- Email: [email protected]
On-chip integrated optical switch based on polymer waveguides
Optical Materials 97 (2019) 109386
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
On-chip integrated optical switch based on polymer waveguides a
a
a
a
a
T a
Minghui Jiang , Daming Zhang , Tianhang Lian , Lilei Wang , Donghai Niu , Changming Chen , Zhiyong Lib, Xibin Wanga,∗ a b
State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699# Qianjin Street, Changchun, 130012, China State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical communication Polymer waveguide Optical switch Electro-optic effect Thermo-optic effect
Optical polymer is one of the promising materials for photonic integrated devices that can be processed with simple, flexible and semiconductor compatible technology. Especially, optical polymer with the advantages of high thermo-optic (TO) and electro-optic (EO) coefficients has been widely applied in optical waveguide switches. However, most of the current optical switches are independent with single function. It is essential to develop an on-chip integrated device with multilayer for signal manipulation. In this paper, we propose an onchip 3-dimensional integrated optical switch based on the EO polymer-clad waveguide incorporate with the TO tunable vertical coupler. The proposed integrated optical switch could realize the function-integration of the adjustable 3D optical connection and the high-speed modulation. A typical fabricated switch shows an insertion loss of 16.3 dB and an extinction ratio of 16.0 dB at the driving power of 48.2 mW. The dynamic characteristics of TO and EO switching functions of the device were also measured in this work.
1. Introduction With the rapid development of optical communication technology, optical fiber communication has become an important method for communication and data transmission. Integrated optical waveguide devices such as optical switches [1,2], optical modulators [3], and optical amplifiers [4,5], etc., have aroused wide concern because of their important roles in optical communication systems. To satisfy the increasing demand for compact and high-density integrated optical devices, the 3-dimensional (3D) integrated photonic structure has become an inevitable trend [6]. To achieve the function of 3D integration, many optical waveguide structures have been proposed, such as the vertically sloped waveguide [7], the microring vertical coupler [8] and the vertical coupling waveguides [9,10]. Among these structures, the vertical coupler has been widely applied in 3D integrated devices due to its flexibility for optical channel selection and the compact size [11,12]. Moreover, the vertical coupler also has important applications for mode-division multiplexing optical interconnects [13,14]. However, most of the current optical devices are independent with single function [15–17]. It is essential to develop an on-chip integrated optical device with multilayer for signal manipulation. The realization of functional integrated device is determined not only by the waveguide structure but also by the waveguide material. Recently, various inorganic and polymer materials have been
∗
developed to fabricate the integrated optical waveguide devices. Compared with inorganic materials, polymer materials have the advantages of low cost, variety and easy processing. Especially, the flexible processing of polymer materials makes them have the unique advantage for fabricating 3D integrated photonic devices. Moreover, polymer materials have relative higher thermo-optic (TO) coefficient and lower thermal conductivity, both of which make them attractive material candidates for realizing high-efficient thermally tunable devices. In addition, the polymer materials also have the advantage of lower dielectric constant and, when doping nonlinear optical chromophores, the synthesized electro-optic (EO) polymer materials present larger EO coefficient [18], which make EO polymer beneficial for the preparation of high-speed EO switches or modulators [19,20]. These excellent properties make the polymer materials a suitable material for the preparation of polymer-based 3D integrated photonic devices with simple fabrication process, low cost, easy integration and high performance. In this paper, we designed and fabricated an on-chip 3D integrated optical switch based on polymeric multilayer waveguides. A vertical coupler was introduced to achieve the optical signal interconnection and switching between the lower waveguide and upper waveguide through the TO effect. In the upper waveguide, a Mach–Zehnder interferometer (MZI) with the EO polymer-clad waveguide was employed to realize the function of high-speed switching or modulating for the
Corresponding author. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.optmat.2019.109386 Received 7 June 2019; Received in revised form 27 August 2019; Accepted 12 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 97 (2019) 109386
M. Jiang, et al.
optical signal. In addition, the quasi-in-plane coplanar waveguide electrodes were introduced and patterned on the surface of the upper waveguide to enhance the poling and modulating efficiency. The EO polymer used in this work is a guest-host material MS-TCF/P(MMAGMA), which was synthesized in our lab. According to the material properties, the waveguide structures and dimensions were carefully designed and simulated, especially for the influences of the crossing angle and the gap thickness between the two waveguide layers on the coupling efficiency of the vertical coupler. Based on the optimized waveguide parameters, we fabricated the device with polymer materials to take advantage of the microfabrication process based on spincoating, photolithography, and inductively coupled plasma (ICP) etching for the construction of the multilayer 3D structures. The microfabrication process allows precise control of the waveguide dimensions, which is essential for achieving good performance.
Fig. 2. (a) Configuration of the device; (b) the top view and (c) the cross-section view of the device.
