Open Access
Add to BibSonomy
Abstract
In this work, thermo-optic tunable 4 × 4 cascaded multimode interference based integrated optical waveguide switching matrices are designed and fabricated using photopolymer lightwave circuits. The materials of the waveguide core and cladding are fluorinated epoxy-terminated copolycarbonate and polymethylmethacrylate, respectively. The driving power that controls matrices for binary encoding of different optical switching states are simulated and analyzed. The measured insertion loss of the actual chip is < 7.1 dB and the maximum crosstalk in adjacent channels is <−30 dB. The switching time is approximately 220 μs and the extinction ratio is obtained as 21.5 dB. This flexible encoding technique can be applied for achieving optical code-division multiple-access network coders.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical switching networks as an effective solution to replace electrical switching networks are attracting more attention in reconfigurable communication systems including cloud computing, high-performance computers, and large datacenters [1–3]. At present, free-space optics and planar optics are the two main types used for achieving various optical cross-connect switching networks. Free-space optical switching devices can provide the switching matrix with large numbers of ports by 3D micro-electro-mechanical systems (MEMS) and free-space beam-steering [4–7]. However, the response time of the device is slow and commonly limited to millisecond-level. Furthermore, the device is bulky and the packaging technology is complicated and expensive. Compared to free-space optical switching structures, planar optical switching networks with higher compactness, complex integration, and fast signal response are more suitable for a monolithic multi-functional photonic module with good compatibility and high efficiency. Particularly, the thermo-optic (TO) switching matrix device with a planar optical waveguide structure is more useful for realizing on-chip planar lightwave circuits as a key part in optical code-division multiple-access (OCDMA) network systems [8,9]. Owing to the ability to operate asynchronously, with increased capacity and enhanced privacy, these highly integrated optical modules for the OCDMA techniques play an increasingly crucial role in contemporary society. Certain typical waveguide structures are used as basic elements of the TO switching matrix, predominantly including Mach–Zehnder interference [10,11], X junction [12], micro-ring resonance [13,14], multimode interference (MMI) [15], and so on. Compared to other types, the MMI waveguide optical switches with more ports can have higher stability, larger fabrication tolerance, and greater feasibility for integration [16,17]. At present, III–V [18,19], silicon [20,21], and polymer waveguide optical switching matrix technologies have been developed and used as wavelength division multiplexing system [22,23]. Compared to III–V and silicon waveguide device, highly flexible structures, well-controlled refractive indices, large thermo-optic (TO) and electro-optic (EO) coefficients are the merits of polymer waveguide chips [24], which are more suitable for realizing monolithic integration in high-speed optical routing networks. Specially, contrast to typical inorganic optical switching matrices, the polymer switching matrices with low cost, low power consumption and high compatibility might be more advantageous for applications to realize Fiber to The Home (FTTH) categories of optical connecting network based on Plastic Optical fiber (POF) system [25,26]. In addition, compared with ordinary Mach-Zehnder interference (MZI) or directional coupling (DC) unit structure, the MMI unit switching matrix can provide more compact architecture and more shorter interactive region between electrode and waveguide based on self-image principle. The MMI switching units might be suitable for achieving large-scale and high-efficient optical matrix system for multi-functional photonic integrated modules.
In this paper, novel TO tunable 4 × 4 cascaded MMI-based integrated fluorinated photopolymer optical waveguide switching matrices are proposed. The epoxy-terminated copolycarbonate (AF-Z-PC EP) and polymethylmethacrylate (PMMA) are the materials used for the core and cladding layers of the waveguide, respectively. The driving power that controls the matrices for binary encoding of different optical switching states is defined. The actual structural properties of the MMI waveguide parts and electrode heater unit region are obtained. The performances of the switching matrices are measured. This technique is suitable for achieving OCDMA network coders.
