiment
3.1 PDMS chemical modification
To limit the light wave to the waveguide core layer with a high EO coefficient, a PDMS material with a refractive index lower than the core layer was used as the upper and lower cladding layers of the MZM. Due to the poor surface wettability and high thermal expansion coefficient of PDMS, the photoresist or metal layer covering the surface of PDMS was prone to cracks and other defects. Therefore, the NLO polymer core layer and metal layer were difficult to apply directly above the PDMS layer. The surface of PDMS was treated with O2 plasma [22] to generate Si-OH bonds on the surface of PDMS. Then, the Si-O-Si bond was formed by a shrinking reaction between 3-glycidoxypropyltrimethoxysilane (GPTMS) and the Si-OH bond on the surface of PDMS, and GPTMS was fixed on the surface of PDMS. The amino hydrophilic group on the other side of GPTMS was used to make PDMS hydrophilic to increase the binding force between the PDMS substrate and the film and improve the surface morphology of the film. Figure 5 shows the comparison of PDMS surface modification steps and droplet contact angle before and after PDMS modification.
3.2 Device fabrication process
The preparation process of the flexible polymer-based EO MZM is shown in Fig. 6. Initially, a layer of PDMS with a thickness of 10 µm was rotated onto an ITO conductive glass substrate, and the ratio of PDMS to the curing agent was 10:1. The cured PDMS was chemically modified, and the NLO polymer core layer was rotated. The NLO polymer was prepared by PMMA doped with 30 wt% CLD-1. The optimal parameters for the spin coating of CLD-1/PMMA were as follows: a spinning speed of 1000 rpm and a spinning time of 30 s; this process yielded a film thickness of 1 µm measured by a probe-type surface profiler (Dektak-XT, Bruker, Germany). For the production of core optical waveguides, we use Au to make the mask, and the core pattern is prepared by ICP dry etching process, and the etching parameters of PMMA layer are RF power 200 W, SF4: O2 = 10: 50 sccm, and the rate is about 140 nm/min. The EO activity of the NLO polymer was activated by the poling process. A layer of Cr metal was rotated above the core layer to connect the positive pole of the HVDC power supply, and ITO film was connected to the negative pole of the power supply. The core layer pattern used an inductive coupled plasma etching process with a power of 400 W. Finally, lift-off technology was used to prepare metal electrodes (200-nm-thick Au) to complete the production of the whole device. The surface appearance of the EO MZM was characterized by scanning electron microscopy (SEM), and images of the main structures of the device are shown in Fig. 7.
Fig. 7. Magnified detailed SEM images of the EO MZM. (a) SEM image of the EO MZM characteristic structure. The structure of the MZM device is symmetric, therefore, one side of the SEM is shown. (b) SEM image of an MMI couple of the input port. (c) SEM image of a waveguide.
3.3 Experimental results
An input light of 10 mW was used in the measurement. To measure the Vπ of the device and its ability to measure weak signals, we initially changed the voltage of the adjustable DC power supply and recorded the output optical power of the EO MZM. We selected 0.2 V as the starting point and 3.4 V as the end point, and the measurement step was 0.2 V. The experimental results showed that the maximum optical power was 6.76 mW when the applied voltage was 3.2 V, and the minimum optical power was 0.39 mW when the applied voltage was approximately 1.6 V. Therefore, the loss of EO MZM was 1.7 dB, and Vπ was approximately 1.6 V. Before measuring its ability to process weak signals, the working point was modulated at the midpoint of the linear workspace. As shown in Fig. 8(a), the bias voltage of the device was approximately 0.7 V. Therefore, a DC bias voltage of 0.7 V was applied before the modulation signal was applied. We selected -5 mV as the starting point and 5 mV as the end point, and the measurement step was 1 mV. The experimental results showed that the applied voltage and output optical power varied linearly within the selected range, and the optical power of the device decreased with increasing external electrical signals. The modulation depth was 0.637 dB (Output optical power maximum−Output optical power minimum)/Input optical power), and the extinction ratio is 16.3 dB. The optical power of the device decreased linearly with the change in the external electrical signal as shown in Fig. 8(b).
Fig. 8. Electro-optic response of the MZM. (a) The Vπ of the device. (b) The relationship between the optical power and the weak electrical signal.
After a lapse of 60 days, the device is tested again for durability. Figure 9(a) shows the loss of the device at room temperature for after preparation and 60 days later. And it can be seen that after 60 days, the device loss has increased by approximately 0.05 dB, which has some durability. The 20 tests 60 days ago had a mean of 1.719 dB, a minimum of 1.665 dB, a maximum of 1.768 dB, and a variance of 0.14565; a maximum of 1.851 dB, a minimum of 1.741 dB, a mean of 1.776 dB and a variance of 0.13603 for 40 tests after 60 days.
Fig. 9. Endurance experiments. (a) Comparison of loss test results before and after 30 days. (b) Half-wave voltage endurance testing of the device.