2. Waveguide materials coupler and transmits in the upper MZI waveguide. When a suitable electric power is applied to the electrode heater, the optical signal can be switched from upper waveguide to lower waveguide. The MZI in the upper layer is an EO polymer-clad waveguide, which consists of an input waveguide, a 3-dB beam splitter, two interference arms, a 3-dB coupler and an output waveguide. The angle of the Y-junction was designed to be 1° to get a low splitting loss. The phase tuning section of the MZI waveguide was 18 mm in length and the separation of the two interference arms was designed to be 35 μm. Meanwhile, the quasi-inplane coplanar waveguide electrodes were employed and formed at the both sides of the MZI arms to enhance the poling and modulating efficiency and achieve the function of the high-speed modulation for the optical signal. For EO polymer switch, the high-speed switching or modulating is mainly due to the EO activity of EO polymer, which is contributed from the movement of the π electrons. Moreover, the smaller difference value of the effective index between microwave and optical wave can also lead to the higher bandwidth and response speed for the device [19,20]. The parameters of the device, including the waveguide dimensions and the crossing angle and gap thickness between two waveguide layers, have important influences on the performances of the device, such as the modulating efficiency and coupling efficiency. We first investigate the influence of the core size of the upper waveguide on the modulating efficiency. For an EO polymer-clad waveguide, the applied electrical field mainly modulates the optical signal by means of modulating the evanescent field leaked into the EO cladding. Therefore, the optical field should be optimized to the EO cladding layer to get a higher modulating efficiency. With the fixed waveguide materials, the optical field distribution of the waveguide with different core size were calculated by the finite-element method (FEM), as shown in the insets of Fig. 3. Fig. 3 shows the relationship between the optical mode in EO cladding and the size of upper core layer. It is clear that the confinement factor of the optical field in the EO cladding layer is decreased with the increase of the waveguide core size. However, the waveguide core with too small size will increase the light-scattering loss. Therefore, the core size was ensured as 2.5 × 2.5 μm2 to provide a trade-off. Based on the fixed core size of the upper waveguide, we further study the influence of the crossing angle and the gap thickness between two waveguide layers on the coupling efficiency of the vertical coupler. The core size of the lower waveguide was also set at 2.5 × 2.5 μm2 to satisfy the phase-matching condition when the heater switch is turned off. Fig. 2(b) and (c) show the top and side view of the vertical coupler. In order to optimize the coupling efficiency of the vertical coupler, the 3D finite-difference beam propagation method (3DFD-BPM, Rsoft) was employed to calculate the coupling efficiency of the device. Fig. 4 shows the relationship between the coupling efficiency and the crossing angle α at different gap thickness Hgap. As expected, the coupling efficiency decreases with the increasing of Hgap and α. At the same time,
In this paper, the epoxy-based negative photoresist SU-8 2002 was selected as the core material, and the cross-linkable polymer poly (methyl-methacrylate-glycidly-methacrylate) (P(MMA-GMA)) with chemical and physical stability properties was selected as the cladding material and the host material of EO polymer. The bisphenol-A epoxy was used as high-refractive index regulator to adjust the refractive index of P(MMA-GMA). The chromophores MS-TCF was synthesized in our lab and doped into the P(MMA-GMA) as the guest material of EO polymer. The formulas of P(MMA-GMA) and MS-TCF are shown in Fig. 1. The physical doping process of the 10 wt% MS-TCF/P(MMAGMA) is as follows. Firstly, MS-TCF was fully dissolved in cyclopentanone and stirred for 5 h. Then the solution was filtered by a disposable filter with 0.22 μm and incorporated with the host material P(MMAGMA) and, then stirred for 24 h. The EO polymer films were prepared to measure the EO coefficient and refractive index. The EO coefficient of the synthesized EO polymer was measured by the reflection method, which was about 20.7 pm/V. The refractive indices of SU-8 and MSTCF/P(MMA-GMA) were measured at 1550 nm by an M − 2000 UI variable angle incidence spectroscopic ellipsometer, which are 1.573 ± 0.001 and 1.503 ± 0.002 respectively. Through regulating the weight percent of bisphenol-A epoxy, the refractive index of P (MMA-GMA) can be adjust to equal that of the EO polymer. 3. Device design and simulation The schematic diagram of the proposed on-chip 3D integrated optical switch is shown in Fig. 2(a), which consists of a vertical coupler and a MZI in the upper layer waveguide. The vertical coupler was introduced to connect the lower waveguide and the upper waveguide to achieve the function of optical signal interconnection and switching between the two-layer waveguides through TO effect. The MZI formed with EO polymer-clad waveguide in the upper layer can realize the function of high-speed switching or modulating for the coupled optical signal from lower layer waveguide. The top-view diagram of the device is shown in Fig. 2(b). An aluminum (Al) electrode heater was deposited along the upper waveguide (i.e., the input waveguide of MZI) to control the coupling efficiency of the vertical coupler, which thus functions as the TO switch. Without applying electric power to the heater, the optical signal is coupled into the upper waveguide through the vertical
Fig. 1. Chemical formula of (a) P(MMA-GMA) and (b) MS-TCF. 2
Optical Materials 97 (2019) 109386
M. Jiang, et al.
Fig. 3. Relationships between the optical mode in EO cladding and the size of upper core layer. The insets show the optical field distribution calculated by finite-element method.
Fig. 6. Optical signal transmission along the propagating direction when (a) and (b) the heater is turned off, and (c) the heater is turned on (the green line is the optical power in upper waveguide and the blue line is the optical power in lower waveguide). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
of the upper waveguide, the temperature underneath will increase and present a gradient distribution. There is a temperature difference between the upper waveguide and the lower waveguide, which will be increased with the increase of the gap thickness. Fig. 5 demonstrates the calculated temperature change in upper and lower waveguides with the increase of the gap thickness. It is clear that the temperature difference between the upper waveguide and lower waveguide will increase with the increase of gap thickness between the waveguides. It is mainly due to the thicker polymer layer below the upper core that is helpful to hinder the transfer of temperature from the upper core layer to the substrate. This means that the power consumption will be reduced with the increase of the gap thickness. Considering the coupling efficiency, power consumption and fabrication tolerances, the gap thickness Hgap and the crossing angle α are chosen to be 2.1 μm and 0.2°. The inset in Fig. 5 shows the steady thermal field distribution. Under this condition, the coupling efficiency can reach to 97.3%. With the above optimized parameters of the device, we simulate the optical signal transmission in the device under different working conditions, as shown in Fig. 6. When there is no electric power applied to the electrode heater, the optical signal is vertically coupled into the MZI waveguide from the lower waveguide, as shown in Fig. 6(a) and (b). Fig. 6(a) shows the optical signal transmission in the upper MZI waveguide, and Fig. 6(b) shows the coupling process between the lower waveguide and upper waveguide (i.e., the optical signal transmission in the section of the square area in Fig. 6(a). On the contrary, when a suitable electric power (~47.3 mW) was applied to the electrode heater, the optical signal can be switched from the upper waveguide to the lower waveguide, as shown in Fig. 6(c).