2. Design and experiments
2.1 Fluorinated photopolymer waveguide material
Fluorinated bis-phenol-A novolac resin (FSU-8) was used as the refractive index (RI) modifier, and diphenyl iodonium salt was added as a photoacid generator to the core waveguide material. The FSU-8 was doped into the AF-Z-PC EPs solution to adjust the RI of the core material. Owing to the heavier fluorine atoms, the vibration overtone absorption signals shifted to a longer wavelength; therefore, replacing C-H bonds with C-F bonds in the polymers could effectively reduce the vibration overtone absorption loss of the materials [27]. The fluorinated photopolymers with cross-linked epoxy structure offered low adsorption loss, high thermal coefficient, and good thermal stability for the integrated optical waveguide device. The molecular structures of AF-Z-PC EP and FSU-8 are shown in Figs. 1(a) and 1(b), respectively. Furthermore, compared with SU-8 2000, as the commercial epoxy photoresist with only C-H bonds [28], the absorption spectrum of the AF-Z-PC EP combined with 20% FSU-8 is illustrated in Fig. 1(c). It can be observed that the fluorinated photopolymer waveguide material exhibits lower absorption characteristics in visible (vis) and near-infrared region (NIR).
Fig. 1 Molecular structure and absorption spectrum of the waveguide material (a) molecular structure of AF-Z-PC EP, (b) molecular structure of FSU-8, and (c) visible and near-infrared absorption spectrum of the waveguide material compared with commercial SU-8 2000.
2.2 Device structure
The cross-sectional structure of the designed fluorinated photopolymer waveguide is illustrated in Fig. 2(a). The polymer waveguide was formed with direct UV writing technique on SiO2 buffer layer; further, it was deposited on the Si substrates lower cladding. The PMMA with bis-phenol-A epoxy resin as RI modifier was spin-coated and thermal-cured on the waveguide forming the upper cladding. The Al electrode heaters were patterned by evaporation and UV lithography process. The thickness of the SiO2 film was 2 μm. Both the width and height of the core waveguide were 3 μm. The thickness of the PMMA upper cladding was obtained as 5 μm. The RIs of the polymer waveguide core and upper cladding were measured as 1.542 and 1.495 at approximately 1550 nm wavelength, respectively. The RI of SiO2 lower cladding was approximately 1.46. Based on the effective index method, the transverse magnetic (TM) mode optical field distribution for the waveguide was simulated by COMSOL software, which is illustrated in Fig. 2(b). It was observed that the size of the waveguide could support single-mode propagation condition at 1550 nm wavelength. The thermal field distribution of Al electrode heater is shown in Fig. 2(c), which was calculated based on the thermal changing profiles in cross-section of the waveguide. The thermo-optic coefficients of the fluorinated photopolymer and PMMA could come up to −1.8 × 10−4 °C−1 and −1.6 × 10−4 °C−1, respectively.
Fig. 2 Structural and optical characteristics of the waveguide and electrode heaters (a) cross-sectional structure of the polymer waveguide, (b) TM-mode optical field distribution for the polymer waveguide, and (c)thermal field distribution of Al electrode heater.
The schematic configuration of integrated optical waveguide cascaded MMI-based 4 × 4 switching matrix is illustrated in Fig. 3. The switching matrix device was composed of five identical cascaded MMI couplers. The entire length of the device was approximately 1 cm. As functional units, each MMI coupler could be controlled by the corresponding electrode heater for achieving channel selection switching characteristic.
Fig. 3 Schematic diagram of the integrated optical waveguide cascaded MMI-based switching matrix module.
The operation of optical MMI coupler is based on the self-imaging principle. In the general interference mechanism, Lπ as the beat length of two lowest-order modes can be defined as
where β is the propagation constant of signal wavelength, neff is the effective refractive index of the waveguide, wem is the effective width of MMI coupler, and λ0 is the free space wavelength.The transfer phases of N × N MMI coupler [29] are set as
where Lmmi = 3Lπ/N, which is the shortest length of MMI coupler. N is the number of input/output ports. i and l represent the ith input channel and lth output channel, respectively.In experiment, the proposed switching matrix is composed of 2 × 2 TO switching MMI coupler units. If φ(2,2) represents the starting phase of the original MMI coupler and φ(2,1) shows the ending phase after thermo-optic modulation, the phase shift ∆Φ can be given as
where Le is the effective operating length of the electrode heater, is the TO coefficient, ΔT is the effective temperature change for the waveguide area, and k = 2π/λ0. When ∆Φ = π/2, the state of MMI unit can be changed from Cross to Bar, which realizes the switching function.The detailed relationship between the effective refractive index of waveguide and heating temperature is shown as Fig. 4. It could be observed that the effective refractive index Neff of the waveguide for fundamental mode decreases with temperature increasing. The slope is obtained as −1.5 × 10−4 °C−1. It could be found that effective TO coefficient for the waveguide can come to same magnitude with that value of polymer waveguide material. It could be guaranteed that the high-efficient TO response characteristics of the chip can be achieved well.
