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Abstract
In this work, thermo-optic (TO) waveguide switches are designed and fabricated based on the bottom-metal-printed technique. Low-loss fluorinated polycarbonate (AF-Z-PC MA) and polymethyl methacrylate (PMMA) are used as core and cladding materials, respectively. The thermal stability and optical absorption characteristics of AF-Z-PC MA are analyzed. The optical and thermal field distributions of the TO switch are simulated. The insertion loss and extinction ratio of the device are found to be 4.5 dB and 19.8 dB, respectively, at a wavelength of 1550 nm. The on-off time of the switching chip is 80 µs. The electrical power consumption is approximately 8.8 mW. The proposed low-loss fluorinated polymer TO waveguide switch realized by bottom-metal-printed fabrication technology is suitable for large-scale integrated photonic circuit systems.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the constant development of relevant techniques in the field of integrated photonic communication systems, the research and evolution of microelectronics technologies and semiconductor materials have reflected a crucial strategic position in such systems [1–3]. As a method of information transmission, optical communication technology has outstanding advantages such as higher information load ability, faster switching response speed, and more substantial anti-electromagnetic interference capability compared with traditional electrical communication technology [4–7]. A thermo-optic (TO) switch in a waveguide structure is a type of crucial device in integrated photonic communication technology that can be integrated into optical cross connections (OXCs) [8], optical add-drop multiplexers (OADMs), and wavelength-division multiplexing (WDM) technology to realize wavelength selectors, single-wavelength or multi-wavelength free control transmission, and network exchange functionality [9,10]. Based on the advantages of the application of TO switch devices, waveguide devices with superior performance, high integration, and strong applicability have become a research hotspot [11–15].
Currently, polymers are a common fabrication material for waveguide devices because of their adjustable refractive index and higher TO coefficient compared with inorganic materials [16–18]. Meanwhile, the processing method for polymer materials is more convenient, making it conducive for large-scale integrated photonic circuit systems. Specially, to reduce optical propagation loss of polymer waveguide effectively, fluorinated polymer waveguide materials with low optical absorption loss at near-infrared communication wavelengths (1310 and 1550 nm) have aroused great interest [19–23]. Compared to other fluorinated polymers, fluorinated polycarbonates with smooth film surfaces and high thermal stability are found to be more suitable for achieved functional integrated optical waveguide devices such as wavelength multiplexers and optical power splitters [24–25]. Therefore, a low-loss fluorinated polycarbonate polymer (AF-Z-PC MA) is synthesized as the core layer of the waveguide in this work. In this kind of fluorinated polycarbonate polymer, the C-H bond molecule is replaced by the C-F bond molecule, which causes the vibration absorption wavelength of the material to move in the long-wavelength direction. Therefore, it reduces the absorption attenuation of the material at near-infrared wavelengths to overcome the high absorption loss in the near-infrared band of conventional polymer photoresists [26]. Meanwhile, compared with traditional polymer materials, the refractive index of AF-Z-PC MA is easy to adjust and the thermal stability is better, which is beneficial for the optimization design and preparation process of waveguide devices.
Compared with traditional waveguide fabrication methods [27–29], waveguide electrodes fabricated by the bottom-metal-printed technique are directly connected to the waveguide core layer, which can decrease heat loss in the air and increase the proportion of the effective heat consumption added to the active region of the waveguide. In addition, the bottom-metal-printed technique is beneficial for improving the heat transfer of the device, which accelerates the response time. Compared with traditional wet etching and reactive ion etching (RIE) [30,31], a waveguide prepared by the bottom-metal-printed technique does not need to process the waveguide core material, so it can greatly reduce the extra loss caused by the preparation technology steps and optimize the fabrication method for a waveguide chip. The electrode is located below the waveguide core layer and isolated from the outside air. Therefore, it avoids the electrode being oxidized in the air, prolongs the operation life of the device, and improves the working performance of the device. Consequently, we use the bottom-metal-printed technique to fabricate a waveguide TO switch that utilizes a metal electrode as cladding to define waveguide structures directly.