The chip was heated, and the set temperatures were room temperature, 50°C, 60°C, 70°C, and 80°C. The heating time was 1 h, and the half-wave voltage of the heated device was tested. The thermal stability test results show that there was no significant change in the performance of the device below 60°C, but after heating at 70-80°C, the half-wave voltage of the device had a significant upward trend, and the overall performance deteriorated (Fig. 9(b)).
The tunable laser was used to input 1550 nm wavelength light; the input optical power was 10 mW, AC signal was a sine wave (along with a 0.7 V DC signal), and the output end was connected to the PD, circuit and oscilloscope through fiber coupling. Figure 10(a) is the input sine wave signal, and the covered sine wave is shown in Fig. 10(b). The covered sine wave has less noise, no distortion, and the period is consistent, and the results show that the device can better restore the input sinusoidal signal. The EO bandwidth was measured as shown in Fig. 10(c). EO 3-dB bandwidth of the proposed modulator is approximately 25 GHz.
Fig. 10. Experiments of the MZM. (a) The normal sine signal. (b) The sine signal recovered by the device. (c) The EO response of the device.
The EO modulator was used as the core component to construct the ECG signal recording system. Figure 11(a) shows a schematic of an ECG detection system based on the MZM. The output light intensity of the modulator varies with the modulation signal, which is converted from the photodetector to the voltage signal and output by the oscilloscope after the signal processing circuit. The measured ECG signals of the MZM are shown in Fig. 11(c). Characteristics of normal human ECG monitoring waveforms are clear and significant, such as QRS-, P- and T-waves. Characteristics of premature ECG signals are also sufficiently measured.
Fig. 11. Schematic diagram of the measurement system and experimental procedure. (a) Diagram showing the ECG measurement system. (b) Normal human ECG. (c) Normal human ECG monitoring waveforms. (d) Standard SBR signal. (e) SBR signal recovered by the device.
The ECG simulator is used to provide sinus bradycardia (SBR) ECG signals, and the waveform is shown in Fig. 11(e) after demodulation by MZM. The system can recognize the demodulation of SBR ECG waveforms and can reproduce the corresponding waveform characteristics; that is, there are no obvious P- and T- waves. The lengths of the two cardiac pacing times are different in the monitored SBR ECG signals.
4. Discussion
Compared with traditional electrical sensors, optical sensors have the advantages of fast transmission speed and strong anti-interference ability. Therefore, it is of great importance to develop an ECG monitoring system based on an electro-optic modulator. Since the amplitude of the ECG waveform is approximately 0.5-2.5 mV, the half-wave voltage of the MZM should be low to effectively record the ECG signal. The material used has an important effect on the half-wave voltage of the device. Table 1 shows the performance comparison of EO modulators based on different materials.
Table 1. Comparison with other materials and structural devices
Experimental results show that the proposed EO MZM has a shorter modulation arm length and a smaller device size while having a similar Vπ compared to the previous 1.4 V lithium niobate modulator [1] proposed by researchers. Compared with EO polymer modulators such as CLD-1/APC [2] and DR1/FPI polymer [18], the proposed EO MZM has a lower Vπ and is more conducive to the detection of weak ECG signals. In addition, the proposed MZM has better flexibility and improves the comfort of wearable ECG monitoring equipment because of the use of PMDS. Compared with the DR1/SU-8 MZM [19], both articles achieved ECG measurements, and this paper has a lower half-voltage and a smaller size. Silicon-based materials have the disadvantage of low electro-optical coefficients. Nevertheless, a polymer material can be introduced into the core layer with a slit waveguide in silicon-based materials, thereby improving the electro-optical coefficient. Compared to silicon-organic MZM [20,21], the silicon-organic MZM has a high electro-optical coefficient and low losses. However, its disadvantages are its relatively difficult preparation process, high cost, and inability to capitalize on the flexibility that is common to all-polymer materials. One of the reasons why we chose all-polymer materials is their flexibility which renders them suitable for contact with the human body in the field of monitoring ECG and their potential for preparing photonic skin in the future. However, due to the small refractive index difference of polymer materials, the integration degree of the device is relatively low; future work will focus on the improvement of this area. Polymer materials have the disadvantages of poor temperature stability and small refractive index matching of optical waveguides. But for producing photonic chips, polymer materials are more suitable for some occasions under specific conditions, such as for wearables.
5. Conclusion
The proposed MZM can detect ECG signals on a chip and has a lower Vπ and higher modulation efficiency. By using the CLD-1 chromophore dispersed into polyurethane, an MZM with a 0.8 cm electrode was fabricated; a Vπ of 1.6 V, a modulation depth of 0.637 dB and a loss of 1.7 dB were achieved at 1550 nm. The low Vπ MZM provides a flexible design for photoelectric sensing of weak ECG signals.
Funding
National Natural Science Foundation of China (61177078, 61675154, 61711530652); Tianjin Key Research and Development Program (19YFZCSY00180); Tianjin Science and Technology Program (20YDTPJC01380).
Acknowledgments
Hongqiang Li acknowledges the support from the Tianjin Talent Special Support Program. Joan Daniel Prades acknowledges the support from the Serra Hunter Program, the ICREA Academia.
Disclosures
The authors declare no conflicts of interest.
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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