The structural parameters of the simple MMI unit were optimized as illustrated in Fig. 5(a). The length and width of the MMI region were 1918 μm and 36 μm, respectively. Similar to output channels, the spacing between the adjacent input channels for MMI was 12 μm. The width at the beginning of the tapered waveguide structure was 7 μm and the ending was 3 μm. The length and width of the electrode heaters were 300 μm and 18 μm, respectively. The relationship of the varying output efficiency between 1st and 2nd output channels of the MMI coupler unit based on different electrode lengths with the increasing power of the heater is shown as Fig. 5(b). It was observed that when the electrode length (Le) was chosen as 300 μm, the extinction ratio between the 1st and 2nd output channels of the unit could reach the maximum value with less than 20 mW power of the electrode heater.
Fig. 5 MMI switching unit defined (a) optimized structural parameters of the simple MMI waveguide and (b) optimal length of the electrode heater simulated.
2.3 Analysis and simulation of encoding functions of the switching matrix
The schematic diagram of the cascaded MMI-based switching matrix with specific location markers is illustrated in Fig. 6. Here, O1, O2, O3, and O4 represent the four output channels. I1, I2, I3, and I4 describe the four input channels, respectively. E1, E2, E3, E4, and E5 represent the electrode heaters acting on the different MMI switching units, respectively. P1, P2, P3, P4, and P5 represent the electric powers loaded on the corresponding electrode heaters, respectively.
When the optical signal is only coupled into I1 channel, A1 is defined to indicate that the optical power is only delivered from one output channel, A2 indicates that equal optical power is produced from two output channels, A3 is from three output channels, and A4 is from four output channels. The encoding schemes of An (n = 1, 2, 3, and 4) related vectors and the corresponding TO tuning transmission light field of the switching matrix are shown in Fig. 7. Similarly, the output results when the optical signal is only coupled into I2, I3, or I4 channels are noted as Bn, Cn, or Dn, respectively. The corresponding vector tables and light-field transmission conditions of Bn, Cn, and Dn are illustrated in Figs. 8, 9, and 10, respectively.
Fig. 7 Encoding schemes of An vectors for the TO tuning transmission light field of the switching matrix.
Fig. 8 Encoding schemes of Bn vectors for the TO tuning transmission light field of the switching matrix.
Fig. 9 Encoding schemes of Cn vectors for the TO tuning transmission light field of the switching matrix.
Fig. 10 Encoding schemes of Dn vectors for the TO tuning transmission light field of the switching matrix.
The related driving power controlling matrices as An, Bn, Cn, and Dn are presented as Eqs. (5), (6), (7), and (8), respectively.