In this work, we present a novel bottom-metal-printed TO waveguide switch prepared with AF-Z-PC MA core layer material. The thermal properties and optical performance of the AF-Z-PC MA material are analyzed. In this process, the structural optimization parameters of the bottom-metal-printed TO waveguide switch are provided. The extinction ratio, electrical power consumption, and switching on-off time of the waveguide chip are measured. The experimental results indicate that the low-loss fluorinated polymer TO switch fabricated by the bottom-metal-printed technique can play a significant role in a large-scale integrated photonic circuit system.
2. Waveguide design and experiment
2.1 Synthesis and properties of the waveguide material
Polycarbonate materials have the advantages of a high modulus of elasticity, dimensional stability with respect to heating, small temperature influence, high temperature of thermal decomposition, resistance to most inorganic and organic acids, and fine substrate adhesion [32]. The proposed fluorinated polycarbonate material (AF-Z-PC MA) was synthesized, and the details of the synthesis process and molecular structural formula are shown in Fig. 1. 4,4’-(Hexafluoroisopropylidene) diphenol (6FBPA), 4,4’-cyclohexylidenebis phenol (BPZ), and bis(trichloromethyl) carbonate (BTC) were used as raw materials. The specific synthesis process step for AF-Z-PC MA was as shown in the following figure: first, raw materials (6FBPA and BPZ) were dissolved in CH2Cl2; then, pyridine and BTC were added to the mixture, which was stirred at zero degrees Celsius for 30 min. The chemical reaction proceeded at normal atmospheric temperature for 6 h. With this method, AF-Z-PC OH was precipitated in methyl alcohol. Then, it was dissolved in tetrahydrofuran. Later, pyridine and acryloyl chloride (C3H3ClO) were added to the mixture at zero degrees Celsius; the mixture was allowed to react for 1 h and then stirred at normal atmospheric temperature for 48 h. Eventually, AF-Z-PC MA was precipitated in methyl alcohol. AF-Z-PC MA was prepared by condensation polymerization, then reacted with C3H3ClO to acquire AF-Z-PC MA terminated by double bonds.
The thermal stability and optical properties of the fluorinated polycarbonate material were then analyzed. The glass transition temperature (Tg) of AF-Z-PC MA was analyzed by differential scanning calorimetry (DSC). The thermal decomposition temperature (Td) was measured by a PerkinElmer thermogravimetric analysis (TGA)-7 analyzer. As shown in Figs. 2(a) and 2(b), the Tg of fluorinated polycarbonate material was 150 °C, and the Td was up to 225 °C. The measurement results mentioned above show that this kind of fluorinated polycarbonate material has excellent temperature stability, which is beneficial for prolonging the service life of the waveguide chip. Furthermore, the low-loss communication window is also a significant characteristic for an optical waveguide chip. The uniform fluorinated polycarbonate solution was formed by 20 wt% AF-Z-PC MA dissolved in cyclopentanone. The polymer solution was filtered and spin-coated onto a quartz tablet at 1000 r/min for 20 s. The solid thin film sample was obtained in 60 °C vacuum oven for 12 h. The UV-vis-NIR absorbance spectrum of the sample was measured by spectrophotometer (SHIMADZU UV-3600), as shown in Fig. 2(c). Because the C-H bond molecule is replaced by the C-F bond molecule, the vibration absorption wavelength of the material moved in the long-wavelength direction. The absorbance of the material at the 1550 nm telecommunication wavelength is low, which can markedly decrease the transmission loss of a waveguide chip fabricated using this material.
Fig. 2. Thermal stability and optical transmission loss of AF-Z-PC-MA. (a) DSC test curves, (b) TGA test curves, (c) the UV-vis-NIR absorbance spectrum.