Fig. 4. Coupling efficiency as a function of gap layer thickness Hgap and crossing angle α.
4. Device fabrication, measurement and discussion Fig. 5. Relationships between the average temperature of core layers and the thickness of middle cladding layers.
The switch was fabricated by using the standard microfabrication technique [21,22]. The detailed fabrication process is shown in Fig. 7. The polymer P(MMA-GMA) was first spin-coated on the SiO2 substrate to form the 5-μm-thick under-cladding. A group of 2.5-μm-wide and 2.5-μm-deep grooves were formed on the under-cladding by the photolithography (ABM Co. Inc., USA) and inductively coupled plasma (ICP) etching (CE-300I, ULVAC Co. Inc, Japan) process. Then the low-
reducing the coupling times between the upper and lower waveguide could increase the process tolerance of the device. However, the gap thickness also has an influence on the power consumption of the coupler switch. When electric power is applied to the heater on the surface 3
Optical Materials 97 (2019) 109386
M. Jiang, et al.
Fig. 7. Fabrication process of the 3D integrated optical switch.
electrodes were simultaneously fabricated by photolithography and wet-etching processes. Fig. 8 shows the scanning electron micrograph (SEM) image of the cross section of the fabricated device. To obtain the function of EO modulation, the chromophore molecular was aligned via the contact-poling process. The device was firstly purged with nitrogen (N2) at 110 °C for 30 min to remove the dissolved oxygen from the polymer. Then the device was poled at 135 °C for 20 min in N2 atmosphere, and the applied poling voltage on the device was about 700 V. The performances of the fabricated device were characterized on a planar optical waveguide testing system. A signal light at 1550 nm from a tunable semiconductor laser (TSL-210, Santec) was coupled into the input port of the lower waveguide via a single-mode fiber (SMF). The output light from the upper waveguide of the device was focused onto an infrared camera with a lens to obtain the near-field image or coupled to an optical power meter through a SMF to measure the optical power. When there is no electric power applying to the heater, the light beam is output from the upper waveguide. The insertion loss of the device was about 16.3 dB, which includes the coupling loss between the device and the fiber, vertical coupling loss, propagation loss, bending loss and splitting loss. It should be possible to reduce the loss of the device by improving the fabrication process and using low-loss polymer materials and polishing the end face of the waveguides. When a suitable electric power is applied to the heater via two probes, the optical signal was switched from the upper waveguide into the lower waveguide. Fig. 9(a) shows the variation of the relative output optical power from the upper waveguide with the driving electric power measured at 1550 nm. As shown in Fig. 9(a), the output optical power decreases with an increase in the driving electric power and the maximum drop in the output optical power is 16.0 dB, which occurs at a driving power of 48.2 mW. Under this condition, the insertion loss of the lower waveguide was measure to be 12.4 dB. To measure the response times of the vertical coupler switch, a square-wave signal at 200 Hz was applied to the electrode heater via two probes and the output light was monitored with a photodetector through a SMF. The output optical signal from the device and the electrical signal from the function generator were simultaneously monitored by an oscilloscope (4104B, Tektronix). As shown in Fig. 9 (b), the upper trace is the square-wave form of the function generator, and the lower trace is the optical response form of
Fig. 8. SEM image of the cross section of the fabricated vertical coupler.
loss polymer SU-8 2002 was spin-coated on the grooves to form the core layer. After the pre-baking process (60 °C for 10 min, 90 °C for 20 min), the coated SU-8 2002 film was exposed to UV light and, then the sample was post-baked (65 °C for 10 min, 95 °C for 20 min) and hard-baked (150 °C for 20 min). After removing the slab layer on the top surface of under-cladding by ICP etching method, 2.1-μm-thick P(MMA-GMA) film was spin-coated on the sample to form the middle cladding (i.e., upper-cladding of the lower waveguide and the under-cladding of the upper waveguide) and baked at 120 °C for 3 h. Next, SU-8 2002 was spin-coated on the P(MMA-GMA) layer and baked at 60 °C for 10 min and at 90 °C for 20 min to form a 2.5-μm-thick film. The SU-8 2002 film was then etched into 2.5-μm-wide strips by the standard photolithography and ICP etching processes. Finally, the synthesized EO material MS-TCF/P(MMA-GMA) was spin-coated on the core layer and baked at 120 °C for 3 h to form a 2-μm-thick upper-cladding of the upper waveguide. After fabricating the waveguides, 0.2-μm-thick Al film was deposited on the upper-cladding by the thermal evaporation, and the electrode heater and the quasi-in-plane coplanar waveguide 4
Optical Materials 97 (2019) 109386
M. Jiang, et al.
Fig. 9. (a) Normalized output optical power form the upper waveguide versus the driving electric power; (b) oscilloscopic display of the waveforms of the input electric signal (upper trace) and the output optical power signal (lower trace).