It was found that there were transferring matrices existing between An and Dn, and between Bn and Cn, respectively. The formulae are presented as Based on the order of the matrix, sharing transformation matrices were found. We defined K1, K2, and K3 as the encoding key matrices when the different operating states of the switching matrix transferred to each other. The related forms of the matrices are presented asThe tunable encoding functions of the cascaded MMI-based switching matrix could solve the critical issues for the data security in the OCDMA system network.2.4 Results and discussion
The detailed fabrication process of the switching matrix device was similar to our previous work [30,31].The actual profile micrographs of the waveguide and electrode region were measured by an optical microscope as shown in Figs. 11(a)–11(c), and by a scanning electron microscope (SEM) as given in and Figs. 11(d). The top views of cascaded output regional and center MMI units are illustrated in Figs. 11(a) and 11(b). It was found that the MMI waveguide structural size designed could be achieved suitably. The partial interactional region between the electrode heater and MMI waveguide is shown in Fig. 11(c). The interactive width of the electrode is 18 μm which matches with the value designed. The input and output width sections of the electrode are set as 50 μm, which are convenient to load lead wires. The side walls of the electrodes were smooth and the structural size of the heaters were controlled satisfactorily. The resistance of the unit electrode was measured as 145 Ω. The cross section of the input waveguide is illustrated in Fig. 10(d). The ridge wall of the waveguide was almost vertical, and the core size could be adequately defined. The interface between core and cladding layer is very clear and there is not any dissolution phenomenon. Furthermore, the surface roughness of the cross-linked fluorinated photopolymer is measured only as 0.393 nm by AFM. All these characteristics are advantageous to reduce scattering loss generated from waveguide structure.
Fig. 11 Actual profile micrographs of the waveguide and electrode region: (a) and (b) images of cascaded output regional and center MMI units measured by optical microscope ( × 500), (c) partial interactional regional images between the electrode heater and MMI waveguide measured by optical microscope ( × 1000), (d) SEM images of the cross-section for the input waveguide.
The actual photograph of the optical coupling testing system using integrated cascaded MMI-based switching matrix chip is illustrated in Fig. 12(a). The 1550 nm signal wavelength from a tunable laser (Santec TSL-210, Japan) was coupled into the chip by an input single-mode fiber, and then the output optical signal was collected by another single-mode fiber into either an optical power meter or an infrared photodetector connected with an oscilloscope (Agilent 86100D, USA). The tunable encoding functions of the device were managed by the electrical controlling system (ECS) through electrode heaters. The five MMI-based switching units were operated by corresponding electrical signals. The entire ECS designed by us consisted of two parts, rectification filter circuits and regulator circuits. Their circuit diagrams are shown in Figs. 12(b) and 12(c). The rectification filter circuits provided ± 24 V DC into the regulator circuit, which consisted of LM317 and LM337 parts, through + Vcc and -Vcc ports by the rectification of the bridge circuit and the filtering effect of the bypass capacitor. The two filter capacitors, C6 and C7, in parallel with small capacitors could eliminate the bypass noise. The variable resistors, VR1, VR2, VR3, and VRA, in the regulator circuit guaranteed high-precision adjustment of the output voltage within the range of 0-24 V DC. By encoding the output electrical signals obtained from five sets of regulator circuits, the corresponding vectors of the switching matrix in different operating states could be achieved. For instance, when the electrode heaters were loaded with electrical signals corresponding to the driving power controlling matrix A1, the related output near-infrared fields of the chip were measured directly by an infrared CCD camera with lens ( × 80) as shown in Figs. 13(a) and 13(b). We determined that the encoding group A1 as [1000], [0100], [0010], and [0001] for the chip can be achieved as shown in Fig. 13(a). The values of P1, P3, and P5 as the driving powers obtained were close to the simulated value of 22 mW. The crosstalk between O1 and O2 was measured as about −32.2 dB and the value between O3 and O4 was obtained as −31.5 dB. The maximum crosstalk in adjacent channels was less than −30 dB. The taper structures of the channel waveguide ports and the spacing distance between O1 and O2, O3 and O4 as only 12 μm tested could reduce the coupling loss between MMI region and channel waveguides, but might also result in increasing crosstalk value to some extent. The comparison of the single-channel output, obtained by encoding A1units, with the other output near-infrared fields obtained by encoding A2 [0011] with P5 = 11.6 mW; encoding A3 [1011] with P3 = 8.6 mW and P5 = 11.2 mW; encoding A3 [1110] with P1 = 13.6 mW, P4 = 11.5 mW, and P5 = 22.3 mW; encoding A4 [1111] with P1 = 11.2 mW, P4 = 11.7 mW, and P5 = 11.5 mW are illustrated in Fig. 13(b). It was demonstrated that the TO tunable encoding function of the chip could be satisfactorily achieved.