Then, the fluorinated polycarbonate thin film with a thickness of 3 µm was spin-coated on a silicon wafer at 1000 r/min using a spin-coater (KW-4A). The refractive index of the thin film was measured by variable angle incidence spectroscopic ellipsometer (M-2000UI) in the near-infrared band, as shown in Fig. 3(a). The refractive index of the material at 1550 nm was 1.536; such a polymer with a high refractive index is suitable to be used as a core layer material for an optical waveguide. The surface roughness was obtained using an atomic force microscope (AFM) (ICON-PT), as shown in Fig. 3(b). The surface morphology of the thin film was quite homogeneous, and the thin film surface roughness was less than 0.875 nm. The polymer film is rather smooth due to aliphatic six-membered ring structure of AF-Z-PC MA. The spin-coated polymeric film was cured at 90 °C for 30 min to remove the solvent. The moderate curing temperature and time is also beneficial to reduce the surface roughness of polymer thin film. By analyzing the thermal and optical properties of the material, it is clear that the AF-Z-PC MA material has high thermal stability, optical transmission characteristics, and film-forming properties; therefore, it is suitable for use as a core layer material for a low-loss polymer optical waveguide device.
Fig. 3. AF-Z-PC MA thin film physical properties. (a) Refractive index of AF-Z-PC MA thin film measured by ellipsometer and (b) surface morphology of AF-Z-PC MA thin film scanned by AFM.
2.2 Waveguide structure and simulation
Three-dimensional and two-dimensional cross-sectional structural diagrams of the designed bottom-metal-printed waveguide are shown in Figs. 4(a) and 4(b), respectively. A 500-µm-thick Si layer was the substrate material for the waveguide TO switch chip, and a 4-µm-thick PMMA layer was spin-coated on it as the lower cladding layer. Then, a 50-nm-thick Al metal self-heating electrode was patterned on the PMMA layer by vacuum evaporation, UV photolithography, and the wet etching technique. Finally, the 3-µm-thick fluorinated polymer AF-Z-PC MA was deposited by the spin-coating method. The upper cladding layer of the waveguide TO switch was air.
Fig. 4. Structural diagrams of the optical waveguide TO switch. (a) 3D schematic diagram of the waveguide chip; (b) 2D cross-sectional structure of the waveguide chip.
We calculated the effective index of the waveguide cross-sectional regions N1 and N2 between the waveguide core layer b according to the Marcatili approximation method using MATLAB software [33]. The refractive indices of the PMMA layer, fluorinated polymer, Al, and air were found to be 1.490, 1.536, 1.347 + 14.133i, and 1.000, respectively, at 1550 nm. The equations for calculating the effective refractive index are shown below:
Fig. 5. Relationships (a) between N1 and core layer thickness b; (b) between N2 and core layer thickness b; (c) mode birefringence between ΔN1 and b for TE0 and TM0 modes.
The mode profile distributions of waveguide core and cladding layers simulated by the COMSOL Multiphysics software are given as the Figs. 6(a)–6(d). It can be observed that there are only TM0 and TE0 fundamental optical modes in the waveguide core region and there are TM−1 surface plasmon polariton (SPP) mode, TE0 mode, and TM1 optical mode in the waveguide cladding regions, respectively. When the core layer thickness of the waveguide is 3 µm, the effective refractive indices of TE0 and TM0 optical modes in the N1 core layer are 1.5231 and 1.5221, respectively. The effective refractive indices of TM−1 SPP mode, TE0 mode, and TM1 optical mode in the N2 cladding layer are 1.5518 + 0.0032i, 1.5174, and 1.5125, respectively. Although lossy hybrid modes exist in the waveguide structure, the imaginary part of the SPP mode is far less than the real part and has little influence on the propagation constant. Moreover, the effective refractive index of TM−1 SPP mode in the N2 cladding layer is larger than that of fundamental modes in the N1 core layer, so there is no mode coupling between them. It can be found that the bottom-metal-printed technique can realize low-loss optical propagation modes in the specific waveguide structure.
Fig. 6. The mode profile distributions of waveguide core and cladding layers simulated by the COMSOL Multiphysics software. (a) TM0 and TE0 fundamental optical modes in the waveguide core region, (b)-(d) TM−1 surface plasmon polariton (SPP) mode, TE0 mode, and TM1 optical mode in the waveguide cladding regions.