Fig. 10. The switching response of the device on a rectangular wave.
the device. The rise time and fall time are 750.6 and 741.7 μs, respectively. Therefore, the optical signal can be switched between upper waveguide and the lower waveguide with an extinction ratio of 16.0 dB at a switching power of 48.2 mW, and the corresponding switching time is about 750 μs When the optical signal is coupled into and transmits in the upper waveguide with EO polymer cladding, it can be modulated by the applied electrical field with a fast response. To demonstrate this function, a square-wave signal with 100 kHz produced by the function generator was applied to the quasi-in-plane coplanar waveguide electrodes with two probes, and the output optical power was coupled into a photodiode detector through a SMF. The driving voltage of the switch and the detected optical response were simultaneously observed on an oscilloscope (4104B, Tektronix), as illustrated in Fig. 10. From Fig. 10, the upper trace is the square wave form of the switching voltage source, and the lower trace is the switching response from the output port of the upper waveguide. The rise time and fall time are 10.87 and 9.54 ns, respectively. As shown above, the optical signal can be switched between the upper waveguide and the lower waveguide through the TO tunable vertical coupler, and also be modulated
through EO effect with a fast response time.
5. Conclusion In conclusion, we have designed and fabricated an on-chip 3D integrated optical switch based on polymer waveguides. The vertical coupler was introduced to achieve the optical signal interconnection and switching between the lower waveguide and upper waveguide through TO effect. A guest-host EO material MS-TCF/P(MMA-GMA) was successfully synthesized and employed as the upper-cladding of the upper MZI waveguide. Meanwhile, the quasi-in-plane coplanar waveguide electrodes were utilized to achieve the high-speed modulation of the switched optical signal from the lower waveguide. The characteristic parameters of the waveguides, electrode heater and modulating electrodes were carefully designed and optimized. The switch was fabricated by the conventional microfabrication process including spincoating, photolithography, dry and wet etching procedures. After poling, the device was measured on the planar waveguide testing system. A typical fabricated switch shows an insertion loss of 16.3 dB at 5
Optical Materials 97 (2019) 109386
M. Jiang, et al.
1550 nm. By applying a driving power of 48.2 mW on the vertical coupler, the optical signal can be switched from the upper waveguide to the lower waveguide, and the extinction ratio is about 16.0 dB. The measured rise time and fall time of TO response are 750.6 and 741.7 μs, respectively. Moreover, the coupled optical signal from lower waveguide into upper waveguide can be modulated at high speed. Under an applied 100 KHz square-wave modulating signal, device presents a fast response time (rise time and fall time are 10.87 and 9.54 ns, respectively). The demonstrated 3D integrated optical switch could realize the functions of adjustable 3D optical connection and the high-speed modulation simultaneously. The proposed integrated waveguide structure and the fabrication process could be developed into other polymer-based photonic integrated circuit and is also suitable for application in the polymer/silica hybrid integrated devices.
[4] M. Zhang, W. Zhang, F. Wang, D. Zhao, C. Qu, X. Wang, Y. Yi, E. Cassan, D. Zhang, High-gain polymer optical waveguide amplifiers based on core-shell NaYF4/ NaLuF4: Yb3+, Er3+ NPs-PMMA covalent-linking nanocomposites, Sci. Rep. 6 (9) (2016) 36729. [5] D. Zhang, X. Li, X. Huang, S. Liu, H. Fu, K. Che, L. Wang, Optical amplification at 1064 nm in Nd(TTA)3(TPPO)2 complex doped SU-8 polymer waveguide, IEEE Photon. J. 7 (5) (2015) 1–7. [6] K. Chen, P. Chu, K. Chiang, H. Chan, Design and fabrication of a broadband polymer vertically coupled optical switch, J. Light. Technol. 24 (2) (2006) 904–911. [7] K.H. Kim, M.S. Kwon, S.Y. Shin, D.S. Choi, Vertical digital thermooptic switch in polymer, IEEE Photonics Technol. Lett. 16 (3) (2004) 783–785. [8] Z.L. Lu, Efficient fiber-to-waveguide coupling through the vertical leakage from a microring, Opt. Lett. 32 (19) (2007) 2861–2863. [9] H. Liang, V.S. Kumar, J.F. Wu, M.B.H. Breese, Ion beam irradiation induced fabrication of vertical coupling waveguides, Appl. Phys. Lett. 102 (13) (2013) 131112. [10] K. Chen, P. Chu, K. Chiang, H. Chan, Design and fabrication of a three-dimensional polymer optical waveguide polarization splitter, Opt. Commun. 250 (2005) 297–301. [11] N. Keil, H.H. Yao, C. Zawadzki, K. Lösch, K. Satzke, W. Wischmann, J.V. Wirth, J. Schneider, J. Bauer, M. Bauer, Hybrid polymer/silica vertical coupler switch with < –32dB polarisation-independent crosstalk, Electron. Lett. 37 (2) (2001) 89–90. [12] K. Chen, P. Chu, H. Chan, K. Chiang, Three-dimensional broadband polymer optical waveguide switch matrix, Appl. Opt. 46 (33) (2007) 8188–8192. [13] G. He, Y. Gao, Y. Xu, L.T. Ji, X. Sun, X. Wang, Y. Yi, C. Chen, F. Wang, D. Zhang, Y. Wu, Design and fabrication of three-dimensional polymer mode multiplexer based on asymmetric waveguide couplers, J. Opt. 20 (5) (2018) 1–9. [14] J. Dong, K. Chiang, W. Jin, Compact three-dimensional polymer waveguide mode multiplexer, J. Light. Technol. 33 (22) (2015) 4580–4588. [15] Y.F. Yan, Z.J. Cao, C.T. Zheng, L. Jin, X.B. Wang, F. Wang, C.S. Ma, D.M. Zhang, Nanosecond bias-free mach-zehnder electro-optic switch based on dye-doped polymer DR1/SU-8 and microstrip line electrode, Fiber Integr. Opt. 31 (4) (2012) 250–262. [16] N. Xie, T. Hashimoto, K. Utaka, Very low power operation of compact MMI polymer thermooptic switch, IEEE Photonics Technol. Lett. 21 (18) (2009) 1335–1337. [17] K.X. Chen, P.L. Chu, H.P. Chan, A vertically coupled polymer optical waveguide switch, Opt. Commun. 