Fig. 12 (a)Actual optical coupling testing system photograph of integrated cascaded MMI-based switching matrix chip, (b) circuit diagrams of the rectification filter circuits,(c)circuit diagrams of regulator circuits.
Fig. 13 Output near-infrared fields of the chip measured by infrared CCD camera with lens ( × 80) (a) encoding group A1 and (b) encoding A2[0011], A3[1011], and A4[1111].
To analyze switching response characteristics, small square wave voltage signals with a frequency of 400 Hz generated from the signal generator were loaded on the electrode heater corresponding to E5 followed by 150 mV DC bias for encoding A2 [1001] state. The TO switching response curves are illustrated in Fig. 14(a). The switching time including rise and fall times was close to 220 μs. At the same encoding of A2 [1001] state, when varied DC power P5 was continuously loaded on the corresponding electrode heaters, the relationship between the driving electrical power and different channel output optical power is shown in Fig. 14(b). It was determined that the encoding of A2 [1001] could be transferred to A2 [1010] state when the value of P5 approaches to 22.5 mW. The extinction ratio was measured as 21.5 dB. The insertion loss of the channel O1 remained around 7.1 dB. Then, to study the operating stability of the chip, the relationship curves between output powers and wavelengths were measured as illustrated in Fig. 14(c). When the input optical power was set as 1 mW, the output optical powers for four encoding state A1 [1000], B1 [1000], C1 [1000], and D1 [1000] from the same O1 channel were measured in C-band (1530–1565 nm).The varying value of the insertion loss of the chip for the same state and channel was less than 1 dB in C-band. Furthermore, the curve for the insertion loss of the chip with changing temperature is shown in Fig. 14(d). The temperature controller was fixed in direct contact with the substrate of the chip. When the switching matrices were set as the encoding state A2 [1100], the output power of O2 channel was measured in the temperature range from 20 to 55 °C. The change of the insertion loss was less than 1.5 dB during this temperature range.
Fig. 14 Actual performances of the chip measured (a) TO switching response curves, (b) relationship between driving electrical power and different channel output optical power, (c) relationship curves between output powers and wavelengths, and (d) insertion loss of the chip with temperature changed.
3. Conclusions
In conclusion, novel TO tunable 4 × 4 cascaded MMI-based integrated optical waveguide switching matrices were proposed and achieved using fluorinated photopolymer lightwave circuits. The materials of the waveguide core and cladding were AF-Z-PC EP and PMMA, respectively. The binary encoding optical switching matrix states of the chip were analyzed and defined by controlling the driving power of the corresponding electrodes. The actual waveguide and electrode structure of the chip was characterized. The insertion loss of the actual chip was measured to be less than 7.1 dB and the maximum crosstalk in adjacent channels was less than −30 dB. The switching time was close to 220 μs and the extinction ratio was obtained as 21.5 dB. The stability of the chip for the wavelength signal and changing temperature was confirmed. The encoding functions of the switching matrix were achieved. This cascaded MMI-based switching matrix encoding technique can be adapted and applied for achieving OCDMA network coders.
Funding
National Key R&D Program of China (2016YFB0402502); National Natural Science Foundation of China (No. 61575076, 61675087); Jilin Provincial Industrial Innovation Special Fund Project (2016C019); Jilin Province Education Department “13th Five-Year” Science and Technology Research Project (JJKH20180157KJ).
Acknowledgments
The authors wish to thank Mingyang Li and Zheng Miao. They are postgraduates in Circuits and Systems from the College of Electronic Science and Engineering, Jilin University. They provided valuable help in achieving driving circuits in the testing system.