Using the above simulation results, optical field propagation of the waveguide was calculated using Rsoft BeamPROP. It can be concluded from Fig. 7(a) that the dimensions can achieve single-mode transmission at 1550 nm. The thermal field distribution of the bottom self-heating Al electrode is illustrated in Fig. 7(b) according to the thermal conductivity profiles in the waveguide cross-section. According to Mach-Zehnder interferometer (MZI) TO switch waveguide theory, the length and distance of the MZI heating arms are 3 mm and 50 µm, respectively. The TO switch polymer waveguide optical field transmission distribution results were simulated by RSoft BeamPROP before and after thermal modulation. The distinct on-state and off-state of the waveguide chip were simulated at 1550 nm, as shown in Figs. 7(c) and 7(d), respectively. The waveguide chip designed with these parameters can successfully realize switching functionality.
Fig. 7. Optical and thermal field simulation results. (a) Optical field distribution state; (b) thermal field distribution state; (c) on-state before thermal modulation; (d) off-state after thermal modulation.
2.3 Waveguide preparation and performance measurement
The specific preparation process for the waveguide TO switch is illustrated in Fig. 8. The spin-coated PMMA layer was cured at 120 °C for 30 min. Later, the Al thin film was deposited onto the PMMA layer by heat evaporation plating in a vacuum. The deposition time and vacuum level were 90 s and 1.1${\times} $10−4 Pa, respectively. Later, the BP-212 photoresist was fabricated on the Al film by spin-coating and prebaked at 90 °C for 20 min to eliminate organic solvents. The Al bottom-cladding layer with self-electrode functional structure was patterned by photolithography and developing. While the Al bottom self-electrode was developing, the photoresist on the exposed region was dissolved in a 5‰ NaOH solution. Then, the remaining BP-212 photoresist on the Al bottom self-electrode was exposed and dissolved in an absolute ethyl alcohol solution. Eventually, the fluorinated polymer AF-Z-PC MA was spin-coated on the Al film at 1000 r/min and cured at 90 °C for 30 min.
The morphologies of the bottom-metal-printed waveguide input segment and electrode, acquired by a standard optical microscope (×50), are illustrated in Figs. 9(a) and 9(b), respectively. The prepared bottom-metal-printed waveguide chip was in accordance with the designed waveguide dimensions, which could decrease the additional loss caused by the preparation process. The waveguide chip prepared by the bottom-metal-printed technique does not need to process the waveguide core material, so it can greatly reduce the extra loss caused by the preparation technology steps and optimize the process steps. The resistance value of the waveguide self-heating electrode heater was 150 Ω. The specific morphologies of the waveguide input segment and branch section were acquired using a scanning electron microscope (SEM) (JSM-7900F), as shown in Figs. 9(c) and 9(d), respectively. The waveguide width was found to be 3 µm, which is consistent with the theoretical design. Therefore, the fabricated waveguide device characteristics were beneficial for reducing device losses from the waveguide structure.
Fig. 9. Micrographs of the waveguide TO switch. (a) and (b) images of the waveguide splitting segments and electrode (×50); (c) and (d) images of the waveguide input and branch sections (×1200).
The actual testing of the waveguide chip coupling detection system is shown in Fig. 10(a). A 1550-nm signal source (Santec TSL-210) was used as the input signal light source, coupled with a tapered optical fiber. The signal output power was acquired by the receiving optical fiber. Then, the optical power and the input/output signal wave signal were measured by an optical power meter (AQQ8203) or a digital oscilloscope (DS4024). The near-infrared field graph of the waveguide chip was collected by a charge-coupled device (CCD) camera. As illustrated in Fig. 10(b), the signal output power was confined to the core layer of the waveguide device. The modulating voltage driving signal from the digital signal generator (SP1642B) was loaded on the self-electrode of the waveguide chip by a probe. Later, the output signal light from the waveguide chip was converted to an electrical signal by a photodetector. After that, the square-wave voltage driving signal produced from the digital signal generator and the modulated square-wave voltage signal generated by the 1550-nm signal source were displayed on the digital oscilloscope.