244 (1–6) (2005) 153–158. [18] J. Luo, S. Huang, Y.J. Cheng, T.D. Kim, Z. Shi, X. Zhou, A.K.Y. Jen, Phenyltetraenebased nonlinear optical chromophores with enhanced chemical stability and electrooptic activity, Org. Lett. 9 (22) (2007) 4471–4474. [19] M. Lee, H.E. Katz, C. Erben, D.M. Gill, P. Gopalan, J.D. Heber, D.J. Mcgee, Broadband modulation of light by using an electro-optic polymer, Science 298 (5597) (2002) 1401–1403. [20] J. Liu, G. Xu, F. Liu, I. Kityk, X. Liu, Z. Zhen, Recent advances in polymer electrooptic modulators, RSC Adv. 5 (21) (2015) 15784–15794. [21] X.B. Wang, M.H. Jiang, S.Q. Sun, J.W. Sun, Y.J. Yi, C.M. Chen, X.Q. Sun, F. Wang, Z.C. Cui, D.M. Zhang, Demonstration of a high-speed electro-optic switch with passive-to-active integrated waveguide based on SU-8 material, RSC Adv. 6 (55) (2016) 50166–50172. [22] W. Jin, K.S. Chiang, Mode converters based on cascaded long-period waveguide gratings, Opt. Lett. 41 (13) (2007) 3130–3133.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 61875069, 61575076 and 61405070), the National Key Research and Development Plan of China (Grant No. 2016YFB0402502), Hong Kong Scholars Program (No. XJ2016026), China Postdoctoral Science Special Foundation (No. 2016T90252), the Science and Technology Development Plan of Jilin Province (Nos. 20190302010GX, 20160520091JH), and the Fundamental Research Funds for the Central Universities (2019jcxk-59). References [1] N. Xie, T. Hashimoto, K. Utaka, Design and performance of low-power, high-speed, polarization-independent and wideband polymer buried-channel waveguide thermo-optic switches, J. Light. Technol. 32 (17) (2014) 3067–3073. [2] A. Densmore, S. Janz, R. Ma, J.H. Schmid, D.X. Xu, A. Delâge, J. Lapointe, M. Vachon, P. Cheben, Compact and low power thermo-optic switch using folded silicon waveguides, Opt. Express 17 (13) (2009) 10457–10465. [3] G.L. Li, I. Tutorial, P.K.L. Yu, Optical intensity modulators for digital and analog applications, J. Light. Technol. 21 (9) (2003) 2010–2030.
6
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
On-chip integrated optical switch based on polymer waveguides a
a
a
a
a
T a
Minghui Jiang , Daming Zhang , Tianhang Lian , Lilei Wang , Donghai Niu , Changming Chen , Zhiyong Lib, Xibin Wanga,∗ a b
State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699# Qianjin Street, Changchun, 130012, China State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical communication Polymer waveguide Optical switch Electro-optic effect Thermo-optic effect
Optical polymer is one of the promising materials for photonic integrated devices that can be processed with simple, flexible and semiconductor compatible technology. Especially, optical polymer with the advantages of high thermo-optic (TO) and electro-optic (EO) coefficients has been widely applied in optical waveguide switches. However, most of the current optical switches are independent with single function. It is essential to develop an on-chip integrated device with multilayer for signal manipulation. In this paper, we propose an onchip 3-dimensional integrated optical switch based on the EO polymer-clad waveguide incorporate with the TO tunable vertical coupler. The proposed integrated optical switch could realize the function-integration of the adjustable 3D optical connection and the high-speed modulation. A typical fabricated switch shows an insertion loss of 16.3 dB and an extinction ratio of 16.0 dB at the driving power of 48.2 mW. The dynamic characteristics of TO and EO switching functions of the device were also measured in this work.
1. Introduction With the rapid development of optical communication technology, optical fiber communication has become an important method for communication and data transmission. Integrated optical waveguide devices such as optical switches [1,2], optical modulators [3], and optical amplifiers [4,5], etc., have aroused wide concern because of their important roles in optical communication systems. To satisfy the increasing demand for compact and high-density integrated optical devices, the 3-dimensional (3D) integrated photonic structure has become an inevitable trend [6]. To achieve the function of 3D integration, many optical waveguide structures have been proposed, such as the vertically sloped waveguide [7], the microring vertical coupler [8] and the vertical coupling waveguides [9,10]. Among these structures, the vertical coupler has been widely applied in 3D integrated devices due to its flexibility for optical channel selection and the compact size [11,12]. Moreover, the vertical coupler also has important applications for mode-division multiplexing optical interconnects [13,14]. However, most of the current optical devices are independent with single function [15–17]. It is essential to develop an on-chip integrated optical device with multilayer for signal manipulation. The realization of functional integrated device is determined not only by the waveguide structure but also by the waveguide material. Recently, various inorganic and polymer materials have been
∗
developed to fabricate the integrated optical waveguide devices. Compared with inorganic materials, polymer materials have the advantages of low cost, variety and easy processing. Especially, the flexible processing of polymer materials makes them have the unique advantage for fabricating 3D integrated photonic devices. Moreover, polymer materials have relative higher thermo-optic (TO) coefficient and lower thermal conductivity, both of which make them attractive material candidates for realizing high-efficient thermally tunable devices. In addition, the polymer materials also have the advantage of lower dielectric constant and, when doping nonlinear optical chromophores, the synthesized electro-optic (EO) polymer materials present larger EO coefficient [18], which make EO polymer beneficial for the preparation of high-speed EO switches or modulators [19,20]. These excellent properties make the polymer materials a suitable material for the preparation of polymer-based 3D integrated photonic devices with simple fabrication process, low cost, easy integration and high performance. In this paper, we designed and fabricated an on-chip 3D integrated optical switch based on polymeric multilayer waveguides. A vertical coupler was introduced to achieve the optical signal interconnection and switching between the lower waveguide and upper waveguide through the TO effect. In the upper waveguide, a Mach–Zehnder interferometer (MZI) with the EO polymer-clad waveguide was employed to realize the function of high-speed switching or modulating for the
Corresponding author. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.optmat.2019.109386 Received 7 June 2019; Received in revised form 27 August 2019; Accepted 12 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 97 (2019) 109386