References
1. N. Dupuis, B. G. Lee, A. V. Rylyakov, D. M. Kuchta, C. W. Baks, J. S. Orcutt, D. M. Gill, W. M. J. Green, and C. L. Schow, “Modeling and Characterization of a Nonblocking4 × 4 Mach–Zehnder Silicon Photonic Switch Fabric,” J. Lightwave Technol. 33(20), 4329–4337 (2015). [CrossRef]
2. Z. Wu, J. Li, D. Ge, F. Ren, P. Zhu, Q. Mo, Z. Li, Z. Chen, and Y. He, “Demonstration of all-optical MDM/WDM switching for short-reach networks,” Opt. Express 24(19), 21609–21618 (2016). [CrossRef] [PubMed]
3. R. S. Tucker, “Green optical communications-Part II Energy limitations in network,” IEEE J. Sel. Top. Quantum Electron. 17(2), 261–274 (2011). [CrossRef]
4. M. Safari, C. Shafai, and L. Shafai, “X-Band Tunable Frequency Selective Surface UsingMEMS Capacitive Loads,” IEEE Trans. Antenn. Propag. 63(3), 1014–1021 (2015). [CrossRef]
5. L. Pelliccia, F. Cacciamani, P. Farinelli, and R. Sorrentino, “High-Tunable Waveguide Filters Using OhmicRF MEMS Switches,” IEEE Trans. Microw. Theory Tech. 63(10), 3381–3390 (2015). [CrossRef]
6. N. Vahabisani and M. Daneshmand, “Monolithic Millimeter-Wave MEMSWaveguide Switch,” IEEE Trans. Microw. Theory Tech. 63(2), 340–351 (2015). [CrossRef]
7. F. Kehl, G. Etlinger, T. E. Gartmann, N. S. R. U. Tscharner, S. Heub, and S. Follonier, “Introduction of an angle interrogated, MEMS-based, opticalwaveguide grating system for label-free biosensing,” Sens. Actuators B Chem. 226, 135–143 (2016). [CrossRef]
8. M. Jelinek, “Functional planar thin film optical waveguide lasers,” Laser Phys. Lett. 9(2), 91–99 (2012). [CrossRef]
9. J. Liu and L. Tao, “Influence of Parametric Uncertainties on Narrow Width Bandpass Optical Filter of Prism Pair Coupled Planar Optical Waveguide,” IEEE J. Quantum Electron. 53(3), 6200105 (2017). [CrossRef]
10. L. Lu, L. Zhou, Z. Li, X. Li, and J. Chen, “Broadband 4×4 Nonblocking SiliconElectrooptic Switches Based onMach–Zehnder Interferometers,” IEEE Photonics J. 7(1), 7800108 (2015). [CrossRef]
11. L. Yang, Y. Xia, F. Zhang, Q. Chen, J. Ding, P. Zhou, and L. Zhang, “Reconfigurable nonblocking 4-port silicon thermo-optic optical router based on Mach-Zehnder optical switches,” Opt. Lett. 40(7), 1402–1405 (2015). [CrossRef] [PubMed]
12. L. Liang, K. Zhang, C. T. Zheng, X. Zhang, L. Qin, Y. Q. Ning, D. M. Zhang, and L. J. Wang, “N × NReconfigurable Nonblocking Polymer/Silica Hybrid Planar Optical Switch Matrix Based on Total-Internal-Reflection Effect,” IEEE Photonics J. 9(4), 4904711 (2017). [CrossRef]
13. J. N. Roy and J. K. Rakshit, “Design of micro-ring resonator-based all-optical logic shifter,” Opt. Commun. 312, 73–79 (2014). [CrossRef]
14. G. Fan, R. Orobtchouk, B. Han, Y. Li, and H. Li, “8 x 8 wavelength router of optical network on chip,” Opt. Express 25(20), 23677–23683 (2017). [CrossRef] [PubMed]
15. H. Yang, P. Zheng, P. Liu, G. Hu, B. Yun, and Y. Cui, “Design of polarization-insensitive 2×2 multimode interference coupler based on double strip silicon nitride waveguides,” Opt. Commun. 410, 559–564 (2018). [CrossRef]
16. Y. Tian, J. Qiu, Z. Huang, Y. Qiao, Z. Dong, and J. Wu, “On-chip integratable all-optical quantizer using cascaded step-size MMI,” Opt. Express 26(3), 2453–2461 (2018). [CrossRef] [PubMed]