Fig. 10. (a) Coupling test photograph of the waveguide chip, (b) output near-infrared field image captured by CCD camera.
The waveguide propagation loss of the TO switch chip was measured by the cut-back measurement method and found to be −0.5 dB/cm at 1550 nm wavelength as given in Fig. 11(a). The calculation equation of the cut-back measurement is shown as:
Fig. 11. Actual performances of the waveguide chip. (a) Waveguide propagation loss measured by the cut-back measurement method, (b) output optical power intensity of the waveguide TO switch.
Figure 11(b) illustrates the curve of the output optical power intensity of the waveguide TO switch with respect to electrical power consumption. The extinction ratio of the waveguide chip was found to be 19.8 dB, and the electrical power consumption of the optical phase reversal was 8.8 mW. The waveguide chip electrode fabricated by the bottom-metal-printed technique was directly connected to the waveguide core layer, which could decrease the heat loss in air and increase the proportion of effective heat consumption added to the active region of the waveguide. Therefore, the electrical power consumption of the chip could be greatly reduced, and the thermal efficiency of the device could be improved. In addition, the structure of the waveguide chip prevents the electrode from being oxidized in air, which could prolong the operation life of the device and improve the working performance of the device.
To analyze the output switching on-off characteristics, a square-wave voltage signal was loaded onto the self-electrode heater with a DC bias voltage of 100 mV. The switching on-off time curves are shown in Fig. 12 at 1550 nm. The measuring switching on-off time was found to be 80 µs, which is reasonably fast for a polymer waveguide TO switch. The waveguide prepared by the bottom-metal-printed technique does not need to process the waveguide core material, so it can greatly reduce the extra loss caused by the preparation technology steps and optimize the fabrication method for the waveguide chip. Additionally, the electrode fabricated by the bottom-metal-printed technique is directly connected to the waveguide core layer, which is beneficial for improving the heat transfer of the device. The thickness of PMMA cladding layer is thin enough and the thermal conductivity of silicon substrate is large, which are conducive to accelerating the heat dissipation of the device and improving the response speed of the device.
The parameter values of the structure, on-off time, and driving power of reported polymer TO waveguide devices, including that in our study, are given in Table 1 [34–36]. Based on the comparison results, the proposed waveguide TO switch chip demonstrates a fast on-off time with lower power-time consumption. Compared with the latest work reported [37–39], it can be found that the 80 µs on-off response speed of the polymer TO switch we fabricated is faster. Furthermore, the bottom-metal-printed waveguide technique, without sophisticated technological processes, is advantageous for the realization of integrated photonic circuit systems.
Table 1. Performances comparison with previously reported polymer TO switch chips.
3. Conclusions
In conclusion, a polymer waveguide TO switch for optical communication based on low-loss fluorinated polycarbonate material was proposed and fabricated using the bottom-metal-printed technique. Low-loss fluorinated polycarbonate (AF-Z-PC MA) and PMMA polymer were selected as the core and under cladding layer materials, respectively. The thermal stability and optical transmission loss of AF-Z-PC MA were obtained. The on-off time of the waveguide switching chip was 80 µs. The insertion loss and extinction ratio of the chip were measured to be 4.5 dB and 19.8 dB, respectively. The electrical power consumption was approximately 8.8 mW. The experimental results indicate that the low-loss fluorinated polymer TO switch prepared by the bottom-metal-printed technique demonstrated superior performance, simple technical process, and high production efficiency. This technique is expected to play a significant role in large-scale integrated photonic circuit systems.
Funding
Department of Science and Technology of Jilin Province (20190302010GX); National Key Research and Development Program of China (2019YFB2203001); National Natural Science Foundation of China (61675087, 61875069).
Acknowledgments
The authors would like to thank Dr. Shuxiang Ding and Dr. Zhanchen Cui from College of Chemistry, Jilin University. They provided valuable help with the synthesis of fluorinated polycarbonate materials.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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