M. Jiang, et al.
optical signal. In addition, the quasi-in-plane coplanar waveguide electrodes were introduced and patterned on the surface of the upper waveguide to enhance the poling and modulating efficiency. The EO polymer used in this work is a guest-host material MS-TCF/P(MMAGMA), which was synthesized in our lab. According to the material properties, the waveguide structures and dimensions were carefully designed and simulated, especially for the influences of the crossing angle and the gap thickness between the two waveguide layers on the coupling efficiency of the vertical coupler. Based on the optimized waveguide parameters, we fabricated the device with polymer materials to take advantage of the microfabrication process based on spincoating, photolithography, and inductively coupled plasma (ICP) etching for the construction of the multilayer 3D structures. The microfabrication process allows precise control of the waveguide dimensions, which is essential for achieving good performance.
Fig. 2. (a) Configuration of the device; (b) the top view and (c) the cross-section view of the device.
2. Waveguide materials coupler and transmits in the upper MZI waveguide. When a suitable electric power is applied to the electrode heater, the optical signal can be switched from upper waveguide to lower waveguide. The MZI in the upper layer is an EO polymer-clad waveguide, which consists of an input waveguide, a 3-dB beam splitter, two interference arms, a 3-dB coupler and an output waveguide. The angle of the Y-junction was designed to be 1° to get a low splitting loss. The phase tuning section of the MZI waveguide was 18 mm in length and the separation of the two interference arms was designed to be 35 μm. Meanwhile, the quasi-inplane coplanar waveguide electrodes were employed and formed at the both sides of the MZI arms to enhance the poling and modulating efficiency and achieve the function of the high-speed modulation for the optical signal. For EO polymer switch, the high-speed switching or modulating is mainly due to the EO activity of EO polymer, which is contributed from the movement of the π electrons. Moreover, the smaller difference value of the effective index between microwave and optical wave can also lead to the higher bandwidth and response speed for the device [19,20]. The parameters of the device, including the waveguide dimensions and the crossing angle and gap thickness between two waveguide layers, have important influences on the performances of the device, such as the modulating efficiency and coupling efficiency. We first investigate the influence of the core size of the upper waveguide on the modulating efficiency. For an EO polymer-clad waveguide, the applied electrical field mainly modulates the optical signal by means of modulating the evanescent field leaked into the EO cladding. Therefore, the optical field should be optimized to the EO cladding layer to get a higher modulating efficiency. With the fixed waveguide materials, the optical field distribution of the waveguide with different core size were calculated by the finite-element method (FEM), as shown in the insets of Fig. 3. Fig. 3 shows the relationship between the optical mode in EO cladding and the size of upper core layer. It is clear that the confinement factor of the optical field in the EO cladding layer is decreased with the increase of the waveguide core size. However, the waveguide core with too small size will increase the light-scattering loss. Therefore, the core size was ensured as 2.5 × 2.5 μm2 to provide a trade-off. Based on the fixed core size of the upper waveguide, we further study the influence of the crossing angle and the gap thickness between two waveguide layers on the coupling efficiency of the vertical coupler. The core size of the lower waveguide was also set at 2.5 × 2.5 μm2 to satisfy the phase-matching condition when the heater switch is turned off. Fig. 2(b) and (c) show the top and side view of the vertical coupler. In order to optimize the coupling efficiency of the vertical coupler, the 3D finite-difference beam propagation method (3DFD-BPM, Rsoft) was employed to calculate the coupling efficiency of the device. Fig. 4 shows the relationship between the coupling efficiency and the crossing angle α at different gap thickness Hgap. As expected, the coupling efficiency decreases with the increasing of Hgap and α. At the same time,
In this paper, the epoxy-based negative photoresist SU-8 2002 was selected as the core material, and the cross-linkable polymer poly (methyl-methacrylate-glycidly-methacrylate) (P(MMA-GMA)) with chemical and physical stability properties was selected as the cladding material and the host material of EO polymer. The bisphenol-A epoxy was used as high-refractive index regulator to adjust the refractive index of P(MMA-GMA). The chromophores MS-TCF was synthesized in our lab and doped into the P(MMA-GMA) as the guest material of EO polymer. The formulas of P(MMA-GMA) and MS-TCF are shown in Fig. 1. The physical doping process of the 10 wt% MS-TCF/P(MMAGMA) is as follows. Firstly, MS-TCF was fully dissolved in cyclopentanone and stirred for 5 h. Then the solution was filtered by a disposable filter with 0.22 μm and incorporated with the host material P(MMAGMA) and, then stirred for 24 h. The EO polymer films were prepared to measure the EO coefficient and refractive index. The EO coefficient of the synthesized EO polymer was measured by the reflection method, which was about 20.7 pm/V. The refractive indices of SU-8 and MSTCF/P(MMA-GMA) were measured at 1550 nm by an M − 2000 UI variable angle incidence spectroscopic ellipsometer, which are 1.573 ± 0.001 and 1.503 ± 0.002 respectively. Through regulating the weight percent of bisphenol-A epoxy, the refractive index of P (MMA-GMA) can be adjust to equal that of the EO polymer. 3. Device design and simulation The schematic diagram of the proposed on-chip 3D integrated optical switch is shown in Fig. 2(a), which consists of a vertical coupler and a MZI in the upper layer waveguide. The vertical coupler was introduced to connect the lower waveguide and the upper waveguide to achieve the function of optical signal interconnection and switching between the two-layer waveguides through TO effect. The MZI formed with EO polymer-clad waveguide in the upper layer can realize the function of high-speed switching or modulating for the coupled optical signal from lower layer waveguide. The top-view diagram of the device is shown in Fig. 2(b). An aluminum (Al) electrode heater was deposited along the upper waveguide (i.e., the input waveguide of MZI) to control the coupling efficiency of the vertical coupler, which thus functions as the TO switch. Without applying electric power to the heater, the optical signal is coupled into the upper waveguide through the vertical
Fig. 1. Chemical formula of (a) P(MMA-GMA) and (b) MS-TCF. 2
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Fig. 3. Relationships between the optical mode in EO cladding and the size of upper core layer. The insets show the optical field distribution calculated by finite-element method.