17. H.-D. Kenchington Goldsmith, M. Ireland, P. Ma, N. Cvetojevic, and S. Madden, “Improving the extinction bandwidth of MMI chalcogenide photonic chip -based MIR nullinginterferometers,” Opt. Express 25(14), 16813–16824 (2017). [CrossRef] [PubMed]
18. D. Melati, A. Waqas, A. Alippi, and A. Melloni, “Wavelength and composition dependence of the thermo-optic coefficient for InGaAsP-based integrated waveguides,” J. Appl. Phys. 120(21), 213102 (2016). [CrossRef]
19. M. Nikoufard, M. K. Alamouti, and S. Pourgholi, “Multimode Interference Power-Splitter Using InP-Based Deeply Etched Hybrid Plasmonic Waveguide,” IEEE Trans. NanoTechnol. 16(3), 477–483 (2017). [CrossRef]
20. K. J. Miller, K. A. Hallman, R. F. Haglund Jr., and S. M. Weiss, “Silicon waveguide optical switch with embedded phase change material,” Opt. Express 25(22), 26527–26536 (2017). [CrossRef] [PubMed]
21. V. Kumar and V. Priye, “3-D Multilayer S-Bend Silicon WaveguideOptical Interconnect,” IEEE Photonics Technol. Lett. 30(11), 1040–1043 (2018). [CrossRef]
22. Z. Zhang and N. Keil, “Thermo-optic devices on polymer platform,” Opt. Commun. 362, 101–114 (2016). [CrossRef]
23. M.-C. Oh, W.-S. Chu, J.-S. Shin, J.-W. Kim, K.-J. Kim, J.-K. Seo, H.-K. Lee, Y.-O. Noh, and H.-J. Lee, “Polymeric optical waveguide devices exploiting special properties of polymer materials,” Opt. Commun. 362, 3–12 (2016). [CrossRef]
24. R. Dangel, A. La Porta, D. Jubin, F. Horst, N. Meier, M. Seifried, and B. J. Offrein, “Polymer Waveguides Enabling Scalable Low-Loss Adiabatic Optical Coupling for Silicon Photonics,” IEEE J. Sel. Top Quant. 24, 8200211 (2018). [CrossRef]
25. Y. Okamoto, Q. Du, K. Koike, F. Mikeš, T. C. Merkel, Z. He, H. Zhang, and Y. Koike, “New amorphous perfluoro polymers: perfluorodioxolane polymers for use as plastic optical fibers and gas separation membranes,” Polym. Adv. Technol. 27(1), 33–41 (2016). [CrossRef]
26. A. Inoue and Y. Koike, “Low-Noise Graded-Index Plastic Optical Fiber for Significantly Stable and Robust Data Transmission,” J. Lightwave Technol. 36(24), 5887–5892 (2018). [CrossRef]
27. Z. Cai, B. Wang, Y. Zheng, M. Li, Y. Li, C. Chen, D. Zhang, Z. Cui, and Z. Shi, “Novel fluorinated polycarbonate negative-type photoresists for thermo-optic waveguide gate switch arrays,” J. Mater. Chem. C Mater. Opt. Electron. Devices 4(3), 533–540 (2016). [CrossRef]
28. http://www.microchem.com/Prod-SU82000.htm.
29. C.-D. Truong, D.-H. Tran, T.-A. Tran, and T.-T. Le, “3×3 Multimode interference optical switches using electro-optic effects as phase Shifters,” Opt. Commun. 292, 78–83 (2013). [CrossRef]
30. C. Chen, X. Niu, C. Han, Z. Shi, X. Wang, X. Sun, F. Wang, Z. Cui, and D. Zhang, “Monolithic multi-functional integration of ROADM modules based on polymer photonic lightwave circuit,” Opt. Express 22(9), 10716–10727 (2014). [CrossRef] [PubMed]
31. C. Chen, X. Niu, C. Han, Z. Shi, X. Wang, X. Sun, F. Wang, Z. Cui, and D. Zhang, “Reconfigurable optical interleaver modules with tunable wavelength transfer matrix function using polymer photonics lightwave circuits,” Opt. Express 22(17), 19895–19911 (2014). [CrossRef] [PubMed]