Fig. 6. Optical signal transmission along the propagating direction when (a) and (b) the heater is turned off, and (c) the heater is turned on (the green line is the optical power in upper waveguide and the blue line is the optical power in lower waveguide). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
of the upper waveguide, the temperature underneath will increase and present a gradient distribution. There is a temperature difference between the upper waveguide and the lower waveguide, which will be increased with the increase of the gap thickness. Fig. 5 demonstrates the calculated temperature change in upper and lower waveguides with the increase of the gap thickness. It is clear that the temperature difference between the upper waveguide and lower waveguide will increase with the increase of gap thickness between the waveguides. It is mainly due to the thicker polymer layer below the upper core that is helpful to hinder the transfer of temperature from the upper core layer to the substrate. This means that the power consumption will be reduced with the increase of the gap thickness. Considering the coupling efficiency, power consumption and fabrication tolerances, the gap thickness Hgap and the crossing angle α are chosen to be 2.1 μm and 0.2°. The inset in Fig. 5 shows the steady thermal field distribution. Under this condition, the coupling efficiency can reach to 97.3%. With the above optimized parameters of the device, we simulate the optical signal transmission in the device under different working conditions, as shown in Fig. 6. When there is no electric power applied to the electrode heater, the optical signal is vertically coupled into the MZI waveguide from the lower waveguide, as shown in Fig. 6(a) and (b). Fig. 6(a) shows the optical signal transmission in the upper MZI waveguide, and Fig. 6(b) shows the coupling process between the lower waveguide and upper waveguide (i.e., the optical signal transmission in the section of the square area in Fig. 6(a). On the contrary, when a suitable electric power (~47.3 mW) was applied to the electrode heater, the optical signal can be switched from the upper waveguide to the lower waveguide, as shown in Fig. 6(c).
Fig. 4. Coupling efficiency as a function of gap layer thickness Hgap and crossing angle α.
4. Device fabrication, measurement and discussion Fig. 5. Relationships between the average temperature of core layers and the thickness of middle cladding layers.
The switch was fabricated by using the standard microfabrication technique [21,22]. The detailed fabrication process is shown in Fig. 7. The polymer P(MMA-GMA) was first spin-coated on the SiO2 substrate to form the 5-μm-thick under-cladding. A group of 2.5-μm-wide and 2.5-μm-deep grooves were formed on the under-cladding by the photolithography (ABM Co. Inc., USA) and inductively coupled plasma (ICP) etching (CE-300I, ULVAC Co. Inc, Japan) process. Then the low-
reducing the coupling times between the upper and lower waveguide could increase the process tolerance of the device. However, the gap thickness also has an influence on the power consumption of the coupler switch. When electric power is applied to the heater on the surface 3
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Fig. 7. Fabrication process of the 3D integrated optical switch.
electrodes were simultaneously fabricated by photolithography and wet-etching processes. Fig. 8 shows the scanning electron micrograph (SEM) image of the cross section of the fabricated device. To obtain the function of EO modulation, the chromophore molecular was aligned via the contact-poling process. The device was firstly purged with nitrogen (N2) at 110 °C for 30 min to remove the dissolved oxygen from the polymer. Then the device was poled at 135 °C for 20 min in N2 atmosphere, and the applied poling voltage on the device was about 700 V. The performances of the fabricated device were characterized on a planar optical waveguide testing system. A signal light at 1550 nm from a tunable semiconductor laser (TSL-210, Santec) was coupled into the input port of the lower waveguide via a single-mode fiber (SMF). The output light from the upper waveguide of the device was focused onto an infrared camera with a lens to obtain the near-field image or coupled to an optical power meter through a SMF to measure the optical power. When there is no electric power applying to the heater, the light beam is output from the upper waveguide. The insertion loss of the device was about 16.3 dB, which includes the coupling loss between the device and the fiber, vertical coupling loss, propagation loss, bending loss and splitting loss. It should be possible to reduce the loss of the device by improving the fabrication process and using low-loss polymer materials and polishing the end face of the waveguides. When a suitable electric power is applied to the heater via two probes, the optical signal was switched from the upper waveguide into the lower waveguide. Fig. 9(a) shows the variation of the relative output optical power from the upper waveguide with the driving electric power measured at 1550 nm. As shown in Fig. 9(a), the output optical power decreases with an increase in the driving electric power and the maximum drop in the output optical power is 16.0 dB, which occurs at a driving power of 48.2 mW. Under this condition, the insertion loss of the lower waveguide was measure to be 12.4 dB. To measure the response times of the vertical coupler switch, a square-wave signal at 200 Hz was applied to the electrode heater via two probes and the output light was monitored with a photodetector through a SMF. The output optical signal from the device and the electrical signal from the function generator were simultaneously monitored by an oscilloscope (4104B, Tektronix). As shown in Fig. 9 (b), the upper trace is the square-wave form of the function generator, and the lower trace is the optical response form of
Fig. 8. SEM image of the cross section of the fabricated vertical coupler.
loss polymer SU-8 2002 was spin-coated on the grooves to form the core layer. After the pre-baking process (60 °C for 10 min, 90 °C for 20 min), the coated SU-8 2002 film was exposed to UV light and, then the sample was post-baked (65 °C for 10 min, 95 °C for 20 min) and hard-baked (150 °C for 20 min). After removing the slab layer on the top surface of under-cladding by ICP etching method, 2.1-μm-thick P(MMA-GMA) film was spin-coated on the sample to form the middle cladding (i.e., upper-cladding of the lower waveguide and the under-cladding of the upper waveguide) and baked at 120 °C for 3 h. Next, SU-8 2002 was spin-coated on the P(MMA-GMA) layer and baked at 60 °C for 10 min and at 90 °C for 20 min to form a 2.5-μm-thick film. The SU-8 2002 film was then etched into 2.5-μm-wide strips by the standard photolithography and ICP etching processes. Finally, the synthesized EO material MS-TCF/P(MMA-GMA) was spin-coated on the core layer and baked at 120 °C for 3 h to form a 2-μm-thick upper-cladding of the upper waveguide. After fabricating the waveguides, 0.2-μm-thick Al film was deposited on the upper-cladding by the thermal evaporation, and the electrode heater and the quasi-in-plane coplanar waveguide 4
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Fig. 9. (a) Normalized output optical power form the upper waveguide versus the driving electric power; (b) oscilloscopic display of the waveforms of the input electric signal (upper trace) and the output optical power signal (lower trace).
Fig. 10. The switching response of the device on a rectangular wave.
the device. The rise time and fall time are 750.6 and 741.7 μs, respectively. Therefore, the optical signal can be switched between upper waveguide and the lower waveguide with an extinction ratio of 16.0 dB at a switching power of 48.2 mW, and the corresponding switching time is about 750 μs When the optical signal is coupled into and transmits in the upper waveguide with EO polymer cladding, it can be modulated by the applied electrical field with a fast response. To demonstrate this function, a square-wave signal with 100 kHz produced by the function generator was applied to the quasi-in-plane coplanar waveguide electrodes with two probes, and the output optical power was coupled into a photodiode detector through a SMF. The driving voltage of the switch and the detected optical response were simultaneously observed on an oscilloscope (4104B, Tektronix), as illustrated in Fig. 10. From Fig. 10, the upper trace is the square wave form of the switching voltage source, and the lower trace is the switching response from the output port of the upper waveguide. The rise time and fall time are 10.87 and 9.54 ns, respectively. As shown above, the optical signal can be switched between the upper waveguide and the lower waveguide through the TO tunable vertical coupler, and also be modulated
through EO effect with a fast response time.
5. Conclusion In conclusion, we have designed and fabricated an on-chip 3D integrated optical switch based on polymer waveguides. The vertical coupler was introduced to achieve the optical signal interconnection and switching between the lower waveguide and upper waveguide through TO effect. A guest-host EO material MS-TCF/P(MMA-GMA) was successfully synthesized and employed as the upper-cladding of the upper MZI waveguide. Meanwhile, the quasi-in-plane coplanar waveguide electrodes were utilized to achieve the high-speed modulation of the switched optical signal from the lower waveguide. The characteristic parameters of the waveguides, electrode heater and modulating electrodes were carefully designed and optimized. The switch was fabricated by the conventional microfabrication process including spincoating, photolithography, dry and wet etching procedures. After poling, the device was measured on the planar waveguide testing system. A typical fabricated switch shows an insertion loss of 16.3 dB at 5
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1550 nm. By applying a driving power of 48.2 mW on the vertical coupler, the optical signal can be switched from the upper waveguide to the lower waveguide, and the extinction ratio is about 16.0 dB. The measured rise time and fall time of TO response are 750.6 and 741.7 μs, respectively. Moreover, the coupled optical signal from lower waveguide into upper waveguide can be modulated at high speed. Under an applied 100 KHz square-wave modulating signal, device presents a fast response time (rise time and fall time are 10.87 and 9.54 ns, respectively). The demonstrated 3D integrated optical switch could realize the functions of adjustable 3D optical connection and the high-speed modulation simultaneously. The proposed integrated waveguide structure and the fabrication process could be developed into other polymer-based photonic integrated circuit and is also suitable for application in the polymer/silica hybrid integrated devices.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 61875069, 61575076 and 61405070), the National Key Research and Development Plan of China (Grant No. 2016YFB0402502), Hong Kong Scholars Program (No. XJ2016026), China Postdoctoral Science Special Foundation (No. 2016T90252), the Science and Technology Development Plan of Jilin Province (Nos. 20190302010GX, 20160520091JH), and the Fundamental Research Funds for the Central Universities (2019jcxk-59). References [1] N. Xie, T. Hashimoto, K. Utaka, Design and performance of low-power, high-speed, polarization-independent and wideband polymer buried-channel waveguide thermo-optic switches, J. Light. Technol. 32 (17) (2014) 3067–3073. [2] A. Densmore, S. Janz, R. Ma, J.H. Schmid, D.X. Xu, A. Delâge, J. Lapointe, M. Vachon, P. Cheben, Compact and low power thermo-optic switch using folded silicon waveguides, Opt. Express 17 (13) (2009) 10457–10465. [3] G.L. Li, I. Tutorial, P.K.L. Yu, Optical intensity modulators for digital and analog applications, J. Light. Technol. 21 (9) (2003) 2010–2030.
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